Bending properties of extruded AZ91-0.9Ca-0.6Y alloy and their improvement through precompression and annealing

Jong Un Lee,Gyo Myeong Lee,Sung Hyuk Park

School of Materials Science and Engineering,Kyungpook National University,Daegu 41566,Republic of Korea

Abstract In this study,an extruded Mg-9Al-1Zn-0.3Mn-0.9Ca-0.6Y (AZXW9110) alloy is subjected to precompression and subsequent annealing(PCA) treatment for improving its bending formability,and the three-point bending properties of the as-extruded material and the precompressed and subsequently annealed (PCAed) material at room temperature are compared.During bending,microcracks formed in undissolved Al2Ca,Al2Y,and Al8Mn4Y particles propagate along the grain boundaries or twin boundaries and consequently form a macrocrack in the tension zones of the bending samples of both the materials.However,the bending formability of the PCAed material is 75% higher than that of the as-extruded material owing to the promoted twinning and slip behaviors in the former.The as-extruded material has a strong normal direction (ND) texture and a weak transverse direction (TD) texture,and the ND-and TD-oriented grains are unfavorable for both {10-12}twinning and basal slip in the tension zone of the bending sample of this material,which eventually results in its low bending formability.The PCAed material has a strong extrusion direction (ED) texture,weak TD texture,and spread ND texture.During bending,{10-12} twins are fully formed in the ED-oriented grains,and this {10-12} twinning effectively accommodates the plastic strain in the tension zone.In addition,the activation of basal slip in the ND-oriented grains of the PCAed material is promoted owing to the spread ND texture of this material.Consequently,this material exhibits substantially superior bending formability because of the vigorous {10-12} twinning in its ED-oriented grains and promoted basal slip in its ND-oriented grains.

Keywords: Magnesium;Bending;Formability;Twinning;Texture.∗Corresponding author.

1.Introduction

Mg is the lightest among commercial structural metals,and it has recently attracted great attention in the transportation industry as a means to meet the demand for vehicle weight reduction.However,Mg alloys have several drawbacks,which have somewhat limited their application to automobile components.For example,the intrinsic problems of low corrosion resistance and low ignition resistance of Mg alloys need to be overcome to enable expansion of their application range in the transportation industry;this need arises because these two properties are highly related to vehicle component durability and passenger safety.Many studies have been conducted to improve the corrosion and ignition resistances of Mg alloys via the addition of large amounts of Ca or rare-earth (RE) elements [1-5] or via surface treatments [6-8].However,these methods have undesirable effects,e.g.,a reduction in ductility because of the formation of coarse Ca-containing second phases,an increase in material cost because of high cost of RE elements,and an increase in process complexity and environmental pollution by surface treatments [1-8].In recent years,impressive outcomes of the combined addition of small amounts of Ca and Y to Mg-Al-based alloys have been reported: such addition considerably improves the ignition and corrosion resistances of the alloys,as well as their strength.Go et al.[9] showed that the combined addition of 0.3 wt.%Ca and 0.2 wt.% Y to a commercial Mg-Al-Zn alloy increases the ignition temperature significantly from 600°C to 820°C because of the formation of double-layered protective oxides on the material surface [10].Baek et al.[11] demonstrated that the addition of Y considerably improves the corrosion resistance of an extruded Mg-Al-Ca-based alloy mainly because of a change in the type of Al-containing intermetallic particles from Al8Mn5to Al8Mn4Y or Al2Y.In addition,Kim et al.[12] reported that the combined addition of 0.5 wt.% Ca and 0.2 wt.% Y to the AZ31 alloy promotes dynamic recrystallization (DRX) behavior during extrusion,which,in turn,causes a significant improvement in the tensile yield strength of the extruded alloy from 248MPa to 290MPa.

