Effects of minor Nd and Er additions on the precipitation evolution and dynamic recrystallization behavior of Mg–6.0Zn–0.5Mn alloy

Bin-Jing Lv,Sen Wng,Tie-Wei Xu,b,Feng Guo,b

a School of Mechanical and Automotive Engineering,Qingdao University of Technology,266520,China

b Key Lab of industrial Fluid Energy Conservation and Pollution Control,Qingdao University of Technology,266520,China

Abstract A new wrought magnesium(Mg)alloy based on Mg–6.0Zn–0.5Mn(ZM60)is developed,which performs excellent combination of high tensile yield strength and good ductility.We investigate the effects of micro-alloying on dynamic precipitation,dynamic recrystallization(DRX)and mechanical properties of ZM60 alloy.The co-addition of minor(0.6wt%)neodymium(Nd)and(0.3wt%)erbium(Er)can accelerate the twinning and DRX process of ZM60 alloy at the initial compression deformation stage.The dynamic precipitation process is also accelerated due to Nd and Er co-alloying.Dislocation accumulation disappears and a higher density of rodβ′1 precipitates and some thickβ′2 precipitates in matrix and fine twins,which inhibits the growth of DRX grains in compressed ZM60–0.6Nd–0.3Er alloy.The as-extruded ZM60–0.6Nd–0.3Er alloy has a yield strength(YS)of 245.8MPa,ultimate tensile strength(UTS)of 347.2MPa and elongation(EL)of 16.3%.The yield strength and tensile strength are improved via minor Nd and Er additions due to fine complete DRX grains,second phase particles and high density of precipitates.The grain refinement,weakened reserved working hardening and weakened basal fiber texture improve the elongation of ZM60–0.6Nd–0.3Er alloy.© 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:Nd and Er;Hot deformation;Dynamic precipitation;Dynamic recrystallization;Texture.

1.Introduction

In the automotive and aerospace industries,the requirements for weight reduction and fuel efficiency promote the development of light metallic materials,and the wrought magnesium(Mg)alloys with high strength and ductility attract much attention in this field[1,2].Mg–Zn–Mn alloys with high zinc content have broad application prospects for their good mechanical properties[3].Zhang et al.[4]reported that ZM61 alloy performed a high ultimate tensile strength(UTS)of 366MPa but a lower elongation(El)of 5% by extrusion deformation and aging strengthening effect.

It is noted that heat treatment,alloying and plastic deformation can improve the precipitation hardening effect for high Zn content Mg–Zn series alloys[4–8].In Mg–Zn alloys,the strengthening precipitates contain rod shapeβ′1phase and oblate shapeβ′2phase.Theβ′1phase distributing along[0001]αdirection has a hexagonal structure(a=0.520nm,c=0.857nm)of MgZn2[9,10].Recently,Gao et al.[11]reported thatβ′1phase had a monoclinic structure(a=2.60nm,b=1.43nm,c=0.52nm,γ=102.5°)of Mg4Zn7.The plate shapeβ′2phase has a MgZn2structure with the lattice parameters ofa=0.520nm andc=0.857nm[9,10].A grain boundary Mg–Zn type precipitate with oblate shape has been observed in Mg–4.0Zn–0.1Ce–0.3Ca and Mg–4.0Zn alloys[12].Alloying appropriate elements have been proved to be an effective way to affect the precipitates of wrought Mg alloys.For instance,micro-alloying Ce and Ca additions can improve hardening in Mg–Zn alloys due to refined rodβ′1and fine basal plate precipitates[6].Plastic deformation is an important way for improving the mechanical properties of Mg alloy.The recent studies on Mg–Zn–Mn alloy showed that the dynamic precipitates form during high strain-rate rolling(HSRR),which displayed excellent hardening effect[8].Due to the hexagonal close-packed(HCP)crystal structure and lower stacking fault energy(SFE)of Mg alloy,DRX occurs easily and plays an essential role in microstructures and mechanical properties during plastic processing[13,14].Wu et al.[7]investigated the dynamic precipitation behavior before DRX in a ZM51 alloy during hot compression under different strain rates.Some researchers found that dynamic precipitation retarded DRX due to the pinning effect of precipitates on grain boundary migration during hot compression in Mg–Al–Zn based alloy and Mg–Gd–Y–Zr alloy[15,16].

