The effects of Ca and Mn on the microstructure,texture and mechanical properties of Mg-4 Zn alloy

2021-10-28 10:09:54ChunqunLiuXinhuChenJioChenAnrejAtrensFushengPn

Journal of Magnesium and Alloys 2021年3期

Chunqun Liu,Xinhu Chen,b,c,∗,Jio Chen,Anrej Atrens,Fusheng Pn,b

a International Joint Laboratory for Light Alloys(MOE),College of Materials Science and Engineering,Chongqing University,Chongqing 400045,China

b National Engineering Research Center for Magnesium Alloys,Chongqing University,Chongqing 400045,China

c Shenyang National Laboratory for Materials Science,Institute of Metal Research,Chinese Academy of Scihences,Shenyang 110016,China

d School of Mechanical and Mining Engineering,The University of Queensland,St Lucia Qld 4072,Australia

Abstract The microstructural evolution,texture and mechanical properties of nine Mg-4Zn-xCa-yMn alloys(x=0.3,0.6,1.0;y=0.2,0.3,0.7wt.%)were investigated systematically.Alloying with Ca and Mn refined the grains of the extruded sheets and increased the unDRX fraction.Mn could be the heterogeneous nucleation site of Ca2Mg6Zn3 phase because of a good atom matching at the orientation relationship of()Mn//(111)Ca2Mg6Zn3,〈5〉Mn//〈1103〉Ca2Mg6Zn3.The traditional texture weakening effect of Ca was strongly decreased for the simultaneously addition of Mn.With increasing Ca and Mn concentration,the strength increased and ductility decreased.Mg-4Zn-0.6Ca-0.7Mn exhibited a good combination of ultimate tensile strength(320MPa),yield strength(286MPa)and elongation(16%).A model of strengthening indicated that grain boundary strengthening and precipitate strengthening made a large contribution to the strength of Mg-4Zn-0.6Ca-0.7Mn.In addition,the dynamic recrystallization,texture modification and the strengthening effect from different parts also have been analyzed in detail.© 2020 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Mg–Zn–Ca–Mn alloy;Texture;Dynamic recrystallization;Particle-stimulated nucleation;Mechanical properties.

1.Introductio n

Wrought magnesium(Mg)alloys have had a resurgence in the past decade,driven by the interest in the potential weight reductions that may possible by the Mg alloy replacing aluminum alloys or steel[1,2].Nowadays,Mg alloys from the Mg–Zn system are of interest for biomaterials engineering as well as for industrial application as lightweight structure materials[3].Alloying with zinc can improve the strength of Mg alloys and suppress the adverse effects of the impurity elements on the corrosion resistance[4].Currently,however the application of Mg–Zn binary alloy has been limited by some poor properties,such as the large crystallization temperature interval and the hot cracking tendency[5,6].Therefore,it is necessary to alloy the Mg–Zn alloy with additional elements in order to improve the microstructure and mechanical properties.

Alloying with rare-earth(RE)elements has been conducted in many studies[7–9].Liu et al.suggested that the addition of Y element can improve the aging hardening behavior and the mechanical properties[7,8].The corrosion resistance of Mg–Zn–Y alloy can be enhanced by the addition of Gd element[9].However,despite the significant improvements of the performance of Mg alloys,the distinct disadvantage of the RE elements is their high cost[10,11].Therefore,it is necessary to research more cost-effective alloying elements which produce mechanical properties improvements similar to those provided by the RE elements.The increasing interest in the Mg–Zn–Ca system has been motivated by the beneficial combination of low cost and desirable properties,such as a relatively high corrosion resistance,strength and plasticity[12,13].Ca improves the corrosion resistance due to the formation of a more-corrosion resistant oxide layer,and weakens the basal texture,achieving a good balance of strength and ductility[14,15].Zn together with Mg may form various Mg–Zn intermetallic compounds.Ca and Zn together in the Mg alloy could form the Ca2Mg6Zn3stable intermetallic compound,which is favorable for elevated temperature applications[16,17].But zinc contents over 4.0wt.% caused grain coarsening of Mg–Zn–Ca alloys,which decreased the mechanical properties[18].Additionally,Ca contents above 1wt.% cause sticking or hot tearing during casting[19].Therefore,alloying research should find out the most appropriate element content to produce high-performance alloys.

