Modifying microstructures and tensile properties of Mg-Sm based alloy via extrusion ratio

Ki Gun,Rui M,Jinghui Zhng,Ruizhi Wu,Qing Yng,?,Jin Meng

a Key Laboratory of Superlight Material and Surface Technology,Ministry of Education,College of Material Science and Chemical Engineering,Harbin Engineering University,Harbin 150001,PR China

bDepartment of Materials Engineering,The University of Tokyo,Tokyo 113-8656,Japan

c State Key Laboratory of Rare Earth Resource Utilization,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun 130022,PR

Abstract Microstructure and tensile properties of a Mg-Sm-Zn-Zr alloy with various extrusion ratios(ERs)of 6.9,10.4 and 17.6 were systematically investigated.It was identified that,greater ER increased dynamic recrystallization(DRX)fraction and coarsened DRX grains,which further suggests weakened basal fiber texture for the studied alloy.This is mainly due to the rising temperature from massive deformation heat when hot-extrusion.As a result,greater ER corresponds to a decreased strength but improved ductility.Finally,transmission electron microscopy(TEM)observations reveal that the dominant intermetallic phase,Mg3Sm,is metastable,and it will transform into Mg41Sm5 during extrusion with high-ER.This transformation leads to the accumulation of surplus Sm and Zn atoms,which induces the precipitation of SmZn3 phase at the surface of Mg41Sm5 matrix.? 2021 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:Magnesium alloys;Extrusion ratio;Microstructure;Phase transition;Transmission electron microscopy;Tensile properties.

1.Introduction

Magnesium(Mg)alloys exhibit broad application prospects in automobile and aerospace,because they own high specific strength,superior damping capacity and low density[1–4].Nevertheless,the relatively lower mechanical strength of traditional Mg alloys restricts their wide application.Rare earths(REs)have outstanding effects on improve the performance of Mg alloys due to their prominent effects such as precipitation,refining grains,solid solution as well as stable intermetallic phases[5-20].Therefore,RE-containing Mg systems have become the worldwide focus.

Samarium(Sm)with relatively lower cost as well as the highest solid solubility in Mg among light RE elements can optimize the alloys’performance via solidsolution/precipitation strengthening[10,11].It suggests that Sm is an appropriate RE element for designing the low-cost RE-containing Mg alloys with high-strength.Thus,an army of researchers are devoted to developing Mg alloys with Sm addition[10–17].For instance,the precipitation evolution and its corresponding hardening effect of Mg-4Sm-xZn-0.4Zr alloy are investigated systematically by Xia et al.[10].Li et al.[12]characterized the precipitates in the aged Mg alloy containing 4wt.%Sm and modified the precipitation sequence.Feng et al.[13]indicated that highly dispersed Mg41Sm5particles could decrease the galvanic corrosion of as-extruded Mg-Sm based alloys.Guan et al.[14]thoroughly researched the influence of different contents of Sm on intermetallic compounds and tensile performance at room temperature(RT)of Mg-Sm based alloys,and demonstrated that the as-extruded alloy with the optimal Sm addition of 3.5wt.% exhibited the highest strength.Recent works indicated that Sm could clearly refine the microstructure and then significantly enhance the tensile properties of extruded ZK60 alloy at RT as well aselevated temperatures[15,16].Furthermore,Lyu et al.[17]developed a superior performance Mg-Zn-Zr alloy via Y/Sm alloying as well as hot-extrusion+aging treatment,and its ultimate tensile strength(UTS),yield strength(YS),and elongation(EL)could,respectively,reach to 465 MPa,413 MPa and～6.5%.

China

It is well known that hot extrusion is an extremely valid process to modify the microstructure and hence optimize the performance of Mg alloys[18–27].Yu et al.[19]and Hagihara et al.[20]showed that extrusion parameters,especially extrusion ratio(ER)and extrusion temperature(ET),can significantly influence the microstructure and consequently determine the mechanical properties.Yang et al.[21]found that increasing ET would result in an increase on DRXed grain size of a Mg-Li-Al-Sr alloy,thus a decrease in alloy’s YS.In regard to ER,the study of Tang et al.[22]implied that among the extrusion parameters,ER shows much more remarkable influences on refining microstructure of AZ31 alloy than extrusion speed(ES)and ET.Zhang et al.[23]pointed out that the microstructure is refined with the increasing ER,and consequently the corresponding YS and UTS are increased in AZ31B alloy.Wen et al.[24]declared that both the YS and EL are increased with the increasing ER in Mg-0.6Zn-0.6Zr alloy with 2.4wt.% Nd addition because of the grain refinement.While according to Feng et al.[25],the grains are refined obviously with the increasing ER in Mg-Li based alloy,but the tensile strength is firstly improved and subsequently decreased.Zhang et al.[26]indicated that the fraction of unDRX is decreased and the texture is reduced with the increasing ER in the extruded Mg-6Sn-2Zn-1Ca alloy.Tong et al.[27]showed that the proportion of DRX increases with the increasing ER,thus weakening the basal texture in Mg-Zn-Ca alloy.

