Xio Liu, Hui Yng, Biwu Zhu,*, Yunzhi Wu, Wenhui Liu, Chngping Tng

a Key Laboratory of High Temperature Wear Resistant Materials Preparation Technology of Hunan Province, Hunan University of Science and Technology,Xiangtan, Hunan 411201, China

bResearch Institute of Automobile Parts Technology, Hunan Institute of Technology, Hengyang, Hunan 421002, China

Abstract Split Hopkinson pressure bar(SHPB)tests were conducted on pre-rolled AZ31 magnesium alloy at 150-350 °C with strain rates of 2150s-1,3430s-1 and 4160s-1.The mechanical response, microstructural evolution and accommodation mechanism of the pre-rolled AZ31 magnesium alloy under high-speed impact loading were investigated.The twin and shear band are prevailing at low temperature, and the coexistence of twins and recrystallized grains is the dominant microstructure at medium temperature, while at high temperature, dynamic recrystallization(DRX) is almost complete.The increment of temperature reduces the critical condition difference between twinning and DRX, and the recrystallized temperature decreases with increasing strain rate.The mechanical response is related to the competition among the shear band strengthen, the twin strengthen and the fin grain strengthen and determined by the prevailing grain structure.The fin grain strengthen could compensate soften caused by the temperature increase and the reduction of twin and shear band.During high-speed deformation, different twin variants, introduced by pre-rolling, induce different deformation mechanism to accommodate plastic deformation and are in favor for non-basal slip.At low temperature, the high-speed deformation is achieved by twinning, dislocation slip and the following deformation shear band at different deformation stages.At high temperature, the high-speed deformation is realized by twinning and dislocation slip of early deformation stage, transition shear band of medium deformation stage and DRX of fina deformation stage.

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Peer review under responsibility of Chongqing University

Keywords: Mechanical response; Pre-twinning; Accommodation mechanism; Pre-rolled AZ31 magnesium alloy; High-speed impact loading.

1.Introduction

The magnesium alloys are widely used to weapons,aerospace, transportation and other field for the lowest density among all structural metals[1-5].In applications,magnesium components, such as transmission system, are unavoidably subjected to high-speed impact loading [6-10].However, the poor plasticity and low strength of magnesium alloy limits its application [11-14].Therefore, how to enhance the mechanical properties of magnesium alloys in harsh working environments has attracted many scholars attention.

The twins, divide the initial grains into small area, could serve as barriers to dislocation motion.These can effectively raise the yield strength of magnesium alloys [15-17].Song et al.[15]proposed that the strength along the rolling direction was improved due to grain refinemen by {10-12}extension twins, introduced by 3 and 5% pre-tensioning in AZ31 magnesium alloys.Wang et al.[16]investigated the high strain rate deformation behavior of pre-twinning AZ31 magnesium alloy and discovered that the yield strength was enhanced due to grain refinemen caused by {10-12} twinning.Song et al.[17]introduced a large number of extension twin boundaries through 3.5% pre-compression along the transverse direction and indicated that the twin boundaries served as barriers to dislocation sliding, leading to the improvement of yield strength in ZK60 magnesium alloy.

Pre-twinning could induce dynamic recrystallization(DRX) during deformation, that could simultaneously enhance ductility and strength in magnesium alloys [18-22].Jiang et al.[18]found that substantial twins, forming during pre-rolling, provided plenty of nucleation sites for DRX during subsequent high strain rate rolling, contributing that an excellent balance of strength and ductility was achieved in ZM61 magnesium alloy.Park et al.[19]introduced a numerous twins by cold pre-forging process in AZ31 magnesium alloy and discovered that the pre-twins provided nucleation sites for DRX during extrusion, leading to the improvement of strength and ductility.Zhang et al.[20]found that the pretwinning increased the number of nucleation sites for DRX during warm compression, resulting in complete recrystallization,finall causing to the higher yield stress and a high elongation of 20.1%.Lee et al.[21]introduced {10-12} twinning by pre-compression, causing the bandability improvement of AZ31 magnesium alloy.

