Microstructure and mechanical properties of an indirect-extruded Mg-8Sn-1Al-1Zn alloy

The microstructure and mechanical properties of an indirect-extruded Mg-8Sn-1Al-1Zn (TAZ811) alloy were investigated and compared with those of a commercial Mg-3Al-1Zn (AZ31) alloy. In the extruded condition, the TAZ811 alloy shows a much smaller grain size but a stronger basal texture than the AZ31 alloy. In addition, the TAZ811 alloy contains fine Mg₂Sn particles in the microstructure, whereas the AZ31 alloy reveals relatively coarse and sparse Al-Mn particles. The TAZ811 alloy showed tensile and compressive strengthening as well as a reduction in yield asymmetry between tension and compression, which is mainly due to grain refinement and the presence of fine Mg₂Sn particles.     

Strength and ductility of novel Mg-8Sn-1Al-1Zn alloys extruded at different speeds

A novel Mg-8Sn-1Al-1Zn alloy developed for high-speed extrusion was successfully extruded at speeds in a range of 2-10 m/min at a temperature of 250 °C. The effect of extrusion speed on the microstructure and tensile properties of the extruded alloys was investigated. Grain size, recrystallization fraction and texture were found to be greatly affected by the extrusion speed, resulting in tensile properties showing lower strength and ductility as the extrusion speed increased. The strength and ductility of the extruded alloys are also discussed in terms of the formation of double twins during the tensile test.     

Microstructural evolution and superplasticity of rolled Mg-9Al-1Zn

Microstructural evolution and superplasticity of a Mg-9Al-1Zn alloy rolled at 673 K were investigated at 573 K and 1.5×10⁻³ s⁻¹. The grain size of the as-rolled Mg alloy was 39.5 μm. However, the grain size of the specimen deformed to a true strain of 0.6 was 9.1 μm. The grain refinement was attributed to dynamically continuous recrystallization during an initial stage of tensile test. Stabilization of subgrain boundaries by fine particles and stimulation of continuous recrystallization by prior warm-deformation were not needed to attain dynamically continuous recrystallization in the Mg alloy. As a result of the grain refinement, the rolled Mg alloy exhibited superplastic behavior.     

Influence of strain rate and crystallographic orientation on dynamic recrystallization of pure Zn during room-temperature compression

This work investigates the strain rate dependence of dynamic recrystallization behaviour of high-purity zinc in room temperature compression under strain rates of 10-4 s-1,10-2 s-1 and 0.5 s-1.Results from electron backscatter diffraction provide insight into the deformation and dynamic recrystallization mech-anisms operative.Continuous dynamic recrystallization,twin-induced dynamic recrystallization,and discontinuous dynamic recrystallization are all active under compressive deformation at room temper-ature.Due to the high stacking fault energy of Zn,continuous dynamic recrystallization is the dominant mechanism while discontinuous dynamic recrystallization only operates in the early stages of compres-sion at 10-4 S-1.Dynamic recrystallization kinetics are enhanced at higher strain rates(10-2 s-1 and 0.5 s-1)due to an increased contribution from twin-induced dynamic recrystallization.The present study reveals that the controlling mechanisms for continuous dynamic recrystallization are basalslip and 2nd order pyramidalslip activity.Because the activation of slip systems is mainly deter-mined by crystallographic orientation,continuous dynamic recrystallization behaviour varies with grain orientation according to their propensity for basal and 2nd order pyramidal slip.     

Recrystallization behavior and microstructure evolution of Mg-5Bi-3Al alloy during very high-speed extrusion