Moreover,manganese(Mn)is a common alloying element in Mg alloys,because it improves the creep behavior and damping capacity,and refined the microstructure of asextruded Mg–Zn,Mg–RE–Zn etc.Mg alloys[20,21].Alloying with Mn can improve the formability,thermal conductivity and modify the microstructure[22].Based on these desired advantages,researchers have begun to focus on the Mg–Zn–Ca–Mn series of alloys.Cho et al.[23]investigated the effect of Mn on the corrosion properties of biodegradable Mg-4Zn-0.5Ca-xMn alloys.Jiang et al.[24,25]studied the effect of speed on high-speed extrusion of Mg-0.2Zn-0.3Ca-0.1Mn on the microstructure,grain orientation and mechanical properties.Tong et al.[26]found in Mg-5.52Zn-0.6Ca-0.3Mn an interesting un-dynamic recrystallization micro-structure,which exhibited a strong basal texture.These existing researches have mainly focused on the influence of a single alloying element or different processing technology on the microstructure and properties of Mg–Zn–Ca–Mn alloys.

There has been no systematic study about the effects of different Ca/Mn ratios on the evolution of microstructure and properties,such as the dynamic recrystallization behavior,texture modification,and mechanical properties.In fact,there is limited understanding of the effect of the addition of a single alloying element on the microstructure and mechanical properties,and moreover it is necessary to add several elements to achieve the required performance.Furthermore,an effect from one alloying element may be changed by the addition of multiple alloying elements,resulting in a promoting or inhibiting effect.For example,excessive addition of Zn can inhibit the anti-corrosion effect of Zr in Mg–Zn–Zr[27,28].In contrast,the simultaneous addition of Y and Ce into Mg–Zn–Zr can mutually promote the precipitation strengthening effect of each element[29].

Then,how do different contents of Ca and Mn affect the structure,the formation of secondary phase and the properties of the Mg–Zn–Ca–Mn alloy system?The interaction between Ca and Mn is still unclear.Understanding these interactions is expected to help to optimize alloying and to achieve a better balance between alloying and performance.Therefore,in this work,different Ca/Mn ratios were used in order to study these problems.This study used nine Mg-4Zn-xCa-yMn alloys(x=0.3,0.6,1.0;y=0.2,0.3,0.7wt.%)to investigate the effect of systematic addition of Ca and Mn on the microstructure,texture and mechanical properties.A good coherent relationship was discovered between the secondary phase and the Mg matrix by FFT analysis.The dynamic recrystallization(DRX),texture evolution and the mechanism of strengthening are studied in detail.

Table 1Chemical composition of the Mg-4Zn-xCa-yMn alloys(wt.%).

2.Experimental method

Alloys of nominal composition Mg-4Zn-xCa-yMn(x=0.3,0.6,1.0;y=0.2,0.3,0.7wt.%)were prepared from pure Zn(99.90wt.%),Mg(99.85wt.%),and Mg-20Ca(wt.%),Mg-2Mn(wt.%)master alloys.Each alloy was melted in an induction heated iron crucible and protected by a gas mixture consisting of CO2(99.99vol.%)and SF6(1vol.%).Upon reaching the temperature 750°C,the melt was stirred for 5min and subsequently held for 15min at 720°C to ensure that all alloying elements had dissolved.The melt was poured into a stainless-steel mold.The compositions of nine alloys are presented in Table 1.

The as-cast ingots were homogenized at 350°C for 24h and 420°C for 12h,followed by quenching into water at 25°C.Thereafter,the ingots were hot extruded at 300°C into cylinder bars with a diameter of 16mm by applying a constant force using a XJ-500 Horizontal Extrusion Machine with an extrusion ratio of 20:1 and a die-exit speed of～12mm/s.The microstructures of the as-cast and as-extruded alloys were characterized using(i)optical microscopy(OM),(ii)a JEOL JSM-7800F field emission scanning electron microscope(SEM)equipped with a HKL Chanel 5 electron backscattered diffraction(EBSD)system,and(iii)transmission electron microscopy(TEM,Zeiss LIBRA 200FEL).The OM and SEM samples were ground finally with 3000 grit SiC paper,polished using 0.25μm diamond paste,ultrasonically cleaned for 10min in acetone and 10min in ethanol respectively.For OM observation,the polished specimens were etched in a solution of 20ml ethanol,1g picric acid,2ml glacial acetic acid,and 2ml distilled water.4mm×6mm×2mm block samples for EBSD observation were electropolished at−30° and 20V for 90s,using a solution containing 100ml isopropanol,800ml ethanol,18.5ml distilled water,10g hydroxyquinoline,75g citric acid,41.5g sodium thiocyanate and 15ml perchloric acid.The EBSD results,includes the grain size,the volume fraction of DRXed area and micro-texture analyses were analyzed by using the Channel 5 software.Thin foil TEM specimens were prepared through mechanical polishing and ion-beam thinning using a Gatan Precision Ion Polishing System at room temperature.The phase analysis was identified with X-ray diffractometer(XRD,a Rigaku D/MAX-2500PC)using Cu-kαradiation with a scanning angle from 10° to 90° and a scanning speed of 2°/min.The macro-texture was also determined by XRD.The tensile specimens with a gauge length of 25mm and a gauge diameter of 5mm were machined from the extruded bar with the tensile axis parallel to the extruded direction(ED).The tensile tests were carried out at room temperature using a CMT5105 material testing machine at a strain rate of 10−3s−1.The yield strength(YS),ultimate tensile strength(UTS)and elongation to fracture were average values of at least three individual repeated tests.