It can be concluded from the above analysis that ER plays a key role in industrial production of Mg alloys,but its influence rule is not very clear in different alloy systems.Based on previous research,Mg-Sm-based alloys are the promising Mg-RE based alloys with excellent performance at low cost,and ER is a significant factor on dominating the microstructure and performance of Mg-Sm-based extruded alloys.Therefore,the microstructure and tensile performance of extruded Mg-Sm-Zn-Zr alloy with different ERs are thoroughly investigated in this work,contributing to develop an in-depth understanding of the role of ER.

2.Experimental procedures

The Mg-3.33Sm-0.55Zn-0.46Zr(wt.%)ingot was fabricated with high purity Mg,Zn,Mg-30 wt.%Zr and Mg-20 wt.%Sm master alloys in an electric resistance furnace.The raw materials were melted at about 755°C under the protection of SF6and CO2(1:99).After being fully stirred,the melt was kept still at 735°C for～0.5h.Eventually,we poured the alloy melt into a permanent mould at about 710°C to produce a cast ingot with dimension ofφ90 mm×800 mm(Fig.1a).The cylindrical billets for following extrusion with dimension ofφ84 mm×100 mm(Fig.1b)were machined from the ingot.After pretreatment at 350°C for 2 h,the above billets were processed at the uniform temperature under the ES of 1 mm/s,and ER of 6.9,10.4 and 17.6,respectively.As shown in Fig.1c,as-extruded bars obtained with different ERs were marked as ER6.9,ER10.4 and ER17.6,respectively.The tensile specimens with dimension ofφ6 mm×36 mm(gauge length)(Fig.1d)were machined from the extrusion bars paralleling to the extrusion direction(ED).

Microstructural characterization of the presented alloys was examined by Olympus-GX71 optical microscope(OM),X-ray diffractometer(XRD,λ=0.15406nm,Bruker D8 FOCUS),scanning electron microscope(SEM,Hitachi S-4800)equipped with EDS,and transmission electron microscopy(TEM,FEI Tecnai G2F20)equipped with EDS conducting at 200 kV.Cuboid specimens for OM and SEM observation were etched in an aqueous mixture of picric acid,ethanol and acetic acid.Thin foils for TEM were milled using argon ion polishing(Gatan 691 PIPS).The grain size of the studied samples was sketchily estimated by Nano Measure 1.2 software.The tensile properties teste was performed on a standard machine(Instron)at RT with a strain rate of 1.0×10?3s?1.

3.Results and discussion

3.1.Microstructures of as-cast alloy

The OM image of as-cast alloy is presented in Fig.2a,suggesting that the relatively uniform microstructure consists of equiaxed grains and some eutectic compounds.The dominant eutectic compounds are recognized to be Mg3Sm(a=0.7371 nm,face-centered cubic crystal structure)based on the analysis of the relevant XRD pattern(Fig.2b).As shown in the typical backscatter SEM micrograph(Fig.2c),some eutectic compounds with various morphologies are mainly located at triangular grain boundaries.Meanwhile,it is worthy to note that the contrast near the grain boundaries and eutectic compounds is slightly stronger than that in the interior of the original grains.In order to further explore the element distribution of the eutectic compounds,EDS mappings corresponding to the network-shaped phase(marked by a yellow box in Fig.2c)was conducted.The result displayed in Fig.2d reveals that obvious Sm and Zn elements segregate in Mg3Sm phase,slight Sm and Zn elements enriched in the area near the grain boundaries and eutectic compounds,which is corresponding to the previous report[28].As shown in the top right of Fig.2a,the average grain size is～39.4μm.A relatively fine microstructure might mainly result from the addition of Zr element.In addition,the constitutional super-cooled region can generally be expanded by the segregation of Sm atoms near the solidliquid interface,which effectively accelerates the nucleation rate during solidification,and finally refines the grains[29].

