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Bingshu WANG,Liping DENG,Adrien CHAPUIS,Ning GUO,Qiang LI
Acta Metall Sin    2015, 51 (12): 1441-1448.   doi:10.11900/0412.1961.2015.00215
Accepted: 06 October 2015

Abstract57)   HTML8)    PDF (1164KB)(396)      

Mg alloy has hexagonal structure and exhibits poor workability at room temperature, which is attributed to the difficulty in activating a sufficient number of independent slips to accommodate the deformation. Twinning plays an important role in plastic deformation of Mg alloys during low and medium temperature to accommodate the imposed strain, especially the strain along the c-axis. Therefore, the twinning behavior of AZ31 Mg alloy during plane strain compression at room temperature was investigated with EBSD in this work. Rectangular specimens with a dimension of 10 mm in length, 9 mm in width and 7 mm in thickness were cut from a hot rolled plate. The results show that {101-2} twinning is dominant when the compression and constraint direction are parallel to transverse direction (TD) and rolling direction (RD) of the plate, respectively. The twinning variant selection mechanism is dominated by the Schmid factor (m) along compression direction, and also related to the constraint direction. The differences of twinning behavior can be interpreted by the twinning strain tensor. For the case when single twinning variant occurs within a grain, the average twinning strain tensor of twinning variant in constraint direction will result in spreading; while for the case there are two or more twinning variants taking place within a grain, the average twinning strain tensor of the variant with higher m will induce spreading in the constraint direction, and that with lower m results in size reducing in the constraint direction. During plane strain compression, different twinning variants coordinate with each other, twinning won't be suppressed until the micro-strain in the constraint direction reaches 0.

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Shaojun LIU, Guangyu YANG, Wanqi JIE
Acta Metall Sin    2015, 51 (5): 580-586.   doi:10.11900/0412.1961.2014.00512
Accepted: 03 March 2015

Abstract155)   HTML9)    PDF (2542KB)(514)      

Mg-Zn-Gd base alloys possess much superiority, such as, high strength, light weight, low cost, etc., and favorable for the application in various airframe components. Two kinds of eutectic phases, such as, W(Mg3Zn3Gd2) and I(Mg3Zn6Gd), can be usually found in Mg-Zn-Gd alloy under the traditional casting conditions. The interface between W phase and α(Mg) is incoherent and thus weak. However, I phase has quasiperiodic lattice leading to a coherent interface between I phase and α(Mg). Therefore, compared with W phase, I phase is more effective to obstruct dislocations slipping and so efficiently strengthening the alloy. So, controlling the solidification path, i.e., controlling the relative amount of I phase and W phase, is critical for increasing the heat resistant of Mg-Zn-Gd magnesium alloy. In this work, the solidification path of Mg-4.58Zn-2.6Gd ternary casting alloy was investigated by experiments and numerical analysis. Experimental results showed that at lower cooling rate (≤0.75 K/s), α+W(Mg3Zn3Gd2) eutectic will be formed first, while at higher cooling rate (≥7.71 K/s), α(Mg)+I(Mg3Zn6Gd) eutectic will be formed first. A numerical model for predicting solidification path of the primary phase in multi-component alloy with considering the effects of solute diffusion in liquid phase and the cooling rate was developed. The thermodynamic data in the computation model was obtained by using the database of Thermo-Calc. The numerical results were in favorable agreement with the experimental ones. The numerical model established in this work provides a direct and easy way to predict solidification path of Mg-Zn-Gd alloy for different casting conditions. The validity of this model was further confirmed by other three different Mg-Zn-Gd alloys, i.e., Mg-3.8Zn-2.0Gd, Mg-5.5Zn-2.0Gd and Mg-5.5Zn-4Gd. It is also found that for Mg-Zn-Gd alloy, the higher Zn-content and the higher cooling rate will promote the formation of I phase. However, higher Gd-content and the lower cooling rate is favorable for the formation of W phase.