Fig.1.(a)XRD patterns of as-cast Mg-4Zn-0.3Ca-0.2Mn,Mg-4Zn-0.6Ca-0.3Mn and Mg-4Zn-1Ca-0.7Mn;(b)corresponding magnified patterns between 20°to 50°.

3.Results

3.1.Microstructure

Fig.1 shows the XRD spectra of Mg-4Zn-0.3Ca-0.2Mn,Mg-4Zn-0.6Ca-0.3Mn and Mg-4Zn-1Ca-0.7Mn in the cast state.Besides theα-Mg phase,Ca2Mg6Zn3and MgZn phase were also been presented.Ca2Mg6Zn3was hardly detected in Mg-4Zn-0.3Ca-0.2Mn due to the rare addition of Ca element.A Mn-rich phase was not detected,which is attributed to the small amount of Mn,so that the amount of the Mn-rich phase did not reach the detection sensitivity of the diffractometer.The optical microstructure examinations(not showing here)indicated that the Mn addition has no obvious refinement effect on the as-cast alloy,because the Mn cannot generate nucleation during the solidification[26,30].Yu et al.[30]reported that Mn and Mg did not form any intermetallic compounds,but the Mn existed only in the form of elementary Mn.In contrast,the grain size decreased significantly when the Ca concentration reached 1.0wt.%,which is consistent with Ref.[31].The SEM micrographs of as-cast Mg-4Zn-0.3Ca-0.7Mn and Mg-4Zn-0.6Ca-0.2Mn are presented in Fig.2.The secondary phase in two alloys exhibited two morphologies:bulk and irregular strips.According to the EDS and XRD analysis,the secondary phase particles were identified asα-Mn(point A),MgZn(point B),and Ca2Mg6Zn3(point C).

Fig.2.SEM images of as-cast Mg-4Zn-xCa-yMn alloys:(a)Mg-4Zn-0.3Ca-0.7Mn,(b)Mg-4Zn-0.6Ca-0.2Mn.

Fig.3 shows the optical micrographs of the as-extruded Mg-4Zn-xCa-yMn alloys.Many irregular bulky,banded secondary phase particles were distributed along the extrusion direction,which may be ascribed to the Ca2Mg6Zn3phase with high temperature stability being crushed during hot extrusion.Fig.4 shows the effect of Ca and Mn on the grain size of dynamically recrystallized(DRXed)grains and the fraction of DRXed.The grain size reduced from 7.1μm(Mg-4Zn-0.3Ca-0.2Mn)to 1.9μm(Mg-4Zn-1Ca-0.7Mn)and the fraction of DRXed area reduced from 95%(Mg-4Zn-0.3Ca-0.2Mn)to 42%(Mg-4Zn-1Ca-0.7Mn).Two elements both have a weakening effect on grain size and the DRXed fraction.In particular,the influence aroused from 0.3 wt% to 0.6 wt%(Ca)and 0.2 wt% to 0.3 wt%(Mn)was obvious.But the effect aroused from 0.6 wt% to 1.0 wt%(Ca)and 0.3 wt% to 0.7 wt%(Mn)was not significant.Zhang et al.also[32]found that the refinement effect became weak when the Ca content reached 1.0wt%.

Fig.3.Optical micrographs of as-extruded Mg-4Zn-xCa-yMn alloys,(a)–(i)represent alloy I-IX,respectively.

Fig.4.The effect of Ca and Mn elements on the grain size of DRXed grain and the fraction of DRXed.