3.2.Microstructure of as-extruded alloys

Fig.1.(a)The permanent mold and a cast ingot with dimension ofφ90 mm×800 mm,(b)cylindrical billets with dimension ofφ84 mm×100 mm,(c)as-extruded bars obtained with different extrusion ratios,and(d)tensile specimens.

Fig.2.(a)OM micrograph,(b)XRD result,(c)SEM micrographs and(d)EDS mappings of the as-cast sample.

Fig.3a-c present the OM images of ER6.9,ER10.4 and ER17.6 alloys.The microstructures of the alloys are clearly refined by extrusion,revealing the occurrence of DRX during this process.But it is also apparent that the microstructures of ER6.9 and ER10.4 samples comprise relatively coarse elongated grains as marked by azure arrows and fine DRXed grains,implying the insufficient DRX at relatively smaller ER(Fig.3a and b).The fractions of the unDRXed regions corresponding to the ER6.9 sample and ER10.4 sample are measured to be approximately 16.3%and 10.7%,respectively.Nevertheless,relatively coarse elongated unDRXed grains disappear almost entirely in ER17.6 alloy,and only fine DRXed grains can be observed,as displayed in Fig.3c.It is evident that the fraction of unDRXed region decreases with increasing ER.In other words,larger ER is conductive to promoting the DRX in Mg-Sm-Zn-Zr alloy.In addition,original eutectic compounds are crushed into numerous intermetallic particles aligned on the ED for all the presented alloys during the extrusion,which is observed more visibly from the subsequent SEM micrographs(Fig.3a1-c1).On the basis of particle stimulated nucleation(PSN)mechanism,numerous shattered intermetallics serve as nucleation sites of DRX during extrusion process,which would significantly promote the DRX.Consequently,the DRXed grain size around shattered particles is much smaller than that in other regions with infrequent distribution of particles.Similar phenomenon was ascribed to the strong drag force on relocating boundaries produced by solute RE atoms near the intermetallic particles of Mg-Gd-Y-based alloy[8].Fig.3a2-c2 display the histograms of the DRXed grain size distribution in samples with different ERs.The average DRXed grain sizes are approximately 1.48μm for ER6.9 sample,1.86μm for ER10.4 sample and 4.19μm for ER17.6 sample,respectively.

Fig.3.(a-c)OM micrographs,(a1-c1)SEM micrographs and(a2-c2)the histograms of DRXed grain size.(a-a2),(b-b2)and(c-c2)represent ER6.9,ER10.4 and ER17.6 alloys,respectively.

Extrusion parameters can significantly influence the Zener-Hollomon(Z)parameter[30-32],which has the crucial effect on the DRXed grain size.Derby[32]reported that the increasedZparameter would refine the DRXed grains effectively,satisfying the following equation:whereZ,dDRXandprepresent the Zener-Hollomon parameter,the average DRXed grain size and the relevant exponent,respectively.Zwas proposed by Watanabe et al.[31]:

whereQ,,TandRrepresent the activation energy(Mg:135 kJ/mol)for lattice diffusion,the strain rate,the processing temperature and the gas constant(8.314 J/(mol K)),respectively.The average strain ratecan be computed by the following equation[30]:

whereDB,DE,VRandERrepresent the diameter of billet,the diameter of extrudate,the ram speed and the extrusion ratio,respectively.According to Eqs.(2)and(3),the values of the average strain rate andZparameter are obtained,which are listed in Table 1.

Table 1Extrusion parameters,the average strain rate and Z parameter of the as-extruded alloys.

Fig.4.XRD analysis of extruded alloys.

It is obvious that both average strain rate andZparameter increase as the ER increases.Based on the Eq.(1),the average size of DRXed grains should decrease with the increasing ER,however,the present experimental results suggest that which increases with ER in the Mg-Sm-Zn alloy.This may result from that the friction heat and deformation heat have been ignored in the above calculation,which generated during hot extrusion with larger ER.Actually,the rising temperature induced by deformation heat has a remarkable effect on the DRXed grain size.The diffusion rate of solute RE atoms and the corresponding driving force for the migration of grain boundaries would be enhanced at higher temperature,which reduces solute drag effect,consequently,DRXed grain size is relatively larger[33].

