Improvement of strength and ductility synergy in a room-temperature stretch-formable Mg-Al-Mn alloy sheet by twin-roll casting and low-temperature annealing

T.Nkt, C.Xu, K.Kie, Y.Yoshid, K.Yoshid, S.Kmdo

a Nagaoka University of Technology, 1603-1, Kamitomioka, Nagaoka 940-2188, Japan

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

c Sumitomo Electric Industries, Ltd., 1-1-1 Koyakita, Itami 664-0016, Japan

Abstract Strength and ductility synergy in an Mg-3mass%Al-Mn (AM30) alloy sheet was successfully improved via twin-roll casting and annealing at low-temperature.An AM30 alloy sheet produced by twin-roll casting, homogenization, hot-rolling, and subsequent annealing at 170 °C for 64 h exhibits a good 0.2% proof stress of 170 MPa and a large elongation to failure of 33.1% along the rolling direction.The sheet also shows in-plane isotropic tensile properties, and the 0.2% proof stress and elongation to failure along the transverse direction are 176 MPa and 35.5%, respectively.Though the sheet produced by direct-chill casting also shows moderate strengths if the annealing condition is same, the direct-chill casting leads to the deteriorated elongation to failure of 23.9% and 30.0% for the rolling and transverse directions,respectively.As well as such excellent tensile properties, a high room-temperature stretch formability with an Index Erichsen value of 8.3 mm could be obtained in the twin-roll cast sheet annealed at 170 °C for 64 h.The annealing at a higher temperature further improves the stretch formability; however, this results in the decrease of the tensile properties.Microstructure characterization reveals that the excellent combination of strengths, ductility, and stretch formability in the twin-roll cast sheet annealed at the low-temperature annealing is mainly attributed to the uniform recrystallized microstructure, fin grain size, and circular distribution of (0001) poles away from the normal direction of the sheet.

? 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; Rolling; Tensile property; Room temperature formability; Twinning.

1.Introduction

To stabilize the concentration of CO2gas in Earth's atmosphere, there is an increasing demand for reducing the weight of transportation vehicles such as automobiles and high-speed trains.Wrought magnesium (Mg) alloys have received great interest because of their low density and high specifi strengths [1]; however, the application of commercial wrought magnesium alloys is usually hampered owing to their poor ductility and room-temperature (RT) formability [1,2].For example, a commercial Mg-3Al-1Zn (mass%, AZ31) alloy sheet shows poor elongation to failure of ～15-20%[3-6], and the Index Erichsen (I.E.) value, an indicator of stretch formability, is only ～3-5 mm at RT [5-7].These values are two to three times lower compared to those of Al-Mg-Si based alloy sheets that are widely used in automotive applications [8-10], and therefore, improvement of both ductility and formability is strongly desired.The poor ductility and formability in commercial magnesium alloy sheets are caused by the strong basal texture in which (0001) poles strongly align parallel to the normal direction of the sheets (ND)[2,11-15].Simultaneous addition of zinc (Zn) and (Ca) elements is reported to be a low-cost method to weaken the basaltexture, and a dilute Mg-1.5Zn-0.1Ca (mass%) alloy sheet exhibits good ductility and stretch formability [16].However,the Mg-Zn-Ca sheet shows poor 0.2%proof stress of 120 MPa along the rolling direction (RD), and the 0.2% proof stress along the transverse direction (TD) is only 73 MPa due to the splitting of (0001) poles to the TD, which results in the ease activation of basal slips [16].To solve this problem, an Al-containing Mg-3Al-1Zn-1Mn-0.5Ca (mass%) alloy sheet exhibiting high 0.2% proof stress and I.E.value of 219 MPa and 8 mm has been developed by Trang et al.[17].Although the sheet has good combination of strengths and formability, the elongation to failure is 17% [18], and this is far below the Mg-Zn-Ca alloy and Al-Mg-Si based alloy sheets.Recently, we have developed a stretch-formable Mg-3Al-Mn(mass%) alloy sheet having a good combination of strengths and ductility by using a conventional process that includes direct-chill casting, homogenization, hot-rolling, and annealing.The 0.2% proof stress and the elongation to failure along the RD are 153 MPa and 28%, respectively, and the alloy sheet has in-plane isotropic tensile properties [18], indicating that the alloy sheet becomes promising materials for automotive body applications.For the further improvement of properties, twin-roll casting is currently the focus of interest, since this casting process contributes to the refinemen of grain structures and secondary phase particles, leading to high mechanical properties in magnesium alloy sheets[17,19].Annealing condition seems to be also important to control mechanical properties of rolled magnesium alloy sheets.This is because deformed microstructures, which results in low plasticity, might be preserved if the annealing is not sufficient while prolonged/high-temperature annealing brings about grain coarsening and deteriorated strengths [20-23].Considering these experimental results, we can expect further enhancement of the tensile properties in our stretch-formable Mg-3Al-Mn (mass%) alloy sheet via twin-roll casting and optimizing annealing conditions, and therefore, in this work,the effects of casting method and annealing conditions on microstructures, tensile properties, and stretch formability of a rolled AM30 alloy sheet have been investigated.