In Mg-Al-based alloys with low Al contents (e.g.,AZ31 and AM30),no precipitation occurs during extrusion;in contrast,fine Mg17Al12precipitates can form in highly alloyed Mg-Al-based alloys (e.g.,AZ80 and AZ91) during extrusion.Therefore,the strength of extruded AZ80 and AZ91 alloys is much higher than that of extruded AZ31 and AM30 alloys owing to the precipitation hardening effect induced by numerous fine precipitates and the enhanced grain-boundary hardening effect caused by precipitate-induced grain refinement in the former two alloys [13,14].For instance,the extruded AZ80-0.3Ca-0.2Y alloy has been reported to exhibit excellent mechanical properties,i.e.,a tensile yield strength of 379MPa,an ultimate tensile strength of 422MPa,and an elongation of 11.3% [9].Extruded metallic materials are usually subjected to the bending forming process to obtain curved final products.Although several studies have recently been conducted on the bending properties of wrought Mg alloys,all of them have been focused on low-alloyed Mg alloys containing no or few second phases,e.g.,AZ31 and AM30[15-28].High-alloyed Mg alloys are expected to have applicability to vehicle body or chassis components,which have a requirement for high strength;therefore,research on the bending properties of such high-alloyed Mg alloys is crucial to facilitate their use in the manufacture of final products having complex shapes.However,no in-depth studies have yet been conducted on the bending properties of high Al (≥8 wt.%)-containing Mg-Al-Zn-Ca-Y alloys with high strength and good ignition and corrosion resistances.An increase in the amount of Al added to Mg-Al-based alloys leads to an increase in the difficulty of movement of dislocations during plastic deformation because of an increase in the amount of solute Al atoms dissolved in theα-Mg matrix,and this difficulty eventually causes the matrix to harden and become brittle.Moreover,when coarse second-phase particles are formed by the addition of a large amount of Al,they can act as crack initiation sites during plastic deformation owing to the excessive stress concentrated on them.On the basis of these facts,it can be expected that high-Al-containing Mg alloys will have lower bending formability than their low-Al-containing counterparts;accordingly,the requirement for improvement of bending formability is greater for the former alloys.Therefore,the present study attempts to investigate the bending properties and deformation behavior of an extruded AZ91 alloy containing small amounts of Ca and Y via three-point bending tests at room temperature (RT).Our previous studies have revealed that the application of precompression and subsequent annealing (PCA) treatment can effectively improve the bending formability of a rolled AZ31 alloy[27,28].Precompression induces a drastic texture change through the generation of {10-12} twins,and subsequent annealing results in the formation of a stable grain structure with a low dislocation density [27].Accordingly,another aim of the present study is to improve the bending properties of the extruded AZ91-Ca-Y alloy through PCA treatment.To this end,an extruded Mg-9Al-1Zn-0.3Mn-0.9Ca-0.6Y (wt.%)(AZXW9110) alloy is first compressed to a plastic strain of 3.0% along the extrusion direction (ED) and then annealed at 400°C,and the bending properties of the as-extruded material and PCA-treated material are systematically analyzed via a comparison of their microstructural evolution and strain accommodation behavior during bending.The twinning and slip behaviors of the two materials during bending are also analyzed in different grain groups with different crystallographic orientations.

2.Experimental procedure

The AZXW9110 alloy was used in this study;the amounts of Ca and Y present in this alloy were optimized values established in a previous study for improving the corrosion and ignition resistances [29-31].A cast billet for extrusion was prepared by melting a commercial AZ91 alloy ingot,high-purity(99.9%) Ca,and a Mg-30Y (wt.%) master alloy in an electric resistance furnace at 750°C in an inert atmosphere comprising a CO2-SF6gas mixture to prevent any oxidation.The molten metal was held at 720°C for 10min to enable its stabilization and then poured into a steel mold preheated to 200°C.The cast billet was homogenized at 440°C for 24h and then water-quenched.A cylindrical sample(Ø70mm×150mm)for extrusion was machined from the homogenized billet and preheated along with dies having an angle of 90°at the extrusion temperature of 350°C for 1h prior to extrusion.The cylindrical sample was directly extruded to a rectangular bar at a temperature of 350°C,a ram speed of 1mm/s,and an extrusion ratio of 22.9.The fabricated extruded bar was 28mm wide and 6mm thick;this bar is hereafter referred to as the as-extruded material.

PCA treatment was performed on rectangular samples machined from the extruded bar.Each rectangular sample had dimensions of 40mm (length)×28mm (width)×6mm(thickness)-which correspond to the ED,transverse direction (TD),and normal direction (ND),respectively-and was compressed to a plastic strain of 3.0% along the ED and then annealed at 400°C for 1h.The precompressed and subsequently annealed sample is hereafter referred to as the PCAed material.For tensile testing,dogbone-shaped specimens with gage dimensions of 10mm (length)×4mm (width)×2mm(thickness) were machined from the central region of the asextruded material and PCAed material.The tensile tests were performed using a Shimadzu AGS-100kNX universal testing machine at room temperature (23°C) and a strain rate of 1×10-3s-1;the loading axis was parallel to the ED.Rectangular samples for bending tests (35mm (length)×25mm(width)×3mm (thickness)) were machined from the PCAed samples.A three-point bending test was performed on an Instron 5985 universal testing machine at a constant crosshead speed of 10mm/min at RT in accordance with the ASTM E290 standard [32].The diameter of the upper and lower rolls of the bending machine was 10mm each,and the distance between the supports was 20mm.The bending test was repeated thrice for each material to ensure repeatability of the results and to verify their consistency;however,for the sake of simplicity,a representative curve for each material was considered for the discussions.

