The influence of temperature on twinning behavior of a Mg-Gd-Y alloy during hot compression

Author’s Accepted Manuscript The influence of temperature on twinning behavior of a Mg-Gd-Y alloy during hot compression S.H. Lu, D. Wu, R.S. Chen, E.H. Han

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S0921-5093(18)31055-4 https://doi.org/10.1016/j.msea.2018.08.004 MSA36775

To appear in: Materials Science & Engineering A Received date: 23 May 2018 Revised date: 31 July 2018 Accepted date: 1 August 2018 Cite this article as: S.H. Lu, D. Wu, R.S. Chen and E.H. Han, The influence of temperature on twinning behavior of a Mg-Gd-Y alloy during hot compression, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.08.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The influence of temperature on twinning behavior of a Mg-Gd-Y alloy during hot compression S.H. Lu 1, 2, D. Wu 1, , R.S. Chen 1, , E.H. Han 1 1

The Group of Magnesium Alloys and Their Applications, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China

2

School of Materials Science and Engineering, University of Science and Technology of China, 72 Wenhua Road, Shenyang 110016, China

Abstract The

microstructure

evolution

and

plastic

deformation

mechanism

of

Mg-6.58Gd-5.7Y-0.55Zr alloy were investigated by uniaxial compression at temperatures ranging from 300 °C to 450 °C and with an initial strain rate of 0.1 s-1, and characterized by optical microscope (OM), scanning electron microscope (SEM) and electron back-scattering diffraction (EBSD). The effect of temperature on twinning behavior was focused in detail. At relatively low temperature, such as 300 °C, multiple twin variant pairs can be activated in one grain, and the twining process was dominated by nucleation. However, at high temperature, most of the twins activated in one grain belong to the single twin variant pair with the highest Schmid Factor (SF), and twin growth seems to represent the twinning behavior at 450 °C. The twinning behavior of Mg-6.58Gd-5.7Y-0.55Zr alloy at temperatures ranging from 300 °C to 450 °C basically follows Schmid law, and can be well 

Corresponding author: D. Wu; Tel: +86-24-23915891; Fax: +86-24-23894149; E-mail: [email protected] R.S. Chen; Tel: +86-24-23926646; Fax: +86-24-23894149; E-mail: [email protected]

understood in terms of variation of stress state and activated deformation modes with compression temperature.

Keywords: Mg-Gd-Y alloy；Hot compression；Twin variant selection；Schmid factor 1. Introduction Mechanical twinning is an important mechanism for plastic deformation of Mg alloys because of their insufficient slip systems [1], particularly at low temperatures and high strain rate deformation. In all of twin modes, {10-12} extension twin was extensively observed in the deformation microstructure due to its lowest critical resolved shear stress (CRSS) [2]. As a hexagonal close packed (hcp) structure, {10-12} twinning has six crystallographically equivalent variants. It is known that {10-12} twin variant selection is usually governed by the Schmid law [3-5]. In six potential {10-12} twin variants, one of them with the highest SF value is tending to be activated [4, 6]. However, recently many non-Schmid twinning behaviors have been reported and the macroscopic SF tends to be influenced by the accommodation work [7, 8]. A plane strain compression was applied on a rolled AZ31B sheet, and twin variant with lower SF can be activated if it needs the minimum accommodation work in its neighboring grains [7]. Previous investigations also reported that existing {10-12} twins may change subsequent {10-12} twinning behavior in neighboring grain [9]. The existing extension twin will lead to paired twins across two neighboring grains or form long twin chains across several neighboring grains, with some twins of quite low SF activated [9]. To our knowledge, all of these non-Schmid twinning

behaviors have been observed in conventional Mg alloys without RE addition, mainly in Mg-3Al-1Zn alloy. Actually, RE addition in Mg alloy may have great effect on its twinning behavior. For instance, compression twinning was usually much less favorable than extension twinning when loaded accordingly in the case of conventional Mg alloy, while Basu et al. [10] found that Mg-1Gd displayed unequivocal activation of both {10-12} extension twinning and {10-11} contraction twinning during plane strain compression of oriented specimens invoking c-axis extension. Moreover, for WE54 Mg alloy, a typically commercial Mg-RE alloy, twin thickening and twin transmission were usually suppressed [11], and a decrease in the gap between the CRSS for twinning and CRSS for the non-basal slip modes were also found [11, 12], which may contribute to its low plastic anisotropy. Due to ultra-high strength properties and excellent high temperature resistance, the Mg-Gd-Y alloy attracted a great many attentions in recent year [13-17], but they usually exhibit quite low ductility and bad workability during thermal mechanical process [18], which may substantially hinder their further application. Recently, an impact forging technique has been proposed and found an effective way to refine the microstructure, modify texture and improve mechanical properties of wrought Mg alloy, with high efficiency and low cost [19]. In our previous study, it has been successfully applied to an Mg-Gd-Y alloy, resulting in its obvious improvement of workability and even super plasticity at 400 °C [20]. Although the plenty of twin induced by the high strain rate loading process play a key role in the microstructure

