A review on the current status of Fe-Al based ferritic lightweight steel

Shivkumr Khple ,Brhm Rju Goll ,V.V.Sty Prsd

aDefence Metallurgical Research Laboratory,Hyderabad,India

b Metallurgical and Materials Engineering Department,National Institute of Technology,Warangal,India

Keywords:Low-density steels Disordered Fe-Al Thermo-mechanical processing Microstructure Properties Structure-property correlation

ABSTRACT There is an ever-growing demand for lightweighting of steel for structural applications,particularly for automobile and transportation applications.It is mainly to improve the fuel efficiency,reduce the CO2 emissions and cater the increased passenger safety.Hence,the main focus is to reduce the density of the steel structure without affecting other properties.This can be achieved by down-gauging of the conventional steel by replacing the steel with higher strength,however,it is limited by dent resistance and stiffness.So,the novel idea is to reduce the density of the steel itself.It is well-known that addition of Al to steel reduces the density of the steel.About 1wt% of Al addition to steel can reduce the density by 1.3%,decreases the elastic modulus by 2% and it improves the strength by about 40 MPa.There is a new class of low-density/lightweight steel with addition of about 6-9 wt% Al to steel.Addition of higher than 9 wt% of Al in steel leads to embrittlement issues due to ordering and environmental effect.These disordered Fe-Al lightweight steels have raised considerable interest due to their low-density,high ductility,costeffectiveness and feasibility for bulk production.The low-density steels are envisaged in the development of an advanced lightweight ground transportation system,huge structures and also for certain defence applications and in thermal power plants.

1.Introduction

Steels are an important class of engineering materials that are produced in large tonnages.In view of their attractive properties such as high strength,good formability,recycling and economical aspects,they are suitable for various engineering applications including automobile,aerospace,rail,marine,structural,heavy duty industrial equipment and machinery and energy sectors etc.[1-5].Despite the evolution of lightweight materials (such as Al and Mg alloys,and various polymer,ceramic and composite materials)and their heavy competition with ferrous materials,yet the steels have been in growing demand because of their excellent combination of properties.Nevertheless,the high density of steels and its poor corrosion and oxidation behaviour restricted some of its widespread applications.Therefore,there is a need and urgent pressure on the steel industry/producers to innovate new grades of steels with lower density and without compromising on its desirable properties.

Requirement for lightweighting of steel for structural application has ever increased,particularly for automobile and transportation applications.This is due to the increasing demand to improve fuel efficiency,reduce the CO2(thus contribute in controlling the global warming),enhance strength to weight ratio,make material suitable for environmental conditions and cater to the increased safety of the passenger [4,5].The major focus of lightweight steel is mainly to reduce the density of the structure without affecting other properties.This can be achieved by downgauging of the conventional steel by the steel with higher strength and stiffness and finding newer steel compositions as per the requirements.However,the down-gauging of the steel sheet is limited by dent resistance and stiffness that it offers.So,the novel idea is to reduce the density of the steel itself.The density of the steel can be reduced by alloying with the addition of light weight elements like Li,Mg,Al,Si etc.Or lighter materials (such as ceramics) as alloying or reinforcement agent.Considering its strong effect on the density reduction,improvement in strength and other engineering aspects such as alloy making and workability,Al has emerged as the single most important alloying element in the development of low-density bulk steels [4,6].

Disordered Fe-Al alloys are an emerging class of low-density/lightweight steels containing 6-9 wt% of aluminium in steel.These Fe-Al alloys have raised considerable interest due to their low-density,high ductility,cost-effectiveness and feasibility for bulk production.These low-density and high strength materials are envisaged in the development of an advanced lightweight ground transportation system,huge structures like bridges,tunnels and also being deemed for certain defence applications like troops carrier and armour etc.[7].Alternatively,they are also considered as potential candidates for steam turbines in thermal power plants[1,8-10].

The Fe-Al ferritic steels received relatively less attention in the early steel literature,probably because of their unappealing combination of strength and ductility [1,2].In the recent past,the various research groups investigated Fe-7 wt% Al lightweight steels(which is single phase ferritic steel among the group of Fe-Al steels) in terms of alloy making (air induction melting with flux cover(AIMFC)etc.),adopting hot/cold rolling and other mechanical working methods to produce steels with better properties [11,12].Nevertheless,these alloys depict large grain size (about 1 mm) in cast condition.They exhibited about 18% tensile elongation and yield strength of 300 MPa at room temperature in the hot-rolled condition.However,the strength and ductility values are still low as compared to other competitive structural materials [1,4,13].Hence,there is a need to further improve the room temperature tensile properties and formability of these lightweight steels.

Al is a strong ferrite stabilizer.Addition of Al to Fe diminishes the austenitic loop and enlarges the ferrite area up to melting point.Addition of above 1 wt% Al to Fe does not show any phase transformation (hardenability) and is basically ferritic in solid solution range.The ferritic Fe-Al steels are generally cold rolled and heat treatment temperature is in the range from 700°C to 1000°C for recrystallisation annealing to control the grain size and texture of the ferrite matrix as well as the precipitation of (κ-carbides).

This paper reviews thoroughly the work done on ferritic lightweight steels.It also presents the developments that are taking place in the lightweight steels [1,4,8,9]more particularly the lightweighting of the components in automobile sector.The present review initially attempts to introduce the general considerations of lightweight steels,and the concepts involved in lightweightening/lowering the density of steel structures.This is followed by a comprehensive review of Fe-Al system,the microstructural phases and different types of low-density or lightweight steels.The advancements that are taking place in the areas of alloy design (effect of alloying addition such as diborides,carbides,nitrides etc.),melting approach,thermo-mechanical processing,effect of microstructural modification etc.Are presented.This is accompanied by highlights on use of recent advancements in the lightweight steels to improve mechanical properties.Some of the important application properties of these lightweight steels are brought out.Finally,the conclusions will bring forth significance of lightweight steels and throw light on future direction in the development of Fe-Al based lightweight steels.

2.General considerations of lightweight steels

Steels are engineering materials that are produced in very large quantities.They find application in various engineering sectors such as automotive,marine,aerospace,construction of buildings,machinery and heavy equipment etc.[1,14].Despite the evolution of lightweight materials such as Al and Mg alloys,plastics,and various competing composite materials,the steels still have been in great demand and extensively used for countless applications because of their excellent combination of strength,formability,affordability,and recyclability [6,15]. However,the density of steel is much higher as compared to the light materials as mentioned above.

3.Importance of lightweightening/lowering the density of steel structures

Requirement for lightweightening of steel for structural applications has increased,particularly for automobile applications.It is reckoned that about 10% reduction in weight of the vehicle can improve the fuel efficiency by 7%.This also implies that there is a reduction of 20 kg of CO2emission for every kilogram reduction in the weight of the vehicle[5,16].Fig.1 explains the concepts used for reducing the weight of the steel part.This can be achieved by down-gauging of the conventional steel by using the steel with higher strength and stiffness (Fig.1(a)).However,the downgauging of the steel sheet is limited by dent resistance and stiffness that the steel offers.Another novel idea is to reduce the density of the steel itself for the given thickness (Fig.1(b)).

Fig.1.The concepts of lightweightening (a) down-gauging of steel with higher strength steel and (b) reducing the density of the steel itself for given thickness.

3.1.Lightweightening and improving strength of steels

In the last three decades,automobile industry witnessed the development of various grades of steel primarily focusing on the improvement of strength and formability,so that the downgauging of the same steel parts can be made.Steels developed in automobile industry based on their strength can be classified as(a)Low-strength steels such as mild steel and interstitial free(IF)steel,(b)Conventional high-strength steels(HSS)such as bake hardening(BH),carbon-manganese (C-Mn) and high strength low alloy(HSLA) steel and (c)Advanced high-strength steels (AHSS)such as dual phase (DP),complex phase (CP),transformation-induced plasticity (TRIP),twinning-induced plasticity (TWIP) and martensitic steels [17].Fig.2 shows the famous banana curve of various steels.

Fig.2.The famous banana curve of various steels used in automobile industry [17].

3.2.Lightweightening by reducing the density of the steel itself

Lightweightening of the steel or reduction in density of the steel can be achieved by alloying the steel with low density alloying elements such as Li(0.534 g/cm3),Mg(1.738 g/cm3),C (2.26 g/cm3),Si(2.33 g/cm3),Al(2.70 g/cm3),Mn(7.21 g/cm3),and Cr(7.19 g/cm3)etc.These elements are expected to contribute to the reduction in density of the steels by affecting the lattice parameter of the steel and also due to their lower atomic masses.However,there is no solubility of lighter elements like Li and Mg in Fe.These elements are highly reactive and flammable.Fig.3 illustrates the density reduction in steel by alloying with lighter elements.For example,addition of 12wt% Al to Fe reduces the density by 17%,of which 10% is caused by lattice dilatation and 7% is contributed by reduction in atomic mass[18,19].Each wt% of Al gives the reduction in density of about 1.3%.For the alloying elements such as C,Si,Al and Mn,a linear decrease in the density of the steel was reported.Above 2% of carbon,graphite phase starts formation,which reduces the strength.Addition of more than 7% of Al in steel tends to form short range ordering,where the ductility starts degrading because of the ordering effect.The effect of Si is similar to Al but almost at half the quantity,which means that at above 3.5 wt% of Si in steel,ordered phase would form to hinder the ductility.The contribution to the density reduction by the addition of Mn and Cr is comparatively less,but these elements increase the strength of the steel by solid solution strengthening in the matrix.

Fig.3.The density reduction in steel by alloying with lighter elements [18,19,28].

Considering its strong effect on density reduction and improvement in strength as well as other engineering aspects such as alloy making and workability,Al emerged as the single most important alloying element in the development of lightweight bulk steels.In all the low density/light weight steels,Al is the main element employed for reducing the density.In some cases,Si is also added along with Al.Although Al can be added in low-density ferritic steels up to 11 wt% [20-23],it has been suggested that the Al content should be restricted to less than about 6.5 wt%[15,21]due to the propensity for short-range ordering at higher Al contents,which increases brittleness and adversely affects formability [2,13,15,21-24]. Carbon and manganese are austenite stabilizers which are added in Fe-Al alloys to get the austenitic phase which is very important in the phase transformation (heat treatment) of steels.Concurrent addition of C and Mn to Fe-Al alloy leads to the stabilization of austenite phase at low temperature.Addition of carbon in Fe-Al leads to the formation of κ-carbide which is an intermetallic phase,it improves strength but leads to reduction in ductility.The addition of Mn to Fe-Al-C leads to the drastic improvement in strength due to solid solution strengthening but reduces ductility due to the formation of bulky κ-carbides.These steels are prone to cracking due to the bulky κ-carbides during hot and cold working [25-27].

