Quasi in-situ EBSD analysis of twinning-detwinning and slip behaviors in textured AZ31 magnesium alloy subjected to compressive-tensile loading

Twinning and detwinning behavior,together with slip behavior,are studied in a textured AZ31 magnesium alloy under compressive and tensile strains along the rolling direction(RD)after each interrupted mechanical test via quasi in-situ electron backscattered diffraction technique.The results show that twinning firstly takes place under the compressive strain along the RD.With the increasing compressive strain,{1012}tensile twins firstly nucleate,then propagate,and finally thicken.While under a reversed tensile strain along the RD,detwinning occurs.No nucleation happens during detwinning.Thus,tensile twins can detwin at lower tensile strain,followed by thinning,shortening,and vanishing.Slips are also activated to accommodate the plastic deformation.In the matrix,prismatic slip can only dominate at relatively high strains.Otherwise,basal slip dominates.While in the twins,prismatic slip can activate at lower strains,which is ascribed to the texture reorientation.

Plastic deformation mechanisms of AZ31 magnesium alloy under high strain rate compression

The twinning and slip activities of AZ31 magnesium alloy sheet at a strain rate of 1 200 s-1 were investigated.Dynamically mechanical properties of various oriented samples were measured using Split Hopkinson Pressure Bar(SHPB).Optical microscope observations reveal that the dominant deformation mechanism is twinning for 90° oriented sample,and is slip for 45° and 0° oriented samples.TEM analysis for samples at a strain of 0.3% shows that the main deformation mechanisms for 90°,45° and 0° oriented sample are and twinning,basal slip and non basal slip,respectively.The main features of the true stress-true strain curves can be explained based on deformation mechanism analysis.

Characteristics of hot tensile deformation and microstructure evolution of twin-roll cast AZ31B magnesium alloys

High temperature tensile properties and microstructure evolutions of twin-roll-cast AZ31B magnesium alloy were investigated over a strain rate range from 10-3 to 1 s-1.It is suggested that the dominant deformation mechanism in the lower strain rate regimes is dislocation creep controlled by grain boundary diffusion at lower temperature and by lattice diffusion at higher temperatures,respectively.Furthermore,dislocation glide and twinning are dominant deformation mechanisms at higher strain-rate.The processing map,the effective diffusion coefficient and activation energy map of the alloy were established.The relations of microstructure evolutions to the transition temperature of dominant diffusion process,the activation energy platform and the occurrence of the full dynamic recrystallization with the maximum peak efficiency were analyzed.It is revealed that the optimum conditions for thermo-mechanical processing of the alloy are at a temperature range from 553 to 593 K,and a strain rate range from 7×10-3 to 2×10-3 s-1.

Effect of twinning on flow stress during initial stage of hot compression of twin roll cast Mg-5.51Zn-0.49Zr alloy

A Thermecmastor-Z hot deformation simulator,optical microscopy,XRD and TEM were employed to characterize the flow stress behavior and microstructure of twin roll cast ZK60 magnesium alloy during initial stage of hot compression at elevated temperature of 300 ℃ and 400 ℃ and a given strain rate of 10-2s-1.The results suggest that flow stress drop during initial stage of hot compression at 300℃,generally led by dynamic recrystallization,is attributed to twinning,correspondingly to dynamic recrystallization as deformation temperature is raised to 400 ℃.

Creep mechanism of Mg-Nd-Zn-Zr alloy at ambient temperature

Magnesium alloy's creep stress exponent and creep mechanism at the room temperature were analyzed by TEM. Relationship among strain,stress and creep time was studied. The creep rate of some mechanisms were calculated. The results show that the dislocation mechanism is possible. The deformation mechanism is dislocation slipping on basal plane,and twinning improves creep deformation.