Table 1Chemical compositions of as-cast alloys.

Previous works showed that adding rare earth ele ments in Mg–Zn alloys exhibited excellent mechanical properties.For instance,adding Nd can enhance texture weakening,and Er addition can improve strength of Mg alloy[17–19].However,until now,the effect of rare earth elements on dynamic precipitation evolution and DRX behavior in the Mg–Zn–Mn alloys during plastic deformation has not been studied.Therefore,in this work,the Mg–6.0Zn–0.5Mn(ZM60)alloy was designed,and then mixing double rare earth elements Nd(0.6wt.%)and Er(0.3wt.%)were added into this alloy.The dynamic precipitation and DRX behavior during compression test,the texture and mechanical properties of as-extruded ZM60 and ZM60–0.6Nd–0.3Er alloys were investigated.

2.Experimental procedures

High purity Mg(99.8wt%),high purity Zn(99.9wt%),Mg–Mn(20wt%),Mg–Nd(20wt%)and Mg–Er(30wt%)master alloy were adopted as the raw materials to prepare the ZM60 and ZM60–0.6Nd–0.3Er alloy under the protection of a CO2(99vol%)and SF6(1vol%)mixed gas atmosphere.The alloy was melted at 750 °C to ensure alloying elements were dissolved.Finally,the ingots were casted into a cylindrical steel mold ofφ110 mm×400mm with a final temperature of 710 °C.The Chemical compositions of ingots were determined by X-ray fluorescence spectrometer(XRF-1800 CCDE)and the results were shown in Table 1.The casting ingots were homogenized at 400 °C for 16h in a 5kW heat treatment furnace,and then machined into cylindrical hot compression specimens with 10mm in diameter and 12mm in length by electro-discharge machining.Hot compression test was carried out on a Gleeble 3500 hot simulation machine at a deformation temperature of 400°C and a strain rate of 1 s−1under different true strains(0.100,0.300,0.600 and 0.900).The specimens were heated at a heating rate of 10 °C/s and held at 400 °C for 3min before hot compression tests,and then deformed to the true strain and immediately quenched in water to keep their microstructure after hot compression.The extrusion was conducted at 400 °C with an extrusion ratio of 26 and a ram speed of 5mm/s.

The compressed specimens were sectioned parallel to the compression axis(section planes)for microstructural observation,and the as-extruded samples were cut vertical to the extrusion direction.A picric-acetic solution(5g picric acid,15mL acetic acid and 100mL ethanol)was used to etch the samples for 45s.The microstructure observation of samples was characterized using a scanning electron microscope(SEM,ZEISS MERLIN Compact)with energy dispersive spectrometer(EDS).The average grain sizes were determined by analyzing SEM micrographs with line-intercept method,and the volume(area)fraction of DRX was measured on SEM micrographs with area method via Image-Pro-Plus(IPP)software.EBSD measurement was conducted using a Hitachi S-3400N SEM instrument equipped with an HKL-EBSD system operating at 20kV.Before observation,samples for EBSD were mechanically polished and then followed by argon ion polishing.Transmission electron microscopy(TEM)analyses were performed on a FEI Tecnai G2 F20 and FEI Titan Themis TEM with energy dispersive X-ray spectroscopy(EDX)operating at 300kV.The length and the density of precipitates were measured in TEM images and the average values were got based on over 500 precipitates using IPP software.Texture of as-extruded alloy was performed using a X’Pert PRO MRD/XL X-ray diffractometer(XRD).Tensile specimens with a gauge section with 8mm in diameter and 40mm in length were machined from the as-extruded alloys with the loading direction parallel to the extrusion direction.Tensile tests were carried out on a universal material test machine(MST-C43)with a constant loading speed of 0.3mm/min at room temperature.

