L.B.Tong,J.H.Chu,W.T.Sun,Z.H.Jing,D.N.Zou,S.F.Liu,S.Kmo,M.Y.Zheng,??

a School of Metallurgical Engineering,Xi’an University of Architecture and Technology,Xi’an 710055,China

b School of Materials Science and Engineering,Harbin Institute of Technology,Harbin 150001,China

c School of Materials Science and Engineering,Jilin University,Changchun 130025,China

d Department of Mechanical Engineering,Nagaoka University of Technology,Nagaoka 940-2188,Japan

Abstract In the current study,a homogenous ultra-fine grained microstructure with average grain size of 1.0μm is achieved in the Mg–Zn–Ca–Mn alloy through the reduplicative equal channel angular pressing(ECAP)at 300°C,and the mechanical properties are remarkably improved,with room-temperature yield strength of 269.6MPa and elongation of 22.7%.The twinning deformation results in a discontinuous recrystallization behavior in the initial stage of ECAP.With further deformation,the continuously dynamic recrystallization contributes to an obvious grain refinement effect.The activation of non-basal slip system leads to the formation of a unique basal texture,which is related to the elevated ECAP temperature and the decreased grain size.Both grain refinement and texture modification derived from ECAP process result in the increase of yield strength,while the cracked secondary phase particles are beneficial to the enhanced ductility,through reducing the stress concentration and hindering premature failure.

Keywords:Mg alloy;ECAP;Microstructural evolution;High strength;Superior ductility.

1.Introduction

In the past decades,as the potential candidate materials,magnesium(Mg)alloys have attracted much attention in automobile,aerospace and biodegradable industries,because of their low density,biocompatibility and excellent biodegradability[1–3].However,compared with conventional Ti,Al alloys or steel materials,Mg alloys usually exhibit the low strength and poor ductility,which seriously hinder their extensive applications.Therefore,it is significant to improve the mechanical properties and achieve the structural stability of Mg alloys,through microalloying technology,thermomechanical processing or heat treatments[4–6].

The commercial Mg–Al series alloys possess the outstanding mechanical properties and good castability,which is related to the solid solution strengthening effect[7,8].Unfortunately,Al element has been proved as a risk factor for Alzheimer’s disease and muscle fiber damage,which is not suitable for the biomedical applications[9,10].Rare earth(RE)elements are widely used in Mg alloys,to improve their mechanical properties or anti-corrosive performances,but the heavy REs(such as Gd,Y)are very expensive,which increases the cost of Mg–RE alloys[11,12].Zn is an abundant nutritionally essential element in the human body and beneficial to improving the strength of Mg alloy,through solid solution and grain boundary strengthening effects[13,14].As a typical grain refiner in Mg alloy,Ca has been proved to effectively accelerate the human bone healing[15].The trace addition of Mn element leads to the improvement of mechanical properties and corrosion resistances of Mg alloys,through achieving the grain refinement and reducing the impurity[16].Therefore,many researchers have increasingly concentrated on the development of low-cost Mg–Zn–Ca–Mn series alloys with superior properties as biomedical materials[17–19].

Compared with the traditional extrusion,rolling or forging,equal channel angular pressing(ECAP)is considered as the most popular severe plastic deformation(SPD)technology[20–22],which can result in a homogenous ultra-fine grained(UFG)structure through the large accumulated strains,and remarkably improve the room-temperature yield strength of Mg alloy.Ge et al.[23]fabricated a homogeneous fine-grained ZM21 alloy through the ECAP processing,and the tensile yield strength was remarkably increased.Furthermore,the specific shear deformation during the repeating ECAP process generally leads to a texture modification effect,which contributes to the enhanced ductility.Zhao et al.[24]reported that the elongation to failure of extruded AZ31 alloy were dramatically increased after ECAP for 8 passes,while the yield strength was decreased,which was attributed to the weakening effect of basal texture.Dumitru et al.[25]investigated the mechanical properties of extruded ZK60 alloy during ECAP,and the yield strength was deteriorated with the increased number of ECAP passes,because the basal plane turned out to be more favorably oriented for basal slip.In contrast,the ductility was remarkably improved,which might be related to the transition from a brittle to a ductile fracture behavior.In general,the ECAP processing will simultaneously cause the grain refinement and texture modification effects,whose influences on the mechanical properties of Mg alloys are extremely complicated.