The microstructural characteristics of the as-extruded and PCAed materials were analyzed by optical microscopy (OM),scanning electron microscopy (SEM),electron probe microanalysis (EPMA),X-ray diffraction (XRD) spectroscopy,and electron backscatter diffraction (EBSD) measurements on the ED-TD plane of the corresponding samples.The types of second phases in the as-cast billet,homogenized billet,and as-extruded material were identified via XRD measurements using Cu Kαradiation at a scan speed of 2°/min in the 2θrange of 20°-80°.The microstructures on the ED-ND plane at the mid-width of the fracture bending samples of the asextruded and PCAed materials were observed by OM,SEM,and EBSD to analyze the fracture and deformation behaviors during bending.All samples for OM,SEM,and EBSD measurements were mechanically polished with progressively finer grades of emery paper (#120-#2000 grit) and then polished with 1μm diamond paste.The samples used for each of these observations were prepared by previously reported methods [27,28].The EBSD measurements were performed using a field-emission scanning electron microscope (Hitachi SU-70) at an acceleration voltage of 15kV.Automated EBSD scans were performed in the stage-control mode using the TexSEM Laboratories (TSL) data acquisition software.The EBSD data were analyzed using the TSL orientation imaging microscopy analysis software;only data with a confidence index greater than 0.1 were used to ensure reliability of the analysis results.

3.Results and discussion

3.1.Second-phase particles in as-extruded material

Fig.1 shows the optical and SEM micrographs of the homogenized billet and as-extruded material.In the homogenized billet,numerous irregular-shaped second-phase particles are present along the grain boundaries and within the grains (Fig.1a and b) because the high melting temperatures of the Ca-or Y-containing phases prevent them from dissolving in theα-Mg matrix during homogenization [9,11,12].These undissolved particles are partially fragmented and rearranged along the direction of metal flow (i.e.,the ED) during extrusion (Fig.1c and d).The as-extruded material exhibits a fully dynamically recrystallized (DRXed) structure comprising nearly equiaxed grains;this complete DRX is attributed to the relatively high extrusion temperature (350°C) and the large equivalent strain applied during extrusion (3.13).The second-phase particles in the extruded material were identified via EPMA mapping of the alloying elements added to prepare the AZXW9110 alloy,i.e.,Al,Zn,Mn,Ca,and Y(Fig.2);here,Mn was added as a component of the AZ91(Mg-9Al-1Zn-0.3Mn,wt.%) alloy ingot.The EPMA results reveal the presence of four types of second phases in the AZXW9110 alloy: relatively coarse (～5-15μm) Al-Ca and Al-Y phases,a fine (～2-6μm) Al-Mn-Y phase,and a very fine (～1-2μm) Mg-Al phase;the particles of the Al-Mn-Y phase are much more abundant than those of the other phases.XRD patterns of the as-cast billet,homogenized billet,and as-extruded material are shown in Fig.3a.Peaks associated with the Al2Ca,Al2Y,Al8Mn4Y,and Mg17Al12phases are detected in the XRD pattern of the as-cast billet,whereas peaks associated with only the Al2Ca,Al2Y,and Al8Mn4Y phases are detected in the XRD pattern of the homogenized billet (that is,peaks originating from the Mg17Al12phase are not detected in this pattern).These XRD results indicate that the Al2Ca,Al2Y,Al8Mn4Y,and Mg17Al12phases are formed during the solidification stage in the casting process and that the formed Mg17Al12phase completely dissolves in theα-Mg matrix during homogenization treatment;however,the other phases remain even after homogenization owing to their high thermal stability.The presence of undissolved Al2Ca,Al2Y,and Al8Mn4Y particles in the homogenized billet is consistent with previous observations in Ca-and-Y-containing Mg-Al-based alloys such as Mg-8Al-0.3Zn-0.3Ca-0.2Y [9],Mg-6Al-1Zn-0.3Mn-0.5Ca-0.25Y [11],and Mg-3Al-1Zn-0.2Mn-0.5Ca-0.2Y [12] (wt.%).On the basis of the XRD results,the Al-Ca,Al-Y,and Al-Mn-Y particles identified from the EPMA results are undissolved Al2Ca,Al2Y,and Al8Mn4Y phases,respectively.The peaks of the Mg17Al12phase reappear in the XRD pattern of the as-extruded material (Fig.3a),which implies that Mg17Al12particles are formed through dynamic precipitation during extrusion.The distribution of the four types of second-phase particles identified in the backscattered electron image in Fig.2 is schematically shown in Fig.3b.The undissolved Al2Ca,Al2Y,and Al8Mn4Y particles are polygonal or irregular in shape,whereas the dynamically precipitated Mg17Al12particles are relatively spherical and smaller than the undissolved particles.After PCA treatment,the size of the DRXed grains increases whereas the distribution of the second-phase particles remains almost unchanged(Fig.1e and f).Because the plastic strain imposed during precompression is as small as 3.0%,the second-phase particles in the extruded material do not occur deform or fragment during precompression.In addition,the undissolved Al2Ca,Al2Y,and Al8Mn4Y particles-which have high thermal stability-do not dissolve in theα-Mg matrix during subsequent annealing.The Mg17Al12particles formed during extrusion at 350°C can dissolve during the subsequent annealing at 400°C,but the amount of dissolved particles may be extremely small because the annealing time is short (1h).Therefore,the PCA treatment has negligible effects on the amount and distribution of preexisting second-phase particles in the as-extruded material.