2.Experimental procedure

AM30 alloys were prepared by a direct-chill casting and a twin-roll casting.The chemical compositions, which were analyzed by inductivity coupled plasma method, are shown in Table 1.They were cut into plates with 100 mm in length,130 mm in width, and 4 mm in thickness, followed by a homogenization treatment at 415 °C for 2 h plus 500 °C for 2 h.After the homogenization, the plates were rapidly cooled by water quenching.The homogenized plates werecontinuously rolled at a roller temperature of 180 °C with a rolling reduction of 20%/pass, and the rolling was repeated 6 times using the roller speed of 5 m/min to obtain sheets with 1 mm in thickness.Before the continuous rolling, the homogenized plates were pre-heated at 180 °C for 30 min, and the rolled sheets were cooled in an air.Annealing was performed in an oil bath using conditions as displayed in Table 2.Note that the sheets were water-quenched after the annealing.Erichsen cupping tests were done to evaluate the stretch formability of the annealed sheets using rectangular specimens with 60 × 60 mm2.The test was performed at a RT on a sheet metal testing machine (ERICHSEN, Model 100), and the punch diameter and speed were 20 mm and ～6 mm/min,respectively.RT tensile tests were also conducted on a universal testing machine (Shimadzu, Autograph AG-50kNI) using dog-bone shaped specimens having gage length of 20 mm and thickness of 4 mm.The initial strain rate was 10-3s-1,and the loading direction was parallel to the RD and TD of the rolled sheets.For reproducibility, all tests were repeated three time.Microstructures and textures were evaluated by a X-ray diffractometer (XRD, Rigaku, MiniFlex600), a scanning electron microscope (JEOL, JSM-7000F) equipped with TSL electron backscattered diffraction (EBSD) apparatus, and a scanning electron microscope (JEOL, IT-500) with an energy dispersive X-ray spectroscopy (EDS) detector.Deformation behavior during the tensile test was also investigated, and the EBSD measurement was done on some tensile test specimens after the plastic strain of 10%.Sample preparations for the microstructural observations were done by mechanical polishing using SiC papers, 0.3 μm-dia Al2O3powder,and 0.04 μm-dia colloidal silica suspension.Microstructural characterizations of sheet samples were done on the RD-ND plane, and the EBSD data was analyzed from the RD-TD plane using Orientation Imaging Microscopy(OIM)v8.5 software.Fracture surfaces of some tensile test specimens were also observed using the IT-500 scanning electron microscope.

Table 1Chemical compositions of AM30 alloys produced by direct-chill casting and twin-roll casting [mass%].

Table 2Summary of annealing conditions operated in this work.