Actually,there exists an intrinsic contradiction between the strength and ductility of Mg alloy,which is derived from the activation of dislocation slip system.Therefore,how to fabricate a high-strength Mg alloy with superior ductility has become a research focus.Especially for the UFG Mg alloy processed by ECAP,the intensive basal texture weakening effect often leads to the deteriorative yield strength[26–28],and therefore it is difficult to achieve a simultaneous improvement of strength and ductility.Furthermore,many researchers have reported that there existed various texture components in the ECAPed Mg alloys,which are related to the deformation routes,temperature and passes[29–31],and the corresponding mechanisms for texture formation also remain controversial and need further understanding.

In the current study,we report a new high-strength Mg–Zn–Ca–Mn alloy fabricated by ECAP processing,which is attributed to both the intensive grain refinement and texture strengthening effects.Especially,it is worth noting that the ductility of this alloy is remarkably increased to～23%.The microstructure evolution,texture formation and mechanical properties during the ECAP are systematically investigated,which will open a new window for the development of wrought Mg alloy with the excellent mechanical properties in the future.

2.Experimental procedures

The Mg–6.0Zn–0.5Ca–0.3Mn(wt%)alloy was fabricated using pure Mg,Zn,Ca and Mg–5.0%Mn master alloy,through a gravity casting under SF6and CO2protective atmosphere.After homogenization treatment at 400°C for 2h,the cast ingot was extruded at 300°C with a ram speed of 0.2mm/s,and the extrusion ratio was 10:1.The as-extruded bars were cut into billets of 10×10×70mm3,which were pressed through an ECAP die with the internal channel angle of 90° and external angle of 37°,respectively(the equivalent strain of each pass was calculated as～1.05).Prior to deformation,the samples were preheated for 5min,and the ECAP was carried out from 1 to 4 passes at 300°C using route Bc(the sample is rotated by 90° in the same direction between each pass),with the punch speed of 0.5mm/s.

The specimens for microstructural observations were machined along the longitudinal planes,which were parallel to extrusion or ECAP direction(ED)and normal direction(ND).After mechanical polishing,all the samples for optical microscopy(OM)were etched in a solution of acetic picral.The average grain size was analyzed through Image-Pro 5.0 software,and the total grain number for each sample was more than 1000.The grain morphologies and secondary phase particles were observed using a scanning electron microscope(SEM,Hitachi S-4800)with a wavelength-dispersive X-ray spectroscopy(WDX)and transmission electron microscope(TEM,FEI Tecnai-G2-F20).The micro-texture and orientation evolution during the ECAP processing were measured by electron backscatter diffraction(EBSD,TSL MSC-2200).

The microhardness was measured on the cross-sectional plane of Mg alloy by a Micro Vikers tester(FM-700)with a load of 200g,and five trials were carried out for each specimen to calculate an average value.Dog-bone shaped flat samples(with gauge dimensions of 6.0×3.0×25.0mm3)were cut along the longitudinal planes(with tensile axis parallel to the ED).The room-temperature tensile tests were conducted on a universal Instron 5569 machine(three measurements for each sample),and the crosshead speed was 1.0mm min?1.Three samples were taken on each tensile condition to calculate the average value and reduced the experiment errors.

3.Results

3.1.Microstructure

Fig.1 shows the optical microstructures of the extruded and ECAPed Mg–Zn–Ca–Mn alloys.The as-extruded alloy exhibits a typically bimodal grain structure,with the elongated grains parallel to ED and fine equiaxed grains,implying that the dynamic recrystallization(DRX)during the extrusion is not completed.The average size of fine recrystallized grain is calculated as～4.2μm.As shown by red dash lines in Fig.1a,many secondary phase stringers and particles with dark contrasts are broken and distributed along the ED.After ECAP for 1 and 2 passes,some deformation bands can be observed,within which there exist a large number of fine DRX grains(～1.3μm).Moreover,many coarse elongated unDRX grains are embedded inside these fine grains,representing an obvious heterogeneous microstructure.With further ECAP processing,the volume fraction of the elongated grain is gradually decreased,while the size of fine grain is not changed.After 3 and 4 passes,the alloys represent the homogenous ultra-fine grained(UFG)structure,with the average grain size of～1.0μm.The accumulated strains during the reduplicative ECAP can provide the sufficient stored energy and gradually achieve a completed recrystallization behavior,which may be beneficial to the grain refinement and the improvement of mechanical properties.Some banded or spherical secondary phase particles can be observed in the as-ECAPed alloys(shown by red dash lines),and the diameter is remarkably decreased,compared with that in the as-extruded alloy,which indicates that these particles are gradually broken during the ECAP.