Twins were pre-introduced into magnesium alloy, which efficientl promote DRX and enhance strength during static loading.However, the mechanical response, microstructural evolution and accommodation mechanism of pre-twined magnesium alloy during high-speed impact loading are still not unveiled.The split Hopkinson pressure bar (SHPB) tests can well simulate the work of magnesium alloy in harsh working environments such as high-speed impact loading and collision.In the present study, the mechanical response, microstructural evolution and accommodation mechanism in pre-rolled AZ31 magnesium alloy were investigated during high-speed impact loading (150-350°C and 2150-4160s-1).

2.Experimental

The commercial as-cast Mg-3Al-1 Zn alloys were applied to prepare pre-rolled AZ31 magnesium alloy samples,and the corresponding chemical composition is given in Table 1.The commercial as-cast AZ31 magnesium alloy samples (123 × 50 × 12 mm) were pre-rolled at 300°C at 12.6 m/min with a reduction of 10% on a two-high laboratory rolling mill withΦ200 mm rolls.Cylindrical compression test samples (Φ8 × 4 mm) were cut along the normal direction (ND).High-speed impact tests were conducted using a SHPB apparatus along ND.The SHPB tests were performed at temperatures of 150-350°C and at strain rates of 2150 s-1, 3430 s-1and 4160 s-1.The high strain rate corresponds to high impact energy.The deformation temperature is accurately controlled by hot air gun, heating refrigeration control box and SHPB temperature controller software.The strain-time data were calculated by a software of the SHPB system.On the basis of 1D wave theory with an assumption that the sample deforms uniformly, the true stress, true strain,and strain rate of each impact were obtained by Eqs.(1)-(3):

Table 1Chemical composition (in wt%) of the AZ31 magnesium alloy.

Table 2Schmid factors (m) of different deformation modes for pre-rolled AZ31 magnesium alloy under a strain rate of 2150s-1 with different temperatures.

Hereεris the reflecte incident bar strain history;εtis the transmitted strain history;Lis the initial length of the specimen;is the mass density of pressure bar)is the wave propagation velocity;Aare the cross-sectional areas of the pressure bar;ASis the cross-sectional areas of the specimen; and theEis the Young's modulus of the materials of the two pressure bars.

Cross-sections along the impact loading axis were machined from the impacted specimens to examine the microstructure.The samples were etched by a mixture of 1 g oxalic acid, 150 ml distilled water, 1 ml nitric acid and 1 ml acetic acid.Microstructures were measured by MR5000 optical microscopy.The macrotexture was detected by D/MAXRB X-ray diffraction.The Electron back scatter diffraction(EBSD) was performed on a SU5000 Hitachi SEM equipped with the Oxford Aztec Nordlys Max data acquisition system.

3.Results

3.1.Initial microstructure

Pre-rolled microstructure is shown in Fig.1.Here,the{10-12}, {10,11}, {10-13} and {10,11}-{10-12} twin boundaries are marked in red, fuchsia, yellow and blue, separately.Substantial {10-12} extension twins are observed.Small amount of {10-13} contraction twins, {10,11} contraction twinning,{10,11}-{10-12} twinning are also detected.This indicates that {10-12} extension twins are preferred during pre-rolling.

3.2.Flow stress

The fl w stress curves of pre-rolled AZ31 magnesium alloy under high-speed impact loading are shown in Fig.2.It is generally known that fl w stresses decrease with increasing temperature [23]and are related to the dislocation generation and formation of twins [24-27].In the present study, the decrease of fl w stresses is not obvious.At a strain rate of 2150s-1, the fl w stress of 150 °C is apparently higher than that of 200 °C, while fl w stresses of 200, 250 and 300 °C are close, and the fl w stress of 350 °C is obviously lower than that of 300 °C.The similar phenomena is also observed at strain rates of 3430s-1and 4160s-1.

As shown in Fig.2(a1)-(a3), the fl w stress under a strain rate of 2150s-1is apparently lower than that under a strain rate of 3430s-1at 150 °C, while the fl w stress under a strain rate of 3430s-1is apparently lower than that under a strainrate of 4160s-1at 150 °C.The similar phenomena is also detected at temperatures of 200-350°C.

Fig.1.EBSD map of AZ31 magnesium alloy after pre-rolling.