3.Results

Fig.1 shows inverse pole figur (IPF) maps and (0001)pole figure of the homogenized samples produced by the(a) direct-chill casting and (b) twin-roll casting.Note that insets in the pole figure are the maximum intensity of the poles.The direct-chill-cast (DC) sample shows coarse grain structures with an average grain size of 348 μm, and the(0001) planes are randomly distributed.The grain size and texture are completely different in the twin-roll-cast (TRC)sample, and the TRC sample forms much fine average grain size of 33 μm with preferential tilting of (0001) poles to the ND.

Fig.1.Inverse pole f gure maps and (0001) pole f gures of homogenized AM30 alloys produced by (a) direct-chill casting and (b) twin-roll casting.Note that insets in the pole figure are the maximum intensity of the poles.

Fig.2 shows backscattered electron (BSE) images of the homogenized samples produced by the (a) direct-chill casting and (b) twin-roll casting.High-magnificatio BSE images and EDS elemental maps obtained from red-rectangles in Fig.2(a,b) are also shown in Fig.2(c-j), and Fig.2(k)summarizes XRD 2Theta-Intensity profile of both homogenized samples.Spherical and rod-shaped secondary phases are distributed in both samples; however, the TRC sample shows much fine distribution of these particles.In the secondary phase particles of the DC sample, Al and Mn elements are enriched, and small amount of Si element could be detected as well.The 2Theta-Intensity profil indicates that they are Al8Mn5phase (rhombohedral,a= 1.2667 nm,c= 0.7942 nm [24]) that contains Si element [25-27].The presence of the Al8Mn5phases in an Mg-3Al-Mn alloy is also demonstrated in [28], and the present XRD measurement agrees with that study.The TRC sample also forms Al- and Mn-enriched particles with rodshaped or spherical-shaped morphology, and as shown in Fig.2(k), they should be Al8Mn5phase.Particles that contain only Si element are also apparent in the TRC sample as indicated by black arrow-heads in Fig.2(g,j).The 2Theta-Intensity profil shows a small diffraction peak that corresponds to Mg2Si phase (cubic,a= 0.635 nm [29]) [30], indicating that the TRC sample forms Mg2Si phase [31]; however,the fraction seems to be limited compared to the Al8Mn5phase as shown in the BSE image (Fig.2(b)).

Fig.3 shows tensile stress-strain curves of the annealed(a, b) DC and (c, d) TRC specimens stretched along the RD and TD, and Table 3 summarizes the ultimate tensile strength (U.T.S.), 0.2% proof stress (P.S.), elongation to fail-ure (El.), and index Erichsen (I.E.) value.The samples annealed at 170 °C for 8 h exhibit high U.T.S.and P.S., and the TRC sample also shows moderate El.over 20%; however,their I.E.values are as low as commercial AZ31 alloy sheets with a typical basal texture [6,7].The annealing at 170 °C for 64 h leads to the substantial improvement of the ductility and stretch formability in both DC and TRC samples, and excellent El.over 33% and high I.E.value of 8.3 mm could be realized in the TRC sample.Although the prolonged annealing decreases the strengths, both DC and TRC samples keep good P.S.of ～160-170 MPa.After the annealing at 200 °C, both DC and TRC samples exhibit enhanced stretch formability and ductility compared to the samples annealed at 170 °C for 8 h.Moreover, the stretch formability in the samples annealed at 170 °C for 64 h is further improved by the high-temperature annealing; however, the annealing at 200°C leads to the decrease of the tensile properties.Overall,all AM30 samples exhibit in-plane isotropic tensile properties as we report recently [18].

Table 3Ultimate tensile strength (U.T.S.), 0.2% proof stress (P.S.), elongation to failure (El.), and Index Erichsen (I.E.) value of annealed AM30 alloy sheets.

Fig.2.Backscattered electron (BSE) images of homogenized AM30 alloys produced by (a) direct-chill casting and (b) twin-roll casting.High-magnificatio BSE images and EDS elemental maps obtained from red-rectangles in Fig.2(a) and (b) are also shown in Fig.2(c-j), and (k)summarizes XRD 2Theta-Intensity profile of the homogenized AM30 alloys.

