搅拌摩擦加工细晶镁合金应力腐蚀机理研究

Grain refinement is an effective method for improving the strength and plasticity of magnesium alloy. However, Grain refinement changes the characteristic of grain boundary, making significant effect on the stress corrosion behavior of magnesium alloy. The characteristic of grain boundary and grain orientation are two key factors in stress corrosion susceptibility of the fine grained magnesium alloy. In this project, firstly, friction stir processing (FSP) will be used to fabricate the solid solution magnesium alloy with different grain size and grain orientation. Microstructure characteristic, such as grain boundary, grain orientation, and dislocation configuration, will be investigated using TEM, SEM, EBSD and XRD et al.. Meanwhile, the effect of grain size and grain orientation on the stress corrosion behavior will be investigated separately. Furthermore, the distribution of the second phase in heat-treatable strengthening magnesium alloy will be changed by ageing treatment, and then the effect of the second phase on the stress corrosion behavior of magnesium alloy will be studied. Based on the above intensive investigation, the effect of the grain size and grain orientation on the stress corrosion in fine grained magnesium alloy will be investigated. Moreover, intrinsic relation between the grain boundary and stress corrosion susceptibility will be revealed, and the effect of the second phase on the stress corrosion behavior will be further interpreted. Meanwhile, control methods of key factors influencing stress corrosion of fine grained magnesium alloy will be obtained. The results of this project will provide theoretical basis for the optimal design on stress corrosion resistant magnesium alloy.

晶粒细化是提高镁合金强塑性的有效方法，但晶粒细化造成晶界特征的变化对镁合金应力腐蚀行为产生显著影响。晶界特征、晶粒取向是决定细晶镁合金应力腐蚀敏感性的关键因素。本项目先以固溶体镁合金为研究对象，采用搅拌摩擦加工（FSP）技术制备出具有不同晶粒尺寸和晶粒取向的细晶镁合金。采用TEM、SEM、EBSD、XRD等技术对晶界特征、晶粒取向、位错组态等进行表征；通过实验手段分离晶粒尺寸、晶粒取向对应力腐蚀行为的单独作用机制。再以时效处理控制可热处理强化镁合金中的第二相分布，分析第二相对细晶镁合金应力腐蚀行为的作用规律。通过这些深入研究，加深对晶粒尺寸和晶粒取向在细晶镁合金应力腐蚀过程中作用的理解，揭示晶界结构与应力腐蚀敏感性之间的本征关系，探讨第二相对应力腐蚀行为的作用机理，寻求细晶镁合金抗应力腐蚀关键因素的控制手段，从而为耐应力腐蚀镁合金材料的优化设计提供理论依据。

晶粒细化是提高镁合金强塑性的有效方法，但晶粒细化造成晶界特征的变化对镁合金应力腐蚀行为产生显著影响。本项目以固溶体镁合金为研究对象，利用搅拌摩擦加工（FSP）和时效热处理对镁合金进行微观结构改性。结果表明，FSP过程中的动态再结晶导致镁合金晶粒细化，加工区晶粒尺寸为2.7μm；FSP后晶粒c轴发生偏转，c轴与横向和加工方向的夹角分别约为55°和25°；FSP和时效热处理双联工艺会影响β相的含量和形貌。.晶粒细化促使FSP镁合金的显微硬度和拉伸强度提高；加工方向的织构软化作用导致拉伸强度低于横向；FSP和时效热处理双联工艺提高了镁合金的拉伸强度，和母相相比FSP+180℃-24h试样的屈服强度、抗拉强度和断面伸长率分别提高了6%、4%和15%。.晶粒细化提高了镁合金的耐蚀性；β相对静态腐蚀行为的影响具有两面性，一方面β相含量较少时，β相和α-Mg基体构成原电池而降低镁合金耐蚀性；另一方面，β相含量较多时，β相发挥腐蚀阻挡作用，并利于形成富铝膜层，提高镁合金耐蚀性。.晶粒细化降低了SCC敏感性；横向的织构强化效应延缓了点蚀的形成，导致横向的SCC敏感性低于加工方向；含量较多β相延缓SCC裂纹的萌生，降低SCC敏感性；SCC行为受阳极溶解控制，氢加速SCC的发生。FSP-MAO试样由于具有更厚、更致密的MAO膜层，且其基体本身比BM更耐蚀，因此具有比BM-MAO试样更优的耐蚀性；慢应变速率拉伸初期MAO和MAOE膜层对腐蚀介质的阻挡作用，可延缓点蚀的发生，从而降低SCC敏感性。