Fig.5 identifies the secondary phase particles in the extruded alloys.According to the EDS analysis,the banded,irregular bulky secondary phase particles were mostly Ca2Mg6Zn3.Some non-fixed-shape elemental Mn particles were attached to the Ca2Mg6Zn3phase particles and there were some small sized MgZn phase particles.These small precipitates(<1μm)were diffusely distributed in the matrix(marked as the yellow circle).Some fine recrystallized grains were distributed around the coarse particles.This is typical characteristic of particle-stimulated nucleation(PSN),an important nucleation mechanism of magnesium alloy during DRX[33].After homogenization,these particles provide more PSN nucleation sites during the deformation process and increase the DRX nucleation rate[34].The coarse particles can accumulate lots of dislocation and storage energy,which provides a favorable condition to nucleate.The DRX fraction of the extruded alloy did not increase,on the contrary,it gradually decreased.The reason is discussed in section four.

Fig.5.SEM micrographs of as-extruded Mg-Zn-xCa-yMn alloys:(a)Mg-4Zn-0.3Ca-0.2Mn,(b)Mg-4Zn-0.6Ca-0.2Mn,and(c)Mg-4Zn-0.3Ca-0.7Mn.

Fig.6.TEM bright field images of as-extruded Mg-4Zn-xCa-yMn alloys:(a and b)Mg-4Zn-0.3Ca-0.2Mn;(c and d)Mg-4Zn-0.6Ca-0.2Mn.

TEM examination was used to further analyze the morphology and composition of the dispersed precipitates in the extruded alloys.Fig.6 shows the TEM bright field images of extruded Mg-4Zn-0.3Ca-0.2Mn and Mg-4Zn-0.6Ca-0.2Mn.Both alloys contained many precipitates,in the form of granules,short rods and irregular blocks.EDS analysis indicated that the precipitates distributed along grain boundaries(GBs)were Ca2Mg6Zn3phase(indicated by the dotted line),which had been also previously observed in many as-extruded Mg alloys[35,36].Du et al.had suggested that Zn and Ca were easily precipitated as Ca-Zn clusters during extrusion due to the lower mix entropy[37].What’s more,grain boundary contains more vacancies and larger distortions,which can provide a more favorable nucleation site for dynamic precipitation during extrusion[38].Therefore,many precipitates were observed along the GBs.

Fig.7.(a,b)TEM bright field images of as-extruded Mg-4Zn-0.3Ca-0.2Mn;(c)corresponding selected area electron diffraction(SAED)pattern of point A;(d)–(f)represent the corresponding EDS analysis of point A～C,respectively.

In order to further analyze these fine precipitates,Fig.7 shows TEM images and corresponding EDS analyses for Mg-4Zn-0.3Ca-0.2Mn.Many fine precipitated particles varying from 10nm to 300nm were located along the grain boundary and within grain interiors.The corresponding EDS analyses(Fig.7c–f)reveal two types of secondary phase particles.The Ca2Mg6Zn3phase was marked as points A and B.It was identified by the selected area diffraction patterns(SADPs),taken along the[101]zone axis.The Ca2Mg6Zn3phase was identified by Tong et al.[39,40]as a hexagonal close-packed structure.Point C detected Mg,Zn,Ca and Mn.However,these elements may be overestimated because of the closeness of the Ca2Mg6Zn3phase.The EDS analysis and the morphology(the darker colored block-shaped particles were surrounded by lighter-colored disk-shaped particles)indicate that point C is elemental Mn.Furthermore,we can predict that Ca2Mg6Zn3grew upon the Mn particle as a heterogeneous nucleus.

Fig.8.(a～c)HRTEM images of Mn phase and Ca2Mg6Zn3 phase;(d～f)corresponding FFT analysis of(a～c).

Table 2JCPDS and experimental parameters of Mg,Ca2Mg6Zn3 and Mn.