refinement in general speaking, it is still not so clear what is the exact twinning behavior and the mechanism of twin variant selection. In present study, the effect of temperature on deformation behavior and twinning characteristic of a typical Mg-Gd-Y alloy (Elektron 675) were investigated by the compressive test at different temperature with a relative high strain rate. The microstructure evolution was systematically characterized, and a detailed grain orientation analysis comparison was also conduct between the activated and inactivated extension twin variants in one grain to detect the related twin variant selection rule. 2. Experimental procedures The material used in the present study was Elektron 675 Mg alloy, termed as E675, developed by Magnesium Elektron Ltd. for wrought applications. The alloy was prepared from pure Mg (99.95 wt. %), Gd (99.5 wt. %), Y (99.5 wt. %) and Mg-30Zr (wt. %) by melting them in an electric resistance furnace at about 780 °C under protection with an anti-oxidizing flux. The melt was poured into a steel mold. After that, the chemical compositions were determined to be Mg-6.58Gd-5.7Y-0.55Zr (wt. %) by using inductively coupled plasma atomic emission spectroscopy. Then the as-cast material was subsequently homogenized for 12 h at 525 °C. The major phases of the homogenized alloy were identified by X-ray diffraction (XRD) (RigakuD/max 2400 X-ray diffractometer) with Cu Kα radiation at 50 kV and 100 mA. Sampling width 0.02º and scanning angle range 20-90º were selected for recording. Cylindrical specimens with 10 mm in height and 8 mm in diameter were machined from the

homogenized ingot and the isothermal hot compression tests were carried out on a Sans type mechanical testing machine at a temperature range of 300-450 °C with initial strain rate of 0.1 s-1. Each specimen was heated to the deformation temperature and held isothermally for 15min before compression tests, and it was deformed up to a true strain of 0.9. A graphite lubricant was used to minimize the friction. Also, to examine the microstructural evolution as a function of strain, some compression tests were interrupted at the true strain of 0.1 and 0.2. The deformed specimens were sectioned in the center parallel to the compression direction. The specimens for optical microscope (OM) observation were polished and etched with a solution of 3 g picric acid, 50 ml alcohol, 20 ml acetic acid and 20 ml water. For electron back-scattering diffraction (EBSD) observation, the sectioned plans were ground using SiC papers with grits from 1000 to 5000 and then electrolytically polished in an electrolyte of 10 % perchloric acid and 90 % ethanol at 15 V. EBSD observation was carried out by a Hitachi Se3400N SEM equipped with an HKL-EBSD system operating at 20 KV. The orientation imaging microscopy was measured at a step size of 1µm for the specimen compressed at 300 °C and 3 µm for specimens compressed at 400 °C and 450 °C.

3. Results 3.1 Initial state of the E675 alloy

Fig. 1. Microstructure of E675 alloy in solutionized condition: (a) OM image; (b) SEM image; (c) energy dispersive X-ray spectra (EDX) result of the cuboid-shaped phase; (d) XRD patterns of E675 alloy.

Fig.1 shows the microstructure of E675 alloy after homogenized at 525 °C for 12 h, including optical microscope (OM) and scanning electron microscope (SEM) pictures. The average grain size was increased to about 150 µm and just a little volume of cuboid phase distributed randomly in the α-Mg matrix. The semi quantitative EDX results in Fig. 1c indicate that the cuboid phase have approximate stoichiometric composition of Mg5(Gd,Y). Since the dissolution temperature of