In our previous study,we extruded Mg-5Bi-3Al(BA53,wt.％)alloy at a very high speed of 70 m/min,and the high-speed-extruded alloy exhibited an unusual fine grain structure.Since dynamic recrystal-lization(DRX)behavior determines the microstructure and corresponding mechanical properties of the extruded alloy,understanding its origin is crucial for further improvement and optimization of alloys.Herein,the DRX behavior during high-speed extrusion of the recently developed BA53 alloy was inves-tigated in detail by analyzing the microstructure and texture of the extrusion butt along the extrusion path obtained by water quenching the remaining part of the billet immediately after extrusion.During the initial stages of extrusion with a high extrusion speed of 70 m/min and a high temperature of 400 ℃,tension twinning predominantly occurred in grains with a high Schmid factor(SF),switching the c-axis of the grains toward the transverse direction of the butt and causing the formation of a ring basal texture.In the intermediate stage,the microstructural changes were dominated by continuous and discontinuous DRX,resulting in the formation of fine recrystallized grains and orientation distribution originated from the parent grain.DRX was completed in the late stage wherein unrecrystallized grains with low SF for basal slip contributed to the late-stage recrystallization,strengthening the ring basal texture.Addition-ally,DRXed grains with favorable orientation for basal slip underwent lattice rotation,contributing to the formation of the obvious ring basal texture.Finally,preferential grain growth occurred after the DRX,leading to an increase in texture intensity and grain size of the extruded alloy.Therefore,the fine grain structure and ring basal texture obtained through high-speed extrusion were primarily attributed to the activated discontinuous DRX facilitated by a high strain rate and temperature.     

Ti-6Al-2Zr-1Mo-1V合金热变形激活动态再结晶的临界条件识别及表征

热压缩实验获得Ti-6Al-2Zr-1Mo-1V合金在温度1073～1323K,应变速率0.01～10s-1条件下的真应力-应变曲线,以此作为识别及表征动态再结晶临界条件的底层数据。对比分析流变应力曲线发现高温、低应变速率下动态回复型软化态势显著;低温、高应变速率下动态再结晶型软化态势显著。引入材料加工硬化率θ,结合θ-σ曲线拐点判据识别了流变应力曲线隐含表征激活动态再结晶的特征参量:临界应变、临界应力。采用含动态再结晶激活能Q的Arrhenius方程求得α、β、n1、n2等材料常数并获得该合金动态再结晶激活能对应变速率及温度的响应图。进一步引入表征动态再结晶临界条件的临界应变模型,获得了临界应变与各热力参数之间的数学关系,验证表明该临界模型预测精度最大为12.9%。     

Cu-Ni-Si合金冷变形及动态再结晶行为研究

研究了时效温度和时效时间对不同冷变形条件下Cu-2.0Ni-0.5Si合金性能的影响。在Gleeble-1500D热模拟试验机上,采用高温等温压缩试验,对Cu-2.0Ni-0.5Si合金在高温压缩变形中的流变应力行为进行了研究。结果表明,合金经900℃固溶,当变形量为40%,时效温度达到450℃时,其显微硬度达到201HV,导电率达到34%IACS。随变形温度升高,合金的流变应力下降,随应变速率提高,流变应力增大。在应变温度为700、800℃时,合金热压缩变形流变应力出现了明显的峰值应力,表现为连续动态再结晶特征。从流变应力、应变速率和温度的相关性,得出了该合金高温热压缩变形时的变形激活能Q。     

Dynamic recrystallization behavior and microstructural evolution of Mg alloy AZ31 through high-speed rolling

High-speed rolling (HSR) is known to improve the workability of Mg alloys significantly, which makes it possible to impose a large reduction in a single pass without fracture. In the present study, dynamic recrystallization (DRX) behavior and microstructural and textural variations of Mg alloy AZ31 during a HSR process were investigated by conducting rolling with different imposed reductions in the range of 20%–80% at a high rolling speed of 470 m/min and 400 °C. High-strain-rate deformation during HSR suppresses dislocation slips but promotes twinning, which results in the formation of numerous twins of several types, i.e., {10–12} extension twins, {10–11} and {10–13} contraction twins, and {10–11}–{10–12} double twins. After twinning, high strain energy is accumulated in twin bands because their crystallographic orientations are favorable for basal slips, leading to subsequent DRX at the twin bands. Accordingly, twinning activation and twinning-induced DRX behavior play crucial roles in accommodating plastic deformation during HSR and in varying microstructure and texture of the high-speed-rolled (HSRed) sheets. Area fraction of fine DRXed grains formed at the twin bands increases with increasing rolling reduction, which is attributed to the combined effects of increased strain, strain rate, and deformation temperature and a decreased critical strain for DRX. Size, internal strain, and texture intensity of the DRXed grains are smaller than those of unDRXed grains. Therefore, as rolling reduction increases, average grain size, stored internal energy, microstructural inhomogeneity, and basal texture intensity of the HSRed sheets gradually decrease owing to an increase in the area fraction of the DRXed grains.     