4.Types of low-density or lightweight steels

Based on the temperature and the relative amount of the alloying additions(Al,C and Mn),the matrix phase of the steel can be ferritic,austenitic or a mixture of both these phases [4,29].Hence,based on the microstructural features,the low-density steels are categorised into four types namely ferritic steels,austenitic steels,ferritic based duplex steel and austenitic based duplex steels [2,6,11,30-36].Table 1 shows the composition range and broad tensile properties of the four types of lightweight/lowdensity steels.The ferritic steels are high in Al whereas the austenitic steels are high in carbon and manganese.In comparison to ferritic steels,low density steels with higher Mn and Al content have a more diverse microstructure and superior mechanical properties.

Table 1Showing the composition range,microstructure and tensile properties of different types of lightweight/low-density steels [4].

(i)Ferritic low-density/lightweight steelshave high amount of Al (up to 12%),Mn up to 4% and low carbon.These steels generally exhibit elongated ferritic microstructure at the hot working temperature.At room temperature,based on the Al and C content,these steels show generally ferritic structure along with minor amount of κ carbide precipitates.

Among all the low-density steels,ferritic steels are comparatively easy to work with the conventional process of hot working and cold working and basically cold worked sheets can be produced of these steels.The ferritic steels have tensile properties similar to that of conventional high-strength low-alloy steels(HSLA)and with an added advantage of density reduction.These steels are also recognised as the first generation of advanced high-strength steel[2,6,11,13,31,33,34,37].

(ii)Austenitic low-density/lightweight steels have high amount of Mn (12% to 30%) and carbon (up to 2%).It can also accommodate high amount of Al (up to 12%) [38-41].These steels show single austenitic phase at the hot working temperature.These steels have mainly austenitic phase with traces of ferrite,κ-carbide and β-Mn phase.In the cast condition,these steels show dendritic microstructure comprising a mixture of austenite and ferrite because of the large amount of segregation caused by the significant quantity of alloying elements.The homogenisation is generally carried out at a reheating temperature of 1100-1250°C for adequate time.The recrystalised microstructure contains equiaxed austenitic grains with annealing twins[42-44].The slow cooling of these steels results in the formation of κ-carbides as well as ferritic phases which most likely appear along the austenitic grain boundaries and also inside the matrix of austenite.These κ-carbides are about 3-6 times bigger in size compared to those resulting from age hardening process which reduces the ductility and toughness.Hence,these steels are generally water quenched from solution temperature (900-1100°C) to avoid the precipitation of coarse κ-carbide.Aging of austenitic Fe-Mn-Al-C steels,in a temperature range of 500-900°C,two types of κ-carbides can be formed,namely intergranular and intragranular κ-carbides.The morphology of these carbides drastically affects the properties of the steel.

The fine κ-carbide precipitate which forms during ageing process enhances the yield strength considerably[15,45-48],whereas the intergranular κ-carbide is very coarse and drastically reduces the ductility.In general,these steels exhibit good combination of strength(600-1700 MPa) and ductility (up to 85%) [4,29].

(iii)Ferritic based duplex low-density/lightweight steelscontain Mn up to 12% and Al up to 7%.They exhibit higher volume fraction of ferrite(greater than 50%)with very small amount of austenite at the hot working temperature[27,49-55].The austenite phase is less stable since the alloying elements are less.Based on the processing condition and also the large variation in C and Mn content,these steels can exhibit a variety of complex transformation at ambient temperature.These steels are cold rolled and then annealed at an intercritical temperature from 700°C to 900°C,followed by austempering.These dual phase steels with plastic deformation of ferrite and retained austenite exhibit superior strength and plasticity compared to the ferritic and HSLA steels [56-61].

(iv)Austenitic based duplex low-density/lightweight steels contain lower amount of Mn and carbon compared to austenitic low-density steels[62-67]. At hot working temperature,these steels exhibit substantial amount of austenite which is a continuous phase with lesser amount of ferrite phase.Due to the higher alloying elements the austenite phase is quite stable.At room temperature,these steels may have stable austenite phase along with κ-carbide precipitates and a small quantity of ferrite phase(these steels are also known as triplex steels owing to the three phases).The tensile properties are much better than ferritic and ferritic based low-density steels [68-72].

5.Phase diagram of Fe-Al alloy

The ferritic Fe-Al steel system is primarily based on Fe-Al binary phase diagram,which is shown in Fig.4[15,73].Al is a strong ferrite stabilizer,which reduces the austenitic loop significantly while zooming the ferritic phase field.This results in a fully ferritic microstructure at room temperature.The solid solubility of Al in bcc iron is up to 10 wt% and it’s solubility in FCC iron is limited to 0.65 wt%.About 1 wt% Al is sufficient to avoid the occurrence of γ phase [74].Addition of above 1 wt% Al to Fe does not show any phase transformation hardenability and is basically ferritic in solid solution range.The ferritic Fe-Al steels are generally cold rolled and heat treatment temperature is in the range from 700°C to 1000°C for recrystallisation annealing to control the grain size and texture of the ferrite matrix as well as the precipitation of κ-carbides.Addition of 34% of Al to pure Fe reduces the solidus temperature from 1538 to 1215°C.It is clear from the Fe-Al phase diagram that above 1 wt% of Al,the hardenability of steel (transformation from austenite to martensite)is ruled out.

Fig.4.The phase diagram of Fe-Al with variation of Al [15,73].

Based on the Al content the main phases observed at room temperature are disordered α A2(Fe-Al),ordered(DO3)Fe3Al and ordered (B2) FeAl.The α ferrite solid solution occurs in Fe-Al system up to 10 wt% of Al.It is BCC solid solution with disordered A2 lattice.The position of the Fe and Al atoms are distributed statistically as shown in Fig.5(a).At higher Al content,the A2 (Fe-Al)lattice gets ordered.Ordered super lattice of Fe3Al occurs with about 12 wt% to 22 wt% Al content.This structure has 12 Fe atoms and 4 Al atoms,with a total of 16 atoms per unit cell which is as given in Fig.5(b).At temperature above 552°C,the first order transformation from Fe3Al to ordered (B2) FeAl occurs.Here the corner positions are occupied by Fe atoms and centre positions are occupied by Al atoms or vice versa (Fig.5(c)).At still higher temperature,(B2)FeAl transforms to disordered α ferrite A2(Fe-Al)as described in the above phase diagram.Fe3Al occurs by the second order transformation from FeAl through ordering of α ferrite A2(Fe-Al) phase below 552°C [15,73].

Fig.5.BCC lattice structures by adding Al to Fe.(a) Dis-ordered A2,(b) Ordered DO3 and (c) Ordered B2 [15].

There are two more areas,which are denoted by K1 and K2 and are known as K-state in the Fe rich part of Fe-Al phase diagram as shown in Fig.6[73-76].This K-state is an intermediate stage which is caused by the complex short-range ordering reactions which is still not clear.It was observed that short-range ordering occurs with 6.2-9.6 wt% of Al while cooling below 400°C [77].It was noticed that at about 250°C,degree of short-range ordering was highest.The disordering to ordering transformation affects tensile properties of the alloy such as ductility and strength.Among all the phases described above,the alloys with disordered α A2 (Fe-Al) show good ductility.The alloys with ordered DO3,B2 and complex K-state show poor ductility and exhibit brittle fracture behaviour at room temperature[78].

Fig.6.The Fe-Al phase diagram,where intermediate regions are marked as K1 and K2[73,75-76].

In Fe-Al system,α(Fe-Al),Fe3Al and FeAl are the important phases from structural application point of view.Fe3Al and FeAl alloys are intermetallic in nature and are well-known as iron aluminide.It was witnessed that alloys based on single phase α(Fe-Al)exhibited good ductility but very poor high temperature strength,whereas iron aluminides offered good tensile and creep properties at intermediate temperature [78-80].Fairly low cost of the material and good corrosion resistance at high temperature are the key properties of Fe-Al alloys.Some physical properties of Fe-Al based alloys are given in Table 2.

Table 2Typical physical properties of alloys based on Fe-Al BCC lattices [78].

5.1.Disordered α-(Fe-Al) ferritic based lightweight/low-density steels

As discussed above,the earlier research on Fe-Al alloy was focused on the development of iron aluminides for high temperature applications,and very limited study on the alloys containing disordered α-phase was reported.Recently,renowned interest is generated in these alloys because of the excellent properties they offer such as reduction in density,lower raw material cost,good ductility and reasonable strength.The corrosion resistance is improved by the presence of Al in the steel.These alloys offer good soft magnetic properties as well [81].They have tremendous potential for automobile application because of the lightweightening of the steels (offered by the reduction in the density of the steel itself).Disordered Fe-Al alloys are an emerging class of lowdensity/lightweight steels containing aluminium in the range of 6-9 wt% in steel(All compositions are in wt%).These Fe-Al alloys have raised considerable interest due to their low-density,high ductility,cost-effectiveness and feasibility for bulk production.

As discussed earlier,addition of Al which being a ferrite stabilizer in iron/steel,reduces the austenitic loop significantly while zooming the ferritic phase field(Fig.4).This results in a fully ferritic microstructure at room temperature.The solid solubility of Al in bcc iron is up to 10 wt% and it’s solubility in FCC iron is limited to 0.65 wt%.About 1 wt% Al is sufficient to avoid the occurrence of γ phase [74].Addition of 34% of Al to pure Fe reduces the solidus temperature from 1538 to 1215°C.The Fe-Al ferritic steels received comparatively less attention in early steel literature.It was probably because of their unappealing high temperature properties such as creep and tensile.During that time research was mainly focused to develop materials for high temperature applications.

The density of the ferritic lightweight steel was examined by various techniques namely levitation,pycnometric and X-ray diffraction technique(theoretical density and lattice constants)for different Al content.The plot of density measurements for several Al content is shown in Fig.7.As the Al content increases there is a reduction in the density of the steel.The density of the steel reduced from 7.8 to 7.25 g/cm3(at about 6 wt% Al),further the steel density approached to 7 g/cm3(at 8.5wt% Al).Hence,more than 10% decrease in density is achieved [1].

Fig.7.The effect of Al content on density of steel measured using different techniques[1,4].