Fig.2.Flow stresses under different deformation conditions.

3.3.Textures

Initial texture and some examples of the {0002} pole fig ures at 2150s-1with different temperatures are illustrated in Fig.3.All the{0002}pole figure display a evident basal texture.The intensities (density maximum) for 150°C, 200°C,250°C, 300°C, 350°C and initial texture are 3.7, 4.2, 3.8,6.1, 5.5 and 4.4, respectively.The basal plane of majority grains for initial sample is almost vertical to ND direction.After deformed at 2150s-1and at a temperature of 150°C,substantial grains rotate toward RD and TD nearly 12 ° In contrast to initial condition, the highest intensity peak is deviated a small angle from ND direction and partial of grainsrotate toward RD and TD nearly 40-50 ° At deformation temperatures of 250, 300 and 350°C at 2150s-1, a majority ofc-axes are titled by approximately 8, 5 and 8 ° from ND direction, separately.

Fig.3.Initial texture and textures produced at different temperatures under strain rates of 2150s-1: (a) Initial texture; (b) 150 °C; (c) 200 °C; (d) 250 °C; (e)300 °C; (f) 350 °C.

3.4.Microstructure

The optical microstructures under different conditions are displayed in Fig.4.As shown in Fig.4(a1),when samples are impacted at 150°C at a strain rate of 2150s-1,black bands are detected, and those that crossed the grain boundaries (GBs)were identifie as shear band.In the meantime, high density twins are also observed.At a strain rate of 2150s-1, samples impacted at 200°C exhibit majority twins and deformation shear bands, inside which substantial deformation twins near grain boundaries accumulate (see the white parallel lines in Fig.4(a2)) and as the deformation temperature increases to 250°C, small recrystallized grains are observed in the shear band, as displayed in the red frame in Fig.4(a3).The shear band with small recrystallized grains is called transition shear band [28].At a strain rate of 2150s-1, when impacted at 300°C (see Fig.4(a4)), abundant dynamically recrystallized grains and few twins appear, while DRX completely occurs at 350°C (see Fig.4(a5)).At a strain rate of 3430s-1and 150°C, intersecting twins and deformation shear bands are onset in Fig.4(b1), while at 200 °C, abundant deformation shear bands appear (see Fig.4(b2))and under temperatures of 250-350 °C, DRX is gradually complete (see Fig.4(b3)-(b5)).At a strain rate of 4160s-1and 150°C, a large number of parallel twins are detected in Fig.4(c1),while at 200°C,both of twins and recrystallized grains are detected in Fig.4(c2)and under temperatures of 250-350 °C, DRX is almost complete (see Fig.4(c3)-(c5)).It can be concluded that twins and shear bands are the dominant accommodation mechanism at relatively low temperature, while DRX plays an important role on uniform deformation at high temperature.

At 150 °C, the twin density at a strain rate of 3430s-1is obviously higher than that at a strain rate of 2150s-1, and the twin density at a strain rate of 4160s-1is obviously higher than that at a strain rate of 3430s-1.At 200 °C, the twin density increases from 2150s-1to 3430s-1, while at 4160s-1,twins and majority recrystallized grains coexist.At 300 and 350 °C, the recrystallization is gradually complete and the average grain size decreases with increasing strain rate.

4.Discussion

4.1.The effects of temperature on microstructure

Under high-speed impact deformation, dislocation slip can not accommodate the early fast plastic deformation.Therefore, twins are favored to coordinate the fast plastic deformation and twin boundaries supply nucleation positions for DRX.To analyze the microstructural evolution,the fl w stress curves at 2150s-1with various temperatures were analyzed by the second derivative method to determine the critical strain and stress for twinning and DRX [29], and the related analyzed results are displayed in Fig.5.One minima is detected at 150 and 200°C.According to microstructure results,the minima is associated with critical stress for twinning and shear band.Two minima are found on the fl w curves of 250,300 and 350 °C.As shown in Fig.4(a3),(a4), both DRX and twins occur.Therefore, the firs minima was the critical stress for twinning and the onset of DRX corresponds to the second minima.

Fig.4.Microstructures of pre-rolled AZ31 magnesium alloy deformed at various impact conditions.