Fig.3.Tensile stress-strain curves of annealed AM30 alloy sheets produced by (a, b) direct chill casting and (c,d) twin-roll casting.Note that the loading directions were parallel to the rolling direction and transverse direction of the sheets (RD and TD).

Fig.4.Inverse pole figur maps and (0001) pole figure of (a-d) direct-chill-cast and (e-h) twin-roll-cast AM30 alloy sheets annealed at different conditions.Note that the maximum intensity is described in the pole figure and enlarged Kernel Average Misorientation (KAM) maps obtained from selected areas are shown on the upper-right in Fig.4(b) and (f).

Fig.4 shows IPF maps and (0001) pole figure of the (a,b, c, d) DC and (e, f, g, h) TRC samples annealed at different conditions.Note that maximum intensity of the pole is described in the pole figure and average recrystallized grain size, (dREC, grains with Grain Orientation Spread less than 0.5° were used for the calculation) and Kernel Average Misorientation (KAM) values are summarized in Table 4.Both DC and TRC samples annealed at 170 °C for 8 h consist of a small fraction of statically recrystallized grains and unrecystallized grains with an elongated shape.Their (0001) poles align ～10° to the RD and show high maximum intensity over 8 MRD(Multiples of Random Distribution).The samples also show large KAM values of ～1.3-1.4°, suggesting that high density of dislocation still exists after the annealing at 170 °C for 8 h.After the annealing at 170 °C for 64 h, although a small fraction of coarse unrecystallized grains still exist as indicated by red arrows in enlarged KAM maps (upper-right insets in Fig.4(b,f)), almost all grains are recrystallized in both DC and TRC samples and then KAM values are de-creased to 0.47°.The prolonged annealing also changes the texture feature, both DC and TRC samples show weakened basal texture with a low maximum intensity of(0001)poles of～4 MRD.The (0001) poles show circular distribution away from the ND, and the tilting angle is about 20-30°, which is beneficia to reduce the yield anisotropy [18].Note that the DC sample also has obvious broadening of (0001) poles to the TD.Annealing at 200 °C for 8 h causes slight grain growth, and the increase of the annealing time also results in the grain growth.After the annealing at 200 °C, the circular distribution of (0001) poles is still retained in both DC and TRC samples, while in the DC sample, the broadening of(0001) poles to the TD is decreased.KAM values are further decreased by the annealing at 200 °C, but the changes are small compared to the samples annealed at 170 °C for 64 h.This suggests that samples are almost recrystallized after the annealing at 170 °C for 64 h and the annealing at 200 °C.At all annealing conditions, the TRC samples form finedRECthan that of the DC samples.

Table 4Average recrystallized grain sizes (dREC) and Kernel Average Misorientation (KAM) values in annealed AM30 alloy sheets.Note that grains with Grain Orientation Spread less than 0.5° were define as recrystallized grains.

To understand the reason why the TRC sample annealed at 170 °C for 64 h exhibits the highest ductility, at first fracture surfaces after the tensile test were observed.Fig.5 shows secondary electron images of the fracture surfaces obtained from the (a, b) DC samples annealed at 170 °C for 64 h,(c, d) TRC samples annealed at 170 °C for 64 h, and (e, f)TRC samples annealed at 200 °C for 64 h.The DC samples show mixed fracture feature, ductile dimples and large cleavage regions (indicated by white arrows) coexist, while in the TRC samples, ductile dimples occupy almost all the fracture surfaces.Deformation behavior during the tensile test was further investigated by the EBSD.Fig.6 shows IPF maps, image quality maps, and (0001) pole figure obtained from the tensile stretched (a, b, c) DC and (d, e, f) TRC samples annealed at 170 °C for 64 h.Note that the loading direction is parallel to the TD.In the image quality maps, boundaries which correspond to double twinning (DTW, 38°＜11ˉ20＞), compression twinning (CTW, 56°＜11ˉ20＞), and tensile twinning(TTW, 86°＜11ˉ20＞) are represented by green-, blue-, and red-colored lines.These boundaries were calculated with 4°tolerance on axis and angle.Fractions of these twins (fDTW,fCTW, andfTTW) are also calculated by following equations,and they are summarized in Table 5.