Mg5Gd was supposed to be about 500 °C [21], obvious lower than the solution temperature in present study, this possibility was excluded. About the cuboid-shaped phase, researchers have obtained different conclusions, i.e. hydride [22], oxide [23] and Mg–RE compounds with different stoichiometric ratios [24, 25]. XRD measurement has also been applied to the homogenized E675 alloy, and the result was shown in Fig. 1d. As we can see, except the Mg matrix and quite a small peak with 2θ≈29.5° appeared, which should correspond to the trace content of cuboid phase. According to PDF card, the small peak may represent Gd2O3 [lattice plane (111), 2θ= 29.04°, a=0.531 nm], GdH2 [(111), 2θ= 29.14°, a=0.531 nm] or YH2 [(111), 2θ= 29.655°, a=0.521 nm]. And no O was detected by EDX (Fig. 1c), so Gd2O3 was also impossible. Recently, Li [21] detected the cuboid-shaped phase as YH2 through a combination analysis secondary ion mass spectrometry (SIMS) and X-ray tomography (XRT). Anyway, it is hardly to be totally removed by the solution treatment in Mg-Gd-Y alloy. Although such cuboid-shaped phases are likely to deteriorate the mechanical property [26], they should not have a significant influence on the deformation mechanism due to their large size and the trace content.

3.2 Flow stress behavior

Fig. 2. True stress-strain curves of E675 alloy during hot compression at different temperature with an initial strain rate of 0.1 s-1.

Table 1 Mechanical properties of E675 alloy during hot compression

Temperature

YS(MPa)

UCS(MPa)

True strain.(%)

300 °C

169

246

27 (Fracture)

350 °C

169

220

33 (Fracture)

400 °C

127

160

83 (No fracture)

450 °C

98

104

83 (No fracture)

YS, yield strength; UCS, ultimate compression strength

True stress-strain curves obtained from uniaxial compression tests at 0.1 s-1 are shown in Fig. 2, and the corresponding mechanical properties were summarized in table. It exhibits that deformation temperature has a significant influence on the compressive flow behavior of this alloy. At low temperature (300 °C and 350 °C), the curves exhibit obvious work hardening behavior after yielding. At first, the stresses

instantly increased to peak stress of 246 MPa and 220 MPa, respectively. And then both of them fracture at a strain of about 0.3. In compression tests at high temperature (400 °C and 450 °C), a steady state-flow can be achieved with a peak stress of 160 MPa and 104 MPa, respectively, which were much lower than that in compression tests at low temperature. In addition, the samples compressed at high temperature exhibit a better ductility and did not crack at a large strain of 83%. Similar observation had been reported by other researchers, and the steady-state flow was usually attributed to a dynamic balance between work hardening and softening effects during hot compression [27].

3.3 Microstructure evolution

Fig. 3. The OM images and SEM images of the specimens compressed to a strain of (a, d, g, j) 0.1 or (b, c, e, f, h, i, k, l) 0.2 with an initial strain rate of 0.1s-1, at different temperature :( a, b, c) 300 °C, (d, e, f) 350 °C, (g, h, i) 400 °C, and (j, k, i) 450 °C.

Fig. 3 shows the optical and SEM microstructure evolution of E675 alloy during the compression tests at different temperature. For the sample compressed at 300 °C and 350 °C, the high density twins were activated in original grains with some lenticular twins nucleated at the original grain boundaries in Fig. 3a and d. As the true strain increase from 0.1 to 0.2, it can be observed that the density of twin increased a

lot (Fig. 3b and e), while the size of twin band kept almost constant. At relative high temperature (400 °C and 450 °C) as shown in Fig. 3(g, h, j, k), the density of twin sharply decreased and twins started to grow thick. Besides, twin boundaries turned to be serrated. The corresponding SEM images in Fig. 3(c, f, i, l) exhibit that except the cuboid phase left after solution treatment, few of precipitate can be observed in the interior of grains or along boundaries.

Fig. 4 (a, b, c) inverse pole figure map, (d, e, f) corresponding grain boundary map and (g, h, i) {0001} pole figure maps marked with the orientation of some typical twinned grains, of specimens compressed to 0.2 at different temperature: (a, d, g) 300 °C, (b, e, h) 400 °C, (c, f, i) 450 °C. CD and TD represent the compression direction and transverse direction, respectively.