Fig.1.OM micrographs of the as-extruded(a)low-magnification,(b)high-magnification and the as-ECAPed alloys for(c)1,(d)2,(e)3,(f)4 passes.

In order to further investigate the recrystallization process during the ECAP for 1 pass,the microstructural evolution of the interrupted sample is shown in Fig.2,and the hardness values at different positions are also measured(Fig.2b).For the initial deformation stage(position 1),the specimen suffers a low-strain compression,the microstructure is almost the same with the as-extruded alloy,except for the zigzag grain boundaries(Fig.2c).With the increased strains,many deformation twins are activated within the elongated unDRX grains at position 2(Fig.2d),to coordinate the local deformation.At the channel corner(position 3 or 4),the sample experiences an intensive shear deformation,and a large number of shear bands are observed(nearly parallel to shear direction).More importantly,the appearance of untwinning and recrystallization behavior leads to the formation of many fine grains within these shear bands.Finally,the volume fraction of fine DRX grain is gradually increased at position 5 and 6,which can be attributed to the continually compressive deformation.In addition,the maximum value of microhardness appears at position 2,indicating that the twinning boundaries possess the intensive strengthening effect.With further deformation,the recrystallization process leads to the softening behavior of Mg alloy,through reducing the deformation stored energy.

Fig.3 shows the SEM micrographs of the as-extruded and as-ECAPed alloys,and the chemical compositions of secondary phase particles are measured by WDX(Table 1).The coarse secondary phase stringers in the as-extruded alloy are distributed along the ED,within which there are a large number of fine secondary phase particles.After ECAP for 4 passes,these coarse particles are broken into some fine particles,due to the large accumulated strains.Furthermore,many fine precipitates are observed on the grain boundaries,which may be related to the aging behavior,because the ECAP temperature is almost the same as the aging temperature of Mg–Zn–Ca–Mn alloy.According to the WDX analysis,the atomic ratio of Mg,Zn and Ca at position A(coarse particle)is calculated as 7:2:5,and Mn elements cannot be detected.Therefore,the secondary phase particles are suggested to be Ca2Mg6Zn3phases,which have been widely reported and identified in ternary phase diagram of Mg–Zn–Ca alloys[32,33].For the fine secondary particle at position B,the atomic ratio of Ca and Zn is similar with that in position A,while the content of Mg element is much higher,because the WDX tests will be influenced by the adjacentα-Mg matrix.In addition,the chemical compositions of the coarse and fine secondary phase particles(position C and D)remain unchanged after ECAP processing.

Fig.2.The sketch map of observation position(a)microhardness evolution(b)and OM microstructures(c)～(h)of position(1)～(6)for the interrupted Mg alloy during ECAP for 1 pass.

Fig.3.SEM micrographs of the as-extruded(a)and the as-ECAPed alloy for 4 passes.

Table 1Chemical composition analysis(WDX)of secondary phase particles(at.%).

Fig.4.TEM micrographs of the as-extruded(a),(b)SAED and as-ECAPed alloys for(c)1 and(d)4 passes.

Fig.4 shows the TEM images of microstructure evolution in Mg alloy during the ECAP.An equiaxed grain structure can be observed in the recrystallized regions of the asextruded alloy,and the dislocation density is very low.The selected area electron diffraction(SAED)analysis proves the secondary phase particle is Ca2Mg6Zn3,which is in accordance with WDX results.After ECAP for 1 pass,some fine grains with their size of～1.5μm can be observed,and a new round of DRX behavior occurs.However,the dislocation density within the fine grain is extremely high(Fig.4c),because the accumulated strains are too low to achieve a completed recrystallization.With increasing ECAP passes,the large shear strains promote the DRX process,and the UFG microstructure can be observed.Moreover,the dislocation density is gradually decreased,and some fine precipitate particles are distributed along the grain boundaries,representing an intensive pinning effect.