3.Results and discussion

3.1.As-cast homogenization-treated alloy

Fig.1 shows the typical microstructures of as-cast homogenization-treated alloys.The original microstructure of homogenized ZM60 alloy is equiaxed grains with a mean grain size of 572μm.The SEM images(Fig.1)and EDS analysis(Table 2)reveal that there are some fine Mg–Zn binary phases and Mn particles distributing inα–Mg matrix.TEM images(Fig.2)with corresponding selected area electron diffraction(SAED)and EDX were carried out to investigate the second phases in the homogenization-treated alloys.Fig.2a shows that few fine rod shape and some plate shape second phases exist inα–Mg matrix(Zoon axis=[113]).Additionally,some large size oblate shape particles distribute in grain boundary(yellow arrows in Fig.2a).According to SAED patterns and EDX results recorded in Fig.2a and b,it is indicated that:(I)Theβ′1phase forms as[0001]αrods;(II)The plate shape and oblate shapeβ′2phases(MgZn2)distribute in the grain and grain boundary,respectively.The TEM image and its corresponding SAED in Fig.2c indicate that Mn phase can be indexed consistently asα-Mn,which has a body-centered cubic structure with the lattice parameters ofa=0.891nm.Besides Mg–Zn binary phase,Mg3Nd2Zn3phase forms in grain boundary and inner grain,and a few Mg12ZnEr particles distribute inα–Mg matrix by adding Nd and Er elements(Figs.1b and 2d).The mean grain size of ZM60–0.6Nd–0.3Er alloy is decreased to 496μm with the refinement effect of Nd and Er elements.

Fig.1.SEM image of as-cast homogenization-treated alloys(a)ZM60 alloy,(b)ZM60–0.6Nd–0.3Er alloy.

Fig.2.Bright–field TEM image and SAED of as-cast ZM60–0.6Nd–0.3Er homogenization-treated alloy(a)Bright-field TEM image and its corresponding SAED ofα–Mg,I is rod shapeβ′1 phase,and II is oblate shapeβ′2 phase distributed in the grain and grain boundary,and(b)EDX results of position A and B in(a).(c)Bright-field TEM image and SAED ofα–Mn,(d)Bright-field TEM image and SAED of Mg3Nd2Zn3 phase.

3.2.True stress–strain curves

The true stress–strain curves and strain-hardening rate vs.true strain curves are provided in Figs.3 and 4.Based on the true stress–strain curves,some hot deformation parameters such as peak stress,DRX critical strain(εc),the relative flow softening(Sr)and strain corresponding to the maximum softening rate(ε∗)were calculated and summarized in Table 3.

Fig.3.True stress–strain curves(a)ZM60 alloy,(b)ZM60–0.6Nd–0.3Er alloy.

Fig.4.True strain-hardening rate vs.true strain curves(a)ZM60 alloy,(b)ZM60–0.6Nd–0.3Er alloy.

Table 2EDS results of as-cast homogenization-treated alloys.

As shown in Fig.3,at early elastic deformation stage of compression,the true stress increases linearly with the increasing deformation strain.The minor Nd and Er additions increase the peak stress(σp)of ZM60 alloy,which suggests that micro-alloying affects the strain-hardening and the DRX softening(critical strain)in an earlier stage of deformation.In general,the flow softening after peak stress in Mg alloys at high temperature is attributed to the nucleation of new DRX grains[20].The turning point of the strain-hardening rate(θ=dσ/dε)with true stress(σ)curves can be used to determine the DRX critical condition of Mg alloy[21].The DRX critical strain(εc)decreases,which is inferred that DRX process is accelerated by adding Nd and Er elements.The value ofSrcan be used to depict the flow softening behavior of all the alloys expressed as:Sr=(σp-σ)/σp[22],in whichσis the minimum stress before a strain of 0.900.The value ofSrincreased from 0.134 to 0.287,which also implies that co-alloying of Nd and Er elements accelerates the DRX.