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Mingfan QI, Yonglin KANG, Bing ZHOU, Guoming ZHU, Huanhuan ZHANG
Acta Metall Sin    2015, 51 (6): 668-676.   doi:10.11900/0412.1961.2014.00523
Accepted: 08 April 2015

Abstract277)   HTML12)    PDF (8320KB)(519)      

Based on the forced convection mixing (FCM) principle, a self-developed FCM semisolid slurry preparation device was successfully developed. Taking AZ91D magnesium alloy tensile parts for example, the rheo-diecasting process that consists of slurry preparation, transportation and forming was achieved by combining with a diecasting machine. Microstructural characteristics of FCM rheo-diecasting parts in different processing parameters were investigated. Mechanical properties of AZ91D alloy parts in different processes were compared. Besides, the formation mechanism and solidification behavior of semisolid slurry were analyzed in FCM rheo-diecasting process. The results show that processing parameters have a great effect on the microstructures of parts, increasing rotation speed or decreasing barrel temperature appropriately is beneficial to optimizing the microstructure. The process not only can produce parts with fine, spherical and uniformly distributed primary a-Mg particles, but also is able to improve mechanical performance of parts significantly. Compared with traditional diecasting, the yield strength remains unchanged, but the ultimate strength and elongation are increased by 12.5% and 80.0%, respectively. Furthermore, compared with parts subjected to T4 and T6 heat treatment, the ultimate strength of the as-cast is the lowest, and the yield strength and elongation are between T4 and T6.

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Tao JING, Sansan SHUAI, Mingyue WANG, Qiwei ZHENG
Acta Metall Sin    2016, 52 (10): 1279-1296.   doi:10.11900/0412.1961.2016.00323
Accepted: 12 September 2016

Abstract100)   HTML4)    PDF (14657KB)(656)      

As a typical hexagonal close-packed structure metal, the dendritic morphology and preferential orientation of Mg would be influenced by many factors. Current investigations still fall short on the thorough description of the diversity and complexity of dendrites growth patterns and their origination, therefore, this paper re viewed recent research progress of this group on 3D characterization of microstructure in solidified magnesium alloys. Using synchrotron X-ray tomography and phase-field modeling, the formation mechanism of the diverse α-Mg (X) dendrites and the affections of alloying element (such as Al, Ca, Zn, and Sn), solute concentration on the growth selection and evolution of α-Mg dendrites during solidification were studied. The results indicate that the alloying elements and solute concentration would impose a significant influence on the morphology and orientation selection of the primary α-Mg dendrites. In Mg-Ca and Mg-Al (hcp-fcc) alloys, dendrites tend to grow with preferred orientation of <112?0> or <224?5> which is in good agreement with the traditional expected direction. The equiaxed growth dendrites in Mg-Sn (hcp-bct) alloys evolve as a structure with 18 branches, six of which grow on the basal plane along <112?0> and the remaining 12 along <112?X> (X≈2) off the basal plane. For the case in Mg-Zn alloys, an orientation transition from <112?0> on the basal plane to <112?1> off the basal plane are observed with the increasing addition of Zn alloying element, a hyperbranched seaweed structure is also revealed with an interim composition. A probable explanation is that the addition of high anisotropy Zn would slightly alter the anisotropy of interfacial free energy in front of the growth interface which results in a dendrite orientation transition (DOT). These findings partially reveal the underlying formation mechanism and origination of the diversity dendritic morphologies and branching structures of α-Mg dendrites in Mg alloys. Furthermore, with the fast X-ray imaging facility, in situ observations of the 3D microstructure evolution in Mg alloys during solidification are also carried out and the evolution of α-Mg dendrites are obtained for further analysis.

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Kaiwen WEI,Zemin WANG,Xiaoyan ZENG
Acta Metall Sin    2016, 52 (2): 184-190.   doi:10.11900/0412.1961.2015.00212
Abstract97)   HTML5)    PDF (3463KB)(419)      