3.3.Intermetallic compounds in the as-extruded alloys

XRD was carried out to investigate the phase compositions of the samples extruded with different ERs.As presented in Fig.4,the results reveal that the dominant intermetallic phase is Mg3Sm in ER6.9 sample.It is worth to note that Mg3Sm is gradually replaced by Mg41Sm5(primitive tetragonal crystal structure)as the ER increases,which implies phase transition occurred during extrusion.This phenomenon may be caused by the tremendous heat produced from the extrusion process with larger ER.

TEM characterization along with EDS analysis was conducted to further study the crystal structure and chemical composition of the intermetallic phases.Fig.5a-c present the typical high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)micrographs,respectively,corresponding to ER6.9,ER10.4 and ER17.6 samples.As shown in Fig.5a,several bright particles with length of about 500 nm and many gray particles with relatively smaller size are observed in ER6.9 sample.Based on the analysis of the relevant selected area electron diffraction(SAED)patterns(Fig.5a1 and a2),the bright particle(marked by A)and the gray one(marked by B)are identified to be Mg3Sm and Mg41Sm5,respectively.The representative HAADF-STEM image of ER10.4 sample is given in Fig.5b.Apart from some bright particles,plenty of gray blocky phases can be clearly observed.The corresponding analysis result of SAED patterns(Fig.5b1 and b2)suggests that the bright intermetallic phase(marked by C)and the gray one(marked by D)are Mg3Sm and Mg41Sm5,respectively.Similarly,based on the analysis of relative SAED patterns(Fig.5c1 and c2),the bright particle(marked by E)and the gray one(marked by F)are demonstrated to be Mg3Sm and Mg41Sm5,respectively.Fig.5a3-c3 display the EDS mappings corresponding to the regions highlighted by yellow dotted boxes in Fig.5a-c.It is obvious that much more Sm and Zn elements are enriched in bright particles,especially Zn.The EDS results also suggest that the bright intermetallic particles in the as-extruded alloys are Mg3Sm while the gray ones are Mg41Sm5.The comparison between Fig.5a,b and c indicates that the proportion of Mg41Sm5phase increases monotonously with the increase of ER,which is accord with the XRD result.The above TEM characterization further confirms that phase transition occurred during extrusion.

Taking the ER10.4 sample as an example,detailed TEM characterization was performed to explore the mechanism of phase transition.Fig.6a gives the characteristic bright-field TEM(BF-TEM)image of ER10.4 alloy.Some disintegrated intermetallic compounds with different size distribute in the DRX area.The coexistence of bright phase and gray one could be obviously found in the corresponding HAADFSTEM image(Fig.6b).The SAED pattern(Fig.6c)obtained from the A region marked by black dotted circle can be simultaneously indexed by two different crystal structures,i.e.Mg3Sm and Mg41Sm5.The high-resolution TEM(HRTEM)micrograph is presented in Fig.6d,and the fast Fourier transform(FFT)patterns obtained from the purple dotted region and yellow dotted region are,respectively,presented in Fig.6e and f.It demonstrates that the bright intermetallic compound is Mg3Sm phase and the gray one is Mg41Sm5,and the orientation relationship(OR)between them is determined as(451)Mg41Sm5about 5.5ofrom(331)Mg3Smand[-540]Mg41Sm5//[01-3]Mg3Sm.These above results reveal that the Mg3Sm phase is metastable and it can transform into Mg41Sm5during the hot extrusion with a large ER.Plastic deformation with a large ER not only results in sufficient DRX but also enhances mass-transfer speed and then promotes phase transition.The similar mechanism of phase transition has been reported in Cu-based alloy[34].The detailed mechanism of the phase transition in the present Mg-Sm based alloys and its influence on performance will be specifically expounded in our immediate future work.In addition,some relatively smaller particles with size of about 0.5μm pinning the DRXed grain boundaries are observed in Fig.6a and b.The relevant SAED patterns obtained from the B region marked by white dotted circle in Fig.6b is shown in Fig.6g.The HR-TEM micrograph and the consistent FFT patterns are presented in Fig.6h and i.The analysis results demonstrate that the particles distributed at grain boundaries are Mg3Sm,which might prevent the further growth of DRXed grain through pinning effect.