Fig.1.(a,c,e) Optical micrographs and (b,d,f) SEM micrographs of (a,b) homogenized billet,(c,d) as-extruded material,and (e,f) PCAed material.ED and TD denote the extrusion direction and the transverse direction,respectively.

3.2.Microstructural characteristics of as-extruded and PCAed materials

Fig.4 shows the microstructural characteristics of the asextruded material and PCAed material,as determined from the EBSD measurements.The as-extruded material has an average grain size of 21.5μm and a two-component basal texture consisting of a strong ND texture and weak TD texture(Fig.4a and b).Because of the use of an extrusion die with a rectangular hole,the compressive force applied to the material during extrusion is imposed simultaneously along the ND and TD;this consequently results in the formation of two texture components (i.e.,the ND texture and TD texture).However,as the ratio of the width (28mm) to the thickness (6mm) of the extruded material is 4.67:1,the compressive force imposed along the ND during extrusion is considerably larger than that along the TD;therefore,the maximum intensity of the ND texture(15.5)is much higher than that of the TD texture(1.9).The PCAed material exhibits a relatively coarse DRXed grain structure with an average grain size of 57.1μm,which is 2.6 times that of the as-extruded material (Fig.4d).The texture of the PCAed material is also substantially different from that of the as-extruded material;the former has three texture components: a strong ED texture,a weak TD texture,and a weak and spread ND texture (Fig.4e).When compressive deformation is applied to the as-extruded material along the ED as the first step of the PCA treatment,{10-12} twins are formed in most grains because the ND-oriented and TD-oriented grains of the as-extruded material are favorable for {10-12} twinning under compression along the ED.This activation of{10-12} twinning leads to weakening of the preexisting ND and TD textures and the formation of a new ED texture.During the subsequent annealing process-which is the second step of the PCA treatment-thermally activated migration of twin boundaries and grain boundaries occurs vigorously because of the high internal strain energy accumulated in the material by precompression;this migration,in turn,causes grain coarsening and the disappearance of twins [33,34].As a result,the PCAed material exhibits a twin-free,coarse grain structure.Details about the microstructural and textural variations and boundary migration mechanisms in wrought Mg alloys during PCA treatment can be found elsewhere [27,28].In the as-extruded material,the fraction of misorientation angles of 20°-40° is relatively higher than those of the other misorientation angles (Fig.4c);this misorientation angle range is typical of wrought Mg materials with a fully DRXed structure [35].In contrast,in the PCAed material,the fraction of misorientation angles of 85°-90° is highest (Fig.4f) because the {10-12} twin boundaries-which have a misorientation angle of 86.3° and are formed by precompression-migrate during the subsequent annealing and eventually become new grain boundaries.

Fig.2.Backscattered electron image and EPMA scanning maps of elements Mg,Al,Zn,Mn,Ca,and Y in as-extruded material.

Fig.3.(a) X-ray diffraction patterns of as-cast billet,homogenized billet,and as-extruded material.(b) Schematic illustration of second-phase particles depicted in backscattered electron image in Fig.2.

Fig.4.Microstructural characteristics of (a-c) as-extruded and (d-f) PCAed materials: (a,d) IPF maps,(b,e) (0001) pole figures,and (c,f) misorientation angle distributions. davg denotes the average grain size.

Fig.5.Tensile engineering stress-strain curves of as-extruded and PCAed AZXW9110 materials.YS,UTS,and EL denote the yield strength,ultimate tensile strength,and elongation,respectively.

3.3.Bending properties of as-extruded and PCAed materials

Fig.5 shows tensile stress-strain curves and corresponding tensile properties of the as-extruded material and PCAed material.The yield strength of the PCAed material (118MPa)is considerably lower than that of the as-extruded material (236MPa),which is attributed to the coarser grains and crystallographic orientations favorable for {10-12} twinning of the former material.The ultimate tensile strength of the PCAed material (303MPa) is also lower than that of the asextruded material (323MPa);however,the difference in the ultimate tensile strength between the two materials is significantly smaller (20MPa) than the difference in the yield strength(118MPa)because the PCAed material exhibits more pronounced strain-hardening behavior during tensile deformation than the as-extruded material.Despite the considerable differences in yield strength and strain-hardening ability,the as-extruded material and PCAed material have similar tensile elongations (11.6% and 12.1%,respectively).