The critical strains were obtained by locating critical stresses on the corresponding fl w curves and are illustrated in Fig.6.Dependence of the critical strains for twinning and DRX on deformation temperature are shown in Fig.7.Both the critical strains for twinning and DRX increase with the decrease of temperature, while the critical strain for DRX is more sensitive to temperature.The difference of critical strain for twinning and DRX decreases with increasing temperature.At high-speed deformation of low temperature,there are two reasons for un-recrystallization: (i) the recrystallization temperature is not achieved; (ii) the impact energy could not arrive the critical strain energy for DRX.At high-speed deformation of high temperature, the critical strain for twinning and DRX reduce, causing that the impact energy easily supplies the enough strain energy for twinning and the following DRX.This indicates that the increment of temperature reduces the degree of relative difficult between twinning and DRX.

4.2.The effect of strain rate on microstructure

As shown in Fig.4, at 150 °C, the twin density increases with applied strain rate in the strain rate range of 2150s-1and 4160s-1, and at 200 °C, the twin density increases from2150s-1to 3430s-1.The twins exhibit a positive strain rate sensitivity, which consist with the result in Ref.[2].When impacted at low temperature, the DRX could not be initiated to accommodate the plastic deformation for the low temperature and strain energy.Generally, the dislocation slip could not satisfy the uniform plastic deformation under high-speed impact loading.Therefore, twinning and shear band, inside which substantial deformation twins near grain boundaries accumulate, are required to participate fast plastic deformation.The high strain rate is corresponding to high impact energy.This means that the number of twins increases with strain rate to accommodate the high-speed deformation and prevent the occurrence of crack, causing that the twin density increases with strain rate.With increasing deformation temperature and strain rate, the critical condition for DRX achieves.DRX facilitates the fast uniform plastic deformation, causing that the requirement for twinning reduces, due to the competitive relationship between twinning/DRX [30].

Fig.5.Dependences of the stress second derivative on stress evaluated at the following 2150s-1 strain rates.

Fig.6.The critical strains determined by double differentiation located on the experimental fl w curves, strain rates 2150s-1.

Fig.7.Dependence of the DRX and twinning critical strains on deformation temperature.

At a strain rate of 2150 s-1, a small amount of DRX could be observed at 250°C, while at a temperature of 250°C and 3430s-1, substantial recrystallized grains are detected, and at a temperature of 200°C and 4160 s-1, recrystallized grains appear.Therefore, the temperature for appearance of recrystallized grain decreases with increasing strain rate.It can be concluded that increase of strain rate could reduce recrystallized temperature.At high-speed impact loading, the thermal conditions gradually change to being adiabatic and the internal temperature of the sample increases,due to the conversion of plastic work into heat [27].The adiabatic temperature rise increases with strain rate.Therefore, the environment temperature required for DRX is reduced with increasing strain rate.

4.3.The effect of microstructure on flo behavior

The mechanical response of material is related to grain size [31], twin [15]and shear band [32].Meantime, the mechanical response is also affected by the deformation condition [33].Due to the fact that the grain boundary could inhibit dislocation motion, the increase of grain boundary area causes the increment of strength in materials [34].The twin boundaries and shear bands could serve as barriers to dislocation motion, leading to the strength improvement of materials[15].When twin, shear band and fin grain coexist, the mechanical response should be related to the competition among the twin strengthen, the shear band strengthen and the fin grain strengthen.Recent research investigated by Gao et al.[35]proposed that the strength of materials was related to the predominant grain structure.