Fig.5.(a-c)HAADF-STEM micrographs,(a1-c1 and a2-c2)SAED patterns and(a3-c3)EDS mappings of extruded alloys,in which(a-a3),(b-b3)and(c-c3)represent ER6.9,ER10.4 and ER17.6 alloys,respectively.

Fig.6.(a)BF-TEM and(b)HAADF-STEM images of the ER10.4 alloy,(c)SAED pattern and(d)HR-TEM corresponding to A region marked in(b),(e-f)FFT patterns corresponding to the purple and yellow dotted region presented in(d).(g)SAED pattern,(h)HR-TEM micrograph and(i)FFT pattern corresponding to B region marked in(b).

As presented in Fig.5b and c,a number of ultra-fine particles sporadically precipitated at gray blocky Mg41Sm5phase in both ER10.4 and ER17.6 alloy.Fig.7a displays the typical BF-TEM image of blocky phase along with numerous ultra-fine precipitated particles(indicated by yellow arrows)in ER10.4 alloy.The analysis of the SAED pattern shown in the inset of Fig.7a implies that the blocky phase is Mg41Sm5.Fig.7b presents the magnified HAADF-STEM micrograph.It is clearly that the precipitated particles are very fine,with the size of 10-40 nm.The HR-TEM micrograph of a typical precipitated particle and the corresponding FFT pattern are,respectively,presented in Fig.7c and d.The FFT pattern can be simultaneously indexed by Mg41Sm5and SmZn3(a=0.669 nm,b=0.4405 nm andc=1.011 nm,primitive orthorhombic crystal structure)[35],and the OR between them is determined as(001)SmZn3//(001)Mg41Sm5and[010]SmZn3//[1-20]Mg41Sm5.As shown in Fig.7e,the FFT pattern corresponding to the purple dotted region in Fig.7c indicates that the precipitated particle is SmZn3.The corresponding EDS mappings of the position indicated with a yellow dotted frame(as shown in Fig.7b)are displayed in Fig.7f.It is obvious that precipitated particles mainly contain Sm and Zn elements,agreeing well with the result from FFT patterns.Strip precipitate around the LPSO phase was roughly identified to be YZn3in terms of the XRD result and EDS spectrum in Mg-Zn-Y alloy produced by semisolid squeeze casting[36].However,the detailed crystal structure provided by TEM observation of this phase in Mg alloys has never been reported in the previously published literature.

Fig.7.(a)BF-TEM micrograph together with SAED pattern,(b)the magnified HAADF-STEM micrograph of blocky intermetallic phase along with numerous ultra-fine precipitated particles,(c)the corresponding HR-TEM micrograph,(d-e)FFT patterns,and(f)EDS mappings corresponding to the yellow dotted square in(b).

Fig.8.(a)Representative HAADF-STEM micrograph of ER17.6 alloy,(b)SAED pattern,(c)HR-TEM micrograph,(d-e)FFT patterns,and(f)EDS mappings corresponding to the yellow dotted square in(a).

In order to reconfirm the formation of the novel phase,comparable TEM characterization was conducted on ER17.6 sample.Fig.8a displays the representative HAADF-STEM micrograph of blocky compounds along with several fine precipitates in ER17.6 alloy.The relevant SAED pattern given in Fig.8b suggests that the coarse blocky phase is Mg41Sm5.The HR-TEM micrograph of the fine precipitated particle is presented in Fig.8c and the corresponding FFT pattern is presented in Fig.8 d.Similarly,the FFT pattern can be simultaneously indexed by two different crystal structures,their OR is determined as(101)SmZn3//(1-21)Mg41Sm5and[13-1]SmZn3//[31-1]Mg41Sm5.As shown in Fig.8e,the FFT pattern corresponding to the purple dotted square in Fig.8c confirms that the precipitated particle is SmZn3.The corresponding EDS mappings of the area indicated with a yellow dotted square(as shown in Fig.8a)are presented in Fig.8f,in which evident Sm and Zn elements enrichment can be obviously observed.The element distribution coincides well with the result from SAED and FFT patterns.Not insignificant,however,the SmZn3particles are only observed on the surface of the Mg41Sm5phase,which implies that the formation of SmZn3phase might be induced by phase transition.As shown in Fig.5,much more Zn element enriched in Mg3Sm compared with Mg41Sm5.The consumption of Sm and Zn elements will be reduced during the transformation from Mg3Sm to Mg41Sm5,especially Zn.The accumulation of surplus Sm and Zn atoms will lead to the precipitation of SmZn3phase at the Mg41Sm5matrix.It should be noted that the precipitated particle with the size being 50-100 nm in ER17.6 alloy is much larger than that in ER10.4 alloy,although both of them are identified to be SmZn3phase.It can be deduced that the growth of the precipitated particles is mainly attributed to the rising temperature induced by deformation heat with a higher ER.