Fig.6a and b shows the bending load-displacement curves and converted bending stress-strain curves,respectively,of the as-extruded and PCAed materials;Table 1 summarizes the corresponding bending properties.For comparison of these bending properties with those of a low-Al-containing Mg-Albased alloy,the bending curves of a rolled AZ31 alloy examined in our previous study [28]-which were obtained at sample dimensions and under bending test conditions identical to those in the present study-are also included in Fig.6.The maximum bending displacement of the as-extruded material before its cracking (i.e.,the limiting bending depth) is quite small,2.5mm,which is half that of the rolled AZ31 material(5.0mm).Although the basal texture of the as-extruded material (strong ND texture+weak TD texture) is similar to that of the rolled AZ31 material (strong ND texture),the former material has a smaller grain size,higher solute atom content,and more abundant second-phase particles than the latter material.These differences are responsible for the higher bending yield strength and lower bending fracture strain of the as-extruded material (Fig.6b).As the load generated during the bending deformation decreases,the required capacity of the bending forming equipment also decreases;accordingly,a material deformable at lower loads is more suitable for the bending forming process in terms of increasing the practical utility of the equipment.Therefore,the as-extruded AZXW9110 material exhibits poorer bending properties than the rolled AZ31 material in terms of both the forming load and the bending formability.The limiting bending depth of the PCAed material (4.4mm) is considerably greater than that of the as-extruded material (2.5mm),and the bending yield strength of the former (175MPa) is lower than that of the latter (260MPa).These results imply that the application of the PCA treatment significantly improves the bending properties of the as-extruded material.Consequently,the bending formability of the PCAed AZXW9110 material is fairly comparable to that of the rolled AZ31 material (the bending fracture strains of the former and latter are 8.78% and 9.95%,respectively;see Fig.6b and Table 1) despite the substantially large difference between the total amounts of alloying elements in them (7.5 wt.%).

Fig.6.(a) Bending load-displacement curves obtained by three-point bending tests of rolled AZ31 material and as-extruded and PCAed AZXW9110 materials and (b) corresponding bending stress-strain curves.

Table 1 Three-point bending properties of extruded AZXW9110,PCAed AZXW9110,and rolled AZ31 materials.

3.4.Bending fracture behaviors of as-extruded and PCAed materials

Fig.7 shows the optical and SEM micrographs of the microstructures on the ED-ND plane at the mid-width of the fracture bending samples of the as-extruded and PCAed materials.It can be observed from the optical micrographs in Fig.7a that in the bending samples of both the materials,a macrocrack is formed on the outer surface and that it propagates toward the inner,centermost region of the sample in a zigzag manner.This occurrence of macrocracking on the outer surface of the bending samples is consistent with previously reported results for wrought Mg alloys with a strong ND basal texture [15,18,27,28].Fig.7b shows the SEM micrographs near the macrocrack tip in the red rectangular region A marked at two positions in the optical micrographs of the two materials in Fig.7a.In both the materials,many cracked or fragmented undissolved particles are observed along and around the macrocrack (see the yellow arrows in Fig.7b)and microcracks are also formed in the undissolved particles present in front of the macrocrack tip in the crack propagation direction (see the magenta arrows in Fig.7b).These observations indicate that the undissolved particles play an important role in the bending fracture behavior and resultant bending formability of the as-extruded and PCAed materials given the brittle nature of these particles and the concentration of stress on them during bending.Microcracks can form not only by the fragmentation of the undissolved particles themselves but also by the deformation incompatibility between the undissolved particles and the adjacent matrix.Accordingly,microcracks are also observed at the particle-matrix interfaces and they are specifically formed only at interfaces perpendicular to the ED,and not at those parallel to the ED (Fig.7c),because the principal stress imposed in the outer region of the sample during bending is tension along the ED.Narrow microcracks are also observed along the grain boundaries and sharp twin boundaries in particle-free regions,but they are much smaller than the particle-induced microcracks (Fig.7c).These observation results of micro-and macrocracks suggest that microcracks are formed in the brittle undissolved particles distributed throughout the materials and that these microcracks combine with each other by propagating along the grain boundaries or twin boundaries.Therefore,as the undissolved particles act as major crack initiation sites,the as-extruded and PCAed AZXW9110 materials both have lower bending formability than the rolled AZ31 material,which has almost no second-phase particles.However,the bending formability of the PCAed material is 75% higher than that of the as-extruded material even though both are identical in terms of the crack initiation sites (i.e.,particle fragments,particlematrix interfaces,grain boundaries,and twin boundaries) and crack propagation behavior.This difference in the bending formability between the two materials can be attributed to the difference between the amounts of plastic strain accommodated in them during bending.The activated deformation mechanisms and resulting strain accommodation are discussed in Section 3.5.

Fig.7.Micrographs showing macro-and microcracks formed in fracture bending samples of as-extruded and PCAed materials: (a) optical micrographs on ED-ND plane at mid-width of fracture bending samples,(b) SEM micrographs near crack tip in red rectangular region A marked at two positions in (a),and(c) SEM micrographs showing crack initiation sites.