Magnesium alloys have attracted more attentions due to their low densities and excellent specific strengths. However, proper manufacturing methods are still needed to promote further applications of magnesium alloys due to the shortcomings of conventional processing methods. As one of the most promising additive manufacturing technologies, selective laser melting (SLM) was utilized to process the most commonly-used AZ91D magnesium alloy in this work. Element vaporization mechanism during the forming process and the influence of element vaporization on chemical composition, microstructure, and mechanical properties of the final products were investigated using OM, SEM, EDS, XRF and XRD. The results show that the relative content of Mg in the SLM-processed samples (86.61%~88.68%) was lower than that in the original AZ91D powders (90.63%) , whereas the relative content of Al in the former ones (10.40%~12.56%) was higher than its counterpart in the latter ones (8.97%). This variation matches well with the calculation by Langmuir model, demonstrating that element vaporization of AZ91D mainly targets at Mg. With the increase of laser energy density (EV), weight ratio of Mg to Al (η) in the SLM-processed samples first increased, then decreased and finally tended to be constant. η of the sample prepared at 55.6 J/mm3 (sample No.8) presented a smallest difference with that of the original powders. A model illustrating analytic relationship between η and EV was established by mathematical regression with the fitting index R2 being 0.858. The sample processed at 166.7 J/mm3 (sample No.1) underwent one of the most remarkable compositional variation and exhibited a typical solidified microstructure similar to the die-cast AZ91D in which net-like β-Mg17Al12 precipitates were distributed around the α-Mg matrix. However, β-Mg17Al12 content as well as solid solubility of Al in α-Mg matrix was much higher in sample No.1. The enhanced tensile strength and micro-hardness as well as the deteriorated elongation of sample No.1 could be attributed to the composition variation during SLM process.

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Jinbao LIN,Weijie REN,Xinyi WANG
Acta Metall Sin    2016, 52 (3): 264-270.   doi:10.11900/0412.1961.2015.00324
Abstract75)   HTML7)    PDF (3583KB)(659)      

Most wrought magnesium alloys exhibit a significant tension-compression asymmetry in yield and work hardening behaviors. To some extent, the widespread implementation of wrought magnesium alloys is hindered due to this disadvantage in some special conditions. In this work, in order to quantitatively analyze the effects of the deformation mechanisms on the tension-compression asymmetry of wrought magnesium alloys, the plastic deformation behavior of the as-extruded ZK60 magnesium alloy under uniaxial tension and compression at room temperature is investigated by the crystal plasticity simulation and experimental methods. The crystal plasticity constitutive model including slip and twinning mechanism is established by modifying the viscoplastic self-consistent (VPSC) model. The activation and evolution of basal slip, prismatic slip, pyramidal slip, {1012}<1011> tensile twinning and {1011}<1012> compression twinning are quantitatively studied during the process of uniaxial tension and compression deformation. Tensile-compression asymmetry of the as-extruded ZK60 alloy with fiber texture is analyzed based on the microscopic plastic deformation mechanism. The results show that the tension and compression twinning in the axial tension-compression process are difficult to active, basal slip is the main deformation mode in the early stage of deformation, but the orientation factor of basal slip is low and has a hard orientation resulting in higher yield stress. With the rotation of grains, the critical shear stress of basal slip reduces, stress continues increasing and prismatic slip becomes the main deformation mechanism, moreover, pyramidal <c+a> slip also has a high activity. At this stage, the strain hardening rate is low and the stress-strain curve is smooth. In the early stage of compression, the tensile twinning has a high activity due to its low critical shear stress (CRSS), leading to a lower yield stress. The tensile twinning gradually saturated after the strain reaches 6.0%. And then, the relative activity decreases rapidly and the hardening rate increases at the same time. Since a large number of twin boundaries hindered the movement of dislocations, slip is no longer the major mechanism. In the later stage, the compression twinning startes activation and its relative activity rises rapidly, the accumulated stress during plastic deformation could be released and then the hardening rate decreases. It can be seen that, the variation in the relative activity of each deformation mode during compression deformation is much more complex than that during tension. The yield asymmetry and different work hardening behavior could be attributed to the combined effects of the strong fiber texture and the polar nature of twinning.

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Wenhui WANG,Di WU,Rongshi CHEN,Changsheng LOU
Acta Metall Sin    2016, 52 (5): 567-574.   doi:10.11900/0412.1961.2015.00333
Accepted: 23 February 2016

Abstract62)   HTML3)    PDF (1179KB)(256)      