The elastic modulus of the steel is affected by the addition of Al.As the Al content increased in the steel,a down-ward trend in the elastic modulus was observed lattice[1,82].The effect of Al content on elastic modulus is shown in Fig.8.This is primarily due to the reduction in the lattice energy of Fe-Al solid solution.It is also caused by the longer distance between Fe and Al atoms in the lattice [1].

Fig.8.Variation of elastic modulus of steel with Al content [1,4].

Although Al can be added in low-density ferritic steels up to 11wt% [13,20,21,23],it has been suggested that the Al content should be restricted to less than about 6.5wt% [15,21]. Due to the propensity for short-range ordering at higher Al contents ＞11% and completely ordered (DO3)Fe3Al aluminde formation at above 12%.Such microstructure increases brittleness and adversely affects formability,particularly the room temperature ductility is less than 5%.

6.Microstructure and mechanical properties of Fe-Al based lightweight steels

Since aluminium content of Fe-Al based ferritic steel is high(6-9 wt%) and it has traces of carbon (＜0.03%) the phase transformation of ferrite to austenite is ruled out at all the range of the temperature.Therefore,for these steels the hot-working process is generally carried out in the single ferrite phase region.Since the addition of Al to steel stabilizes the ferrite region up to the melting point of these steels,over and above it increases the temperature of recrystallisation of ferrite matrix.For this reason,the adequate grain refinement cannot be achieved by dynamic/static recrystallisation.This results in more of elongated grains of the ferrite matrix in the direction of rolling.Generally,a columnar single-phase ferrite microstructure is observed in the as-cast steels.Depending on the cooling rate from the hot working condition,κ-carbides are formed in the ferrite grain boundaries [6,13].These κ-carbides are generally elongated,thick and shape is of rod-type.Moreover,they are semi coherent with the α Fe-Al matrix [60].

The tensile properties of Fe-Al solid solution are moreover affected by even minute quantity of carbon as it leads to the generation of harmful κ-carbide precipitates along the grain boundaries.In binary Fe-C and ternary Fe-Al-C alloy system,the solubility of carbon(below 500°C)in αFe and α(Fe-Al)is less than 13 and 50 ppm,respectively.Carbon content more than that mentioned above leads to the formation of κ-carbide precipitates.These are basically carbon stabilised ordered solid solution and exist over a range of composition in the form of Fe4-yAlyCxwhere x is 0 ≤x ≤1 and y is 0.8 ≤y ≤1.2.They are of perovskite structure related to L12’type.The size of the κ-carbide precipitates increases with increase in the carbon content.Hence,the work on Fe-Al(ordered or disordered alloys) system was carried out with high purity raw materials with negligible carbon [2,13,31,83].

2003,Herrmann et al.studied the mechanical behaviour of the as-cast samples with the Al content in the range of 2-9.6 wt%.All the alloys showed a single phase with very coarse grain (approx.1 mm)microstructure[2].It was observed that long duration heat treatments(14 days/320°C)resulted in the formation of κ-carbides(thin to thick rod/plate type) on the grain boundaries.The mechanical properties are shown in Fig.9.It is noticed that the strength and ductility of these steels are dictated mainly by solid solution strengthening of Al.Though the contribution from the short-range ordering is only minor but it also depends on the quenched in excess vacancies resulting from heat treatments.It was observed that quenching from high temperature results in excess vacancies which add on to hardening.Post annealing treatment to soften the material by eliminating the excess vacancies is very slow process.It has also been observed that as the Al content increases in the solid solution range(disordered-A2 Fe-Al)up to about 13wt%,the yield strength also increases due to solid solution strengthening according to Suzuki’s theory of solid solution strengthening.Further increase in the amount of Al lead to the formation of ordered intermetallics,namely Fe3Al (DO3) and FeAl (B2) causing drastic decrease in strength as well as ductility.

Fig.9.Variation of tensile properties of Fe-Al alloy with different Al content.Lattice structure and strengthening caused due to solid solution is indicated.TE-total elongation,UE-Uniform elongation [1,2,4].

The strength of the ferritic iron-aluminium steels can be increased by work hardening during deformation.The rate of work hardening for Fe-Al alloys estimated with respect to different Al content is shown in Fig.10.In case of α(Fe-Al)alloys with Al up to 7wt%,the work hardening rate is very low.With further increase in Al to 16%,there is a steep rise of the work hardening rate due to the change from the disordered to K1 state to ordered state and drops to a reasonable rate above 20%.In Fe-Al steels,the deformation is mainly due to generation of dislocation and slips within the grains.For a complete disordered state (6wt% Al),dislocation glide bands were noticed besides dislocation tangles among them.For K1 state(8 wt% Al),straight segments of paired dislocation with narrow mechanical antiphase boundaries were observed at low strain rate.At high strain rate,dense dislocation bundles were seen in the ferrite.In addition,some undeformed B2 regions were observed signifying that the deformation of matrix is not uniform.It is proposed that strain hardening of the steel is primarily governed by shearing of the ordered phases through super-dislocation.The deformed structure at room temperature was observed for different Al content.It was reported the dislocation structure exhibited an evolution from a single bowed dislocation to paired superdislocations with a strong inclination to form straight screw segments when the phases changes from disordered to complete ordered phase.Due to the inadequate dislocation mobility,deformation twinning takes place more easily at higher Al content and lower temperature [2].

Fig.10.The plot of rate of work hardening with Al content.The influence on lattice structure is also indicated [1,2,4].

Hot torsion process was used to understand the role of various deformation parameters (such as strain,strain rate and temperature) on Fe-8 wt% Al low density steel.The test was carried out at three temperature namely(900°C,1000°C and 1100°C,with a stain rate of 0.1-10/s.In this alloy,the dynamic recrystallisation observed has been of two types.Continuous dynamic recrystallisation(CDRX)is noticed at low strain rate and low temperature and on the other hand discontinuous dynamic recrystallisation(DDRX)is noticed at high strain rate and high temperature which is shown in Fig.11.In DDRX,the orientation of the newly recrystalised grains is random.Whereas in CDRX,since the original grains are elongated in the direction of the shear,subgrains start nucleating inside the grains,favourably near the previous grain boundaries.This microstructure exhibits strong intensity of cube texture,which comparatively has a low formability.There is a condition in which the deformation changes from CDRX region in the plot to DDRX region based on the strain rate and the temperature at which the deformation is carried out.Hence formability of these ferritic steels can be increased by carrying out deformation of the steel basically hot working at very high temperature and high stain rate which preferably may produce random orientated recrystallized grains.These orientated recrystallized grains may aid to combat with surface defects such as roping [34].

Fig.11.A map of strain rate vs.temperature showing the areas of mechanisms in Fe-8wt% Al ferritic steel [34].

2013,Rana et al.have studied the Fe-Al solid solution steels et al.content of 6.8 wt%,8.1 wt% and 9.7 wt% [13].To avoid the detrimental effect caused by the presence of κ carbide,high purity interstitial free raw materials were used.The damaging effect of addition or the presence of small amount of carbon is avoided.All the melts were taken by vacuum induction melting and hot worked to 3 mm sheets after 85% reduction.Steel with 6.8% Al could be successfully cold worked to 1 mm whereas the steel with 8.1 wt% and 9.7 wt% Al cracked during cold rolling.Hence,these steels were subjected to warm rolling to get 1 mm sheets.All these steels were annealed and properties were evaluated.It has been observed that as the Al content increased from 6.8 wt% to 9.7 wt% the decrease in density from 7.21 to 6.802 g/cm3is observed.The average grain size was large(70-90 μm)for alloys with 6.8 wt% and 8.1 wt% Al when compared to Fe-9.7 wt% Al (smaller grain size 17.5 μm) [13].

The strain hardening exponent of Fe-6.8 to 9wt% Al steel is 0.17 to 0.13 [13].The value drops as the amount of Al in the solution increases,which is due to solid solution strengthening.The strain hardening exponent for DP steels(DP450,DP500,DP600)is 0.16 to 0.14.It is also reported that the formability of Fe-Al lightweight steel is inferior to that of IF steel,but possess higher tensile strength.These lightweight steels are more formable than the DP steels Hence,lightweight Fe-Al-based steels have slightly greater deep drawing capabilities than DP steels.As a result,developing Fe-Al solid-solution alloys to replace several existing DP grades is a possibility.In terms of processing,the Fe-Al steels have a major advantage over DP steels.When compared to DP steels,the Fe-Al steels have a significant advantage in terms of processing.It is also reported that Fe-6.8Al and Fe-8.1 A l lightweight steels are highly beneficial as compared to IF,DP 450,DP600 steels for automobile components with respect of stiffness,dent resistance and crash resistance in terms of weight saving and performance improvement [13].

The tensile properties of Fe-6.8wt% Al (YS: 342 MPa and % El:31.1)are similar to the properties reported by Brüx et al.,2002[6].On the other hand,the tensile properties particularly;ductility is much better compared to the similar composition reported by Baligidad et al.,2007 [84]with small amount of carbon in Fe-Al alloys.Light weight steels with 6.8 and 8.1wt% Al have shown better tensile properties compared to the corresponding dual phase steels,DP450 and DP500,respectively.Here,450 and 500 indicates the strength values of these steels respectively.In terms of weight savings and performance improvement also these steels have exhibited noticeable improvement compared to interstitial free steel and dual phase (DP450 and DP500) steels for automobile applications.

2014,Pramanik and Suwas have investigated the Fe-Al lightweight steels with Al ranging from 5 to 9 wt%[23].As the Al content increased about 10% reduction in density was observed for these steels.Further,the increase in the strength in this composition range of lightweight steels was primarily attributed to solid solution strengthening.Detailed analysis of the different mechanisms involved in strengthening of low-density Fe-6.8 wt% Al steels was studied [10].The fine scale tools such as Mössbauer spectroscopy,X-ray line profile analysis,small angle X-ray scattering(SAXS),and transmission electron microscopy(TEM)were used for the analysis.In this alloy the contribution to the increase in strength is ascribed to various mechanisms.The mechanisms which are considered are based on the grain size,dislocations arising during deformation,formation of ordered phase,lattice frictional stress and the solid solution from Al atoms.The yield strength of alloy in the cold rolled condition has resulted in 1800 MPa which reduces drastically upon subsequent annealing to 265 MPa.The extent of strengthening contribution from each mechanism for Fe-6.8 wt% Al steel under different processing condition is plotted as in Fig.12.It is clear that the contribution by solid solution strengthening,lattice frictional stress and the formation of ordered phase is constant across all deformation condition indicating that heat treatment has least effect on the strengthening by these mechanisms.It can be noticed that in cold rolled sample,the contribution for increase in the strength is predominantly by gain size and dislocations.However,with the increase in the annealing period the contribution from these mechanisms to strengthening of the alloy reduces drastically(Fig.12).