After reaching a maximum value(peak stress),the flow stress gradually reduces with the increase of deformation strain caused by dynamic softening effects,which offsets the work hardening caused by dislocation accumulating.When the true strain reaches about 0.600,the flow stress decreases to a steady state,which is the balance between hardening and softening.In ZM60 alloy,the flow stress exhibits a slight increasing beyond the strain of 0.800.In contrast,the flow stress of ZM60–0.6Nd–0.3Er alloy is increased evidently with the increase of strain between 0.629 and 0.900.This is attributed to the significant strain hardening induced by dislocation pining of precipitates and second phase particles.

The true stress-strain curves of two alloys display different mechanisms under the same deformation condition,which indicates different hot deformation behavior and the microstructure evolution(discussed in detail in part 3.3).

3.3.Microstructure evolution of compression test

The microstructure graphs of compressed samples at different true strains are presented in Fig.5.It can be seen that the DRX occurs in both alloys along original grain boundaries at a true strain ofε=0.100 after the critical strain of DRX.Moreover,some DRXed grains appear along original grain boundaries,and some coarse twins and twin-induced DRX(TDRX)grains occur in the microstructure of ZM60–0.6Nd–0.3Er alloy(Fig.5b).When the deformation strain increases from 0.100 to 0.300(Fig.6a and c),the EBSD analysis results show that new fine DRX grains are mainly formed along original coarse grain boundaries as“necklace”structure in ZM60 alloy.This indicates that the DRX mechanism is continuous dynamic recrystallization(CDRX)[23].Because Mg and its alloys have limited slip systems,deformation twinning becomes another important deformation mechanism[24,25].In ZM60–0.6Nd–0.3Er alloy,it can be seen that more{102}tensile twins(86°)and some{101}compression twins(56°)distribute in matrix,and some fine DRX grains form in coarse twins in Fig.6b and d.Therefore,in addition to CDRX,TDRX is another important DRX mechanism after adding Nd and Er elements.Furthermore,the nucleation of new fine twins in coarse original grains(Fig.6)at a strain of 0.300 can be explained by an arresting of twin boundaries[26].Twin boundary mobility decreases due to boundary stabilization by the oblate shape dynamic precipitates in twin boundary,as shown in Fig.7a and b.It can be inferred that co-addition of Nd and Er can accelerate the twinning during hot compression.Firstly,the co-alloying of Nd and Er increases the kinds and amount of second phases,and it becomes an effective barrier for dislocation movement,which promotes the twin nucleation[27,28].Secondly,it was reported that Nd and Er elements could decrease the SFE,but Zn element slightly increased the SFE with a lower solute concentration according to density functional theory(DFT)calculations[29,30].The EDS results of as-cast homogenization-treated alloys(Fig.1 and Table 2)indicate that the Zn content in Mg matrix decreases due to the formation of second phases by adding Nd and Er.Some Nd and Er atoms may be dissolved into Mg matrix after homogenization treated,but EDS cannot be detected due to the low content.Thus,the SFE decreases more or less,and twin is prone to occur in ZM60–0.6Nd–0.3Er alloy[31].

Table 3Hot deformation parameters of the compressed alloys.

At the strain of 0.600,some coarse original grains can be found in Fig.5e and the volume fraction of DRX grains is calculated as 57.8%,indicating that the DRX did not completely occur in ZM60 alloy.Due to the accelerated twinning and DRX process by Nd and Er co-alloying,original grains were almost replaced by the fine DRX grains in ZM60–0.6Nd–0.3Er alloy(Fig.5f)and the volume fraction of DRX grains is as high as 80.5%.DRX process became easier with the increase of strain due to the increasing driving force.Therefore,the DRX fraction of ZM60 alloy increases to 69.1%while the true strain reaches 0.900.However,as presented in Fig.5h,the coarse original grains disappear and the microstructure consists of all fine equiaxed DRX grains,which implies that complete DRX is accomplished in ZM60–0.6Nd–0.3Er alloy.