Dynamic tensile behavior of AZ31B magnesium alloy at ultra-high strain rates

The samples having {0001 } parallel to extruding direction （ED） present a typical true stress-true strain curve with concave-down shape under tension at low strain rate. Ultra-rapid ten- sile tests were conducted at room temperature on a textured AZ31B magnesium alloy. The dynamic tensile behavior was investigated. The results show that at ultra-high strain rates of 1.93 × 10^2 s^-1 and 1.70 × 10^3 s^-1, the alloy behaves with a linear stress-strain response in most strain range and exhibits a brittle fracture. In this case, {10-12} 〈 10-11 〉 extension twinning is basic deformation mode. The brittleness is due to the macroscopic viscosity at ultra-high strain rate, for which the external critical shear stress rapidly gets high to result in a cleavage fracture before large amounts of dislocations are activated. Because { 10-12} tension twinning, { 10-11 } compressive twinning, basal 〈 a 〉 slip, prismatic 〈 a 〉 slip and pyramidal 〈 c ＋ a 〉 slip have different critical shear stresses （CRSS）, their contributions to the degree of deformation are very differential. In addition, Schmid factor plays an important role in the activity of various deformation modes and it is the key factor for the samples with different strain rates exhibit various mechanical behavior under dynamic tensile loading.

The roles of stress state and pre-straining on Swift effect for an extruded AZ31 Mg alloy

The Swift effect of Mg alloy is sensitive to initial texture.However,dislocation slip is the main deformation mechanism during torsion of Mg alloy.The underlying relation of Swift effect and dislocation slip is still not clarified.The effect of stress state and pre-straining on Swift effect was studied experimentally during free-end torsion for an extruded AZ31 alloy.The free-end torsion was performed with axial tension and compression stress which is lower than yield stress.It is found that the transition of axial deformation from contraction to elongation occurs when the axial stress changes from negative to positive.The pre-dislocations introduced by pre-tension promote axial shortening during torsion.While the pre-twins introduced by pre-compression are inhibition of axial shortening.The change of axial deformation is attributed to competition between twinning and prismatic slip.The axial shortening of extruded Mg alloy is generated by tensile twinning leading to c-axis strain.In contrast,the axial elongation can be generated by the activation of prismatic slip.The magnitude of axial strain generated by twinning is larger than that by prismatic slip.Moreover,the occurrence of detwinning results in axial elongation at low shear strain.

AZ31镁合金板室温弯曲压直过程中的组织及力学性能

研究AZ31镁合金板在室温弯曲压直过程中组织演变及其对力学性能的影响。结果表明:室温弯曲压直工艺预制拉伸孪晶不仅有助于镁合金的塑性变形,对材料的强度的提升也有积极的影响。室温弯曲压直后板材产生拉伸孪晶切割晶粒,导致晶粒细化和位错在晶界及孪晶密集处能量聚集造成位错强化,使镁合金板抗拉强度提升。同时,预制拉伸孪晶弱化基面织构可激活更多种类的滑移系统,导致室温弯曲矫直后的板材断后伸长率增加。

ZM6合金室温蠕变机理研究

采用单轴拉伸法对Mg-Nd-Zn-Zr(ZM6)合金在308K的蠕变行为进行了测试。在低于屈服强度的应力下,随应力的增加,蠕变量增加。通过蠕变量、应力与时间的关系,获得了蠕变速率及蠕变应力指数。通过比较各种蠕变机制的蠕变速率,确定合金室温蠕变机制为位错滑移。TEM分析表明,镁合金蠕变过程中发生了基面滑移,也存在极少量的孪生。

镁合金板材温热变形机理及温热成形技术

镁合金温热成形工艺具有较好的应用前景,是实现轻量化的重要途径,但镁合金温热成形机理需要进一步深入研究。通过电子背散射衍射(Electron back scatter diffraction,EBSD)原位跟踪观测方法,针对100～230℃范围,对轧制镁合金板材在单向压缩和单向拉伸变形时的变形机理进行系统研究和定量分析。分析镁合金板材在不同条件下的力学性能、织构转变特点、孪晶与滑移系启动规律,揭示不同变形条件下镁合金板材的塑性变形机理。研究结果表明,镁合金板材在变形过程的力学性能变化、织构演化和晶粒取向变化在很大程度上取决于孪晶参与变形的比例。镁合金板材在170℃具有较高的塑性成形能力,该温度下的大量锥面滑移系启动有利于协调轧板在厚度方向的变形。根据已获得镁合金板材变形机理,为镁合金板件冲压成形工艺提出建议。提出镁合金板件温热成形工艺,开发若干典型镁合金板件产品。