Fig.12.Advanced technologies used to characterise Fe-6.8Al steel.(a) Mössbauer spectra,(b) Bhf distribution,(c) 2D-SAXS profile,(d) Dark field TEM micrograph and (e) Plot showing the cumulative contribution of different mechanisms in strengthening of Fe-6.8wt% Al lightweight steel [10].

One of the major intrinsic problems of totally ferritic Fe-Al lightweight steel is about their large grain size.These steels cannot be refined by phase transformation as they are stable up to their melting point without γ phase formation.Hence,recrystallisation is the only process to get fine grains.However,it has been discovered that as the Al concentration of the steel increases,the recrystallisation temperature decreases,reducing the number of rolling passes required to achieve grain refinement via recrystallisation.Hence,the properties of α(Fe-Al)steels are seriously relying on as-cast microstructure.Therefore,greater focus is required to reduce the as-cast grain size.Slater et al.,in 2019 evaluated the effect of alloying elements (Si,Al,Mn) on the solidification parameters particularly liquidus temperature,mushy zone width and segregation in the steels [85].Al has been varied in the range of 2.9-10.7 wt% whereas Si varied in between 2.5 wt% and 6.2 wt%.One melt with 6.1Si-0.3 P and two melts were cast with 7Al-0.5Mn and 7Al-0.5Mn-1.5Si.The average gain size obtained for various composition of lightweight steel is shown in Table 3.The steel with all levels of Al(2.9 wt%-10.7 wt%)have shown larger average grain size compared to the steels with Si addition.Addition of 0.35 wt% P to Fe-6.1 wt% Si alloy resulted in the minimum average grain size(260 μm)among all the alloys studied.The average grain size of less than 500 μm could be achieved by the addition of small amount of Si and C to Fe-7Al-0.5Mn alloy.

Table 3The average grain size obtained for various lightweight ferritic steels [85].

2010,Khaple et al.have investigated the effect of melting process,vacuum induction melting (VIM),Air induction melting with flux cover(AIMFC-)and different Al(7.9 wt% and 16 wt%)amounts[11].A schematic diagram of the melting process(AIMFC/VIM)is as shown in Fig.13.Alloys made by AIMFC process have shown lower in sulphur and higher in hydrogen content at each level of Al.The effect of these parameters on microstructure and mechanical properties was analysed.The melting process of all alloys made by both the process(AIMFC/VIM)showed single phase microstructure(Fig.14).It was reported that as Al content increased,the hardness of the alloys increased irrespective of the melting process.Higher the Al content,more is the hydrogen content found in the alloys.This hydrogen in the alloys leads to hydrogen embrittlement.The hydrogen pick-up by Al was not only during melting but also during processing (to make plates/sheets from ingot) and fabrication process (such as cutting and machining).In Fe-Al alloys,the Al of the alloy reacts with the hydrogen present in the moisture even at room temperature and leads to hydrogen embrittlement.

Fig.13.A schematic diagram showing the induction melting in air (a) set up,(b) melting process with flux cover (c) melting process in vacuum [11,33].

Fig.14.Optical micrographs of AIMFC processed(a)Fe-16 wt% Al,(b)Fe-9 wt% Al(c)Fe-7 wt% Al and VIM processed(d)Fe-16 wt% Al,(e)Fe-9 wt% Al and(f)Fe-7 wt% Al steel[11].

In case of Fe-16 wt% Al alloy made by AIMFC process,tensile samples could not be prepared as samples cracked during fabrication itself.However,alloy made by VIM process for the same composition could be fabricated,but failed during tensile testing at very low stress indicating very low ductility.For 9 wt% Al alloy,the samples could be fabricated successfully for the alloys made by both the process.On the other hand the Fe-9 wt% Al samples made by AIMFC process fractured at low stress showing low ductility.Nevertheless,at 7 wt% Al alloys made by AIMFC and VIM process have shown similar properties with reasonable ductility.It is attributed to the reason that at 7 wt% Al,the hydrogen embrittlement is not affected,whereas above 7 wt% Al i.e.higher(9 and 16)Al content,the hydrogen embrittlement is more severe.It affects not only during melting,solidification,fabrication but also during tensile testing[11,33].

7.Addition of carbon to lightweight steels

Alloys based on Fe-Al system with excellent high strength to weight ratio,oxidation resistance,low density and inexpensive raw material costs are expectant to cater for many structural applications.From the literature on Fe-Al alloys,it is clear that these alloys have lower ductility and toughness at room temperature.The vulnerability of Fe-Al to hydrogen embrittlement is the foremost reason for these poor properties.Addition of carbon to ironaluminium alloys is observed to be appropriate for decreasing the hydrogen embrittlement.The numerous benefits of alloying carbon to steel along with high Al is given in Table 4.It also leads to the improved strength because of the carbide formation.Among all the Fe-Al alloys,FeAl based B2 alloys have been studied initially because of its surpassing properties such as low density,excellent corrosion and oxidation resistance.But in these alloys,addition of carbon leads to formation of graphite,which reduces its performance as high temperature structural material.

Table 4Benefits of carbon alloying to steel containing Al in the melt.

7.1.Lightweight steels based on Fe-Al-C system

In case of Fe-Al alloy,it is recently conceived that addition of carbon is more favourable.Based on the Al content,addition of carbon generates different kind of microstructures.It becomes significant to examine the effect of carbon and unravel the mechanisms which results in the strength improvement of the alloy.It is manifested that the Fe-Al alloy properties depend on the type of carbide,amount of volume fraction,distribution of the carbides and their formation process.

Carbon addition to Fe-Al low density steel is more advantageous.It results in the reduction of the cost of raw materials as steel scrap can be used which is the cheapest among different supplies of iron.Conventionally available low-cost melting processes such as induction/arc melting can be used.Hence,use of steel scrap and commercial purity Al reduces the production cost of these alloys compared to the presently available automobile steels such as dual phase,complex and advance high strength (AHSS) steels.

The addition of carbon leads to considerable improvement in machinability by decreasing the hydrogen mobility.As the solubility is less than 0.045 wt% C in these alloys,carbon precipitates out as perovskite type Fe3AlC0.5phase known as κ-phase.The isothermal section of Fe-Al-C system at room temperature is shown in Fig.15 [86].The schematic diagram of Fe3AlC0.5carbide(κ-phase) with the arrangement of the atoms is shown in Fig.16.Homogenous distribution of these carbides operates as traps for hydrogen which is the reason for machinability improvement.Additionally,these fine carbides aid in obstructing grain growth during hot working.This is contrary to the very poor machinability because of surface cracking noticed in case of Fe-Al alloys containing low carbon.

Fig.15.The isothermal section of Fe-Al-C system at temperature [86].

Fig.16.A schematic diagram of κ-carbide unit cell.Al is at the corner position,Fe occupies the face centered position and carbon atom sits at the centre,which is also an octahedral site of Fe and Al atoms [87].

7.2.Low-density/lightweight ferritic steels based on Fe-Mn-Al-C system

To get austenite in Fe-Al alloys,high amount of Mn and carbon are added which are austenite stabilizers.By the addition of Mn to Fe-C system,there is a gradual shift of the high temperature peritectic reaction to lesser carbon content as Mn content is increased.Further with Mn greater than 13%[4],the peritectic reaction fades away completely.Al being ferrite stabilizer,addition of it to Fe-Mn-C system expands the ferrite region and reduces the single-phase region of austenite.A typical equilibrium phase diagram of Fe-Mn-Al-C system at fixed level of carbon (0.85%),aluminium(7 wt%)with varying Manganese is as shown in Fig.17.The quaternary Fe-Mn-Al-C alloy phase equilibrium have been examined by many authors [88-91]. Mainly austenite,ferrite,cementite and κ-carbide are the four phases which are found in these alloys at hot working temperatures.The other phases that are reported in these alloys are M3C,M23C6,M7C3carbides and β-Mn.The stability of the phases depends on the alloy composition and the temperature.κ-Carbide forms in these steels below 950°C and its stability reduces as the carbon content decreases.With the increase in the Mn content,the single-phase region of the austenite is stretched towards lower temperature,the solvus line of κ-carbide is also reduced and β-Mn phase appears.It is reported that the β-Mn phase observed with Mn higher than 20%.The stability of β-Mn phase is more with the higher Mn content [92,93].Even 2% Al is adequate to form κ-carbides in Fe-Al-C system whereas at least 5% Al is required in Fe-Al-Mn-C system to form κ-carbide with 2% Mn content.This implies that Al restricts the formation of M3C,M23C6,M7C3type of carbides whereas Mn restricts the formation of κ-phase.It is also reported that disorder-order (BCC_A2 (α) to BCC_B2) transformation happens at higher than 2% Al.As the content of Al and Mn is increased the transition temperature of disordered to order increases.Whereas with the increase in carbon content this transition temperature decreases.This suggests that increasing the concentrations of Al and Mn enriches the formation of ordered phases within the ferrite,on the other hand the ordered phase formation is restricted by increasing the carbon content.The thermodynamic calculation and the experimental results revealed that the austenite single phase region is somewhat narrowed as the temperature is dropped from 1200°C to 900°C.

Fig.17.A typical equilibrium phase diagram of Fe-Mn-Al-C system at fixed level of carbon (0.85%),aluminium (7 wt%) with varying manganese [88-91].

The isothermal section of the phase diagrams at 900°C has considerable importance in the alloy design as this is generally the hot working temperature practised during industrial production of the components.In this system,austenite is not a stable phase at lower temperature but numerous heat treatment processes can be adopted to obtain different phases with various morphologies to get better combination of properties.Aluminium addition to Fe-Mn-C steel leads to the generation of short-range ordering(SRO),κ-carbide precipitate and also increases the stacking fault energy (SFE).Fe-Al-Mn-C steels having high SFE containing SRO the plastic deformation is largely influenced by planar glide.Deformation mechanisms such as twinning-induced plasticity(TWIP) and transformation-induced plasticity (TRIP) are used to correlate the microstructure with properties of Fe-Al-Mn-C steels.The advanced deformation mechanisms are operating in high Mn austenitic Fe-Al-Mn-C steels.The essential criteria for the alloy design can be hot working in the absence of κ-carbide,forbid the formation of coarse of κ-carbide on the grain boundaries and avoiding the formation of M23C6,M3C,M7C3type of carbides at lower temperatures.