3.2.Grain orientation and texture evolution

Fig.5 shows the orientation image microscopy(OIM)of extruded Mg–Zn–Ca–Mn alloys before and after ECAP,with the reference direction of transverse direction(TD).The pure red,blue and green colors represent the〈0001〉,〈010〉and〈20〉crystallographic directions parallel to TD,while these grain orientations are equivalent to the basal texture with(0001)planes parallel to ED.Similar with OM observation,there exist two distinct regions(coarse unDRX grains and fine DRX grains)in the as-extruded alloy(Fig.5a).After ECAP for 1 pass,except for the grain refinement,the various colors within the coarse unDRX grains prove that some sub-grain structures with different orientations can be formed(Fig.5b).A large number of low angle grain boundaries(LAGBs,≤15°)appear within both coarse and fine grains.The volume fraction of the fine grains with〈0001〉//TD orientation(red color)is dramatically increased,implying a remarkable texture evolution.With further deformation,a typical UFG structure is observed in the as-ECAPed alloy for 4 passes.Moreover,many fine grains with〈0001〉//TD orientation(with most of basal planes parallel to ED)are gradually generated(Fig.5c),which will play an important role on the mechanical properties through texture modification.

In order to further investigate the recrystallization behavior during the ECAP,the evolution of misorientation angle of Mg–Zn–Ca–Mn alloy is shown in Fig.6.The volume fraction of LAGBs in the as-extruded alloy is calculated as～15.5%,because the equivalent strain during the extrusion process is not enough to achieve a completed recrystallization,leading to a large number of sub-grain structures.After ECAP for 1 pass,the volume fraction of LAGBs is increased to～25.6%,which may be attributed to the new stage recrystallization behavior.Most of dislocations generated during the ECAP cannot be absorbed and gradually transformed to many sub-grains,which promotes the formation of LAGBs.For the as-ECAPed alloy for 4 passes,the volume fraction of LAGBs is remarkably decreased to～18.9%,exhibiting an obvious transformation process from LAGBs to high angle grain boundaries(HAGBs).Combined with the results from Fig.1,Fig.4 and Fig.5,it can be concluded that the recrystallization behavior is gradually completed with increasing the ECAP passes(the accumulated strains),and a homogenous equiaxed grain structure is accomplished.

Fig.5.OIM images of the as-extruded(a)and as-ECAPed alloys for 1(b)and 4(c)passes.

Fig.6.Misorientation distribution of the as-extruded(a)and as-ECAPed alloys for 1(b)and 4(c)passes.

Fig.7 shows the texture evolution of extruded Mg–Zn–Ca–Mn alloy during the ECAP from EBSD analysis.The asextruded alloy exhibits a traditional basal texture,with(0002)plane and〈100〉crystallographic direction parallel to ED(Fig.7a),and the maximum texture intensity is 17.5 MRD(multiples of random distribution).Meanwhile,there are some basal planes inclined to ED from 0 to 15°,because the Ca microalloying usually results in the appearance of non-basal texture component,through reducing the c/a value ofα-Mg matrix[34].

The texture component intensity is remarkably decreased in the as-ECAPed alloy for 1 pass,and a typical dual-texture distribution is observed,with basal plane parallel and inclined 30–45° to ED(Fig.7b).With further ECAP,it is worth noting that the non-basal texture components gradually disappear,while the basal texture is regained to a certain degree.After ECAP for 4 passes,an intensive basal texture can be observed,which is in accordance with grain orientation in Fig.5 but quite different from many previous studies on the texture of ECAPed Mg alloys[35,36].The reduplicative shear deformations usually lead to a non-basal texture component,because the main sliding plane will be gradually parallel to the shear plane.The formation of this unique texture may strongly influence the mechanical properties of the as-ECAPed Mg–Zn–Ca–Mn alloy,which will be discussed in the following section.

Fig.7.Pole figures of the extruded(a)and ECAPed alloys for 1(b),2(c)and 4(d)passes.

3.3.Room-temperature mechanical properties

Fig.8 shows the microhardness and tensile properties of the as-extruded and as-ECAPed alloys after different passes,and the corresponding data are listed in Table 2.Although the dynamic recrystallization process during the ECAP for 1 pass results in the intensive grain refinement(Fig.5b),the hardness value is remarkably decreased.During the hardness tests,the basal texture is weakened(Fig.7b),and the indentation stress is inclined～45° to ED,which increases the value of Schmid factor and results in the decrease of hardness.After ECAP for 2 passes,both the grain refinement and the recovery of basal texture(Fig.7c)contribute to the improvement of hardness.With further deformation,the hardness value almost retain unchanged(from 2 to 4 passes).Especially,the hardness values of the ECAPed alloys are lower than that of the extruded alloy to a certain degree,because the elongated unDRX grains with the intensive basal texture(act as the reinforced phases)in the as-extruded alloy hinder the activation of basal slip during the indentation tests and increase the hardness value.