3.4.Texture evolution in the as-extruded alloys

Generally,typical(0001)fiber texture parallel to the ED will be created in extruded Mg alloys.Fig.9 gives the XRD patterns of the samples with different ERs.Distinctly,the(0002)Mgpeak intensity is much higher than both(100)Mgand(101)Mgpeak intensities in ER6.9 and ER10.4 samples,which indicates that strong(0001)basal fiber texture has been formed.Texture intensity was frequently semi-quantitatively determined by the relative intensity(I%)[37]:

TheI%(0002)values i.e.69.54 %,50.36% and 34.43% are corresponding to the ER6.9,ER10.4 and ER17.6 alloys,respectively.GreaterI%(0002)value ordinarily indicates stronger basal texture.The above values demonstrate that smaller ER corresponds to stronger basal fibers in the present alloys.Basal fiber texture in extruded Mg alloys might be mainly from the unDRXed grains,while random texture in DRXed regions[19].As described in Fig.3,the volume percentage of DRXed regions increases as the ER increases in the studied alloys,increasing ER to 17.6 results in complete DRX.Therefore,the intensity of(0001)basal fiber texture decreases significantly with increasing ER,which would be conducive to enhancing the ductility but reducing the texture strengthening along the ED[38-39].

Fig.9.Relative intensity of(0002)diffraction peaks of the as-extruded alloys.

For Mg alloys,non-basal slip systems generally own a much higher critical resolved shear stress(CRSS)than that basal slip system.With greater ER,matrix is subject to larger strain,which will activate the non-basal slip readily.However,the CRSS for non-basal slip will decline as the temperature increases.Then,non-basal slip would be activated during hot-extrusion,and more easily with higher ERs due to the increased temperatures by deformation and friction heat.What’s more,stress concentration at grain boundary in DRX region can be facilitated by grain refinement,which is advantageous to activating non-basal slip[22].The higher activation rate of non-basal slip is beneficial to the random distribution of grains,which thus weaken the intensity of(0001)basal fiber texture.

3.5.Tensile properties

Fig.10a and b present the typical engineering stress-strain curves and true stress-strain curves of the extruded alloys tested at RT.The detailed tensile performance i.e.EL,YS and UTS,are shown in Table 2.The ER6.9 alloy exhibits optimal mechanical properties,whose engineering EL(ELE),engineering UTS(UTSE)and engineering YS(YSE)are 12.1%,347 MPa and 340 MPa,respectively.When ER increases to 17.6,the ER17.6 alloy’s UTSEand YSEdecrease to 241 MPa and 202 MPa,respectively.Meanwhile,the ELEincreases to 29.7%,which increases by approximately 145.5 % compared with ER6.9 alloy.

Table 2Tensile performance of as-extruded alloys.

Fig.10.Representative(a)engineering tensile stress-strain curves,and(b)true stress-strain curves.

It is generally considered that the size of grains significantly influence Mg alloys’mechanical properties.As described in Fig.3f,the DRXed grain size increases significantly because of the distinct rising temperature induced by the increasing ER.Following Hall-Petch equation[39]:

wherekanddrepresent a constant and the average grain size,respectively.The relatively fine grains can actually enhance the grain boundary strengthening(Δσgs).The average grain sizes of ER6.9,ER 10.4 and ER 17.6 samples are approximately 1.48μm,1.86μm and 4.19μm,respectively.Thus,as-extruded alloys with small ER exhibit much better grain boundary strengthening because of their much finer grain size.Furthermore,large number of shattered particles distributed in the matrix can efficaciously prevent the migration of grain boundaries and dislocations by pinning effect during deformation,and sequentially enhance the YS of the as-extruded alloys.Moreover,texture is one of the key determinants affecting the mechanical properties in deformed Mg alloys.Under tension paralleling to the ED,a relatively strong(0001)fiber texture generally enhances the activation stress for the basal slip,which can effectively improve the YS of extruded alloys.Therefore,the texture strengthening in the investigated alloys is weakened with increasing ER due to the decreasing intensity of(0001)fiber texture.