To have a further understanding about the twinning behavior of E675 alloy during hot compression, a detailed analysis of grain orientation and twin orientation was employed under the help of EBSD characterization. Fig. 4 shows EBSD results of E675 alloy compressed at 300 °C, 400 °C and 450 °C. Ordinary grain boundaries (low angle grain boundaries and high angle grain boundaries) and special grain boundaries ({10-12} extension twinning, {10-11} contraction twinning, {10-12}-{10-11} double twinning and {10-12} twin variants) are marked in different colors. The angular deviation to identify the twin boundaries is within 5° of the ideal values. For the sample compressed at 300 °C, although most of the twins were identified as {10-12} extension twin, with some special boundaries between extension twin variants of 60.0° {10-12}-{10-12} <10-10> and 60.4° {10-12}-{10-12} <8-1-70> as depicted in Fig. 4d, a few of {10-11} contraction twinning and {10-11}-{10-12} double twinning can also be observed in its microstructure as shown in Fig. 4d. It has been reported that the contraction twin and double twin may lead to a shear failure at low strains [28]. Relatively at high temperature of 400 °C and 450 °C, it can be observed that only {10-12} extension twin was activated as shown in Fig. 4e-f. Besides, the thickness of these extension twin bands is larger than that in the microstructure of sample compressed at 300 °C, which is consistent with that observed by OM. Some typical twinned grains (G01-G24) were selected and highlighted by grain symbol in Fig.4a-c, and the matrix orientations of these twined grains were presented at {0001} pole figure map in Fig. 4g-i. As we can see, the c-axis of most of these

twinned grains were located perpendicular to the load direction with a little deviation of 15° indicated by two red lines. For basal slip of these selected grains, SF can be obtained through the function of Schmid law and the value is in the range of 0~0.25. In contrary, the SF for extension twinning is in the range of 0.30~0.49. Given the similar CRSS of both basal slip and extension twin [29], it is very favorable for nucleation of {10-12} extension twin in these selected grains. It also means that the extension twinning behavior of E675 alloy, during hot compression at temperature ranging from 300 °C to 450 °C, generally obey Schmid factor criterion. The fraction of 86° misorientation boundaries and average thickness of twin band were counted and shown in Fig. 5. The average thickness of twin band was determined by acquiring the maximum distance between two adjacent local peaks about 86° within one twin. With increasing compression temperature, the fraction of 86° misorientation boundary decreased from 4.8 % to 3.1 %, whereas the average thickness of twin band was sharply increased from ~6 um to ~72 um, which indicates that nucleation dominated the extension twinning process at 300 °C and growth of extension twins was significantly enhanced at 450 °C.

Fig. 5 Fraction of boundaries with 86° misorientation and average thickness of twin bands obtained from the EBSD results of specimen compressed at different temperature.

3.4 Detail characterizations of twin variant selection

Fig. 6 (a) Twinning morphology of typical grain G01 selected from Fig. 4(a). (b) Line profile of the misorientation angle along the white arrow AB in Fig. 6(a). (c) The orientation of parent matrix (black square) and corresponding theoretically six {10-12} twinning variants (triangles or circles) in a {0001} pole figure, where the activated and inactivated twin variants were indicated by red circle and dark triangle, respectively, and corresponding three-dimensional crystallographic relationship between the parent grain and the selected variants (T1, T2, T3). Table 2 SF of six theoretical twin variant of some grains selected from Fig. 4. Twin

300 °C

400 °C

450 °C

variant

G01

G07

G09

G10

G11

G14

G15

G16

G24

T1

0.4945

0.3999

0.2707

-0.0323

0.0167

0.1092

0.1139

0.1990

0.1805

T2

0.4819

0.3586

0.3221

-0.0733

0.0515

0.0840

0.1276

0.2243

0.1277

T3

0.1432

0.0946

0.0423

0.0814

0.2783

0.3501

0.4706

0.4031

0.0716

T4

0.1346

0.0711

0.0132

0.1426

0.2242

0.3919

0.4455

0.3712

0.1188

T5

0.0960

0.0320

-0.0149

-0.1451

-0.0812

0.0152

0.0744

-0.0164

-0.1412

T6

0.1017

0.0142

-0.0371

-0.1653

-0.0619

0.0013

0.0859

-0.0089

-0.1356

To have a detailed analysis on the twin variant selection of E675 during hot compression, some typical grains of crystallographic orientation map were selected from Fig. 4, and were shown in Fig. 6-10, where M and T represent matrix of grain and twin band, respectively. The orientation of parent matrix (black square) and corresponding theoretically six {10-12} twinning variants (triangles or circles) were marked in a {0001} pole figure, where the activated and inactivated twin variants were indicated by red circle and dark triangle, respectively. Two twin variants with the smallest misorientation of about 7.4°, i.e., T1 and T2, T3 and T4, T5 and T6 form a twinning pair. Complicated extension twin morphology of sample compressed at 300 °C was observed in Fig. 6a. The misorientation profile along white arrow AB exhibits a sharp orientation variation in this seriously twined grain as shown in Fig. 6b. A large number of local peaks about 86° indicates many extension twins were activated, and some local peaks about 60° represent two extension twins from different twin variant pairs [30], intersecting with each other. As shown in Fig. 6c, it can be observed that the activated multi-extension twins (T1, T2, and T3) in grain G01 are from different twin variant pairs, i.e. T1 and T3 are in meta-position, and T1 and T2 are in para-position [8]. The SF of six potential extension twin variants were calculated and displayed at the Table 2. Those font-weight numbers highlighted in yellow correspond to the activated twin variants. It can be observed that both high SF twin variants (T1 and T2) and lower SF twin variant T3 were activated in grain G01.