Fig.8 shows the HRTEM images of the Mn particle and the Ca2Mg6Zn3phase as well as corresponding FFT patterns.For comparison,the experimental values as well as the relative standard parameters of Mn and Ca2Mg6Zn3are presented in Table 2.The above measuredd-spacing valves were confirmed to be valid for identifying the secondary phase particles.The orientation relationships between the two kinds of secondary phases(Mn and Ca2Mg6Zn3)and the Mg matrix were exhibited as(5)Mn‖(101)Mg,〈13〉Mn〈11〉Mg;(01)Ca2Mg6Zn3//(101)Mg,〈101〉Ca2Mg6Zn3〈11〉Mg.Therefore,the relationship between the Mn particle and the Ca2Mg6Zn3phase can be speculated as(5)Mn//(01)Ca2Mg6Zn3,〈13〉Mn//〈101〉Ca2Mg6Zn3.The misfit(δ)between(5)Mnand(01)Ca2Mg6Zn3was calculated as 59%,which indicated that the Mn particle was not an effective heterogeneous nucleation site for the Ca2Mg6Zn3phase at this deductive orientation relationship.But we have observed the Ca2Mg6Zn3phase attached on the Mn particles in Fig.7(b),so a direct interface relationship is necessary.As shown in Fig.8(c and f),FFT patterns gives an orientation relationship of()Mn//(111)Ca2Mg6Zn3,〈5〉Mn//〈1103〉Ca2Mg6Zn3.The misfit at this condition was calculated as 9.3%.Combined with the morphology observation from Fig.7(b),it is reasonable to consider that the elemental Mn particles can indeed act as a heterogeneous nucleation site for the Ca2Mg6Zn3phase,promoting the precipitation of the Ca2Mg6Zn3particles from the magnesium matrix.

Fig.9.Pole figures of as-extruded Mg-4Zn-xCa-yMn alloys,(a)～(i)represent alloy I-IX,respectively.

3.2.Texture

Macro-texture is presented in the form of pole figures,as shown in Fig.9.The pole intensity point migrated from the(0001)plane(Mg-4Zn-0.3Ca-yMn)to the(100)plane(Mg-4Zn-0.6Ca-yMn and Mg-4Zn-1Ca-yMn).With increasing Mn content,the pole intensity of the Mg-4Zn-0.3Ca-xMn alloys slight decreased,from 1.8 to 1.6.However,the pole intensity of(100)plane in other alloys increased with increasing Mn content,from 1.8 to 2.6 in the Mg-4Zn-0.6Ca-yMn alloys and from 1.9 to 4.5 in the Mg-4Zn-1.0Ca-yMn alloys.As shown in Fig.9(d～i),the pole intensity position of the(100)texture became closer to the center,especially in Mg-4Zn-0.3Ca-0.7Mn and Mg-4Zn-1Ca-0.7Mn alloys.In contrast,the pole position of(0001)texture gradually moved to near the edge.

To analyze the texture modification and the mechanisms of dynamic recrystallization,IPF maps(on the left hand)and Schmid factor maps(on the right hand)are presented in Fig.10.The Mg-4Zn-0.3Ca-xMn alloys with high volume fraction of DRXed grains had relatively random orientations(Fig.10a-c).But the alloys with higher Ca and Mn concentrations had coarse unDRX grains with(010)planes parallel to the ED.The increasing fraction of blue grains was consistent with the trends of macroscopic texture presented in Fig.9.The critical resolved shear stress(CRSS)and the Schmidt factors(m=cosα×cosβ,whereαandβare the angles of tensile stress axis with respect to the optimal slip direction and basal plane normal,respectively)of basal slip controlled the degree of difficulty of plastic deformation,as the basal slip is the dominant deformation mode in Mg alloys at room temperature.The reddish grains in the Schmidt factor map indicate that the Schmidt factor was close to 0.5,and the color of blue grains were close to 0.There were many red-DRXed grains in the three Mg-4Zn-0.3Ca-xMn alloys,which indicated that basal slip dislocations were favorably operated in most grains when tension along the ED.For the other alloys,Fig.10e–g show that the Schmidt factor of the coarse unDRX grains was close to 0.So,basal slip was suppressed due to the hard-orientation in many grains,which was not conducive to improving the plasticity of these alloys.

Fig.11 shows the micro-texture and unDRXed area distribution of the Mg–Zn–Ca–Mn alloys.White areas represented unDRXed grains,and DRXed grains were blue.The fraction of white areas increased with increasing concentration of alloy elements,consistent with Fig.4.The micro-texture revealed the grain orientation of three areas:the entire region,DRXed region and unDRXed region.In the entire region,the smallest and strongest pole intensity appeared on Mg-4Zn-0.3Ca-0.7Mn and Mg-4Zn-1Ca-0.7Mn respectively.The pole intensity gradually increased with increasing alloying content,and the basal pole dispersed along the TD direction.In the DRXed region,the pole intensity showed a downtrend in Mg-4Zn-1Ca-yMn alloys.UnDRXed regions exhibited a relatively strong basal texture with a peak intensity as high as 23.7(Mg-4Zn-1Ca-0.7Mn),and Mg-4Zn-0.3Ca-0.7Mn was the smallest(11.91).Detail texture modification will be discussed in part4.