3.3.1.Dynamic precipitation evolution during compression test

Compared with as-cast homogenization-treated alloys,more precipitates appear in both compressed samples at a strain of 0.600(Fig.7).It can be seen that the fine rod shape phase is the main dynamic precipitates,and few thick precipitates distribute in Mg matrix.Grain boundary dynamic precipitates with oblate shape appear to be similar in both alloys in Fig.7a and b.

The EDX mapping scanning of the area(Fig.8b–d)shows that the rod and oblate shape precipitates are Mg-Zn binary phases.As presented in Fig.8e and f,the fine rod precipitates areβ′1phases parallel to the[0001]αdirection.Fig.9a shows a bright field TEM micrograph of ZM60–0.6Nd–0.3Er alloy from[110]αdirection of Mg matrix.The high resolution transmission electron microscope(HRTEM)image and its corresponding fast transformation(FFT)in Fig.9b indicate thatβ′1phase is Mg4Zn7phase,which has a monoclinic structure(a=2.60nm,b=1.43nm,c=0.52nm,γ=102.5°).The thick precipitate lying on(0001)basal planes isβ′2phase.Besides,the oblate shape precipitates distributing on the grain boundary are investigated by SAED with zone axis=[113]αon the side of the matrix(Fig.9c).The HRTEM image and its FFT pattern were applied for further investigation of the oblate precipitate in Fig.9d.The interplanar spacing of(20)oblate precipitate is 0.26nm,which agrees well with MgZn2phase with the lattice parameters ofa=0.520nm andc=0.857nm.The orientation relationships between the oblateβ′2precipitate and the other side of theα–Mg matrix arezone axis.It suggests that the oblate shape grain boundary precipitate is not coherent with the orientation of theα–Mg matrix.

At a true strain of 0.600 in ZM60 alloy,the number of rodβ′1dynamic precipitates within the unit area in theα–Mg matrix is 172/μm2,and the mean length of rodβ′1precipitates is 59.2±19.3nm.By adding Nd and Er,more intermetallic phase particles form,which consumed more Zn element,and it further reduced the Zn content in the Mg matrix(Table 2).However,more intermetallic particles can generate more nucleation sites with increasing strain,such as dislocations,which promotes the diffusion of solute atoms[32,33].As a result,the density of rod dynamic precipitates improves obviously after adding Nd and Er,especially in fine twins(Fig.7a and b).The number of rodβ′1precipitates within the unit area in the matrix of ZM60–0.6Nd–0.3Er alloy increases to 362/μm2.Additionally,the rapidly increasing number of dynamic precipitates consume much more solute Zn atoms after alloying,which further decreases the mean length of rodβ′1precipitates(46.6±21.1nm)in ZM60–0.6Nd–0.3Er alloy.

Fig.5.SEM micrographs of compressed samples under different true strains(a,c,e and g)ZM60 alloy,(b,d,f and h)ZM60–0.6Nd–0.3Er alloy.

Furthermore,as mentioned earlier,at the initial compression stage(e.g.ε=0.100),twin nucleation occurs more easily at high strain rates attributed to Nd and Er co-alloying.With the increasing of true strain,high density dislocations are observed in twins in ZM60 alloy,as shown in the area depicted by the yellow dotted line(Fig.7a).However,there is no high density dislocation area observed in twins in ZM60–0.6Nd–0.3Er alloy as shown in Fig.7b.During the compression test,increasing strain generates more dislocations(Fig.7a)and SFs(white arrow in Fig.7c),which can accelerate the diffusion of solute atoms and provides more heterogeneous nucleation sites for precipitation[34–36].Stacking faults(SFs)generate more easily due to the decreasing of SFE after RE addition,but not in ZM60 alloy.Moreover,as shown in Fig.7d,the misfit dislocation(“⊥”)reported in the previous study was observed in{101}compression twinning(56°)boundary[37],which might become the heterogeneous nucleation site for oblateβ′2precipitates in twin boundary.The accelerated twinning process and the increased density of twins lead to a significant increment on the volume fraction of precipitates.Therefore,there are fewer precipitates inner twins in ZM60 alloy,but the density of rodβ′1precipitates inner twins of ZM60–0.6Nd–0.3Er alloy reaches 468/μm2.Dynamic precipitation reduces the density of dislocations and local stress concentration.As a result,when the high densityβ′1andβ′2precipitates after alloying,the high-density dislocations disappear in twins as shown in Fig.9b.