Effect of initial texture on dynamic recrystallization of AZ31 Mg alloy during hot rolling

Wedge-shaped AZ31 plates with two kinds of initial textures were rolled at 573 K to investigate the effect of initial texture on dynamic recrystallization (DRX). The results indicated that the initiation and nucleation of DRX were closely related to the initial texture. The initiation and completion of DRX in the TD-plate were significantly retarded compared with that in the ND-plate. Twin related DRX nucleation was mainly observed in the ND-plate samples; while gain boundary related DRX nucleation was mainly observed in the TD-plate samples. The different DRX behavior between the TD- and ND-plates was attributed to the different deformation mechanism occurring before DRX initiation. For the ND-plate, dislocation glide was considered as the main deformation mechanism accompanied with {1 0 −1 1}-{1 0 −1 2} double twin, which led to the increment of a faster increasing stored energy within the grains. And {1 0 −1 1}-{1 0 −1 2} double twin was mainly found to be DRX nucleation site for the ND-plate. For the TD-plate, {1 0 −1 2} extension twin was the dominant deformation mechanism which resulted in a basal texture with the c-axis nearly parallel to ND. The stored energy caused by dislocation motion was relatively small in the TD-plate before a basal texture was formed, which was considered as the main reason of that DRX was retarded in the TD-plate compared with that in the ND-plate. Based on the difference in deformation mechanism and DRX mechanism caused by the different initial texture, the variation in grain size, micro-texture and misorientation angle distribution in the ND and TD plates were discussed.     

Microstructure evolution and texture development during high-temperature uniaxial compression of magnesium alloy AZ31

Texture development in magnesium alloy AZ31 was studied by uniaxial compression tests at temperatures, strain rates and final strains ranging from 573 to 773 K, 1.0 × 10⁻³ to 5.0 × 10⁻⁵ s⁻¹ and −0.2 to −1.5, respectively. Fiber texture was formed in all of the deformation conditions. The main component of the texture varied depending on deformation conditions; it appeared about 33–38° away from the basal pole after the deformation at higher temperatures and lower strain rates. This can be attributed to the increased activity of the secondary pyramidal slip system. With a decrease in temperatures and an increase in strain rate, the tilting angle of the main component (compression plane) from the basal pole decreased down to about 20°. Construction of a basal fiber texture was detected after deformations at the lowest temperature and high strain rates.     

Sb对Mg-3Sn-1.5Al-1Zn-1.2Si合金组织性能的影响

在熔炼过程中以单质形式加入Sb,研究了0~1.8%(质量分数,下同)范围内不同含量的Sb对Mg-3Sn-1.5Al-1Zn-1.2Si合金显微组织和力学性能的影响。结果表明,Sb能与Mg基体优先生成Mg3Sb2相,加入1.0%的Sb对Mg2Si相的汉字状结构变质作用显著,Mg2Si中的Si能与Sn发生取代反应,生成富Sn的Mg2(Si,Sn)。随着Sb的增加,铸态合金和挤压态合金的延伸率逐渐减小,而抗拉强度呈现先增加后降低的趋势,塑性和强度的最佳配合点约为1.0%,Sb含量的增加有利于改善Mg-3Sn-1.5Al-1Zn-1.2Si合金的耐热性能。     

Effects of initial texture on deformation behavior during cold rolling and static recrystallization during subsequent annealing of AZ31 alloy