8.Effect of carbon on microstructure and mechanical properties of lightweight steels

Very limited work has been reported in case of Fe-Al alloys with lower Al content(Al ＜7 wt%).Since the Al content is comparatively low,the oxidation resistance offered is less and its high temperature properties are not so good compared to the iron aluminides and other heat resistant steels.The distinct advantage of these low Al containing Fe-Al alloys is that these alloys are less prone to hydrogen embrittlement.Therefore,these alloys are easy to make and process.Additionally,these alloys exhibit better tensile ductility compared to iron aluminides.In the recent trend for lightweightening of structural materials,particularly with respect to automobile application,these alloys have gained more prominence.The reduction in density offered by these alloys with about 7 wt% Al is about 10%,which is the desirable requirement with reasonable strength.In majority of the research for Fe-Al ferritic steels,the high purity iron is used where carbon content is less than 0.05wt% [2,13,23]. This was basically to avoid the formation of κcarbide (perovskite carbide,Fe3AlC0.5) which is a brittle phase which affects the ductility.However,recent research on Fe-Al alloys by few authors[84,94,95]have shown that carbon addition to Fe-Al alloys have more beneficial effects.Uniform distribution of the κ-carbide improves the ductility by reducing the hydrogen embrittlement effect.It increases machinability,additionally;the cost of the raw material can be reduced.Moreover,this alloy can be made by the conventional melting processes such as air induction melting.

Very few authors have reported the effect of carbon on Fe-Al alloy with Al in the disordered range [32,94,96-98]. Baligidad et al.,in 1998 have studied Fe-Al alloys in the range of 8.5 to 16 wt% with carbon in the range of 0.1 to 1 wt%[98].The room temperature ductility in tensile condition is plotted for different Al and carbon content and is shown in Fig.18.Higher the Al content in Fe-Al alloys,lower is the ductility with similar level of carbon content.However,the creep,tensile strength at high temperature as well as the room temperature ductility of the Fe-Al-C alloys are better than corresponding binary Fe-Al alloys.2011,Jiménez and Frommeryer have studied the effect of 1 wt% carbon on Fe-Al alloys with Al varying in the range of 2 wt% to 6 wt%[96].In case of Fe-6 wt% Al,the carbon content was varied from 1 to 1.8wt%.Here the objective was to examine different carbides formed in the system.It was observed that ferrite,ferrite+Fe3C pearlite,κ-carbides and M23C6phases were present in the alloys.It was also reported that as the Al content increases the formation of carbides (Fe3C and (Fe,Al)23C6)is reduced.Further,it is noticed that graphite formation has been reported with higher carbon (C ＞1.2 wt%) in Fe-6 wt% Al alloy.However,the effect of these phases on the mechanical properties is not reported.

Fig.18.The plot of tensile properties(a)strength and(b)ductility with variation in Al and carbon content in steel [84].

Recently various researchers have reported the effect of different carbon content on Al containing steel[27,32,33,84,96-101]. In fact,the carbon content for 7 wt% Al was varied in the wide range from 0.01 wt% to 2.2 wt%.The Thermo-Calc software was used to predict the phases which is reported to match with the experimental findings [101].These steels could be successfully hot-rolled without any cracks.The Fe-7Al steel with 0.012 wt% C show single ferrite phase whereas increasing the carbon up to 6.5 wt%,a dual phase (ferrite and κ-pearlite) microstructure is formed and at 1.5 wt% C only κ-pearlite is found and with increase in carbon to 2.2 wt%,additionally graphite phase is formed (Fig.19).The microstructural phase evolution with the addition of carbon on Fe-7Al steel was correlated well with the increase in the mechanical properties [98](Fig.20).However,the ductility of Fe-7Al-0.35C steel in hot-rolled and annealed condition was observed to be more than 12% which is the minimum requirement for the formability operation of the steels in automobile applications.

Fig.19.Optical micrographs showing the microstructure of Fe-7 wt% Al based steel with (a) 0.012,(b) 0.35,(c) 0.65,(d) 1.5 and (e) 2.2 wt% carbon [98].

Fig.20.Comparison of yield strength and ductility of carbon containing lightweight steels [27-^,32-*,84-! 98-$].

Most of the research carried by Baligidad on Fe-Al-C alloys was made by air induction melting process followed by electroslag refining (ESR) process.Fe-Al-C alloys made by ESR process resulted in better properties because of uniform distribution of κcarbide precipitates in the alloys.However,these alloys were prone to decarburisation as they exhibit poor oxidation at higher temperature.Nevertheless,decarburisation seems to be problematic only at temperature higher than 800°C.The microstructure which consists of α(Fe-Al)ferrite and κ-carbide is stable up to 600°C.By having fine κ-carbides and homogenously distributing these carbides in low Al containing Fe-Al-C ferritic steels can be one of the candidate materials for lightweightening of various components particularly in automobile industries.

9.Effect of alloying elements on microstructure and mechanical properties of lightweight steel

Even small quantity of κ-carbides affects the mechanical properties of the steel particularly the ductility.In order to avoid the detrimental effect of κ-carbides formation,microalloying elements such as Ti,Nb and B are used to lock the free interstitial free elements including carbon.2002,Brux et al.studied the effect of microalloying elements on the microstructure and mechanical properties of Fe-6Al ferritic steel [6].They found that addition of microalloying elements(Ti,Nb and B)refined the ferritic grains up to an average grain size of 40 μm.They have reported that the optimum combination of the microalloying elements have displayed better tensile properties which is due to grain refinement of the ferritic grains.

Researchers from Pohang University,Pohang Iron and Steel Company (POSCO),South Korea have done extensive study on Mn containing ferritic low-density steels.Addition of Mn to Fe-Al based steel stabilizes the austenite phase as Mn is an austenite stabilizer.It also opened up the window of heat treatment as different phases can be obtained by following various schedules of heat treatment.There is solubility of Mn in the matrix of Fe-Al and also in the precipitate of κ-carbide (Fe,Mn)3AlC0.5as well.This increases the strength and hardness of the matrix and the precipitate in Fe-Mn-Al-C steels compared to Fe-Al-C steels.Addition of Mn to Fe-Al-C ferritic steel enhanced the mechanical properties considerably.However,the workability of Fe-Mn-Al-C steels is poor compared to similar composition of Fe-Al-C ferritic steels as these Mn-containing steels were prone to cracking in both the hot and cold rolled condition [27,50,51].The researchers from South Korea focused on examining the size,shape and distribution of κcarbides in the ferritic steels to derive the maximum benefits.

Lee et al.,2013 studied the effect of variation of alloying elements (Al,Mn and C) on the microstructure evolution in Fe-Mn-Al-C ferritic low-density steel [102].They attempted to relate the microstructure evolution of these steels with the prediction of the phases and their volume fractions obtained by calculation based on thermodynamics.The microstructure of each alloy composition is systematically analysed experimentally after the thermo-mechanical processing from ingot to sheet.They ascertained that the thermodynamic calculation based on thermodynamic data can be used to predict the microstructure evolution.This will help the researchers in designing new steels with the required microstructure.

2010,Shin et al.reported cracking in the Mn containing Fe-Al-C ferritic steel during hot rolling[27].Steel with higher than 6% Mn and 7.5% Al displayed cracking behaviour compared to the steel with lower 3%-4% Mn and 5.5°C 6.5% Al.They observed that ferrite bands or the densely populated coarse κ-carbides in the secondary bands did not play crucial role in the formation of cracks.They found that steel containing higher Mn and Al content cracked during hot rolling because of the formation of higher volume fraction of continuous thin κ-carbide film.The main reason for the cracking of these steels is due to the crack initiation in continuously formed film type κ-carbide precipitates between the ferrite and secondary phase bands.Cracks formed and propagated (in a cleavage manner) by combining with many voids at the interfaces between the bands.Some of the guidelines suggested to address the cracking problem are (i) Improvement in the solidified structure by controlling the casting process,(ii) Increasing the homogenisation time,(iii) increasing the hot rolling temperature,(iv)Reducing the alloying elements added to the steel such that the combined Al+Mn content should be less than 10%.

Research on cracking phenomena taking place in Mn containing ferritic steel during cold rolling has been extensively studied.Han et al.,2011 studied the cracking phenomenon taking place during cold rolling of Mn containing ferritic low-density steels with different carbon content[103].The carbon content varied from 0.1 to 0.3%.They found that Fe-5±1Mn-7±1Al steel with 0.1% carbon cracked whereas steel with 0.3% C did not crack during cold rolling.Only some of the steel with 0.2% C cracked.They observed that the cracks initiated at continuously formed coarse κ-carbides along the band boundaries and also noted the abrupt propagation of the cracks in the ferrites in a cleavage manner in steel with lower carbon content (0.1%).The secondary bands densely populated with discontinuous κ-carbides in the form of fine lamellar(which is mostly present in steel with higher carbon content) did not play significant role in the cracking process.Effect of Al(4%-6%)content on the cracking behaviour of Fe-(0.3 ± 0.1)C-(3 ± 1) Mn steel is reported by Sohn et al.,2014 [25].Al being the ferrite stabilizer affects the morphology,particularly the size and the volume fraction of the κ-carbides formed in the steel.Both the centre and the edge cracks occurred in steel with higher (6 wt%) Al content.The tendency of cracking in lower Al(4 wt%)content steel was very less.Cracking in steel with higher Al content was caused by continuously formed κ-carbides which were long and lamellar,whereas in lower Al (4wt%) steel the k-carbides were thin,short and discontinuous which prevented from cracking.

In ferritc (Fe-Al-C) low-density Mn containing steel,with 6-8 wt% of Al,cracking phenomena is observed due to the formation of κ-carbides.With an aim to eliminate κ-carbide formation and maintain the density of the steel,Al content was reduced to 5wt% and effect of Si study was studied by Heo et al.,2012[8].The Fe-5Al-8Mn-0.2C lightweight steel with Si(1 wt% and 2 wt%)was processed and characterised.They found that the base alloy with 5 wt% Al does not show any κ-carbide precipitates instead very complex cementite comprising Fe,Mn and C was reported.The ductility of the base alloy is reasonably good (15%) whereas addition of Si to the base alloy drastically decreases the ductility to less than 5%.Si in the Fe-5Al-8Mn-0.2C lightweight steel form undesirable complex hard and brittle carbides.Hence,density reduction by the alloying with Si to Fe-Al-Mn-C based steels is not recommended.