Fig.6.OIMs,grain boundaries and twin boundaries of compressed samples at a true strain of 0.300(a and c)ZM60 alloy,(b and d)ZM60–0.6Nd–0.3Er alloy.

The precipitates can significantly suppress twin growth due to the back stress or misfit stress when shear-resistant precipitates are embedded in a twinned matrix[38].The occurrence of twinning(Fig.4d)reduces the volume fraction of dislocations and stress concentration in Mg matrix,and this inhibits the dynamic precipitation behavior during compression deformation.As a result,in ZM60–0.6Nd–0.3Er alloy,the density of rodβ′1precipitates in Mg matrix(362/μm2)is smaller than that in twins(468/μm2).Furthermore,the more dynamic precipitates form in twins,the more solute Zn atoms are consumed.Due to a lower density of dynamic precipitates in matrix,there are more Zn solute atoms for continued growth of dynamic precipitates,which increases the length of rodβ′1precipitates.Thus,as shown in Fig.7b,the length of rodβ′1precipitates in twins(lT=21.8±7.1nm)is significantly lower than that in matrix(lM=46.6±21.1nm)of ZM60–0.6Nd–0.3Er alloy.

3.3.2.Dynamic precipitates on DRX behavior during compression test

It is commonly accepted that coarse second phases stimulate nucleation of dynamic recrystallization and the fine precipitates inhibit DRX process during hot deformation[39].Interestingly,for the alloys in this study,the dynamic recrystallization process has accelerated due to Nd and Er coalloying.

When the true strain reaches 0.600,the microstructure of ZM60 alloy is not recrystallized completely with the average grain size of 37.2±15.5μm(Fig.5e).At the initial compression stage,only a small density of fineβ′1phases precipitated in both alloys after homogenization treatment(Fig.2a).However,as shown in Figs.1 and 5,numerous large intermetallic particles distribute in the matrix and grain boundaries in ZM60–0.6Nd–0.3Er alloy,which easily stimulate nucleation of DRX grains according to the particle stimulated nucleation(PSN)mechanism[38,40].Furthermore,a large number of twins can provide favorable conditions for the nucleation of DRX,which accelerate the DRX process by adding Nd and Er elements at the initial stage of deformation(Fig.5a–d).Thus,the DRX nearly completes before the formation of high density dynamic precipitations,and the dynamic precipitates hinder the DRX process weakly.As a result,the average grain size of ZM60–0.6Nd–0.3Er alloy reduces to 18.4±9.7μm.

Fig.7.Bright–field TEM image of compressed samples at a true strain of 0.600(a)ZM60 alloy,(b)ZM60–0.6Nd–0.3Er alloy,(c)High magnification of area‘A’,(d)High magnification of area‘B’.

In previous studies[41],after all original microstructures transform to new DRX grains,strain hardening and DRX softening reaches a dynamic equilibrium and then gets a steady state of flow stress.However,in this study,with the increase of deformation strain,the flow stress increases after a minimum value at a true strain about 0.600,especially for ZM60–0.6Nd–0.3Er alloy.In general,the flow hardening with strain is attributed to the dynamic grain growth during hot deformation in Mg alloys[42,43].With strain ranges from 0.629 to 0.900,the DRX completes and the number of dislocations increases with the increasing strain.The movement of grain boundary can be pinned by the dynamic precipitation in ZM60–0.6Nd–0.3Er alloy.The high density of dynamic precipitates distributes in the DRX grains,which can effectively pin the dislocation and restrict the grain boundary mobility by Zener pinning effect.Therefore,for ZM60–0.6Nd–0.3Er alloy during the strain range of 0.600–0.900,the DRX softening effect decreases,but the hindering effect on the growth of DRX grains improves due to the high density dynamic precipitates and broken second phase particles.Thus,the flow stress is increased evidently after the stable stress.In Nd and Er free alloy(ZM60),the increase of flow stress is not significant because of the incomplete DRX with the strain from 0.600 to 0.900.