Among the traditional cast magnesium alloys system, Mg-3Nd-1Zn alloy with high strength and heat resistance, has been widely applied in aeronautics such as in engine box and wing rib of airplane. In present research, the as-cast Mg-2.7Nd-0.6Zn-0.5Zr (NZ31) alloy was solution treated and then aged at temperatures ranging from 200 ℃ to 300 ℃. The microstructures and mechanical properties of the aged specimens especially at relatively high temperature (225~300 ℃) were systematically characterized by OM, SEM and TEM. A new kind of precipitates distributed in line was clearly found in the specimens aged at high temperature (225~275 ℃) for a short time (15~30 min), which corresponded to a significant enhancement of hardness and tensile strength at room temperature. The TEM results showed that the precipitates distributed in line had a composition of Mg12Nd and a granular shape, and mainly formed along the (0001)Mg basal plane. The special distribution of the granular Mg12Nd precipitates was effective barriers to the basal slips and may probably restrain the intergranular coordination during tensile deformation, leading to a large strengthening effect, that is, the yield strength and ultimate tensile strength, of the NZ31 alloy aged at 250~275 ℃ for 20~30 min, increased by about 70% and 29% respectively, in comparison with only the solutionized one.

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Yanqiu WANG,Kun WU,Fuhui WANG
Acta Metall Sin    2016, 52 (6): 689-697.   doi:10.11900/0412.1961.2015.00500
Accepted: 06 April 2016

Abstract62)   HTML1)    PDF (939KB)(484)      

The effects of second phases on microarc oxidation (MAO, also named plasma electrolytic oxidation-PEO) behavior of Mg base materials were investigated and the related mechanism was discussed. The formation of barrier layer and its influence on sparking discharge behavior were characterized and analyzed on the base of systematic selecting and designing substrate materials. The variation of second phases at the early MAO stage was observed and analyzed by SEM and EDS, and then the effect mechanism of second phases on MAO behaviors was revealed. Voltage evolution trend during MAO were recorded to study the formation state of the barrier layer on the different Mg base materials. According to the growth mechanism of MAO film, the film growth process can be simplistically considered as a repeated breakdown and reconstruction process of a capacitor. Accordingly, the growth process of MAO film on multiphase metal materials and the effects of second phases were discussed. The results show that different second phases in substrate materials have different effects on formation process of MAO films, depending on their own characteristics. For the second phases which have the characteristics of valve metals, although selective sparking discharge occurs at the early stage of MAO, the second phases will not hinder the growth of MAO film since barrier layer can form on the second phases, and they will not induce structural defects into the film-substrate interface. If the second phases have not the characteristics of valve metals, their conductivity property will be an important influencing factor to affect the MAO behaviors. For the elecinsulating second phases which have not the characteristics of valve metals, sparking discharge just occurs on Mg matrix in the substrate, while doesn't occur on the second phases; the second phases exist in the MAO film as heterogeneous phases, do not react in MAO process, and will not hinder the growth of MAO film. For the semi-conductive second phases which have not the characteristics of valve metals, they delay the growth of MAO film because they destroy the integrity of barrier layer. For the electroconductive second phases which have not the characteristics of valve metals, they seriously hinder the growth of MAO film.

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Shuaiju MENG,Hui YU,Huixing ZHANG,Hongwei CUI,Zhifeng WANG,Weimin ZHAO
Acta Metall Sin    2016, 52 (7): 811-820.   doi:10.11900/0412.1961.2016.00039
Accepted: 04 May 2016

Abstract74)   HTML5)    PDF (1824KB)(759)      

Due to the increasing demand of low density and high strength Mg alloys for the automobile, railway, and aerospace industries, the exploration of cost-effective RE-free Mg alloys becomes more and more attractive. Instead of Mg-Sn based system, the Mg-Bi alloy system seems to satisfy this requirement as a potential candi date, since it shows typical precipitation-type phase equilibrium and contains thermal stable Mg3Bi2 phases, which exhibit a high melting temperature of 821 ℃ comparable to those in RE-bearing Mg alloy. In addition, the fine Mg3Bi2 plates on the prismatic plane were reported to be more effective than the more commonly observed basal plates for precipitation-hardening. In this work, pure Mg with/without 6% Bi (mass fraction) additions were extruded, and the corresponding microstructure and mechanical properties were investigated. The results show that dynamic recrystallization (DRX) occurs in both alloys after extrusion and these two kinds of specimens exhibit similar extrusion texture. However, the as-extruded Mg-6Bi alloy represents finer and homogenous microstructure, and the average grain size (AGS) decreases from 30 μm to 4 μm when 6% Bi added. In addition, the Mg-6Bi alloy contains strip-like fragmented Mg3Bi2 particles along the extrusion direction and fine Mg3Bi2 precipitates, and demonstrates superior mechanical properties with tensile yield strength of 189 MPa, ultimate tensile strength of 228 MPa, and an elongation of 19.9%. There is a large number of nano-scale Mg3Bi2 particles in the tensile fracture surface of Mg-6Bi alloy. And there is a large number of twins in the microstructure of compression fractured pure Mg sample; while for the Mg-6Bi alloy specimen, with a large number of second phase particles on the α-Mg matrix, little twins are observed. Moreover, the Mg-6Bi alloy also gives a low tension-compression yield asymmetry with yield asymmetric ratio of 1.01. These significantly improvement of mechanical properties are mainly attributed to the combined effects of grain refinement and large quantity of co-exist micro/nano-size Mg3Bi2 particles.