The EBSD microstructures corresponding to a strain rate of 3430s-1at various temperatures are illustrated in Fig.8.Here,the red line is corresponding to{10-12}extension twin,the yellow line is related to {10-13} twin, the fuchsia line corresponds to {10,11} twin and the blue line represents to{10,11}-{10-12} twin.Shear band is marked by the white line [36].The sub-grain boundary (＜2 °) is also marked by the green line in Fig.8(a1).As shown in Fig.8(a), the{10-13} twins, {10,11} twins and {10,11}-{10-12} twins are detected and {10,11}-{10-12} twins are the dominant one.Meantime, substantial shear bands happen at 200 °C.Ac-cording to Fig.8(a) and (a1), substantial sub-grains form in initial coarse grains and are benefi for DRX, leading to grain refinemen during further plastic deformation.In shear bands,some small grains are observed.EBSD recrystallization distribution map for shear band area is also displayed in Fig.8(a1).Here, the red color represents deformation grain, the yellow color corresponds to substructured grain and the blue color is corresponding to recrystallized grain.It can be seen that only several small grains inside shear bands are recrystallized grain.At 250 °C, {10-13} twins, {10,11} twins and{10,11}-{10-12} twins are also observed and large amounts of dynamic recrystallized grains occur.At 350 °C, DRX is complete.Combining with Fig.4, it is indicated that at low temperature impact loading, shear bands and twins are the dominant grain structure, while fin recrystallized grains and twins coexist at medium temperature,and at relative high temperature, fin recrystallized grains completely replace initial grains, twins and shear bands.

Fig.8.EBSD maps determined at a strain rate of 3430s-1 and the following temperatures: (a-a1) 200 °C; (b-b1) 250 °C; (c-c1) 350 °C.

Accordingly, at low temperature, the mechanical response depends on the twin strengthen and the shear band strengthen.At medium temperature, the mechanical response is decided by the twin strengthen and the fin grain strengthen, while at high temperature, the fin grain strengthen is the dominant strengthen mechanism.In current study, the fl w stresses at 250C and 300°C with a strain rate of 2150s-1are close,while at 3430s-1, the fl w stress of 200 °C approaches to that of 250 °C, and the fl w stresses at 4160s-1are also detected the same phenomenon at some temperatures.Under these condi-tions, the predominant grain structure changes from twin and shear band to fin recrystallized grain.Therefore, under high temperature, the fin grain strengthen is the main strengthen mechanism and compensates for soften caused by the increment of temperature and the reduction of twin and shear band.

Fig.9.-ODF section of pre-rolled AZ31 magnesium alloy.

It can be seen from Fig.4 that the twin density increases with strain rate at low temperature.Therefore, the twin strengthen is enhanced with increasing strain rate, contributing that the fl w stress increases with strain rate at same temperature.At medium temperature, shear band and twin density increase from 2150s-1to 3140s-1,while at 4160s-1,the recrystallized grain is the dominant grain structure and coexists with twin.Accordingly, the twin strengthen and the shear band strengthen are improved from 2150 s-1to 3140 s-1,while at 4160 s-1, the twin strengthen and the fin grain strengthen both determine the material strengthen.At high temperature,the recrystallization fraction increases and the average grain size decreases with increasing strain rate, leading to the enhancement of fin grain strengthen.This finall causes the fl w stress increases with strain rate.

4.4.The identificatio of dominant deformation modes

The predominant deformation mechanism during deformation can be analyzed by calculating the SF combined with the CRSS [29,37].Aiming to determine the SF, the 4-index e.g.{h1k1i1l1}＜u1v1t1w1＞, which describing the burgersvector for slip systems and twins in magnesium alloys, was firs changed into the 3-index cubic format as follows:

SF calculation method is as follows:

here,mis the SF, (φ1,Φ,φ2) is the Euler angle.The Euler angle is obtained from the strongest pole density point in the constantφ2-ODF cross-section.For magnesium alloys with hexagonal close packed, the cross-sectional with orientation ofφ2=0 andφ2=30 ° only need to be considered.The correspondingφ2-ODF sections of initial texture for pre-rolled AZ31 magnesium alloy are listed in Fig.9.As shown in Fig.9, Euler angles in the strongest pole density point for the initial texture are (30, 2, 30).The correspondence SF of different slip systems and twins are displayed in Table 2.It should be noticed that the negative SF of twinning cannot occur.

According to the literature[38],the CRSSes for various deformation modes under different temperatures are illustrated in Table 3.According to the CRSS values of pre-rolled AZ31 magnesium alloy for deformation modes, the (CRSS/m) ratios for slip systems and twins were obtained and are shown in Table 4.At 150, 200 and 250°C, basal slip has a pretty low value of CRSS/m.It is concluded that the dominant deformation mechanism is basal slip.Contraction twinning and2nd-order pyramidal slip are the minor deformation mechanism.At deformation temperatures of 300 and 350°C, 2ndorder pyramidal slip is the predominant deformation mode,and contraction twinning and basal slip are the secondary deformation mechanism.