Table 5Fractions of double twinning (fDTW), compression twinning (fCTW), and tensile twinning (fTTW) in AM30 alloy sheets annealed at 170 °C for 64 h after applying tensile plastic strain of 10% along the transverse direction.

Fig.5.Secondary electron images of tensile fracture surfaces obtained from the (a, b) direct-chill-cast AM30 alloy sheets annealed at 170 °C for 64 h,(c, d) twin-roll-cast AM30 alloy sheets annealed at 170 °C for 64 h, and (e,f) twin-roll-cast AM30 alloy sheet annealed at 200 °C for 64 h.

The fractions of the TTW are very similar in both samples; however, in the DC sample, the fractions of the DTW and CTW are more than twice as in the TRC sample.This means that the formation of DTW and CTW is significantl suppressed in the TRC sample.

Fig.6.Inverse pole figur maps, image quality maps, and (0001) pole figure obtained from tensile stretched (a-c) direct-chill-cast and (d-f) twin-roll-cast AM30 alloy sheets annealed at 170 °C for 64 h.Note that loading direction is parallel to the transverse direction of the sheets, and in the image quality maps,boundaries which correspond to double twinning, compression twinning, and tensile twinning are represented by green-, blue-, and red-colored lines.

Fig.7.Summary of 0.2% proof stress and elongation to failure along (a) rolling and (b) transverse directions of magnesium alloy sheets with Index Erichsen value larger than 8 mm.Note that the data of twin-roll cast AM30 alloy sheet are also plotted in Fig.7.RD and TD represent rolling and transverse directions of the sheets.

4.Discussions

In this work, we have successfully improved the strengthductility synergy in an AM30 alloy sheet.The AM30 alloy sheet produced by twin-roll casting, homogenization treatment, hot-rolling, and subsequent low-temperature annealing at 170°C for 64 h exhibits excellent combination of strengths,ductility, and stretch formability.The P.S.and El.along the RD are 170 MPa and 33.1%, and the alloy sheet exhibits the high I.E.value of 8.3 mm.The alloy sheet also exhibits in-plane isotropic tensile properties; the P.S.and El.along the TD are 176 MPa and 35.5%, respectively.Fig.7 summarizes P.S.and El.along (a) RD and (b) TD of various magnesium alloy sheets with the I.E.value larger than 8 mm[16,18,32-43].The P.S.and El.of the TRC AM30 alloy sheet annealed at 170 °C for 64 h are also included in Fig.7.Recently developed RT-formable magnesium alloy sheets have inverse relation between strengths and ductility,and the sheets with high P.S.generally show low El.as indicated by dot lines.The AM30 alloy sheet designed in this work exhibits better combination of P.S.and El.,and some magnesium alloy sheets,age-hardenable dilute Mg-Al-Ca-Zn-Mn alloy and Mg-Zn-Y-Zr alloy sheets, still have comparable properties as the AM30 alloy sheet along the RD; however, the poor strengths along the TD become bottleneck in these sheets [35,40]and then we believe that the AM30 alloy sheet becomes promising material for automotive sheet applications where high tensile properties, excellent room-temperature formability, and in-plane isotropic properties are essential.

Fig.8.Relationship of inverse square root of average recrystallized grain size and 0.2% proof stress of AM30 alloy sheets.Note that blue-colored line and the slope are obtained by a least-square method using the data except for AM30 alloy sheets annealed at 170 °C for 8 h.RD and TD represent rolling and transverse directions of the sheets.