2020,Pramanik and Suwas studied the effect of Cr and Mn addition on the microstructure and mechanical properties of Fe-Al ferritic steels [104].Addition of 6.3 wt% of Cr to Fe-6.5 wt% Al and addition of 3.1 wt% of Mn to Fe-3.7 wt% Al lead to the decrease in the density of 7 and 7.5 g/cm3,respectively compared to density of pure iron(7.8 g/cm3).The elastic modulus of Fe-6.5Al-6.3Cr steel is 186 GPa,whereas Fe-6.5Al-3.1Mn steel is 178 GPa.The steel with Cr and Mn alloying additions displayed higher elastic modulus than the elastic modulus(175 GPa)of the base(Fe-6.5Al)alloy.It is also reported that addition of Cr to Fe-6.8Al improved tensile properties marginally (YS-529 and % El-27%) when compared to tensile properties(YS-494 and% El-25%)of Mn added Fe-6.8Al steel.The better properties are shown by Mn containing steel is due to the grain refinement effect [104].

10.Strength improvement by grain refinement

It is well known that refinement is most preferred method to simultaneously improve both the strength and plasticity of any metallic material.Grain refining of steel can be achieved during or/and after solidification process.Generally,the methodology adopted for achieving grain refinement is by (a) agitation and stirring during solidification,(b) addition of grain refiners in the melt,(c)rapid solidification and (d) severe plastic deformation.Fine ferrite grains can be achieved by offering additional nucleation sites for ferrite or by increasing the driving force for the transformation of ferrite to austenite.

Addition of alloying elements to Fe-Al binary alloys can improve the strength.The alloying elements such as Cr,Co and Mn can improve the strength by solid solution strengthening.Alloying elements such as B,Nb,Ta,Hf and Zr have demonstrated to enhance the tensile strength by precipitation hardening [78,83,105,106]. It was also reported that addition of oxides also improves the strength by oxide dispersion strengthening (ODS) [107,108].

Strength can also be improved by addition of carbides such as SiC,TiC,ZrC,and diborides such as ZrB2and TiB2in the ferritic lowdensity steels.These metallic particles/precipitates can be added as reinforcements or formed in-situ during the melt.The morphology of these precipitates has tremendous effect on the properties of the steel.The size,shape,volume fraction and distribution of the particulates considerably affect the mechanical behaviour of the lightweight steels.The addition of boron (B) has been tried to improve the ductility of Fe-Al-based alloys.

11.Types of reinforcements/particulates

A low-density hard phase with high stability and high-elastic modulus can be effectively used in enhancing the properties of the of lightweight Fe-Al steels by numerous methods.The important property required for the particulate is that it should be thermally and chemically compatible with the matrix.Various particulates such as carbides,oxides and borides can be potential candidates to improve the strength.Carbides and borides are found to be more effective which will be discussed in the following sections,whereas the oxides have very limited use.The selection of the particulates is decided based on the morphology,the availability and its compatibility with the matrix of the steel.Since the development of lightweight steels are based on Fe-Al system(because of their low cost),the cost of the reinforcement along with its lower density is also the main criteria to be considered.

Since the reinforcement(ceramic)is generally brittle,the matrix needs to accommodate the dimensional mismatch occurring due to the difference in strain of matrix and the reinforcement during plastic deformation.The reinforcements need to be highly stable,fine sized and uniformly distributed to improve the properties of the steel.The selection of the composition and their process control to achieve the above parameters play very important role[109,110].

Vast majority of the processes bank on the formation of the reinforcement by eutectic or precipitation process.The method used for having the reinforcement in Fe-Al alloys is dependent of the type (fibre or particulate etc.) and size of the reinforcement.Particulate reinforcement is the process which is significantly used in Fe-Al alloys.Powder or melting process can be used to make these steels with reinforcement.Mechanical alloying,reaction synthesis and melting process rely mainly on the in-situ formation of the reinforcements.Particulate reinforcements can also be added externally in the form of powders in the melt or in powder processing routes.Fe2O3particulates have been introduced by displacement reaction in Fe-Al alloy matrix to achieve oxide dispersion.

11.1.Oxide reinforced Fe-Al alloys

Powder processing of Fe-Al alloys particularly,in aluminides is the most general route opted for oxide reinforced composites.Liquid phase sintering process was used to make Fe-40at% Al alloy composite [111]. Pre-alloyed powders of Fe-40wt% Al were mixed with 30wt% ZrO2of ＜45 μm or 15vol% Al2O3of ＜38 μm.Due to the thermodynamic instability of ZrO2in the molten matrix of Fe-Al alloy,it was found to be unsuitable by liquid phase sintering process.The Al2O3powders could not be wetted by the Fe-Al matrix.Wettability with Fe-Al matrix was achieved after addition of an equal quantity of TiC powders.It is important to note that Ti addition to Fe-40wt% Al has improved the wettability of the alloy[112].

11.2.Boride reinforced Fe-Al alloys

The stability of borides of Zr,Ti,Nb and Ta in Fe-Al alloys was examined by Doucakis and Kumar 1999 [113]. They observed that TiB2and ZrB2displayed extreme resistance to coarsening.The density of these dispersoids is 4.52 and 6.08 g/cm3,respectively.Considerable amount of strengthening is obtained by the addition of borides to Fe-Al based alloys without impairing the ductility.At high temperature,Iron boride dissolves in the Fe-Al matrix,hence its use as a dispersoid is not recommended.

Powder metallurgy processing of Fe-Al aluminides with TiB2dispersoids has shown good elongation of 11% and yield strength of about 1 GPa [114]. Rapid solidification process is by melt spinning and rapidly solidifying to get ribbons.The rapidly solidified process generally resulted in finer dispersoids of submicron size.The ZrB2addition has resulted in improved strength and ductility though their distribution was not uniform [115]. The powders of Fe,Al,Ti and B were used to prepare Fe-Al-TiB2alloy by reactive synthesis using a pseudo HIP process[116].The usage of pressure during the synthesis resulted in the improvement of ductility probably by reducing the porosity.The powders when sintered in vacuum produce the alloy with reduced volume fraction of pores and also the size of the TiB2dispersoids was smaller.In this case,it was noted that the alloy had a dispersoid size in the range of 1-7 μm.

Thus,both the ingot and powder processing routes have been used to prepare boride containing Fe-Al alloys.Various stable and more compatible metallic refractory diborides have been used to strengthen Fe-Al alloys.These dispersoids are impervious to coarsening and also expected to refine the matrix grains.Moreover,these dispersoids do not impair ductility.It is expected that the presence of boron may help to improve ductility in Fe-Al alloys.

However,TiB2or ZrB2powders addition to pre-alloyed Fe-Al powders is not very beneficial because of limited solubility of borides in iron aluminides.This may result in debonding at the interface of boride/matrix at high temperature even in alloys processed by liquid phase sintering thus limiting their potential as structural materials.

Khaple et al.,2017 have recently reported the effect of TiB2or ZrB2in Fe-7Al steel.Addition of these borides resulted in one order of improvement in grain refinement(895-78 μm)in cast condition[37].Further hot-rolling and annealing also resulted in grain refinement.They observed that very good ductility (from 19% to 38%) can be obtained by the addition of these borides.This improvement in ductility with slight improvement in strength is mainly attributed to the grain refinement.

11.3.Carbide reinforced Fe-Al alloys

These constitute the most investigated of Fe-Al alloys and also the most promising.The low cost of these Fe-Al based alloys is the main advantage because of the low-cost sources of raw materials basically steel scrap and commercial grade Al which can be used.The benefits of addition of carbon to ferritic steels have already been addressed in the previous sections.With the addition of carbon in Fe-Al system lead to the formation of stable Fe3AlC0.5κcarbide precipitate,which has a preovskite structure [86].This carbide is more stable at low Al content in the ferritic low-density steels.Alloy carbides of high atomic weight or high density such as WC,TaC and MoC are avoided because of their higher density than that of steel.Since density reduction is the main aim with which development of low-density steels started.

The alloying elements distribution in the steel depends on the concentration of the carbon and the corresponding amount of carbide forming element.If the carbon content in the steel is relatively higher than the alloying element,the later would be completely used up for the formation of respective alloying element carbide.On the other hand,if the carbon content is less compared to the alloying element,then the remaining extra quantity of alloying element after forming carbide will be present in the steel matrix as solid solution or in the form of Laves phase.Alloying elements such as Cu,Ni and Co do not form carbides whereas Mn is weak carbide former.Addition of Si and Mn to Fe-C steel leads to the formation of cementite rather than forming respective metal carbide.Hf,Zr,Nb,Ta,Ti and V are very strong carbide formers even minor quantity of these alloying elements with carbon leads to the formation of carbides whereas Cr,Mo and W are good carbide formers.All these alloying elements Hf,Zr,Nb,Ta,Ti,V Cr,Mo and W have relatively higher enthalpies than iron carbide[117].Generally,six forms of metallic carbides are reported in steel such as MC,M2C,M3C,M23C6,M2C3,M7C3and M6C,Here M represents respective metallic element.Carbides of the type MC and M2C (TiC,NbC,WC and Mo2C) have simple crystal lattice.Whereas other carbides are more complicated(Cr23C6and Fe3C).It is to be noted that contribution of carbides in the strength of the steel relies upon the morphology (size and shape) and volume fraction of the precipitates.The finer the size of the precipitate,the higher is its strengthening contribution.The fineness of the carbides is detected by the activation energy barrier for nucleation of the precipitate.This in turn depends on interfacial energy,the free energy of formation and the misfit of the carbide with matrix.Generally,the finest carbides are obtained by MC and M2C type of carbides (such as TiC,VC,NbC etc.) which are close-packed intermetallic compounds.On the other hand,carbides such as M23C6,M2C3,M7C3possess complex crystal structure and the heat of formation is also low.These carbides are reasonably coarser in size[117].

The volume fraction of the carbides formed depends on the solubility of the metal carbide in the austenite phase as compared to its solubility in ferrite phase.The solubility of Cr,Mo and V carbides is highest in the austenite phase.Hence,the volume fraction of these precipitates is larger in ferrite.It is regarded that when strong carbide formers are present in the steel in sufficient quantity,their carbides would form in preference to cementite.Small amount even in micro content of alloying elements such as Al,Ti,Nb and V are very effective in delaying the grain growth since these elements are present as highly dispersed carbides and they require very high temperature to dissolve the carbides into the solution.It was observed that the solubility of carbides in the austenite increases in the order of NbC,TiC and VC [118].

