孪晶引致的变形镁合金低周疲劳微观损伤行为研究

Lightweight magnesium alloy moving parts are very useful to lose weight and saving energy, but tend to fatigue failure under cyclic loading, therefore, it is very important to study clearly the fatigue failure behaviors and enhance fatigue performance of magnesium alloys. In this project, extruded AZ31B and GW83 wrought magnesium alloy are chosen for study. Different volume fraction of static twins, dynamic repeatedly twinning-detwinning twins and the residual cumulative twins is introduced into the extension-compression low cycle fatigue process by controlling initial texture, pre-deformation and tension and compression cycle strain amplitude. By using some advanced detection methods, such as high-resolution scanning electron microscopy and electron contrast technology (SEM-ECC) and electron backscatter diffraction technology (EBSD) and confocal microscopy (OM) and so on, some basic questions, for example, micro-crack initiation mechanism from various types of the twins, its competition relationship with persistent slip bands and grain boundaries, twin induced mirco-crack propagation and coalescence behavior, and competitive propagation along the twin boundary and grain boundary, will be systematically studied. The quantitative contribution of twin induced micro-cracks to the initiation fatigue life and propagation fatigue life will be made clear. And it is hopeful to establish twins induced fatigue crack initiation and propagation physical models. The research results may provide theoretical foundation for improve the wrought magnesium alloy fatigue life based on the twin, which is greatly connected with the alloy composition, texture, grain size and precipitations.

轻质镁合金运动零部件减重效应大，在循环载荷作用下往往以疲劳的方式失效，研究清楚镁合金的疲劳失效行为和提升其疲劳性能意义重大。本项目选取AZ31B和GW83挤压变形镁合金，通过初始织构、预变形和拉压循环应变幅值等控制不同体积分数的静态孪晶、动态孪生-去孪孪晶和残余累积孪晶参与低周疲劳变形和损伤过程，利用高分辨扫描电镜及其电子衬度技术(SEM-ECC)和电子背散射衍射技术(EBSD)，以及共聚焦显微镜(CM)等先进观测手段，深入研究各种孪晶诱发疲劳微裂纹机制，孪晶与驻留滑移带、晶界之间竞争萌生微裂纹机制，孪晶引致微裂纹的扩展和合并行为，裂纹沿孪晶界和晶界竞争扩展机制等科学问题，获得孪晶微裂纹对裂纹萌生寿命和扩展寿命的量化贡献，期望建立镁合金孪晶疲劳裂纹萌生和扩展物理模型，为将来基于合金成分、织构、晶粒度和析出相等来调控孪晶体积分数、进而调控变形镁合金疲劳寿命奠定理论基础。

轻质镁合金运动零部件减重效应大，在循环载荷作用下往往以疲劳的方式失效，研究清楚镁合金的疲劳失效行为和提升其疲劳性能意义重大。本项目系统研究清楚了在低周循环载荷下挤压AZ31、ZK60 和GW83镁合金的疲劳寿命，孪生主导变形的AZ31和ZK60合金体现出两段式疲劳寿命曲线，而孪生参与变形的Gw83合金体现出三段式疲劳寿命曲线。ZK60合金的循环变形机制为：当应变幅值小于0.35%时，循环变形机理是基面滑移。当应变幅值在0.35%-5%之间时，孪生-去孪行为参与循环变形。GW83合金循环塑性变形机制：当应变幅值低于拐点应变幅值，滑移为主要变形机制；当应变幅值大于拐点应变幅值，滑移和孪生为主要变形机制。AZ31合金的疲劳损伤机制为：在1%应变幅值时的疲劳损伤主导机制为滑移带裂纹，孪晶对疲劳损伤也有贡献。在0.3%应变幅值时的疲劳损伤主导机制为滑移带裂纹。GW83合金疲劳损伤机制：应变幅值为0.5%时，微裂纹主要萌生于驻留滑移带和晶界，然后沿驻留滑移带缓慢扩展；临近疲劳失效时基体中存在大量的微裂纹，而无明显主裂纹。应变幅值为1.2%时，微裂纹主要萌生于晶界，疲劳裂纹萌生寿命占总疲劳寿命的50%；后半疲劳寿命阶段微裂纹沿晶界迅速长大，合并其它小裂纹形成主裂纹；未发现表面孪晶引致的表面裂纹。预制孪晶大幅度降低AZ31合金的疲劳性能，首先萌生少量的孪晶界裂纹，随着循环周增加，孪晶致引裂纹大量萌生，但是几乎不扩展；晶界裂纹和母晶粒的PSB裂纹没有明显增加。疲劳末期，主裂纹迅速产生并扩展，且遭遇孪晶裂纹或PSB，会发生偏折。孪晶内部形成PSB裂纹，显著降低镁合金疲劳寿命。目前已发表7篇高水平学术论文。研究结果对于丰富镁合金的塑性变形理论、疲劳机理和调控镁合金的疲劳性能具有重要意义。