Fig.8.Mechanical properties of the as-extruded and as-ECAPed alloys:(a)microhardness,(b)tensile curves.

Table 2Mechanical properties of the as-extruded and as-ECAPed Mg alloys.

After ECAP for 1 and 2 passes,the tensile yield strength(TYS)of extruded Mg alloy is dramatically decreased,while the value of elongation to failure(EF)is increased,representing a superior ductility.The grain refinement derived from ECAP deformation cannot improve the yield strength of Mg alloy,which is attributed to the texture modification.With further ECAP process,the TYS and UTS values are significantly increased to～324MPa and～270MPa(4 passes),from～305MPa and～239MPa in the as-extruded alloy,due to the grain refinement and texture strengthening effects.More importantly,the EF values of the as-ECAPed alloys after 3 and 4 passes are increased to 15.5% and 22.7%,respectively.Therefore,ECAP processing successfully improves the mechanical properties of wrought Mg alloys,which will provide an opportunity for the design of high-strength Mg alloys with superior ductility in the future,and the corresponding mechanisms need to be further discussed.

4.Discussion

Actually,there exists an intrinsic contradiction between the strength and ductility of metallic materials,which is related to the ability of dislocation motion.The grain refinement can effectively improve the yield strength of Mg alloy,at the cost of sacrificial ductility.Meanwhile,the weakening effect of basal texture derived from ECAP usually deteriorates the yield strength.Moreover,the dislocation accumulation and residual stress seriously hinder the improvement of ductility.Therefore,how to accurately control the grain size,texture type,dislocation distribution,and obtain an optimized microstructure,has become a critical factor of achieving the fabrication of high-performance Mg alloy.The current study achieves the simultaneous improvement of yield strength and ductility for extruded Mg alloy,through a multi-pass ECAP processing,and the strengthening and toughing mechanisms will be discussed as follow.

4.1.Effect of ECAP on the microstructure evolution

Generally speaking,both deformation temperature and total strains can influence the recrystallization behavior of wrought Mg alloys.Therefore,the large accumulated strains during the consecutive ECAP contribute to the grain refinement effect.Additionally,the initial grain size influences the recrystallization process of Mg alloy during the hot deformation.In this study,the original as-extruded alloy exhibits two distinct regions:elongated unDRX and equiaxed DRX grains,which are shown in Fig.9a.The extrusion process cannot provide the sufficient strains for a fully completed recrystallization,leading to the formation of this heterogeneous grain structure.Therefore,many sub-grains or dislocation accumulation behaviors are remained within the unDRXed grains,representing a high volume fraction of LAGBs.

During the initial ECAP process(before reaching the ECAP corner),the sample mainly experiences a typical compression,and the twinning behaviors favorably occur within some coarse unDRX grains with specific orientations(Fig.2d and Fig.9b).In contrast,the twinning activations are severely hindered in the fine DRX grains,due to the ultra-high value of critical resolved shear stress(CRSS),which will be dramatically increased with decreasing grain size[37].Moreover,the dislocation accumulation phenomenon or sub-grain structure will gradually happen within the coarse unDRX grains,in order to coordinate the ECAP deformation.With increasing strains(the sample passes through the channel angle),the discontinuous recrystallization induced by twinning behaviors can be observed in the unDRX grains(Fig.2e and f,Fig.9c).Meanwhile,these elongated coarse grains are gradually segmented into fine grains,which restrict the twinning activation.The dislocation accumulations are relieved through a new round recrystallization process,resulting in that partial sub-grain structures convert to HAGBs.The twinning activation hardly occurs during the subsequent ECAP,due to this grain refinement effect.Therefore,the main deformation mechanism is proved as continuous dynamic recrystallization.Many LAGBs are gradually changed into HAGBs(Fig.6),exhibiting an obvious fine-grained structure.According the combining of OM and EBSD analysis,the discontinuous recrystallization induced by twinning activation occurs during the initial ECAP.With the increase of ECAP pass,the continuous dynamic recrystallization gradually becomes the main deformation mechanism.The intensive grain refinement derived from ECAP processing will play an important role on the yield strength of wrought Mg alloy.

Fig.9.Schematic diagrams of recrystallization behaviors in the extruded Mg alloy during the ECAP.