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Zhenliang LI,Fei LIU,Aiping YUAN,Baoyu DUAN,Xiaowei LI,Yiming LI
Acta Metall Sin    2016, 52 (8): 938-944.   doi:10.11900/0412.1961.2015.00592
Accepted: 20 April 2016

Abstract56)   HTML1)    PDF (828KB)(506)      

Mg alloys have been applied widely as structural materials over the past decades, with low density, high specific strength, stiffness, specific elastic modulus, and high recycling rate. However, their features of poor ductility and formability at room temperature have limited their application due to hexagonal close-packed crystal structure with less independent slip systems. Grain refinement and texture randomization are two means to activate other slip systems. In this work, the billets of Mg-9Al-3Zn-1Mn-6Ca-2Nd alloy produced by spraying deposition method (the Osprey process) were studied in order to analyze the effect of rolling deformation at 350 ℃ and pass reduction ε =20%, 25% and 30% on texture and microstructure evolution of an extruded size-asymmetry Mg alloy by SEM, TEM and XRD. The results show that under the condition of reduction of ε =20% at 350 ℃, a long-period stacking ordered phase with 24R structure was formed in (Ca, Nd)Al2 phase (C15 Laves phase ). All of basal texture (0002), prismatic texture {100}<0001>, and pyramidal texture {102} were activated, with pole density level weakened while pass reduction increased (ε =20%, 25% and 30%), namely, texture randomization achieved in Mg alloy, with main causes of nanometer-sized dispersed C15 phase impeding dislocation movement and sub-cells inducing the process of recrystallization.

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Feng WANG,Dezhi MA,Zhi WANG,Pingli MAO,Zheng LIU
Acta Metall Sin    2016, 52 (9): 1115-1122.   doi:10.11900/0412.1961.2016.00048
Accepted: 27 May 2016

Abstract65)   HTML2)    PDF (1343KB)(264)      

As the lightest metallic structural material, magnesium alloys were widely used in automotive, aerospace, electronic equipment and other fields. Among commercial magnesium alloys, AM series were commonly used due to excellent ductility and energy absorption. However, their relatively poor strength greatly restricted their extended use. In order to improve mechanical properties of AM50 alloy, the Zn and Y elements were added into the AM50 alloy in the form of atomic ratio of 6∶1 by the permanent mold casting. The microstructure, solidification behavior and mechanical properties of AM50-x(Zn, Y) (x=0, 2, 3, 4, 5, mass fraction, %) alloys were investigated by OM, SEM, EDS, XRD, thermal analysis and tensile tests. The results indicated that addition of Zn and Y elements with an atomic ratio of 6∶1 to AM50 alloy, the microstructures were obviously refined, and the quasicrystal I-phase(Mg3Zn6Y) cannot form. In addition, the granular Al6YMn6 phase and fine Al2Y phase were formed in the microstructure, and the size of Al6YMn6 phase increased with increasing the Zn and Y content. The Φ-Mg21(Zn, Al)17 phase with lamellar structure was formed around β phase when x≥3, and its amount increased with increasing the Zn and Y addition. Thermal analysis results show that the Φ-Mg21(Zn, Al)17 phase was formed at 354 ℃ by the peritectic reaction, in which the precipitation temperatures of α-Mg and β phase were decreased with the increase of x content. Due to the formation of Al6YMn6, Al2Y and Φ-Mg21(Zn, Al)17 phases, the size and amount of the β phase was decreased. For AM50-4(Zn, Y) alloy, the microstructure was greatly refined, and the ultimate tensile strength, yield strength and elongation of the alloy reached to the maximum, 206.63 MPa, 92.50 MPa and 10.04%, respectively.

