镁合金棘轮效应机制及对疲劳寿命影响研究

It is known that the fatigue-life becomes difficultly to be predicted when ratcheting effect is involved for magnesium alloys subjected to asymmetric-stress-control cyclic deformation process. In an attempt to investigating the ratcheting effect, in this project, the AZ31 is selected as the target alloy, and a series of cyclic deformation experiments are conducted under the asymmetric-stress-control conditions. By using in situ EBSD observation technique and other relevant methods, the cyclic structural evolution and the associated mechanism during deformation process are investigated systemically with the points on the cyclic hardening/softening process, the relationship between the cyclic hardening/softening process and the ratchet strain rate, the influence of ratcheting process on the Bauschinger effect and the resultant material damage caused by the ratcheting strain and so on. On the basis of these results, a theoretical prediction of the fatigue-life by taking into account of the ratcheting effect of magnesium alloys is presented by elucidating the effects of ratcheting process on the variously mechanism-dominated cyclic deformation and the weight of each influencing factors.

非对称应力循环过程中的棘轮效应使得对镁合金疲劳寿命的预测非常困难。本项目针对循环变形中棘轮效应，选取AZ31B镁合金为研究对象，系统开展非对称应力控制下的循环变形试验，结合EBSD原位、准原位观测技术及相关测试、表征方法，研究在不同变形条件下，棘轮变形过程中循环变形机制、变形组织的演化过程；深入分析在不同变形条件下棘轮变形过程中循环硬化/软化机制、棘轮应变率与材料循环硬/软化的相互关系、棘轮对Bauschinger效应的影响以及棘轮应变引起的材料损伤等问题；阐明在不同变形机制主导下棘轮对镁合金疲劳寿命的影响机理及各影响因素所占的权重，为棘轮变形下疲劳寿命预测提供理论支持。

非对称应力循环变形过程中，织构、棘轮应变、疲劳实验参数（平均应力、应力幅和循环频率等）的叠加使得挤压镁合金的疲劳过程变的异常复杂。本项目通过研究平均应力、应力幅和循环频率等因素对循环变形机制、组织的演化以及棘轮应变的影响，分析了循环硬化/软化机制及来源，棘轮应变率与材料循环硬/软化的相互关系，不同变形机制主导下棘轮对镁合金疲劳寿命的影响机理。取得的主要研究成果有：1）揭示了位错和压缩孪生/去孪生机制在循环变形过程中所占比重对棘轮应变及疲劳寿命的影响规律；2) 阐明棘轮应变的频率效应取决于滑移和拉伸孪生机制的权重，与应变速率有关。疲劳寿命与棘轮应变有关，与位错和孪生引起的循环硬化差异关联较少；3) 发现了在较大应力幅下，大量柱面<a>滑移的启动影响了循环过程中应变硬化率，并抑制了循环过程中的棘轮应变。相同的平均应力下应力幅与棘轮应变不存在正比的关系。4）阐明了棘轮应变与变形机制对疲劳寿命的影响具有不确定性。一般来说，棘轮应变带来了损伤但提高了硬化率。孪生机制在循环过程中促进了塑形变形，却有利于裂纹的萌生。本项目的研究成果丰富和发展了镁合金在应力控制循环过程中变形机理，提供了确定镁合金构件应用过程中棘轮应变及寿命的极值点方法，为镁合金构件安全可靠性的评价提供新思路和理论指导。