Fig. 7 (a) Twinning morphology of typical grain G07 selected from Fig. 4(b). (b) Line profile of the misorientation angle along the white arrow AB in Fig. 7(a). (c) The orientation of parent matrix (black square) and corresponding theoretically six {10-12} twinning variants (triangles or circles) in a {0001} pole figure, where the activated and inactivated twin variants were indicated by red circle and dark triangle, respectively, and corresponding three-dimensional crystallographic relationship between the parent grain and the selected variants (T1, T3).

Fig. 8 (a) G09, (b) G10 and (c) G11 were selected from Fig. 4(b). Their respective twinning morphology and crystallographic relationship between the parent grain and the selected variants were analyzed.

A typical grain G07 was selected from the microstructure of the specimen deformed at 400 °C in Fig. 4(b), and its inverse pole figure map was shown in Fig. 7a, where two extension twins T1 and T3 were activated. In Fig. 7b, it can be observed that the angular relationship between two activated twin variants is about 60°, which is consistent with the special boundary 60.0° {10-12}-{10-12} <10-10> [31], and it indicated that T1 and T3 are from different twin variant pairs, i.e. T1 and T3 are in meta-position (Fig. 7c). Similar analysis has also been done for other three typical twined grains (G09-G11), selected from Fig. 4b and as shown in Fig.8. As we can see,

all of the four selected grains contain two kinds of activated twin variant coming from two different twin variant pairs, and with the SF also ranking in top three (Table 2).

Fig. 9 (a) Twinning morphology of typical grain G24 selected from Fig. 4(c). (b) Line profile of the misorientation angle along the white arrow AB in Fig. 9(a). (c) The orientation of parent matrix (black square) and corresponding theoretically six {10-12} twinning variants (triangles or circles) in a {0001} pole figure, where the activated and inactivated twin variants were indicated by red circle and dark triangle, respectively, and corresponding three-dimensional crystallographic relationship between the parent grain and the selected variants (T1, T2).

Fig. 10 (a) G14, (b) G15 and (c) G16 were selected from Fig. 4(c). Their respective twinning morphology and crystallographic relationship between the parent grain and the selected variants were analyzed.

In the microstructure obtained from the specimen compressed at 450 °C, many grains (G21-G24) were almost consumed by their thick extension twin band with the thickness of ~ 100 um as shown in Fig. 4c. A typical grain G24 was selected as shown in Fig. 9. It can be observed that a local angle peak (≈7°) appeared at the interior of twin band. The crystallographic relationship between the parent grain and the activated variants (T1, T2) is highlighted in Fig. 9c. The results exhibit that the activated twin variants T1 and T2 are in Para-position, belonging to the same one twin variant pair. Similar analysis has also been done for other three typical twined grains (G14-G16), selected from Fig. 4c and as shown in Fig.10. As we can see, although more than one twin can be found in one grain, all of them belong to one single twin

variant pair. According to the calculated Schmid factors of six possible {10-12} twin variants in the Table 2, it can be clearly observed that twin variants with the highest Schmid factor were activated. As we mentioned before, generally speaking, the nucleation of extension twin was governed by Schmid law during the hot compression of E675 Mg alloy, at temperature range of 300-450°C.