4.2.Influence of ECAP on the texture evolution

The poly-crystal texture is mainly influenced by strain path and plastic deformation model,and therefore the underlying deformation mechanisms can be demonstrated by texture evolution.For the ECAP processing with different temperatures,routes and passes,the deformation mechanisms are complicated,which can achieve various crystallographic texture components and affect the mechanical properties of metallic materials.In general,the microcosmic slip plane(direction)tends to be aligned with the macroscopic shear plane(direction)during the ECAP.Therefore,the texture evolution can be ascribed to the activation of main slip systems.

The main slip systems of Mg alloys during the deformation include the{0002}〈110〉(basal〈a〉),{100}〈110〉(prismatic〈a〉)and{11}〈113〉(pyramidal〈c+a〉),with various critical resolved shear stresses(CRSSs),respectively.Fig.10 shows the schematic diagram of the relationship between the texture evolution and the activation of different slip systems,the conventional basal texture in the as-extruded alloy splits into two components after ECAP for 1 pass,implying that the basal〈a〉slip system is dominant,which has been proved in the previous studies[38,39].Many basal planes are gradually inclined about 45° to ED,a typical shear texture can be observed(defined as B-type,green circular region in Fig.10(a).The orientations of partial basal planes retain unchanged(Atype texture),due to the low equivalent strain in single ECAP pass(～1.05),exhibiting a remarkable texture memory effect(blue rectangular region).In addition,although some coarse grains can be deformed through twinning activation,the subsequent recrystallization weakens the influence of twinning on the formation of crystallographic texture.

Fig.10.Schematic diagrams of the texture evolution and activation of slip systems during the ECAP.

After ECAP for 2 passes,the Mg alloy gradually recovers the conventional basal texture with(0002)plane parallel to ED,which is rarely reported in the Mg–Zn series processed by ECAP.This unique texture component can be attributed to the increased activation of the prismatic slip system,which has been demonstrated by the viscoplastic self-consistent(VPSC)simulation and observed in the ECAPed binary Mg–Li alloys[40].The relations between the orientation evolutions of crystallographic planes and the activations of the corresponding slip systems are revealed in Fig.10b.Most of(100)planes are parallel to the shear plane,resulting in the formation of two-type texture components(P1and P2).With further ECAP processing(4 passes),the shear texture derived from basal slip almost disappears,and the basal fiber texture(especially P2-type texture)is dramatically enhanced.The activation of prismatic slip can be ascribed to the following factors:(1)the high ECAP temperature(300°C)dramatically decreases the CRSS value of prismatic slip system,(2)the obvious grain refinement leads to the increased plastic grain-boundary compatibility stress(when two deformed grains are bonded at the grain boundary,there will be the additional shear stress)and promotes the non-basal slip[41].After ECAP for 1 pass,the average grain size is remarkably decreased to～1.3μm,and the alloy represents a distinct deformation mechanism,the extensively prismatic〈a〉slip occurs.This intensive non-basal texture is also beneficial to the activation of non-basal slip system.

Fig.11.Schmid factor values of Mg alloys:(a)the as-extruded and as-ECAPed for 1(b)and 4(c)passes.

4.3.Strengthening and ductility improvement mechanisms

According to the conventional Hall–Petch relationship[42],the room-temperature yield strength of polycrystalline material will be remarkably increased through the grain refinement,because a large number of grain boundaries can effectively hinder the dislocation slip.Many researchers have tried to improve the strength of Mg alloy through ECAP process.Unfortunately,the typical shear deformation mode during the ECAP favorably leads to the weakening of conventional basal texture in the wrought Mg alloy,which is beneficial to the activation of basal slip system.The ECAP usually decreases the yield strength and improves the ductility of extruded Mg alloys,representing a reversed Hall–Petch effect.The competitive relationship between the grain refinement and texture modification from ECAP determines the values of yield strength and elongation to failure,and therefore the evolutions of mechanical properties of Mg alloys are complicated and remain controversial to some degree.