It’s interesting to note that the average size of DRXed grains increases as ER increases,but the ductility of the investigated alloys increases.In the present work,the enhanced ductility is mainly attributed to the reduction of unDRX regions.The EBSD map of grain boundary distribution along with the corresponding misorientation distribution of ER6.9 alloy is shown in Fig.11a,in which the black line and green one represent high angular grain boundary(HAGB,θ≥15o)and low angular grain boundary(LAGB,θ≤15o),respectively.It’s obvious that numerous LAGBs are mainly distributed in the unDRXed regions,implying the dislocation density in unDRXed regions is much higher than that in DRXed grains.This can be further confirmed by the subsequent TEM characterization.As presented in Fig.11b,a mass of dislocations are distinctly observed from the typical TEM image within the unDRX region viewed along[11-20]direction with g=[0002]of ER6.9 alloy.It can be deduced that the dislocation density of the extruded alloys decreases with increasing ER because of the decreased fraction of unDRXed regions(as shown in Fig.3).It has been reported that the ductility of processed materials are basically controlled by hardening capacity(Hc),which can be expressed as the ratio of(UTST?YST)to YST,where UTSTand YSTrepresent the true UTS and the true YS,respectively[40-42].As shown in Table 2,the evaluated values ofHcare 0.039,0.143 and 0.429 in ER6.9,ER10.4 and ER17.6 alloys,respectively.This indicates the hardening capacity is improved by the increased ER due to the increased grain size and the weakened texture[40-43].Accordingly it can be inferred the larger grain size and the lower initial dislocation density provide large space for dislocation generation and storage during the tensile test,which enhances the deformation hardening effect and consequently improve the ductility of the extruded alloys with large ER.

In addition,Park et al.[44-45]reported that tensile deformation along the ED leads to the formation of{101}-{102}double twins(DTs)in as-extruded Mg alloys at ambient temperature.These DTs may result in crack initiation because of the dislocation pile-ups around the interface between twins and Mg matrix,and accordingly decrease the ductility.Jin et al.[46]confirmed the above theory in AZ31 alloy and suggested that the intensity of texture will influence the twinning volume fraction.Compared with fine DRXed grains,{10-11}-{10-12}DTs are more effortlessly formed among relatively elongated unDRXed grains.As shown in Fig.3,an increase of the ER causes a decrease of the unDRX fraction.Therefore,the ER increase makes double twins difficult to be generated,and consequently increases EL.

Fig.11.(a)EBSD map of grain boundary distribution along with the corresponding misorientation angle distribution,and(b)the typical TEM image within the unDRX region viewed along[11-20]direction with g=[0002]of ER6.9 alloy.

It is thus clear that ER can remarkably affect the extent of DRX,the average grain size of DRXed region,the hardening capacity,and hence influence the YS,UTS and EL of the present alloys.

4.Conclusions

In this work,the effect of ER on the microstructure and tensile properties of Mg-Sm-Zn-Zr alloy was investigated in detail.The main conclusions are obtained as follows:

1.DRX occurs during the extrusion and the microstructure of the Mg-Sm-Zn-Zr alloy is effectively refined.As ER increases,the intensity of(0001)basal fiber texture decreases significantly due to the decreasing fraction of un-DRX,the average DRXed grain size increases simultaneously because of the rising temperature generated during hot extrusion with larger ER.

2.The alloy with the ER of 6.9 exhibits the satisfactory YSEand ELEof 340 MPa and 12.1%,respectively.With increasing ER,the strength of the extruded alloys decrease owing to the combined effect of increased average DRXed grain size and weakened(0001)fiber texture.On the contrary,the ductility of the extruded alloys is improved by increasing ER,which is mainly attributed to the weakened texture and the decreased dislocation density.

3.Mg3Sm phase is metastable and that vast deformation heat produced during the hot extrusion process with large ER leads to a phase transition to Mg41Sm5.The accumulation of surplus Sm and Zn atoms resulted from phase transition results in the precipitation of SmZn3phase at the surface of Mg41Sm5matrix.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(No.52071093,51871069,51701200),Fundamental Research Funds for the Central Universities(3072020CF1009),the Open Funds of the State Key Laboratory of Rare Earth Resource Utilization(RERU2020012),and Grant-in-Aid for JSPS Fellows from the Japan Society for the Promotion of Science.