3.5.Deformation behavior in tension zones of as-extruded and PCAed materials

Because macrocracking occurs in the outer region of the bending samples,the bending formability of the materials is governed by the amount of strain accommodated in the tension zones of the samples.For analysis of the deformation behaviors and amounts of strain accommodated during bending,EBSD measurements were conducted in the tension zones of the fracture bending samples of the as-extruded and PCAed materials;the exact measurement positions are illustrated by the blue rectangular region B marked at two positions in the optical micrographs of the two materials in Fig.7a.The inverse pole figure (IPF) maps and boundary maps in the examined tension zones are shown in Fig.8.The amount of deformation imposed on the material is largest on the outer surface of the bending sample,and it decreases with increasing depth from the outer surface;hence,in both the materials,the undetected area (shown in black color) in the bottom region of the IPF maps is larger than that in the top region (Fig.8a and c).Numerous twin traces are observed within the grains of both the materials (black lines in the grains in Fig.8a and c).However,most of the formed twins are undetected in the EBSD measurements (Fig.8b and d) because they are expected to be{10-11}contraction twins and{10-11}-{10-12}double twins,which have high dislocation densities.The total length per unit area of twin boundaries detected in the PCAed material(0.012 μm-1)is six times that in the as-extruded material (0.002 μm-1);moreover,almost all the detected twins are identified as {10-12} extension twins (Fig.8b and d).It should be noted that the IPF maps of the as-extruded material before and after bending have almost the same color (Figs.4a and 8a),which means that the crystallographic orientations of the grains do not change during bending.In contrast,the IPF maps of the PCAed material before and after bending are completely different in color (Figs.4d and 8c),which indicates a significant variation in the crystallographic orientations of the grains during bending.For confirmation of the texture change,the (0001) pole figures of the as-extruded and PCAed materials before bending are compared with those in the tension zone after bending (Fig.9).In the as-extruded material,the TD texture weakens slightly (its maximum intensity decreases from 1.9 to 1.5) and the ND texture strengthens (its maximum intensity increases from 15.5 to 17.1) after bending;however,the variation in the distribution of the overall texture is insignificant (Fig.9a).In the PCAed material,in addition to a slight weakening of the TD texture (its average maximum intensity decreases from 2.65 to 2.2),the initial ED texture disappears completely and the ND texture strengthens substantially (its maximum intensity increases from 6.1 to 11.9) after bending (Fig.9b).Consequently,unlike before bending,after bending,the overall textures of the as-extruded and PCAed materials become similar: they comprise a strong ND texture and weak TD texture.The drastic change in the texture of the PCAed material during bending is a result of lattice reorientation caused by {10-12} twinning in the EDoriented grains.Detailed twinning and slip behaviors during bending are analyzed in two grain groups in the as-extruded material(i.e.,TD-oriented grains and ND-oriented grains)and in three grain groups in the PCAed material (i.e.,TD-oriented grains,ED-oriented grains,and ND-oriented grains).

Fig.8.(a,c) IPF maps and (b,d) corresponding boundary maps in tension zones of fracture bending samples of (a,b) as-extruded and (c,d) PCAed materials.EBSD measurements were conducted in the blue rectangular region B marked at two positions in Fig.7a. ltwin denotes the length of twin boundaries per unit area.

Fig.9.(0001) Pole figures in tension zones of bending samples of (a) as-extruded and (b) PCAed materials before and after bending test.

3.5.1.Twinning behavior in TD-oriented grains

Fig.10 shows IPF maps illustrating the TD-oriented grains in the EBSD measurement areas and the crystallographic orientations of both the twinned TD-oriented grains (marked as M) and the twins formed in the TD-oriented grains (marked as T).In total,48 and 44 TD-oriented grains are observed in the as-extruded and PCAed materials,respectively;their area fractions are 5.3% and 8.8%,respectively.Among these 48 and 44 TD-oriented grains,twinning occurs only in five grains of each material (grains A-E in the as-extruded material and grains F-J in the PCAed material;see Fig.10).All the twins formed in the TD-oriented grains are identified as{10-12} extension twins,and the basal poles of the twinned areas are oriented almost parallel to the ND owing to a lattice reorientation of 86.3° caused by {10-12} twinning.The{10-12} twinning can be active under two loading conditions: compression perpendicular to thec-axis of the hexagonal close-packed (hcp) lattice or tension parallel to thec-axis.The TD-oriented grains are unfavorable for {10-12} twinning under tension along the ED because theirc-axes are almost perpendicular to the loading direction(i.e.,the ED).However,unlike in the uniaxial tensile test,in the three-point bending test,tensile stress is imposed in not only the longitudinal direction but also the width direction in the tension zone of the bending sample,although it is higher in the former direction than in the latter direction [15].Imposition of the tensile stress in the width direction (i.e.,the TD) can induce the formation of {10-12} twins in the TD-oriented grains.Because the occurrence of{10-12}twinning in the TD-oriented grains causes a change in the crystallographic orientation of the basal pole from the TD to the ND,this twinning leads to weakening of the TD texture and strengthening of the ND texture.However,because the area fractions of twins formed in the TD-oriented grains are extremely small (0.6% and 1.0% for the as-extruded and PCAed materials,respectively),their contribution to the change in the overall texture during bending is insignificant.