In this study,the strength and ductility of extruded Mg alloys are simultaneously improved,which can be ascribed to the unique texture modification effect and the enhanced homogeneous deformation.Fig.11 shows the average values of Schmid factor for basal slip systems in the as-ECAPed alloys for different passes.The Schmid factor in the as-extruded Mg alloy with an intensive basal texture is only 0.173,and the activation of basal slip is severely restricted.After ECAP for 1 pass,the appearance of non-basal texture results in that the Schmid factor is dramatically increased to 0.244,which promotes the basal slip.Therefore,the mechanical properties of the as-ECAPed alloy for 1 pass are primarily influenced by texture evolution rather than the grain refinement,the weakening effect of basal texture leads to the reduced yield strength.Moreover,the formation of non-basal texture can contribute to the improved ductility,because the homogenous deformation to large fracture strains is possible.With further ECAP,the unique basal texture components can be gradually formed,leading to an ultra-low Schmid factor(0.136),which effectively hinders the basal slip system and improves the yield strength of the as-ECAPed alloy.Both grain refinement and texture modification contribute to the remarkably increased yield strength,through hindering the dislocation slip(the basal slip system is favorably activated at room temperature)of the as-ECAPed alloy for 4 passes.It is worth noting that the as-ECAPed alloy after 3 passes exhibits the ultra-low elongation,which is related to the grain boundary strengthening.The most grain refinement effect can be observed in the as-ECAPed alloy for 3 passes,which seriously restricts the activation of basal slip system and reduces its ductility.In addition,the low ductility of the as-ECAPed alloy for 3 passes is related to the relatively heterostructure,compared with that for 4 passes,because some unDRX grains can be observed.The recrystallization process is not fully completed,and partial stress concentration behavior may occur,which leads to its reduced elongation.

Fig.12.Fracture morphologies of(a)as-extruded and as-ECAPed alloys for 1(b),2(c),3(d),4(e)passes.

More importantly,the high-strength ECAPed Mg alloy in this study exhibits an excellent ductility,with its EF value of～22.7%.Generally speaking,when the main slip systems for Mg alloys are restricted,the ductility will be dramatically worsened.Many faceted structures and hierarchical morphologies can be observed on the fracture surface of the as-extruded alloy,exhibiting a typical cleavage fracture characteristic(Fig.12a).Both the coarse secondary phase particles and heterogeneous grain structures in the as-extruded alloy will result in the premature failure behavior,because the stress concentration easily occurs around the hardening phases[43]and the elongated unDRX grains with intensive basal texture,representing a poor ductility.Fig.12b～e show the fracture morphologies of the as-ECAPed alloys,all the samples exhibit the obvious ductile-fracture features,with many fine dimples.In addition,the hardening secondary phase particles can effectively improve the yield strength of Mg alloy to a certain degree,but the coarse Ca2Mg6Zn3particles in the as-extruded alloy will accelerate the stress concentration and deteriorate the ductility.A large number of cracked Ca2Mg6Zn3particles can be observed on the fracture surfaces(shown by blue arrows),which indicates that they carry the load transferred byα-Mg matrix.Therefore,the refinement of coarse Ca2Mg6Zn3particles in the as-extruded alloy during the ECAP processing promotes the homogenously tensile deformation and contributes to the enhanced ductility.

5.Conclusions

In the current study,a high-strength Mg–Zn–Ca–Mn alloy with superior ductility is fabricated through the ECAP process at 300°C,which is related to the grain refinement and unique texture modification.The microstructure evolution,strengthening and ductility improvement mechanisms derived from the reduplicative ECAP deformations are discussed,which will pave a new avenue for the design and processing of high-performance wrought Mg alloys in the future.The main conclusions are summarized as followed:

(1)The twinning-induced discontinuous recrystallization occurs within the coarse grains during the ECAP for 1 pass,representing a heterogeneous microstructure.With increase of ECAP passes,the continuous dynamic recrystallization behavior results in an obvious grain refinement.

(2)A typical basal texture can be gradually formed with increasing ECAP passes,which is ascribed to the activation of prismatic〈a〉slip system.Both the elevated temperature and fine-grained structure contribute to the transformation from basal slip to non-basal slip during the ECAP processing.

(3)The enhanced yield strength of the as-ECAPed alloy is attributed to the strengthening effects from the grain refinement and texture modification,while the improved ductility is related to the refinement of secondary phase particles during the ECAP.

Declaration of Competing Interest

None.

Acknowledgments

The authors are grateful to the financial aid from the National Natural Science Foundation(Grant nos.51771178,51671152 and 51874225),the Key Research and Development Program of Shanxi Province(Grant no.2018ZDXMGY-149),the Youth Innovation Team of Shanxi Universities and the Natural Science Foundation of Jilin Province(Grant no.20180414016GH).

Data availability

All data generated or analyzed during this study are included in this article.