4. Discussion 4.1 The effect of temperature on twin nucleation and twin growth

As we know, twinning process usually consists of twin nucleation, propagation and twin growth [32,33]. Nucleation and growth are driven by stress states with different length scales. The nucleation of a twin is usually activated by local stress, while twin growth needs a long range intergranular stresses [34]. When the local stress was larger than CRSS of twin, twin nucleation followed by propagation will happen. After that, twin growth may appear when a long range intergranular stresses was applied. Twin growth accompanying migration of the coherent twin boundary usually comprises the motion of defects on the twin boundary. It is accepted that such migration is motivated by a twin resolved shear stress (TRSS) on the twin plane and direction [11]. When the specimen was compressed at 300 °C in present work, the deformation temperature was lower than equicohesive temperature E675 alloy [35, 36], so grain boundaries were stronger than the intragranular part [37]. In other words, the stress

concentration around grain boundaries facilitates twin nucleation. Furthermore, it has been reported that the CRSS of basal slip and {10-12} extension twin are insensitivity to deformation temperature, whereas non-basal slips including prismatic and pyramidal slip are quite temperature-dependent [38]. At relative low temperature, high CRSS value of non-basal slips limits their activation, so {10-12} extension twin and basal slip still play a key role during compression. As we know, basal slip can only provide two independent slip systems, which are insufficient to meet homogeneous deformation and can not supply amount of strain along the c-axis. Therefore, twinning is a very important deformation mechanism for E675 Mg alloy compressed at relative low temperature. What’s more, twinning usually reflects much more swiftly than a dislocation slip [39]. It has been found that strain rate enhances the activation of {10-12} extension twinning [40], and large volume fraction of extension twins has been activated in some as-cast alloys [41, 42] when deformed at a strain rate not higher than 0.001s-1. Thus, the relative high initial strain rate of 0.1s-1 in present study may also contribute much to the flourished nucleation of extension twin of E675 Mg alloy compressed at relative low temperature. When the compression was applied at 450 °C, the deformation temperature was higher than E675 alloy equicohesive temperature [35, 36] and grain boundaries developed weaker regions compared with grains [37]. Therefore, the stress state in the microstructure favors a long range intergranular stresses, which may assist twin growth. Besides, {10-12} twin boundaries consists of sequential {10-12} coherent twin boundaries (CTBs) and parallel Basal-Prismatic planes serrations (BPs or PBs)

[43, 44]. The migration of twin boundary is usually through the glide-shuffle of twin dislocations and the climb-shuffle of interface dislocations with respect to the CTBs and the PBs/BPs [45]. Moreover, the stress around {10-12} extension twin tip was accommodated through the emission of (c + a) 1/3<11-23> [46], which means TRSS of extension twin will decrease with the increasing compression temperature. Therefore, the growth of {10-12} extension twin will be improved at high deformation temperature. 4.2 Schmid factor analysis on twin variant selection In previous works, many non-Schmid twinning behaviors have been reported, which were usually attributed to the influence of accommodation work in the neighboring grains [7] and existing twin [8]. However, most of these non-Schmid twinning behaviors have been observed in rolled Mg-3Al-1Zn alloy sheet with a strong basal texture and compressed at room temperature. For such alloy sheet, c-axis of most of grains was parallel to normal direction of the sheet. When a compression strain was applied along transverse direction or rolling direction at room temperature, basal slip and non-basal slip were hard to be activated due to low SF or high CRSS. Thus, extension twinning will become dominant deformation mode. Transfer efficiencies of twin–twin and twin–slip between neighboring grains has been explained by geometric compatibility factor (m′) [47]. The value of m′ increases with the decrease of misorientation between two neighboring grains [48]. Misorientation of most of grains in a Mg alloy sheet with strong basal texture was usually quite low, theoretically in the range of 0-30° [4], which therefore result in a

noting transfer efficiencies of twin–twin and twin–slip and an obvious non-Schmid twinning behavior. However, in present work, most of activated twins were identified as extension twin in a cast and solutionized Mg-RE alloy with a random orientated microstructure and deformed at 300-450°C. The value of m′ should be relatively low due to weak texture, and the basal slip and non-basal slip will participate more in deformation due to high SF of more grains of random orientation and low CRSS at high temperature. So, non-Schmid twinning behavior rarely can be observed during the hot compression of E675 Mg alloy. 5. Conclusions The effect of temperature on twinning behavior of E675 has been investigated through compression tests at temperature range 300-450 °C with initial strain rate 0.1 s-1. The {10-12} extension twin was almost exclusively observed in microstructure, except extremely a few of {10-11} contraction twinning and {10-11}-{10-12} double twinning observed at 300 °C. The extension twin behavior of E675 alloy was generally governed by Schmid law. Twinning behavior of E675 Mg alloy transformed from nucleation-dominated process to growth-dominated process with increasing compression temperature from 300 °C to 450 °C. In one grain, multiple twin variant pairs usually appeared at relative low temperature such as 300 °C, while single twin variant pair was often activated at 450°C. Acknowledgements This work was funded by the National Key Research and Development Program of China through Project No. 2016YFB0301104, the National Natural Science

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