AZ31镁合金蠕变初期的变形特征

通过蠕变曲线的测定和TEM的形貌观察,研究了AZ31镁合金在蠕变初期的变形特征及组织演化规律.结果表明,蠕 变初期的变形特征是:大量形变产生的<A>位错在合金的基面和非基面滑移, <A+c>位错在锥面滑移.其中<a>位错通过位错 分解反应可由一非基面交滑移至另一非基面.随蠕变进行,高密度的形变位错发生动态回复,可进一步束集形成位错胞和位错墙. 蠕变初始阶段,在应力的作用下,适当取向的晶体发生孪生,并作为一种附加的变形机制而改善合金的韧性.

镁合金变形的晶体塑性有限元分析

晶体塑性理论已被广泛应用于现有的有限元分析中,从微观角度来模拟和预测晶体材料的宏观各向异性力学行为及塑性变形过程中织构的演化与发展。现有的晶体塑性理论框架核心主要基于由滑移引起的塑性变形机制,在预测由滑移和孪晶引起塑性变形的材料力学响应方面还不够完善。本文以具有密排六方(HCP)结构的变形镁合金塑性变形过程为例,综述了以滑移和孪晶为核心的晶体塑性理论的理论研究和应用现状,重点评述了现有孪晶的数值实现方法,并预测了相应理论的发展方向。

变形镁合金室温应变硬化行为的研究进展

阐述了镁合金应变硬化行为的研究现状;概括了组织参数对镁合金应变硬化行为的影响;提出此研究领域尚需解决的科学问题。镁合金的应变硬化行为与激活变形模式的类型和数量有密切的关系。此外,织构、固溶元素、析出相和晶粒尺寸等组织参数对滑移和孪生主导变形的应变硬化行为都有较大的影响。目前,组织参数与应变硬化行为的关系还需要更加系统的研究。此外,有必要研究和开发适用于镁合金的应变硬化理论模型。

动态析出对Mg-5Zn-2Al(-2Y)合金蠕变行为的影响

在175℃,50 MPa 条件下对 Mg-5%Zn-2%Al 及含2%Y(质量分数,%)的两种合金的固溶态与铸态蠕变行为进行了研究,TEM 观察与分析表明：蠕变到12h 时固溶态合金已经发生了动态析出,动态析出相为棒状 MgZn_2相,沿基体 {11(?)0}_α面并垂直于基面(0001)析出；蠕变曲线的对比表明,动态析出过程可使稳态蠕变速率明显减小．TEM 分析证实,在蠕变过程中基面 a-位错交滑移至棱面{01(?)0}_α,计算表明,阻碍基面 a-位错交滑移的主要因素为基面扩展位错的束集能,而棒状 MgZn_2相可促使基面扩展位错束集．随着蠕变时间的延长,动态析出相逐渐由棒状 MgZn_2相向颗粒状平衡相 MgZn 相转变．添加 Y

镁合金塑性变形机制的研究进展

综述镁及镁合金室温塑性变形的滑移和孪生机制的研究进展,总结镁合金塑性变形机制的研究状况。结果表明,强化孪生、降低c/a比值和细化晶粒,能提高变形镁合金的室温塑性。通过对现有研究成果的归纳和总结,指出提高镁合金塑性变形性能的有效途径,对高性能变形镁合金材料的研制及镁合金加工工艺优化具有指导意义。