3.4.Microstructures and mechanical properties of as-extruded alloys

Fig.10 presents the microstructures of both as-extruded alloys.There are also some coarse original grains in ZM60 alloy due to the incomplete DRX as shown in Fig.10a,and the average grain size is 9.6±8.7μm.After adding Nd and Er elements,the DRX process was accelerated and the DRX grain growth was hindered due to the dynamic precipitates.It results in completed equiaxed DRX grains without coarse original grains(Fig.10b),and the average grain size decreases to 6.2±2.9μm.From our studies,the microstructure evolution of as-extruded alloys agrees well with the compression sample observations.

Fig.8.TEM images and EDX mapping scanning results of the ZM60–0.6Nd–0.3Er alloy at a true strain of 0.600(a–c)EDX mapping scanning results,(d)HAADF image of EDX field,(e)Bright–field TEM micrograph and(f)its corresponding SAED pattern.

Fig.11 shows the(0002)and(100)pole figure obtained from ND-TD plane of the as-extruded alloy sample.The asextruded ZM60 alloy exhibits a typical basal fiber texture with the intensity of 3.3 as presented in Fig.11a.Compared with ZM60 alloy,more randomized textures appear and the density of textures decreases from 3.3 to 2.4 due to the co-addition of Nd and Er.Inverse pole figures(IPF)with the axes in the ED of hot extruded alloys(Fig.12)are used to analyze the orientation distribution of grains.From Fig.12a,without micro-alloying,<100>direction ofα–Mg parallel to the ED indicates that a fiber texture with basal planes preferentially is parallel to the ED.The IPF in Fig.12b shows that clustering of the orientations of ZM60–0.6Nd–0.3Er alloy are near<113>and<110>−<111>,which is similar to the reported rare earth texture(RE texture)[44].

The fine broken second phase particles accelerate the DRX behavior and provide favorable orientations for the DRX nucleation,which leads to the random orientations and the further decrease in basal fiber planes texture intensity in ZM60–0.6Nd–0.3Er alloy.On the other hand,the dynamic precipitates at the DRX regions restrict the rotation of grains toward the basal orientation,which can effectively pin the dislocation and restrict the grain boundary mobility by Zener pinning[8,45].In addition,the co-addition of Nd and Er can decrease SFE of ZM60 alloy[29,46].Generally,the decreased SFE may improve the formation of stacking faults on the corresponding slip planes,which possibly changes the CRSS of basal and non-basal slips and then affect their relative activity[47,48].Therefore,the basal fiber texture of ZM60 alloy is weakened by minor Nd and Er additions.

The typical engineering stress-strain curves at an ambient temperature of the as-extruded alloys are shown in Fig.13.The ZM60 alloy exhibits a YS of 210.9MPa and an UTS of 298.5MPa.After adding minor Nd and Er elements,the YS and UTS increase to 245.8MPa and 347.2MPa,respectively.It is well known that grain size,second phase particles,solid solution and texture have significant influences on the mechanical properties of Mg alloys[49,50].In this study,the contents of Zn dissolved in the matrix of these two alloys are almost the same,which cannot cause differences in mechanical properties.According to the well-known Hell–Petch equation(σy=σ0+kd−1/2)[51],the grains are refined and more uniform due to the completed DRX by Nd and Er additions,which benefits the strength of the as-extruded ZM60–0.6Nd–0.3Er alloy.Furthermore,compared with ZM60 alloy,more fine broken second phase particles distributing in the matrix play a role of dispersion strengthening,and the Orowan looping of a higher density of dynamic rod-shaped precipitations further improves the strength by micro-alloying[52].Consequently,the precipitation strengthening effect can be promoted in ZM60–0.6Nd–0.3Er alloy.