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Yun CAI,Chaoyang SUN,Li WAN,Daijun YANG,Qingjun ZHOU,Zexing SU
Acta Metall Sin    2016, 52 (9): 1123-1132.   doi:10.11900/0412.1961.2016.00051
Accepted: 27 May 2016

Abstract103)   HTML2)    PDF (1457KB)(484)      

Magnesium alloys are considered as one of the lightest structural metallic materials with excellent properties such as high specific strength, superior damping characteristics and electromagnetic shielding performance. In order to improve the mechanical properties of magnesium alloys, the hot rolling, hot extrusion and other hot forming processes are often introduced to produce the high performance parts. Both of the two softening mechanisms, dynamic recovery and dynamic recrystallization (DRX), occur during the hot deformation. As an important softening mechanism in hot processing, DRX is beneficial to obtaining fine grains structure, eliminating defects and improving mechanical properties for magnesium alloys. In this work, isothermal compression tests of AZ80 magnesium alloy were conducted on Gleeble thermo-mechanical simulator in the temperature range of 200 to 400 ℃ and strain rate range of 0.001 to 1 s-1. In view of the dynamic hardening and softening mechanisms, the softening behavior of AZ80 magnesium alloy, dominated by dynamic recrystallization, was depicted. Dynamic recrystallization volume fraction was introduced to reveal the power dissipation during the microstructural evolution which was indicated by the strain rate sensitivity value based on the dynamic material model. To quantify the dynamic recrystallization softening behavior by the strain rate sensitivity (SRS) value, the SRS value distribution maps were constructed depending on various temperatures and strain rates. Therefore, the critical conditions and evolution process were studied in terms of temperatures and strain rates, while features of the SRS value distribution maps at different strains were deeply investigated. It can be concluded that the value of dynamic recrystallization critical condition decreases and dynamic recrystallization volume fraction increases when the temperature increases and strain rate decreases during the deformation. The strain rate sensitivity was positive correlated with the dynamic recrystallization volume fraction. It has been verified effectively by the analysis of microstructure that the region in which the strain rate sensitivity value is above 0.21 corresponds to the higher dynamic recrystallization volume fraction and lower strain rate.

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Study on the Microstructural Evolution and Mechanical Properties Control of a Strong Textured AZ31 Magnesium Alloy Sheet During Cryorolling
Yaqiong YAN,Jinru LUO,Jishan ZHANG,Linzhong ZHUANG
Acta Metall Sin    2017, 53 (1): 107-113.   doi:10.11900/0412.1961.2016.00134
Accepted: 28 October 2016

Abstract69)   HTML7)    PDF (5483KB)(465)      

A strongly basal textured AZ31 magnesium alloy sheet with the normal direction (ND) perpendicular to the c-axis has been cryorolled at the liquid-nitrogen temperature to the strain of different amount to analyze the influence of cryogenic rolling temperature. The microstructure and texture of the cryorolled samples have been investigated by using SEM, EBSD and XRD. And the mechanical properties of the cryorolled sheets have also been tested under quasi-static uni-axial tension at the ambient temperature along the rolling direction (RD) and transverse direction (TD) respectively. The microstructural and textual evolutions of the strongly basal textured AZ31 magnesium alloy sheets during cryorolling and the relationship between mechanical properties and the microstructural and textural evolutions of cryorolled samples has also been discussed in this work. The results show that a lot of twins have been observed in cryorolled sheets, and they were found to be {101?2} tension twins. {101?2} tension twins were the dominant twinning type of the AZ31 magnesium alloy sheet during cryogenic rolling. With the increase of cryogenic rolling pass, new texture component with the c-axis paralleled to the normal direction (ND) was strengthened and the breadth of {101?2} tension twins was also increased. Grains were separated by the twin grain boundaries after cryorolling. The mechanical test results show that the strength of the sheets increased while the ductility decreased after cryogenic rolling. The strength of the sheets along RD was higher than that along TD.

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