Depending on the composition of the alloying elements Al,Mn and C in Fe-Mn-Al-C ferritic steel,κ-carbide phase generally forms below 800°C.In the earlier sections,it has been already presented how the morphology,volume fraction and distribution of the κ-carbide have profound effect on the workability and mechanical properties of these ferritic steels.Effect of various alloying elements such as Si,Mn,Ti,Nb,Mo,Zr and W on the Fe-Al-C alloys with 10.5 wt% to 11 wt% Al was investigated[84].A combination of air induction melting and electroslag refining process was used to melt these alloys,hot-rolled to plates and examined.Considerable solubility was noticed in κ-carbide precipitate as well as ferrite matrix of the alloy for Mn,Mo and W.On the other hand,solubility of Si was very much higher in κ-carbide precipitate.It was observed that Ti,Nb,Mo and Zr lead to the formation of their respective carbides.Addition of Mn,Mo and Zr marginally improved the room temperature ductility.On the other hand,Ti,Nb and W have resulted in poor ductility.Si addition has severely impaired the ductility of the alloy rendering it highly brittle.

The effect of carbide forming alloying elements(Ti,C,Nb and Ta)with respect to strengthening and its stability was investigated on Fe-(7.16 and 13.6)Al-0.216C steel [30,119,120]. The long duration homogenisation treatment of Fe-7.16Al-0.216C alloy with Nb and Ta have shown the formation of (Fe,Al)2Nb and (Fe,Al)2Ta Laves phases.The steel with Ti addition has only shown TiC precipitates whereas steel with vanadium addition has displayed V4C3type of carbide precipitate.The phase formation was predicted for the Fe-7.16Al-0.216C alloy with Nb,Ta,Ti and V as the alloying elements and was noticed to agree with the experimental results.

The role of Nb(1.5 wt% and 3.5 wt%)on Fe-8.5Al and 10.5Al with low carbon was investigated [121,122]. It was observed that addition of Nb leads to the formation of Nb2C carbide precipitate in addition to κ-carbide precipitates in the ferritic matrix.Additionally,for the alloy with 3.5% Nb,Fe2Nb Laves phase was noticed.

The influence of Nb and carbon additions on the ferritic Fe-8Al-5Mn steel was investigated [36,123]. Precipitates such as NbC and κ-carbide in ferritic matrix are noticed.Solubility of Mn is observed in the κ-carbide precipitate and matrix.During hot rolling of the steel,these precipitates(NbC and κ-carbide)are expected to hinder the dynamic recrystallisation of the ferrite grains resulting in elongated grains in the rolling direction.Higher the carbon content(0.05),more is the elongation of the grains compared to the un-recrystallized elongated grains in lower carbon content (0.005 and 0.02) alloy.During annealing process,the κ-carbide precipitates acts as nucleation sites and promote static recrystallisation and help to refine the grains by particle-simulated nucleation(PSN) mechanism [124].Hence,the alloy with 0.05C show uniformly distributed fine grains as these precipitates retard the growth of the recrystallized grains [123].

Khaple et al.,2020[125]have recently reported the in-situ effect of Nb(0.2 to 1 wt%)addition on Fe-7Al-0.35C steel.They reported that Nb addition lead to the in-situ formation of niobium carbide and κ-carbide precipitates which caused grain refinement from 320 to 80 μm.The presence of these phases was predicted by using ThermoCalc programme which have been reported to agree well with the experimental results [125].Earlier attempts to add Nb to Fe-Al-C steels resulted in very low ductility [30,120-123]which may be related to susceptibility of these alloys due to hydrogen embrittlement.Their main objective was to improve the high temperature properties of higher Al containing Fe-Al alloys[30,120-122]with higher Nb content to have niobium carbide and lave phases.All the alloys reported by Khaple et al.,2020 [125],exhibited significant (20% or more) tensile elongation.Also,about 80% increase in the yield strength (430-732 MPa) was witnessed with the Nb addition (Table 5).

Table 5The tensile properties of the various Fe-Al-C based lightweight steels.

In recent times,TiC has also been considered as one of the most essential reinforcing carbide in composites based Fe and Fe-Al matrix because of its excellent combination of properties such as,high modulus,low density,high temperature stability and high hardness.These high modulus and hardness TiC precipitates in Fe-Al based composites drastically improved the important properties such as high temperature strength and wear resistance.

Significant research has been carried out to understand the solidification aspects of TiC precipitates in Fe based alloys and it has been noted that on the whole the evolution of TiC reinforced composite microstructure can be controlled by parameters such as cooling rate in the formation of TiC particle morphology.Various studies have been carried out to prepare composites based on Fe3Al-TiC and FeAl-TiC in vacuum using arc melting or induction melting process [126-128].They focused mainly on the growth morphology of TiC particles.The studies on the in-situ formation of TiC precipitates in Fe-Al ordered and disordered solid solution is very limited[30,99,120,129].It is expected that micro-alloyed high strength steels are basically steels containing small amount of alloying additions such as V,Nb and Ti [130].These alloying elements operate as solid solution or form precipitates to suppress the recrystallisation and grain growth behaviour of the austenite.Micro-alloying of carbon steels is usually practised.Grain refinement can be enhanced by using these micro-alloying elements to get better combination of mechanical properties.As the cost of alloying elements such as Nb and V is much higher than Ti,the development of lightweight steels with Ti as alloying element is recently getting more attention.The effect of Ti (0.05-0.23 wt%)addition on carbon containing steel has been investigated [130].It was observed that Ti addition causes considerable grain refinement which is attributed to the formation of TiC or TiN which retards the grain growth during hot processing resulting in decreased ferritic grains.Good improvement in the mechanical properties such as hardness,strength and impact energy was detected by the addition of titanium in the steel.

Effect of Ti and C on as-cast ferritic lightweight steel has been examined [129].In this study,the Ti and C content was taken in stoichiometrically quantity such that only TiC precipitate forms in the steel.No precipitate is observed in the base steel,whereas primary and secondary TiC precipitates were observed in all the other steels within the ferritic matrix.It was noted that as the carbon and titanium content increased in the alloy,the size and the volume fraction of the TiC precipitate increased.The increase in compressive yield strength is attributed to the composite strengthening caused by higher volume fraction of harder TiC precipitates.

The effect of 0.5 wt% Ti on Mn containing low-density steel(Fe-7Al-32Mn-2.2C) was examined in the cast condition [131].JMatPro software was used to predict the physical properties and the phase evolution,which was validated successfully with the experimental results.In this steel a density reduction of 16.3% was reported compared to the density of the conventional steels.

Khaple et al.,2018 have reported the work on in-situ effect of Ti(0.2 to 1wt%) addition on Fe-7Al-0.35C stee [132].They have reported the presence of two carbide precipitates namely,TiC and κcarbides in ferrite matrix which lead grain refinement from 162 to 48 μm.The presence of these phases was predicted by using ThermoCalc programme which have been reported to agree well with the experimental results [101].Earlier attempts to add Ti to Fe-Al-C steels resulted in very low ductility [30,121,122]which may be related to lower susceptibility of these alloys to hydrogen embrittlement.All the alloys reported by Khaple thesis 2021 [101]containing Ti with Fe-7Al-035C exhibited significant (18% or more) tensile elongation.The yield strength of steel increased moderately from 408 MPa to 568 MPa with the Ti addition from 0.2 to 1.0.These lightweight steels with all the above alloying additions show a reduction in density of～10% compared to the conventional steel(7874 kg/m3) (Table 5).

12.Application based properties of ferritic low-density steels

12.1.Magnetic properties

Commercially available soft magnetic materials comprise of high purity iron,low-carbon steel,iron-silicon alloys,ironaluminium alloys,iron-aluminium-silicon alloys,iron-nickel alloys,iron-cobalt alloys and soft ferrites [134-136]. Iron-silicon alloys are by far the widely used magnetic materials based on the consumption.The iron-silicon(3-5 wt% Si)alloys can be employed in both ac and dc applications.More sophisticated cold rolled grain oriented(CRGO)silicon steels are used for large generators,power and distribution transformers.The non-grain oriented silicon steels are generally employed in small motors,generators,small power transformers and relays.Commercially high purity iron is used in electromagnet cores and relays.Iron-nickel (50wt% Ni) are expensive and are used in high frequency power and current transformers,magnetic amplifiers,sensitive relays,telephone receivers etc [134,135,137].Generally,these soft magnetic materials have poor mechanical properties particularly at elevated temperatures,and they tend to lose their soft magnetic properties at high temperatures[136-138].This has limited their application at elevated temperatures.However,many industrial applications(e.g.,rotors in high-speed generators,magnetic bearings in engines,auxiliary power units,etc.)require soft magnetic materials which can retain their soft magnetism at high temperature [134,135,137]and can with stand high stresses at elevated temperatures,i.e.,they need good mechanical properties at elevated temperatures in addition to retaining their soft magnetic properties at these temperatures.Currently,such inexpensive soft magnetic materials are not commercially available.

Recently,renewed interest has been shown on the research of iron-aluminium based lightweight steels with lower level of Al(1-7) wt% for automobile applications.It has been shown that these steels have shown good ductility (30%) [4,13,33]and reasonable high temperature properties at elevated temperature[32,97]. These steels display large grain size which make them a potential candidate as soft magnetic material [11,33,139].

Fe-Al alloys containing low level of Al ＜7 wt% can be a potential soft magnetic material for room and elevated temperature application due to (a) reduced eddy current losses,(b) superior mechanical properties at room and elevated temperature (c) higher resistance to oxidation at elevated temperature and(d)low density.These alloys are also suitable to the special environment such as nuclear radiation,shocks acceleration etc.Thus,in recent years,there has been a significant interest in Fe-Al alloys as soft magnetic materials.Some of the soft magnetic properties of Fe-Al alloys in comparison to other soft magnetic alloys is given in Table 6[134-138].

Table 6Soft magnetic properties of Fe-Al lightweight steel with other soft magnetic alloys [134-138].