3.5.2.Twinning and slip behaviors in ND-oriented grains

Fig.10.Twinning behavior of TD-oriented grains in tension zones of fracture bending samples of (a) as-extruded and (b) PCAed materials: IPF maps of twinned grains and hexagonal close-packed (HCP) unit cells of matrix (denoted by M) and twins (denoted by T) formed in the grains (top panels) and crystallographic orientations on (0001) pole figure of the matrix and twins (bottom panels).TT denotes {10-12} tension twinning.

Fig.11.Twinning behaviors of ND-oriented grains and ED-oriented grains in tension zones of fracture bending samples of (a,b) as-extruded and (c,d)PCAed materials: IPF maps of twinned grains (left) and crystallographic orientations and HCP unit cells of matrix (denoted by M) and twins (denoted by T,T1,and T2) formed in the grains (right).TT,CT,and DT denote {10-12} tension twinning,{10-11} contraction twinning,and {10-11}-{10-12} double twinning,respectively.

In the as-extruded material,almost all grains other than the TD-oriented grains are ND-oriented grains because the material has a weak TD texture and strong ND texture.Because the ND-oriented grains undergoc-axis contraction deformation under in-plane tensile stress conditions (e.g.,tension along the ED and tension along the TD),{10-11} contraction twinning and subsequent {10-11}-{10-12} double twinning can occur in these grains in the tension zone of the bending sample.From the analysis of the partially detected twins formed in the ND-oriented grains,it is confirmed that {10-11} contraction twins or {10-11}-{10-12} double twins are formed in the ND-oriented grains of both the as-extruded and the PCAed materials(Fig.11a-c).Because of the low mobility of a {10-11} twinning dislocation,growth of {10-11} contraction twins and {10-11}-{10-12} double twins formed within the contraction twins is difficult [36].Therefore,these contraction and double twins barely contribute to the accommodation of plastic deformation;however,they can cause void formation and cracking owing to the local concentration of high stress induced by the excessive dislocations accumulated in them [36,37].In fact,the formation of the contraction and double twins in the ND-oriented grains does not have a favorable effect on the bending formability of the as-extruded and PCAed materials.However,the basal slip behavior in the ND-oriented grains is expected to be different in the two materials.The ND texture of the as-extruded material is highly concentrated along the ND,whereas that of the PCAed material is somewhat spread from the ND (Fig.9) because the PCA treatment causes weakening and dispersion of the ND texture [27,28,38].This spread distribution of the ND texture of the PCAed material facilitates the activation of basal slip in the ND-oriented grains.Fig.12 shows Schmid factor (SF)maps for basal slip under tension along the ED for the NDoriented grains of the as-extruded and PCAed materials.The average SF value of the PCAed material (0.27) is higher than that of the as-extruded material (0.16);therefore,the promoted activation of basal slip in the ND-oriented grains of the PCAed material partially contributes to the improvement in its bending formability.

Fig.12.Schmid factor (SF) maps for basal slip under tension along ED for ND-oriented grains of (a) as-extruded and (b) PCAed materials. SFbasal denotes the average SF value for basal slip.

3.5.3.Twinning behavior in ED-oriented grains

Unlike the as-extruded material,the PCAed material has many ED-oriented grains.These grains are favorable for {10-12} twinning in the tension zone during bending because theirc-axes are aligned almost parallel to the principal tensile loading direction (i.e.,the ED).Because the critical resolved shear stress (CRSS) for {10-12} twinning is much smaller than that for {10-11} twinning [36],{10-12} twins form at lower stress levels under favorable loading conditions.In addition,the width of a {10-12} twinning dislocation is approximately six times that of a {10-11} twinning dislocation[39];therefore,a formed {10-12} twin can easily grow to fill a grain,which a formed {10-11} twin cannot do.Therefore,the activation of {10-12} twinning in the ED-oriented grains causes a decrease in the bending flow stress and an increase in the bending fracture stain because the deformation imposed in the tension zone during bending is accommodated by this twinning.Fig.11d shows an originally ED-oriented grain in the fracture bending sample of the PCAed material,from which it is clearly observed that this grain almost completely transforms into a {10-12} twinned area after bending (T1 in Fig.11d);only a very small part of the original grain remains unchanged after bending (M in Fig.11d).The basal poles of the {10-12} twins formed in the ED-oriented grains are oriented almost parallel to the ND owing to the lattice reorientation of 86.3°.Accordingly,the extensive {10-12} twinning in the ED-oriented grains leads to a significant variation in the overall texture of the PCAed material (from the ED texture to the ND texture),as shown in Fig.9b.After{10-12} twinning occurs throughout an ED-oriented grain,contraction and double twins can form in the fully {10-12}twinned grain;this twin formation is confirmed by the presence of a contraction twin in the {10-12} twinned area (T2 in Fig.11d).