This study demonstrates that the initial texture of Mg alloy significantly affects the microstructure developed during cold rolling and the recrystallization behavior during subsequent annealing. In a sample with a texture oriented toward the normal direction(ND sample), thick and large-sized shear bands are intensively formed during cold rolling and the deformation is strongly localized along these bands. In contrast,in a sample with a texture oriented toward the transverse direction(TD sample), many {10-12} twins are formed at the early stage of cold rolling, and then, numerous small-sized shear bands are formed in these twins. Results of nanoindentation tests reveal that the cold-rolled ND sample has internal strain energy that is substantially concentrated along the shear bands, whereas the cold-rolled TD sample has a large amount of internal strain energy that is homogeneously distributed throughout the material;this latter behavior is strongly related to the extensive {10-12} twinning and the resultant microstructural variations in the TD sample. During subsequent annealing, recrystallization occurs locally along the shear bands in the ND sample, which leads to the formation of a bimodal grain structure comprising fine recrystallized grains and coarse unrecrystallized grains. In contrast, during subsequent annealing of the TD sample, static recrystallization occurs homogeneously throughout the material, which results in the formation of a uniform grain structure that mostly comprises equiaxed recrystallized grains.     

Role of initial texture on the plastic anisotropy of Mg-3Al-1Zn alloy at various temperatures

The effect of initial texture on the anisotropic properties of AZ31 Mg alloys was investigated on the basis of microstructure evolution after compression tests and Lankford parameter (r-value) experiments at various temperatures. Two kinds of sheets were used: one is the cast-and-rolled sheet, and the other is the strip-cast sheet. Compression tests were conducted up to a strain of 0.3 at temperatures of 200-400 °C, and tensile tests were performed to obtain the r-value at various temperatures (25-450 °C). The results showed that, at all test temperatures, the average r-value of the RD plane were greater than those of the ND and TD planes in the cast-and-rolled material. When comparing the average r-value of the RD plane, the cast-and-rolled material revealed much higher values than those of strip-cast material. It was observed that the cross-sectional shape of RD compressive specimens (the compression axis was parallel to the rolling direction) of cast-and-rolled materials changed from an initial circular shape to an ellipsoidal shape due to the plastic anisotropy. Compression processes of specimens were simulated using a finite element method where the Hill's anisotropic yield criterion was adopted. The simulated results were in a good agreement with experimental data.     

均质化制度对Mg-Li合金显微组织和机械性能的影响

研究了超轻镁锂变形合金热处理后的合金显微组织和机械性能。镁锂合金在室温条件下具有非常良好的延展性，Mg—Li—Zn系变形镁合金铸锭经不同温度、时间均匀化退火后的组织、硬度以及冷轧性能进行了研究。结果表明。Mg．9％Li-2％Zn-2％Ca合金在573K温度均匀化退火12h后合金铸锭组织均匀；而对于Mg-9％Li-2％Zn合金，523K温度均匀化退火24h后组织较好。     

Constitutive modeling for the dynamic recrystallization evolution of AZ80 magnesium alloy based on stress-strain data

In order to improve the understanding of the dynamic recrystallization (DRX) behaviors of as-cast AZ80 magnesium alloy, a series of isothermal upsetting experiments with height reduction 60% were performed at the temperatures of 523 K, 573 K, 623 K and 673 K, and the strain rates of 0.01 s⁻¹, 0.1 s⁻¹, 1 s⁻¹ and 10 s⁻¹ on a Gleeble 1500 thermo-mechanical simulator. Dependence of the flow stress on temperature and strain rate is described by means of the conventional hyperbolic sine equation. By regression analysis, the activation energy of DRX in the whole range of deformation temperature was determined to be Q = 215.82 kJ mol⁻¹. Based on dσ/d? versus σ curves and their processing results, the ?ow stress curves for AZ80 magnesium alloy were evaluated that they have some characteristic points including the critical strain for DRX initiation (?_c), the strain for peak stress (?_p), and the strain for maximum softening rate (?*), which means that the evolution of DRX can be expressed by the process variables. In order to characterize the evolution of DRX volume fraction, the modified Avrami type equation including ?_c and ?* as a function of the dimensionless parameter controlling the stored energy, Z/A, was evaluated and the effect of deformation conditions was described in detail. Finally, the theoretical prediction on the relationships between the DRX volume fractions and the deformation conditions were validated by the microstructure graphs.     