Table 3Critical resolved shear stresses of different deformation modes in magnesium alloy at different temperatures (MPa).

Table 4Dependence on temperatures of (CRSS/m) for various deformation modes.

Fig.10.The disorientation distribution and the corresponding inverse pole figur with rotation axis for Fig.8(c).

Fig.11.Typical pre-twins in grains 1-3: (a)-(c) inverse pole figur map; (a1)-(c1) the position of the extension twins in the pole figure and (a2)-(c2) the selected twins (green color) and matrix grains 1-3 (dark color) on {0002} pole figure

The disorientation distribution and the corresponding inverse pole figure with a rotation axis of Fig.8(c) are illustrated in Fig.10.Majority of grains boundaries are with disorientations of 38 ° about the 〈 -12-10 〉 axis and 56 ° about 〈-12-10〉.According to previous research [39],it can be concluded that substantial recrystallized grains nucleate in boundaries of {10,11}-{10-12} double twins and {10,11} contraction twins.Therefore, during impact deformation, the contraction twins are favored and cause{10-12} extension twins, finall leading to the formation of {10,11}-{10-12} double twins.These contraction twins and double twins provide nucleation positions for DRX at relative high temperature in pre-rolled AZ31 magnesium alloy.

4.5.The influenc of pre-twins on deformation modes

Inverse pole figur maps of grains (1-3) in Fig.1 and pre-twins (T1-T4) corresponding orientation, represented on{0002} pole figures are shown in Fig.11.It can be seen from Fig.11(a1) to (c1) that pre-twins T1-T3 are at the location of RD-TD great circle, while the orientation of twin T4 is within 45 ° of the ND axis.The twins T1-T4 could be identifie to(-1102)[1-101],(01-12)[0-111],(-1102)[1-101]and (0-112)[01-11], separately (see Fig.11(a2)-(c2)).To unveil the influenc of pre-twins on deformation modes,the Schmid factor for the initiation of slip systems, extension twin and contraction twin was calculated and is displayed in Table 5.The contraction twin hardly occurs in (-1102)[1-101]pre-twin T1, while extension twin is not favored in (01-12)[0-111]pre-twin T2 and (0-112)[01-11]pre-twin T4.The dependence of CRSS/m on temperature for pre-twins T1-T4 is illustrated in Table 6.During high-speed impact loading,basal slip is easily activated in pre-twins T1-T4 under various temperatures,and contraction twin is relatively easily initiated in (01-12)[0-111]pre-twin T2 and (0-112)[01-11]pre-twin T4 in the temperature ranges from 150 to 200°C,while extension twin is easily onset in (-1102)[1-101]pre-twins T1 and T3.In addition, the non-basal slip, such as 2nd-order pyramidal slip and prismatic slip, is favored to accommodate the uniform deformation in pre-twins.Accordingly, (-1102)[1-101]pre-twin is benefi for the onset of extension twin, basal and non basal slip, while (01-12)[0-111]and (0-112)[01-11]pre-twins induce contraction twin, basal and non basal slip at low temperature (≤200°C) and only easily activate basal and non basal slip at high temperature.

Table 5Schmid factors (m) of twinning for pre-rolled AZ31 magnesium alloy.

Table 6Dependence on temperatures of (CRSS/m) for various deformation modes.

4.6.The influenc of twinning and recrystallization on texture

As shown in Fig.8(a),{10,11}-{10-12}double twins were mainly initiated and were responsible for the twinning rotation of 38 ° and the contraction twin is the secondary deformation modes at low temperature deformation.This type of twinning occurred and rotated the ND components towards the orientation which deviates a small angle from ND direction (see Fig.3(b)).At high temperature deformation, the contraction twin would firstl occur and then induce recrystallization with increasing strain.The contraction twins at low strain are responsible for the texture change that the ND components towards the orientation which deviates a small angle from ND direction.The following DRX induced by double twins and contraction twins leads to small change on texture.