镁合金轧制板材低温变形行为与微观机制

研究AZ31镁合金轧制板材在低温(室温至423K)拉伸条件下的力学性能,并通过高温金相显微镜的拉伸附件观察动态微观组织的变化过程。结果发现:镁合金的塑性随温度的升高和应变速率及负载的降低而增加,表现出一定的应变速率敏感性。常温下的主要变形机制为{1011}压缩孪生,其形核和长大速度很快,但同时也存在少量的基面滑移,不同类型的孪晶在晶粒中彼此相交;而随着温度的升高,a+c非基面滑移的临界剪切应力迅速下降,大量非基面滑移系被激发,板材塑性性能明显改善;当应变速率较高时(423K,应变速率10?2/s),断裂方式为有一定塑性特征的解理形貌;随着应变速率的降低(423K,应变速率10?3/s),断裂方式逐渐变为塑性断裂。

高应变速率下Mg-Gd-Y镁合金动态拉伸性能与失效行为

为了研究Mg—Gd—Y镁合金在高应变速率下的动态拉伸性能及失效机制，采用分离式Hopkinson拉杆（SHTB）装置在室温下且应变速率为1400～3000S。范围内对其进行了动态拉伸实验，并利用光学显微镜和扫描电子显微镜对动态拉伸后的镁合金试样进行了分析．结果表明，在动态拉伸载荷作用下，Mg—Gd—Y镁合金沿ED方向呈现连续屈服现象．应变速率增大后，Mg—Gd—Y镁合金具有正应变速率效应．在动态拉伸载荷作用下，Mg-Gd-Y镁合金的断口形貌呈解理断裂特征，镁合金的变形方式为孪生和滑移，且滑移为主要变形方式．

挤压AZ31B镁合金的低周疲劳性能(英文)

通过外加总应变幅控制的拉-压对称疲劳试验,研究常温下挤压AZ31B镁合金在不同应变幅下的疲劳性能。结果表明,除了在低应变幅0.5%外,样品均呈现循环应变硬化;应变幅为0.5%时,样品在初始阶段呈现循环硬化,随后保持应力恒定;在压缩过程中孪晶的产生以及随后的卸载和反向拉伸过程中的去孪晶行为导致了高应变幅下的滞回环形状拉-压不对称现象,而低应变幅0.5%下的滞回环形状基本对称,说明低应变幅下孪生-去孪生现象不明显。在整个疲劳过程中,高应变和低应变下的应力—应变曲线呈现2种不同的滞回环形状,这是由不同疲劳阶段孪生和位错滑移2种不同的变形机制所导致。

不同取样方向AZ31B镁合金应变率效应

采用分离式Hopkinson压杆获得了冲击压缩方向与热轧态AZ31B镁合金板材法向分别成90°、45°及0°的动态应力—应变曲线,利用光学显微镜及透射电子显微镜对微观组织演化过程进行了分析。结果表明:应变率约为1 200 s-1,压缩方向与板面法向成90°时,孪晶的面积分数随应变量的增大逐渐增加,孪生是主要变形机制,其曲线为凹型;压缩方向与板面法向成45°及0°时,其塑性变形以位错滑移为主,曲线形状分别为近似线型和凸型;应变率约为2 800 s-1,压缩方向与板面法向成90°时,孪晶面积分数迅速增加,大多数晶粒取向发生了改变,有利于滑移,其曲线由较低应变率的凹形变为凸型。

挤压态MB15镁合金的显微组织与力学性能研究

针对MB15镁合金在挤压温度为648K,应变速率0.001s-1,挤压比9∶1的条件下进行挤压变形研究,通过对其变形后的组织特征分析和拉伸性能测试,光学显微组织研究表明MB15镁合金在挤压过程中发生动态再结晶;透射电镜组织研究表明机械孪晶、位错滑移和动态再结晶是材料变形的典型特征;扫描电镜组织研究表明挤压后的材料塑性明显增强。挤压变形后的MB15镁合金沿挤压方向的抗拉强度提高39.5%,屈服强度提高89.4%,延伸率提高25%,弹性模量提高6.7%。