Fig.9.TEM images of precipitates in ZM60–0.6Nd–0.3Er alloy at a true strain of 0.600(a)Bright-field TEM micrograph and SAED pattern ofα–Mg,(b)HRTEM image and corresponding FFT ofβ′1 phase.(c)Bright-field TEM micrograph and FFT ofα–Mg on one side of the grain boundary,(d)HRTEM image and corresponding FFT ofβ′2 phase andα–Mg on the other side of the grain boundary.

Fig.10.SEM micrographs of as-extruded alloys(a)ZM60,(b)ZM60–0.6Nd–0.3Er.

It is known that the ductility of the alloy is influenced by some factors.In the present study,after adding minor Nd and Er elements,the elongation of ZM60 alloy increases from 11.8% to 16.3%(Fig.13).Firstly,the average grain size of ZM60 alloy with Nd and Er additions is reduced from 9.6±8.7μm to 6.2±2.9μm,and the grains become more uniform due to fully DRX.Grain refinement can improve the plastic deformation compatibility of the alloy,which inhibits the initiation and propagation of cracks[53].Additionally,reserved working hardening is caused by the high density of dislocations due to incomplete DRX in ZM60 alloy,and it is weakened by the additions of Nd and Er.Finally,the weakened basal texture may initiate the activation of a non-basal slip in the system.As a result,owing to the grain refinement,weakened reserved working hardening and texture modification,the ductility of as-extruded ZM60 alloy is improved via minor Nd and Er additions.

Fig.11.(0002)and(100)textures of the as-extruded alloys(a)ZM60,(b)ZM60–0.6Nd–0.3Er.

Fig.12.IPF of as-extruded alloy(a)ZM60,(b)ZM60–0.6Nd–0.3Er.

Fig.13.Mechanical properties of as-extruded alloy.

4.Conclusion

In this work,effects of minor Nd and Er additions on microstructures and mechanical properties of ZM60 alloy were thoroughly investigated,and the dynamic precipitates,DRX behavior and texture were analyzed and discussed.The main specific conclusions can be drawn as follow:

1)The minor co-addition of Nd and Er accelerates the DRX due to the large fraction of twins and PSN mechanism at the initial stage of compression.Continuous DRX and twin-induced DRX are the main DRX mechanisms for ZM60–0.6Nd–0.3Er alloy.More intermetallic particles,twins,dislocations and SFs increase the nucleation sites for dynamic precipitation by adding Nd and Er,resulting in larger density ofβ′1(Mg4Zn7)precipitates than that in ZM60 alloy.A small fraction ofβ′2(MgZn2)and a higher density ofβ′1precipitates in twins compared to that in matrix,and this is attributed to the reducing of dislocations and stress concentration in Mg matrix.

2)Compared with the incomplete DRX and a low density dynamic precipitation in ZM60 alloy,the DRX is nearly completed in ZM60–0.6Nd–0.3Er alloy at a strain of 0.600.The hindering effect on the DRX grains growth is enhanced owing to second phase particles and high densityβ′1dynamic precipitates,which evidently increases the flow stress at a strain range of 0.600–0.900 by adding Nd and Er.

3)With Nd and Er co-addition,the intensity of basal texture is weakened mainly due to more fine broken second phase particles and high density dynamic precipitates.The coaddition of Nd and Er improves the YS and UTS of ZM60 alloy from 210.9 MPa and 298.5 MPa to 245.8 MPa and 347.2 MPa,respectively.The enhancement in strength is mainly attributed to the fine grain strengthening,dispersion strengthening of broken second phase particles and dynamic precipitation strengthening.The elongation increases from 11.8% to 16.3%,which is mainly attributed to the grain refinement,weakened reserved working hardening and RE texture weakening effect.

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.

Acknowledgments

The authors are grateful to the support of the Major Research Development Program of Shandong Province of China(Project No.2019GGX102060),the Chinese Postdoctoral Science Foundation(Project No.2017M612224)and the Natural Science Foundation of Shandong Province of China(Project No.ZR2016EMQ08).