The good P.S.in the TRC AM30 alloy sheet annealed at 170 °C for 64 h may be attributed to its fin grain structures with the average recrystallized grain size (dREC) of 6.2 μm.In metal materials, the P.S.increases by decreasing the grain size (d).This is known as the Hall-Petch relationship, and the relationship is expressed by a following equation;

whereσ0is the fl w stress andkyis the hall-petch coeffi cient.Fig.8 summarizes thed-1/2RECand P.S.of the AM30 alloy sheets obtained in this work.Except for the sheets annealed at 170 °C for 8 h, the AM30 alloy sheets follow the Hall-Petch relationship as indicated by a blue-colored line,and thekyis calculated to be 302 MPa?μm1/2.The obtained value is similar to that of other wrought magnesium alloys[44-47], and this supports that the grain refinemen contributes to the good P.S.in the TRC alloy sheet after the annealing at 170 °C for 64 h.Furthermore, it should be mentioned that the alloy sheet shows in-plane isotropic P.S., and this would be attributed to the circular distribution of (0001)poles as shown in Fig.4 [18].The unexpected high P.S.after the annealing at 170 °C for 8 h should be caused by the presence of high fraction of unrecrystallized grains which contributes to dislocation strengthening (Table 4) [48,49].

The TRC alloy sheet annealed at 170 °C for 64 h also forms fine and more uniform grain structures than that of the DC alloy sheet, and this may result in the excellent ductility in the TRC alloy sheet.The DC alloy sheet annealed at 170 °C for 64 h shows weak distribution of (0001) poles, and spreading of the (0001) poles in the DC alloy sheet is more obvious than the TRC alloy sheet annealed at the same condition.However, the El.of the DC alloy sheet is still lower than the TRC alloy sheet.As shown in Fig.5, the DC alloy sheet shows large fraction of cleavage regions after the tensile test, while the tensile fracture surface of the TRC alloy sheet mainly consists of ductile dimples.Also, as indicated by the EBSD observation from the cross-section of the 10%deformed samples, the DC alloy sheet forms higher fractions of CTW and DTW than those in the TRC alloy sheet (Fig.6 and Table 5).They indicate that the formation of CTW and DTW is facilitated due to the presence of coarse grains in the DC alloy sheet, resulting in the limited ductility along the TD despite of the spreading of(0001)poles[50,51].Although the fractions of TTW in both DC and TRC alloy sheets are similar (Table 5), as demonstrated by Ando et al.[51], TTW has little effect on crack initiation and then the TRC alloy sheet keeps excellent ductility.The finel dispersed secondary phase particles in the TRC alloy (Fig.2) may also contribute to the better ductility [52-54].The TRC alloy sheet annealed at 200 °C for 64 h also shows ductile dimple fracture feature after the tensile test; however, the El.is lower than that of the TRC alloy sheet annealed at 170 °C for 64 h.This may be due to the enhanced prismatic slips in the fin grained TRC alloy sheet annealed at 170°C[47,55].Note that a small fraction of unrecrystallized grains still exists after the annealing at 170 °C for 64 h (Fig.4(b,f)), the fraction seems to be limited and then the annealing at 170 °C for 64 h could bring about the highest ductility in both DC and TRC alloy sheets.Otherwise the presence of some amount of recrystallized grains may help to improve the ductility as demonstrated in [56,57].

Except for the sheets annealed at 170 °C for 8 h, the AM30 alloy sheets show good stretch formability, and this is also attributed to the weakly aligned basal texture and fin grain structures [18].Compared to the DC alloy sheets, the TRC alloy sheets exhibit the high I.E.value if the annealing condition is the same, and the much fine distribution of secondary phase particles in the TRC alloy sheet would contribute to the improvement [52-54].Unlike the El., the annealing at 200 °C further improves the stretch formability in both DC and TRC alloy sheets.There is a possibility that high work-hardenability and ease activation of TTW in coarse grained magnesium alloys would have a preferable effect on the stretch formability [6,37,44,58,59], and further in-depth microstructural characterization needs to be done.