3.3.Mechanical properties

Fig.12 shows the typical tensile engineering stress-strain curves obtained from the studied samples.For Mg-4Zn-0.3CayMn(wt.%)alloys,with increasing Mn content,the YS increased from 165MPa to 206MPa,an increase of 41MPa;the UTS increased from 270MPa to 289MPa,and the elongation(EL)decreased from 30% to 26%.For Mg-4Zn-0.6Ca-yMn(wt.%)alloys,with increasing Mn content,the YS increased from 213MPa to 286MPa,an increase of 63MPa;the UTS increased from 278MPa to 320MPa,an increase of 42MPa,and the EL decreased from 24% to 16%.Mg-4Zn-1Ca-0.7Mn shows a much higher YS and UTS compared to the other alloys,with a YS of 327MPa and a UTS of 332MPa,and a poor elongation of 9%.In general,both of UTS and YS increased with increasing Ca content;the increment in YS was more significant than for the UTS,especially in Mg-4ZnxCa-0.7Mn(wt.%)alloys,the YS increased from 206MPa to 327MPa,an increase of 121MPa,and the UTS increased from 289MPa to 332MPa.Elongation decreased with increasing Ca content,the most significant decrease was for Mg-4ZnxCa-0.7Mn(x=0.3,1.0wt.%)alloys,from 26% to 9%,far more than for other alloys.The ductility of the Mg–Zn–Ca–Mn alloys gradually decreased with increasing concentrations of Ca and Mn.But it should be noted that,although not reported here,a trace of Ca addition improved the elongation of extruded Mg-4Zn and achieved a good balance between strength and ductility[41].After the simultaneously addition of Ca and Mn elements,such an improvement did not occur.A higher concentration of alloying elements decreased elongation and improved strength.

Fig.10.EBSD IPF maps(on the left hand of each part)and corresponding Schmid factor distribution maps(on the right hand)of as-extruded Mg–Zn–Ca–Mn alloys:(a～d)represent alloy I-IV,(e～g)represent alloy VII-IX.

4.Discussion

4.1.Dynamic recrystallization

In order to understand the microstructural evolution,we took the Mg-4Zn-0.6Ca-0.7Mn as an example to illustrate the dynamic recrystallization process,as shown in Fig.13.

(a)Before deformation,the coarse particles were located at the grain boundaries and within the grains.

(b)During the hot extrusion,these coarse secondary phase particles,consisting mainly of the Ca2Mg6Zn3phase,were crushed to many clusters,which heavily impeded the dislocation movement[42].Clusters of particles are more effective than single particles in causing classical PSN during recrystallization[43].Therefore,high density of dislocation around these particles can provide a great driving force for recrystallization.But,due to the non-uniform distribution of secondary phase particles,the dislocation density was lower in some regions but higher in other regions.This process is illustrated in Fig.13(a,b).

(c)The formation of sub-grains was caused by the accumulation of dislocation[44].Meanwhile,some particles precipitated during extrusion,as previously mentioned that the Mn acted as a heterogeneous nucleation site for Ca2Mg6Zn3phase particles,promoting the precipitation of Ca2Mg6Zn3from the magnesium matrix.These precipitates could hinder the movement of grain boundaries[45].In addition,grain boundary bulging was occurred,as shown in Fig.13c(indicated by the white arrows).This phenomenon was also observed by Ref[46],which named as strain induced boundary migration(SIBM).Due to the non-uniformity of the deformation,the original flat interfaces migrated to the side with a high dislocation density to form a crystalline core.These bulges may have later developed into new recrystallized grains as they grew into the grain mantle region[43].

(d)With the continued movement of dislocations,the misorientation was continuously increased,which provided conditions for the transition from low-angle grain boundaries(LAGBs)to high-angle grain boundaries(HAGBs)[47].This was a typical refinement induced by dislocation movement,via expending dislocation to acquire the transition of LAGBs to HAGBs(cDRX).Meanwhile,these bulges were also migrating further,and crystalline cores could grow later.

(e)Finally,the DRX process had completed,but the un-DRXed regions also existed(Fig.13d).This is because that some micro-areas with insufficient dislocation density can not drive the recrystallization[48].On the other side,the nanoscale precipitates seriously hindered the movement of grain boundaries and the expanding of the DRX area[49].Compare with the DRXed region,the unDRXed area included a greater number of dislocations because the transition process was suspended.

Fig.11.UnDRXed area distribution maps and corresponding micro-texture of as-extruded Mg–Zn–Ca–Mn alloys:(a～b)represent alloy III and IV,(c–e)represent alloy VI-IX,respectively.