4.7.The accommodation mechanism

The achievement of high-speed impact deformation in pretwined magnesium alloy is attributing to microstructural evolution in pre-twins and grains.The pre-twin is mainly belong to extension twin.Extension twins, basal and non basal slip are favored in (-1102)[1-101]pre-twins, while (01-12)[0-111]and (0-112)[01-11]pre-twins are good for the occurrence of contraction twins, basal and non basal slip at low temperature (≤200°C) and activate basal and non basal slip at high temperature.According to the analysis results of texture,a great quantity of grains' orientation are(30,2,30)and mainly benefi for contraction twins, basal slip and 2nd-order pyramidal slip.The schematic diagram for microstructural evolution under different temperatures is shown in Fig.12.The typical grain structures are illustrated in Fig.12(a)-(c).(01-12)[0-111]and (0-112)[01-11]pre-twins are displayed in Fig.12(a), while (-1102)[1-101]pre-twin is described in Fig.12(b) and the grain with dominant orientation (30, 2, 30)is shown in Fig.12(c).At low temperature deformation, contraction twins, basal and non-basal slip happen in (01-12)[0-111]and (0-112)[01-11]pre-twins (see Fig.12(a1)), while extension twins,basal and non-basal slip occur in(-1102)[1-101]pre-twins(see Fig.12(b1))and contraction/double twins,basal and non-basal slip are onset inside majority grains at early deformation stage (see Fig.12(c1)).With increasingdeformation, the number of twins increases.At fina deformation, deformation shear bands form (see Fig.12(a1)-(c1)).At high temperature deformation, (01-12)[0-111]and(0-112)[01-11]pre-twins are good for dislocation slip (see Fig.12(a2)), while (-1102)[1-101]pre-twins firstl induce extension twins, basal and non-basal slip (see Fig.12(b2)),and contraction/double twins, basal and non-basal slip initiate inside majority grains (see Fig.12(c2)) at early deformation stage.At medium deformation stage, substantial twins form and induce DRX,followed by the initiation of transition shear bands (see Fig.12(a2)-(c2)).At fina deformation stage, fully DRX occurs.Therefore, during high-speed deformation, various twin variants, introduced by pre-rolling, activate different deformation mechanism to coordinate plastic deformation and are good for non-basal slip.At low temperature, the highspeed deformation is accommodated by twinning, dislocation slip and the following deformation shear band at different deformation stages.At high temperature, the high-speed deformation is realized by twinning and dislocation slip at early deformation stage, transition shear band at medium deformation stage and DRX at fina deformation stage.

Fig.12.The schematic diagram for microstructural evolution under different temperatures.

5.Conclusions

(1) Under high-speed impact loading, as deformation temperature increasing, the transition of predominant microstructure happens and the predominant microstructures firstl are intersecting twins, then become deformed shear bands or coexistence of twins and recrystallized grains, and finall are complete recrystallization.

(2) The increment of temperature reduces the degree of relative difficult between twinning and DRX and the increase of strain rate could drop recrystallized temperature due to the adiabatic temperature rise.

(3) The mechanical response is related to the competition among the shear band strengthen, the twin strengthen and the fin grain strengthen, and depends on the dominant grain structure.At low temperatures, the twin strengthen and the shear band strengthen determine the mechanical response.At medium temperature, the mechanical response depends on the twin strengthen and the fin grain strengthen.While at high temperature, the fin grain strengthen is the dominant strengthen mechanism and compensates for soften caused by the reduction of twin and shear band and the increment of temperature.

(4) During high-speed deformation, different twin variants,introduced by pre-rolling, have different accommodation mechanism and are benefi for non-basal slip.At low temperature, the high-speed deformation is accommodated by twinning, dislocation slip and the following deformation shear band at different deformation stages.At high temperature,the high-speed deformation is realized by twinning and dislocation slip of early deformation stage, transition shear band of medium deformation stage and DRX of fina deformation stage.

Acknowledgments

This work was financiall supported by the National Natural Science Foundation of China (Nos.52071139, 51905166,52075167) as well as from the Natural Science Foundation of Hunan Province(2020JJ5198)and the Open Platform Fund of Hunan Institute of Technology (KFA20014).