Dynamic deformation behavior and microstructural evolution during high-speed rolling of Mg alloy having non-basal texture

The dynamic deformation behaviors and resultant microstructural variations during high-speed rolling(HSR) of a Mg alloy with a non-basal texture are investigated. To this end, AZ31 alloy samples in which the basal poles of most grains are predominantly aligned parallel to the transverse direction(TD) are subjected to hot rolling with different reductions at a rolling speed of 470 m/min. The initial grains with a TD texture are favorable for {10–12} twinning under compression along the normal direction(ND); as a result, {10–12} twins are extensively formed in the material during HSR, and this consequently results in a drastic evolution of texture from the TD texture to the ND texture and a reduction in the grain size. After the initial grains are completely twinned by the {10–12} twinning mechanism, {10–11} contraction twins and {10–11}-{10–12} double twins are formed in the {10–12} twinned grains by further deformation.Since the contraction twins and double twins have crystallographic orientations that are favorable for basal slip during HSR, dislocations easily accumulate in these twins and fine recrystallized grains nucleate in the twins to reduce the increased internal strain energy. Until a rolling reduction of 20%, {10–12}twinning is the main mechanism governing the microstructural change during HSR, and subsequently,the microstructural evolution is dominated by the formation of contraction twins and double twins and the dynamic recrystallization in these twins. With an increase in the rolling reduction, the average grain size and internal strain energy of the high-speed-rolled(HSRed) samples decrease and the basal texture evolves from the TD texture to the ND texture more effectively. As a result, the 80% HSRed sample, which is subjected to a large strain at a high strain rate in a single rolling pass, exhibits a fully recrystallized microstructure consisting of equiaxed fine grains and has an ND basal texture without a TD texture component.     

预处理工艺对双辊铸轧3003铝合金再结晶行为的影响

对双辊铸轧3003铝合金板材进行了3种预处理退火,研究不同预处理工艺下的冷轧板材在380～500℃退火时晶粒组织和再结晶织构的变化规律。结果表明:最优化预处理工艺为610℃/12h+460℃/12h,高温阶段第二相尺寸发生粗化,低温阶段基体中Mn的过饱和固溶度显著降低,两者均有利于提高后续退火时的再结晶形核率。500℃退火时,在粗大第二相的附近产生了粒子诱发形核机制,降低了再结晶织构强度;并且退火时几乎不存在析出,析出相对再结晶形核的抑制作用甚微,从而得到了晶粒细小、织构弱的再结晶组织。     

Effect of crystal orientation on corrosion behavior of directionally solidified Mg-4 wt% Zn alloy

Microstructure and crystallographic orientation of directionally solidified Mg-4 wt% Zn alloy were characterized by X-ray computed tomography (XCT) and electron backscatter diffraction (EBSD) in this study. Results reveal that Mg-4 wt% Zn alloy with dendritic microstructure exhibits typical $\left\{0002\right\}$ basal texture along growth direction. Based on this, the effect of grain orientation on corrosion behavior of directionally solidified Mg-4 wt% Zn alloy in 0.9 wt% NaCl solution was investigated. Result shows that $\left\{0002\right\}$ oriented planes have better corrosion resistance than $\left\{11{\displaystyle \overline{2}}0\right\}$ and $\left\{10{\displaystyle \overline{1}}0\right\}$ ones, which is attributed to a synergistic effect of surface energy, atomic packing density and the stability of oxidation film.     

Effect of Pd on microstructures and tensile properties of as-cast Mg-6Al-1Zn alloys

The effects of Pd on the microstructure and mechanical properties of Mg-6Al-1Zn alloys were investigated. Mg-6Al-1Zn-xPd (x = 0-6 wt.%) alloys were prepared using a permanent mould casting method. The microstructure of the as-cast alloys was characterized by the presence of Mg₁₇Al₁₂ and Al₄Pd phases. The volume fraction of the Al₄Pd phase was increased by the addition of 1-6 wt.%Pd but the volume fraction of the Mg₁₇Al₁₂ phases decreased. At room temperature, the tensile strength increased with increasing Pd addition up to 2 wt.%Pd, and the elongation to fracture decreased with a concomitant increase in the aggregation of the coarse Al₄Pd phase. At 150 °C, the tensile strength increased with the addition of Pd. Therefore, the room and elevated temperature tensile properties of as-cast Mg-6Al-1Zn alloys can be improved by Pd addition.