12.2.Damping properties

Improvement in the damping properties of the components and the system with respect to acoustic and vibration can be achieved by using damping alloys [140,141]. There is a continuous demand for the development of damping materials possessing high damping capacity,high strength,low-density along with low cost[140-144].These damping materials find various usage in domestic appliances,automobile industry,defence vehicles such as warships,seismic operations etc.To reduce the unwarranted noise and vibrations [140,141,145]. Some of the generally used high damping alloys in the industry are Mg-Zr based alloy,Cu-Zn-Al alloy,Mn-Cu based alloy,Cu-Al-Ni,Ti-Ni,Ni-Co based alloy,Fe-Mo based alloys [140-146].Fe-Al based low density steels have been assessed to display high damping properties because of their behaviour of high saturation magnetostriction in a relatively low magnetic field and are of relatively less costly compared to the other damping alloys used.It was reported that these steels offer exceptionally good damping properties,more particularly the logarithmic vibration decrement exceeding,δ＞30%[145-147].

Several attempts were made to develop the optimum composition,processing route and various heat treatment conditions to get the maximum damping performance of these low-density steels [143-147].Yasuda et al.,2009 have reported the strength and damping properties of Fe-(6.8-12.3)wt% Al[142].Addition of very small amount of Ni resulted in very high yield stress (above 1 GPa) and magneto-mechanical damping capacity of 0.032.Emdadi et al.,2015 have studied the effect of grain size and cooling rate on damping properties of Fe-5.4Al-0.05Ti steel [144].They found that annealing for longer duration or at higher temperature improves the damping capacity.They also reported that the heat treatment schedule of the rolled sheet has a profound effect on the damping properties of the alloy.

12.3.Oxidation properties

Fe-Al based lightweight steel have good oxidation resistance due to the formation of thin adherent alumina layer similar to the formation of chromium layer in case of ferritic stainless steel.Fe-Al based alloys with higher Al content(above 12 wt%)have displayed excellent oxidation resistance up to moderate temperature of 600°C.These alloys have shown better sulphidation resistance compared to the alloys industrially used in this environment[148-151].Fe-Al alloys with lower Al content have shown moderate oxidation resistance.The oxidation resistance of lightweight steel decreases with the increase in Al content.Addition of Cr have improved the oxidation resistance.

For Fe-6Al steel,when oxidised at 800°C for 25 h,has shown weight gain of 0.2 mg/cm2whereas Fe-8Al with 1wt% C has shown increased weight gain of 2 mg/cm2(an order of magnitude oxidation) for similar condition [148,152].The difference is mainly attributed to the presence of carbon.Shankar et al.,2003 have shown (using scanning Auger electron spectroscopy analysis) that the carbides in Fe-Al alloys undergo preferential oxidation even when the Al content of Fe-xAl-1C is as high as 16%[153].But the deterioration was more significant in Fe-8% Al alloy.Presence of carbon in lightweight steel declines the oxidation properties of the steel.

The oxidation behaviour of Fe-5Al(wt%)alloy was studied from 500 to 1000°C under 1 atm partial pressure of oxygen[151].It was reported that highest oxidation rate was at 800°C and the lowest oxidation rate was observed at 1000°C.This inversion of oxidation at 1000°C was attributed to the formation of Al2O3.Similar trend was reported in case of Fe-4.94Al (wt%),when oxidised between 450°C and 900°C at 700 Torr oxygen partial pressure[152].It was reported that the oxidation rate increased with temperature up to 570°C and then decreased to a minimum at 850°C He showed that a scale composed of Fe2O3,Fe3O4and FeAl2O4formed on Fe-4.9Al alloy(below 570°C).This is attributed to the high oxidation rate at this temperature compared to the poor barrier offered by these oxides to the migration of Fe from the substrate to the surface[152,153].Hence,for the lightweight ferritic steels to be exposed to high temperature,the Al content should be at 7wt% and above can be coated to improve the life of the component.

12.4.Weldability

Welding dissimilar combinations of different lightweight steels to light weight metals is becoming increasingly popular[154-156].The aerospace vehicle,chemical,thermal power plant,and petrochemical industries are the key drivers of this demand.This is motivated only by the desire to lighten components by dissimilar welding.Fe-Al based steels have mechanical qualities comparable to dual phase steel while having a 10% lower density [13,101].Fabrication of lightweight steel components with other steels is critical to achieving these benefits [4].

Understanding the microstructure of these different joints is a key problem in the joining of ferritic lightweight steels with other steels [3-5,156-158].The disparity in melting points,reactivity,metallurgical,and thermal properties of dissimilar materials causes a challenge in welding [154-162].Various studies have been reported on the degraded weldability of lightweight steels[157-162]but extensive exploration is required to find the cause and engineering solution.Arc welding,laser welding,friction stir welding,plasma arc welding,electron beam welding [158,159,162,163],and other welding processes[157,160,162]have all been utilised to join lightweight steels to other steels.Porosity,micro fractures,lack of fusion,segregation,undercut,distortion,and other weld flaws can occur during welding.The creation of intermetallic compounds and the development of residual stress are two further unavoidable welding faults [157-162].

The heat affected zone in arc welding process is due to very high heat input as compared to EBW which ends up within the formation of various brittle Fe-Al intermetallics[160-164].Compared to conventional fusion welding processes such as gas metal arc welding,laser welding and electron beam welding(EBW)of ferritic lightweight steels exhibits great advantages such as low power input,high efficiency,very high power density,low deformation,low heat affected zone (HAZ),high depth to width ratio,low distortion of microstructure,less contamination and less defects[165-167].EBM has been used to weld ferritic lightweight steel with other steels.Beam oscillation was altered in EBW to control weld microstructure,porosity,and intermetallics,as well as improve joint mechanical performance.Dissimilar welding studies on ferritic steel with ordinary carbon steel were reported by Dinda et al.,2016[165].Welding was done at various speeds and with and without beam oscillations.The finer grain size has resulted from the increased welding speed.The welding method with beam oscillation produced a microstructure that was uniform and homogeneous.The application of beam oscillations improves the weld quality and performance of EBW dissimilar steel to Fe-Al junctions,according to this study.2016,Dinda et al.have successfully used laboratory source XCT techniques with sub-micron resolution to visualize and measure porosity in weld beads under diverse weld production circumstances [165].Because EBM is so expensive,various traditional welding processes for the manufacture of lightweight steel must be evaluated before being adopted on a large scale.

12.5.Formability

Formability refers to a material's ability to change shape without necking failures,which is vital for automotive and other strip applications.The strain hardening exponent and the normal anisotropy factor are commonly considered the most relevant markers for sheet formability.

Stretch-forming and deep-drawing abilities are also assessed using other tests such as bending and hole expansion.The strain hardening exponent of the Fe-6.8 to 9wt% Al steel is in the range of 0.17 to 0.13 [4,13].The value decreases with increasing Al content which is due to solid solution strengthening.The strain hardening values of Fe-7Al with diboride (TiB2and ZrB2) resulted high ductility of about 38% and a strain hardening exponent of 0.165[37,101].The tensile ductility reported by addition of Nb(0.2-1.0)to Fe-7A-0.35C steel is in the range of 30.2 to 20.1 whereas the strain hardening exponent is in the range of 0.161 to 0.140 [125].The tensile ductility reported by addition of Ti(0.2-1.0)to Fe-7A-0.35C steel is in the range of 28.6 to 18.4 whereas the strain hardening exponent is in the range of 0.174-0.187[101].By the addition of Nb/Ti the strength has improved and ductility also (＞18%) which is much above the (12%) minimum ductility that required for the formability operation of the steels [168].

The normal anisotropy values for the Fe-6.8 to 9 wt% Al steel are(1.37-1.16) drastically lower than that of the IF steel (2.3).The planar anisotropy values for the Fe-6.8 to 9wt% Al steel are(1.33-1.357) drastically higher than that of the IF steel (0.67).The strain hardening exponent for the DP steels(DP450,DP500,DP600)is in the range of 0.16 to 0.14,whereas the normal anisotropy is about～1.0 [4,13].It is clear that the formability property of Fe-Al lightweight steel is poor compared to the IF steel,but these steels have higher tensile strength.These lightweight steels have superior formability than the DP steels mentioned above.Hence,lightweight Fe-Al-based steels have slightly better deep drawing capabilities when compared to DP steels.As a result,the development of Fe-Al solid-solution alloys to replace some existing DP grades is a viable option.When compared to DP steels,the Fe-Al steels have a significant advantage in terms of processing.

13.Conclusions

The steels have been in growing demand because of their excellent combination of properties despite the evolution of lightweight materials(such as Al,Mg and Ti alloys,and various polymer,ceramic and composite materials).Nevertheless,the high density of steels and its poor corrosion and oxidation behaviour restricted some of its widespread applications.

Disordered Fe-Al alloys are an emerging class of low-density/lightweight steels containing 6-9 wt% of aluminium in steel.These Fe-Al alloys have raised considerable interest due to their low-density,high ductility,cost-effectiveness and feasibility for bulk production.

The Fe-Al ferritic steels received relatively less attention in the early steel literature,probably because of their unappealing combination of strength and ductility.In the recent past,the various research groups investigated Fe-7 wt% Al lightweight steels(which is a single phase ferritic steel among the group of Fe-Al steels).The addition of aluminium (to the extent of 7 wt%) to Fe reduces its density by 10%.Fe-7Al alloy exhibit single phase ferritic microstructure.

In particular,reduction of grain size and subsequent improvement in the mechanical properties of single phase ferritic Fe-7Al alloy was achieved with the addition of inoculants such as diborides(ZrB2and TiB2)and strength improvement with the addition of carbon.It was reported that strong carbide formers such as Nb and Ti resulted better strength and moderate ductility of Fe-7Al-0.35C based steel.

It is also reported that the formability of Fe-Al lightweight steel is inferior to that of IF steel,but possess higher tensile strength.However,these ferritic lightweight steels are more formable than the DP steels.Hence,lightweight Fe-(6.8-8.1)Al-based steels have slightly greater deep drawing capabilities than DP steels.Also,the Fe-Al steels have a significant advantage in terms of processing when compared to DP steels.It is also reported that Fe-6.8Al and Fe-8.1 A l lightweight steels are highly beneficial as compared to IF,DP 450,DP600 steels for automobile components with respect of stiffness,dent resistance and crash resistance in terms of weight saving and performance improvement.

The focus of future work on the ferrite lightweight steels should be carried out to fulfil the gap areas of research on understanding the microstructure,physical properties,formability,weldabiliy and elastic modulus.Future work on these alloys should concentrate on improving the Young’s modulus and the deep drawability aspect of lightweight steels.

The other aspect should be to scale-up the production of these ferritic lightweight steels in terms of melting,processing,and other fabrication issues on industrial scale.Intensive application tests,such as formability,fatigue,coatings,weldability are needed to understand the relationship between the application,properties,the governing microstructural parameters and provide proper insight for the use of these steels as automotive part.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.