4.2.Texture modification

The most interesting finding about texture modification in the present alloy systems is that traditional weakening effect of Ca on basal texture was disappeared.Previous study indicated that Ca additions can change the stacking fault energy and the axial ratio of the alloy,which results in the splitting of basal poles and weakens the texture intensity[50].But the addition of Mn in the present study caused this phenomenon to disappear.Taking Mg-4Zn-xCa-0.7Mn alloys as an example,the pole intensity point of Mg-4Zn-0.3Ca-0.7Mn was located at(0001)plane(Fig.9c).But the pole position had migrated to(100)plane in Mg-4Zn-0.6Ca-0.7Mn and Mg-4Zn-1Ca-0.7Mn.Meanwhile the pole intensity had increased from 2.6 to 4.5(Fig.9f and i).The enhancement of macro(100)texture indicated that more grains had an orientation relationship with the basal surface parallel to the extrusion direction.In extrusion Mg alloy bars,such orientation relationship was widely observed,which was denoted as“fiber texture”[24,26].As for the micro-texture,as shown in Fig.11(g,i,l),the basal texture intensity had increased from 3.44(Mg-4Zn-0.3Ca-0.7Mn)to 16.73(Mg-4Zn-1Ca-0.7Mn),which indicated the same grain orientation with macro-texture.Two texture results both indicated that Ca aroused an improvement on fiber texture rather than traditional weakening effect.Similarly,Mn also did an improvement effect on the orientation relationship of(0001)plane parallel to the extrusion direction.Taking Mg-4Zn-1.0Ca-yMn alloys as an example,the pole intensity of(100)plane had increased from 1.9 to 4.5 and the micro-texture of(0001)plane increased from 11.11 to 16.73.The increased unDRXed fraction would be the most powerful evidence for explaining the improvement effect.Combined with IPF maps and micro-texture analyses,the large unDRXed area marked as blue grains has an orientation of(0001)plane parallel to the extrusion direction,which indicated that the unDRXed area did a large contribution for the improvement of fiber texture.Compared with Mn element,Ca owned a more significant effect on the fraction of DRXed due to the large line interspace,as shown in Fig.4(c and d).It’s reasonable to consider that the traditional weakening effect of Ca element had been concealed by the improvement of unDRXed fraction,which caused the increased intensity of basal texture.

4.3.Mechanical properties

Strengthening of metals can be generally attributed to mechanisms such as grain-boundary strengthening,precipitate strengthening,dislocation strengthening,and solid-solution strengthening[29,51].Fig.14 plots the yield strength against D−1/2and against the unDRXed fraction for the nine alloys.The large deviation of the data indicated that the yield strength was not in a good linear relationship with D−1/2and the unDRXed friction,indicating that the total strengthening effect in the Mg-4Zn-xCa-yMn alloys is no single from contribution by the grain size and/or the fraction of DRXed region.The Mg-4Zn-0.6Ca-0.7Mn exhibited a bimodal microstructure composed of fine DRXed grains(56%)and coarse unDRXed grains(44%)and a good combination of strength and elongation.Therefore,this alloy is taken as example,to analyze the effects of different parts of the microstructure.Our previous discussion in Section 4.1 indicated that the DRXed area included fine recrystallized grains and a lower dislocation density,whereas the unDRXed region contained coarse grains and a large dislocation density.The general strengthening effect can be approximately expresses by the linear relationship[52,53]:

Fig.12.Mechanical properties of as-extruded Mg-4Zn-xCa-yMn alloys.

where the unit of yield strengthσyis MPa,Δσgbis the strengthening contribution by grain boundary strengthening,Mis the Taylor factor,τ0is intrinsic critical resolved shear stress,Δτs,Δτp,Δτdare solid solution strengthening,precipitation strengthening,and dislocation strengthening,respectively.For the present study,Zn,Ca and Mn were not all dissolved in the Mg matrix completely because they formed many secondary phase particles.It is approximately concluded that solid-solution strengthening provided a limited contribution to the total strength of as-extruded alloy.Hence,the solid solution strengthening and the intrinsic strengthM(τ0+Δτs)is combined asσ0,which is taken as the as-cast yield strength of 85MPa.TheMΔτdis neglected in the DRXed regions due to the low dislocation density,theΔσgbcan also be ignored owing to the coarse grains in the unDRXed regions.Thus,Eq.(4-1)can be written as follows for the DRXed region and the unDRXed region,respectively:

Grain boundary strengthening is an important mechanism in magnesium alloys,according to Hall–Petch[54,55]relationship:

wherekyis the locking parameter representing grain boundaries as an obstacle to slip(N/m3/2),ky≈272MPaμm1/2[56],and the unit of the average grain sizeDisμm.The Hall-Petch relationship is typically an indication of the boundary strengthening,and takes grain boundaries or twin boundaries as barriers to intergranular dislocation motion,in order to improve the resistance of plastic deformation[57].The DRXed grain size was 2μm,so the strengthening effect produced by the grain boundaries in the DRXed region is calculated as 192MPa.TheΔτpfrom the secondary phase particles can be evaluated using the following equation[42,58]:

Fig.13.Schematic illustration of the dynamic recrystallization process during hot process.

whereΔτpis the increase of critical resolved shear stress(MPa),Gis the shear modulus(17GPa),bis the Burger vector(0.32nm),the unit of interspacing of precipitates(λ)is nm,and the unit of diameter of particles(r)is nm.Fig.15 shows the fine precipitates in DRXed and unDRXed area of the extruded Mg-4Zn-0.6Ca-0.7Mn.The average diameter of precipitates in the DRXed region was about 163nm and the interspacing of the precipitates was about 521nm.TheMdepends on texture which ranged from 2.1 with very strong texture and activated pyramidal slip,to 4.5 with random texture.According to Du and Liu,the value ofMDRxwas approximately 4.0 since the DRXed regions exhibited a basal texture intensity of 4.67,while the value ofMunDRxwas taken as 2.5 due to the much stronger basal texture intensity of 18.93[29,42].Finally,the increasedMDRxΔτpin the DRXed region was calculated to be 33MPa.The average diameter of precipitates in the unDRXed region was about 63nm and the interspacing of the precipitates was about 259nm.Thus theMunDRxΔτpfor the unDRXed regions was estimated to be 28MPa.

TheΔτddislocation strengthening follows the general relation with the dislocation density,which is calculated by the following equation[52]:

whereαis a constant with a value of 0.2,andρis the dislocation density.The main effect on the dislocation during deformation process is the strain,although other factors such as temperature,alloy elements and processing method may also influence it[59].It is not possible to accurately measure the dislocation density of the unDRXed region by the X-ray line method since there were different fraction of un-DRX and DRXed areas.Pesicka et al.adopt a transmission electron microscopy method to calculate the dislocation density[60].Four horizontal and four vertical lines are crossed in the TEM picture containing dislocation lines,as shown in Fig.16.The densityρis then obtained as[60,61]:

wheretis the foil thicknesses with a value of 80nm,ΣNvandΣNhare the numbers of intersections of dislocations with the vertical and horizontal grid lines respectively.ΣNhandΣLhare the total lengths of the horizontal and vertical test lines from all micrographs(nm).Compared to the research of Du[42],we calculated the dislocation density in the unDRXed region as 2.2×1014m−2,which is little higher but reasonable since the higher extrusion ratio and rate.ThusMunDRXedΔτdis estimated to be 40MPa.

Theσy,DRXedandσy,unDRXedpredicted by Eqs.(4-2)and(4-3)are 310MPa and 153MPa,respectively.The DRXed and unDRXed fraction were 56% and 44%,respectively.Then,the yield stress of the Mg-4Zn-0.6Ca-0.7Mn is evaluated as 310×56%+153×44%≈241MPa,which is only somewhat smaller than the measured value(286MPa).This might be impacted by the uncomprehensive consideration about the dislocation strengthening in the DRXed region and the grain boundaries strengthening in the unDRXed region.Besides that,the interaction between the DRXed and unDRXed grains should not be ignored.Mg alloy,with an HCP crystal structure,has a complicated plastic deformation behavior.Basalslip and twin system{10-12}are two typical deformation mechanisms due to their relatively low CRSS at room temperature[62].Fine DRXed grains with more randomly orientation distributed around the unDRXed grains,which can facilitate the coordinated plastic deformation of adjacent grains.However,the plastic deformation of the unDRX region was hard to activated due to the hard orientation and their large grain size.It is reasonable to consider that the existed unDRXed grains can weaken the contribution to plasticity aroused from these fine DRXed grains.The fraction of the unDRXed grains was significantly increased with the alloying,thus the plastic deformation resistance was improved.And a homogeneous microstructure in the Mg-4Zn-0.3CayMn alloys was beneficial to activate the basal slip in adjacent grains,consequently Mg-4Zn-0.3Ca-yMn alloys exhibit high elongation,but a low yield strength.