The amount of plastic strain accommodated by {10-12} twinning (i.e.,twinning strain) can be calculated asεtwin=ftwin··γtwin,whereεtwin,ftwin,,andγtwindenote the twinning strain,area fraction of the twinned region,average SF for {10-12} twinning under deformation conditions,and {10-12} twinning shear (0.129),respectively[40].The complete disappearance of the ED texture of the PCAed material after bending (Fig.9b) indicates that all the ED-oriented grains transform into fully twinned areas by{10-12} twinning during bending.Fig.13 shows the IPF maps and distributions of SF for {10-12} twinning of grains favorably oriented for this twinning under tension along the ED (these grains correspond to the ED-oriented grains) for the as-extruded and PCAed materials.Very few grains of the as-extruded material (area fraction: 1.8%) have a favorable orientation for {10-12} twinning;however,numerous grains of the PCAed material (area fraction: 51.9%) are favorable for {10-12} twinning (Fig.13a and b).The average SF values for {10-12} twinning under tension along the ED for the grains of the as-extruded and PCAed materials are 0.44 and 0.43,respectively (Fig.13c and d).From the area fractions and average SF values of the twinning-favorable grains of the as-extruded and PCAed materials,the total amounts of strain accommodated by complete {10-12} twinning in these grains during bending are calculated as 0.10%(=1.8%×0.44×0.129) and 2.88% (=51.9%×0.43×0.129),respectively.As shown in Fig.6b,the bending fracture strains of the as-extruded and PCAed materials are 5.02%and 8.78%,respectively.This difference of 3.76% between the bending fracture strains is 74% of the difference of 2.78%in the above-calculated amounts of strain accommodated by{10-12} twinning in the two materials.This result suggests that the improved bending formability of the PCAed material is attributable mainly to the accommodation of strain by{10-12} twinning in the ED-oriented grains,which constitute more than half the total grains of the material.

Fig.13.Characteristics of ED-oriented grains of (a,c) as-extruded and (b,d) PCAed materials: (a,b) IPF maps and (0001) pole figures and (c,d) distributions of SF for {10-12} twinning under tension along ED. fED and SFtwin denote the area fraction of ED-oriented grains and the average SF value for {10-12}twinning,respectively.

Consequently,the as-extruded AZXW9110 material has poor bending formability owing to a combination of the presence of numerous undissolved particles that act as crack initiation sites and the unfavorable crystallographic orientations for both basal slip and {10-12} twinning in the tension zone during bending.In contrast,even though the PCAed AZXW9110 material also contains undissolved particles,it exhibits substantially improved bending formability owing to the vigorous occurrence of {10-12} twinning in the ED-oriented grains and the promoted activation of basal slip in the ND-oriented grains.The results of the present study demonstrate that the application of a combined process of plastic deformation(precompression) and subsequent heat treatment (annealing) prior to a bending test can significantly improve the bending properties of a high-Al-containing Mg-Al-Zn-Ca-Y alloy,which already has high strength,high corrosion resistance,and high ignition resistance.

4.Conclusions

In this study,PCA treatment is applied to an extruded AZXW9110 alloy to improve its bending properties,and the bending deformation behavior and resultant bending formability of the as-extruded material and PCAed material are investigated.In both the materials,undissolved Al2Ca,Al2Y,and Al8Mn4Y particles act as the main crack initiation sites during bending;however,the twinning and slip behaviors during bending are considerably different in the two materials.In the as-extruded material,which has a strong ND texture and weak TD texture,{10-11} contraction twins or {10-11}-{10-12}double twins are formed in the ND-oriented grains and very few {10-12} twins are formed in the TD-oriented grains.In contrast,the PCAed material has substantially different crystallographic orientations and its overall texture is composed of a strong ED texture,weak TD texture,and spread ND texture.During bending,{10-12} twins are fully formed in the EDoriented grains,and this vigorous {10-12} twinning accommodates a large amount of strain.In addition,although the degree of {10-12} twinning in the TD-oriented grains of the PCAed material is almost the same as that of the as-extruded material,the activation of basal slip in the ND-oriented grains is more pronounced in the former because of its spread ND texture.Consequently,the bending formability of the PCAed material is 75% higher than that of the as-extruded material.This study demonstrates that the RT bending properties of highly alloyed wrought Mg materials can be improved significantly through control of their texture via PCA treatment.

Declaration of Competing Interest

The authors declare that they have no conflict of interest.

Acknowledgments

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government(MSIP,South Korea;No.2019R1A2C1085272).