In spite of the lower stretch formability of the DC alloy sheets, it is still surprising that even the DC alloy sheets exhibit moderate stretch formability after the rolling process if the sheets are almost recrystallized.During hot-rolling, it is widely recognized that commercial Mg-Al based alloy sheets form strong basal texture due to dynamic recrystallization,and the basal texture becomes stronger with increasing the total rolling reduction; however, in this work, we used 4 mmthick samples even for the direct-chill cast alloy, and this is much thinner than conventional cast slab reported in the literature [18,60,61].This decreases the total rolling reduction and may help to retard the formation of strong basal texturein the present AM30 alloy sheets, resulting in the moderate formability.

The AM30 alloy sheets annealed at 170 °C for 8 h exhibit high P.S.over 200 MPa; however, they suffer from poor stretch formability.Also, these alloy sheets show deteriorated ductility compared to the sheets annealed at different conditions, and especially, the El.of the DC alloy sheet are only 8.9% and 14.9% for the RD and TD, respectively.The combination of high strengths, low ductility, and poor formability may be responsible for the large fraction of unrecystallized grains exhibiting slight tilting of (0001) poles of ～10°to the RD (Fig.4(a,b)).The TRC alloy sheet annealed at 170 °C for 8 h also shows poor I.E.value; however, it is surprising that the sheet exhibits moderate El.of ～21% in both RD and TD, and the stretch formability is slightly improved compared to the DC alloy sheet (Table 3).This suggests that large plasticity can be obtained if the unrecystallized grains are finel distributed.Since the presence of unrecystallized grains contributes to high strengths [62,63], the effect of bimodal microstructures on tensile properties and formability should be quantitatively evaluated for further improvement of properties in room-temperature formable magnesium alloy sheets.

Finally, we would like to mention the beneficia effect of twin-roll casting for the improvement of strengths and ductility synergy in the AM30 alloy sheet.As shown in Fig.1, the homogenized TRC alloy shows much fine grain structures than that of the DC alloy, and the average grain size in the TRC alloy is about one-tenth of the DC alloy.The fin average grain size in the homogenized state mainly contributes to the uniform and fin grain structures after the hot-rolling and subsequent annealing in the TRC alloy sheet[64,65],realizing the excellent combination of strengths and ductility.Further,the secondary phase particles become fine via the twin-roll casting due to the high cooling rate [66,67], and the fin distribution may contribute to the better ductility and formability in the TRC alloy sheets to some extent.As shown in Table 1,the TRC alloy contains slightly higher content of Si element,and this may lead to the formation of Mg2Si phases (Fig.2);however, the fraction seems to be limited and then the high ductility could be maintained in the TRC sheets.It should be noted that the homogenized TRC AM30 alloy shows basal texture feature; however, the hot-rolling and annealing could eliminate the alignment of (0001) poles to the ND, and the weakened texture feature would be a main reason for the high formability in the AM30 alloy sheets.The modificatio of texture feature in the AM30 alloy sheets needs to be further investigated for establishing the basis of texture formation in wrought magnesium alloys.

5.Conclusion

In this work, we have developed a room-temperature formable Mg-3Al-0.4Mn (AM30) alloy sheet with high strengths and ductility synergy via twin-roll casting and lowtemperature annealing.The balance of the 0.2% proof stress and elongation to failure is higher than that of recently designed magnesium alloy sheets with high stretch formability.Furthermore, the AM30 alloy sheet has in-plane isotropic tensile properties, which is indispensable to structural components in automobiles.From microstructural characterization,we have also found that uniform and fin grain structures with circular distribution of (0001) poles mainly contribute to the excellent properties.

Acknowledgment

This work was supported by JSPS KAKENHI Grant Numbers JP19K15321,JP18H03837,The Amada Foundation(AF-2019037-C2), Advanced Low Carbon Technology Research and Development Program (ALCA), 12102886, National Natural Science Foundation, Grant Number 51971075, and Nagaoka University of Technology (NUT) Presidential Research Grant.