预低周疲劳变形对金属单晶体静态力学行为影响的机理性研究

Several kinds of typical face centered cubic (fcc) metallic crystals, including Cu single crystals with different orientations, Cu-161t.%Al alloy single crystals with low stacking fault energy, Al-4wt.%Cu alloy single crystals containing precipitates, etc. are selected as the target materials in the present research project. Firstly, fatigue pre-deformation to different stages (i.e. to different accumulative plastic strains) are performed on these single crystals within a certain range of plastic strain amplitudes, and uniaxially tensile deformation tests are subsequently carried out on these pre-fatigued crystals at a fixed strain rate to obtain the basic data of their static mechanical properties. Optical microscope, the electron channeling contrast (ECC) technique in scanning electron microscopy (SEM) together with transmission electron microscopy (TEM) are adopted to observe fatigue deformation characteristics and fatigue dislocation structures, as well as tensile deformation features, fracture surface morphologies and corresponding dislocation structures of pre-fatigued crystals. By making comparisons to the experimental data obtained from direct tensile deformation tests of these single crystals, the micro-mechanisms for the effect of fatigue pre-deformation on the static mechanical behavior of fcc metallic crystals will thus be clearly revealed as a general rule, focusing on the detailed influences of crystallographic orientation, stacking fault energy, precipitates, etc. on such micro-mechanisms. In a word, the present research project makes important theoretical and practical senses to the deep-going understanding of the strengthening mechanism by low-cycle fatigue training as well as the micro-mechanisms for the degradation in static mechanical properties due to fatigue damage of metallic crystals, and it is expected to provide some valuable references for residual fatigue life prediction after fatigue damage of metallic crystals.

本项目拟采用不同取向Cu单晶体、低层错能Cu-16at.%Al合金单晶体以及含析出相的双相Al-4wt.%Cu合金单晶体等fcc晶体作为研究材料，在一定塑性应变幅范围内对其进行不同程度的（即不同累积塑性应变量）预疲劳变形试验后，再在一定应变速率下进行单向拉伸得到它们的基本静态力学性能数据。利用光学显微镜、扫描电镜电子通道衬度技术和透射电镜等观察分析预疲劳变形后的变形（损伤）特征、位错结构以及预疲劳后拉伸变形特征、断口形貌特征和位错结构的变化，通过与直接拉伸变形的相应结果进行系统比较，进而清晰地揭示预疲劳变形对fcc金属晶体静态力学行为影响的一般性规律与微观机理，重点考察晶体取向、层错能和沉淀相等因素对机理的具体影响。该工作对深入认识金属晶体的低周疲劳锻炼强化机制及其在疲劳损伤后静态力学性能下降的微观机理具有重要的理论和实际指导意义，并且可为金属晶体疲劳损伤后剩余疲劳寿命预测提供有益的参考。

本项目采用了具有不同层错能面心立方(FCC)金属晶体（如：不同取向Cu单晶、纯Al和Cu-Al合金）以及时效强化型Al-4wt.%Cu合金作为研究材料，系统研究了预疲劳变形对它们静态力学性能的影响规律及其微观机理，揭示了层错能、晶体取向、外加应变幅、循环累积塑性应变量以及沉淀相等在影响机理中所起的具体作用。得到以下主要创新性研究结果：(1)预疲劳变形对高层错能纯Al单向拉伸力学行为的影响不甚显著，主要与变形过程中位错易发生动态回复有关。但是，预疲劳变形后可适当减弱其拉伸力学行为的尺寸效应，并提高其拉伸塑性；(2)预疲劳变形对较高层错能不同取向铜单晶单向拉伸力学行为具有明显影响。只有在合理的塑性应变幅下进行预疲劳处理后引入适当的波状滑移位错结构（如[017]临界双滑移取向的迷宫/PSB类楼梯结构、[-233]共面双滑移取向的位错胞结构、[-112]共轭双滑移取向的少量PSB楼梯结构、以及[011]多滑移取向的初级形变带内的PSB楼梯结构），同时不引入明显的疲劳损伤条件下，才能有效提高铜单晶体单向拉伸性能；(3)对于Cu-Al合金（尤其是最低层错能的Cu-16at.%Al合金），预疲劳过程中可引入许多平面型可动位错以及随后拉伸时能形成纳米尺寸形变孪晶，因此预疲劳对其单向力学性能具有最为显著的影响，表现出明显的低周疲劳锻炼强化效应；(4)对于时效强化型Al-4wt.%Cu合金，在预疲劳损伤等级D（= Ni/Nf，Ni为预循环周次，Nf为疲劳寿命）为20%的预疲劳变形下，可导致固溶加160℃/16h时效Al-4.0wt.%Cu合金拉伸过程中位错与theta''相发生较强相互作用，削弱了位错在晶界处塞积引发的应力集中，再加上拉伸过程中小韧窝的连接，导致预疲劳后在强度提高的同时扔保持了较好的塑性。这与单相金属中由于位错滑移方式以及位错组态的变化导致的预疲劳对静态力学性能的改善有所不同。上述结果深入揭示了FCC金属晶体的低周疲劳锻练强化机制及其在疲劳损伤后静态力学性能下降的物理微观机制，对深入理解预疲劳变形对单相和含沉淀相FCC金属单向拉伸力学行为影响的一般规律及微观机理具有重要的指导意义。基于项目研究，共发表学术论文30余篇，其中SCI检索文章30篇，参加学术会议30余人次，做邀请报告11次，培养博士生2名、硕士生5名。较好地执行并完成了项目的研究计划。