铝合金残余应变及裂纹萌生与扩展的原位电子显微学研究

The encountered fracture during servicing of the wrought aluminum alloy mainly involves the initiation and propagation of the cracks, and, the non-coherent precipitates existing in the aluminum alloy are responsible for initiation of the cleavages, furthermore, the residual strains owing to the precipitate and the deformation to the alloy play an important role in the initiation and propagation of the cracks. In the program, the electron backscatter diffraction (EBSD) technology will be employed to explore the distribution of the residual strain, the microstructure of the phase interfaces and the crystal orientations in an aluminum alloy, then, the spatial distribution features of the residual strain along different orientations will be studied chiefly, the microstructures in the fields initiating cracks will also be researched, and the inherent relationships between the cleavage initiations and the residual strains, the precipitate particles, the microstructures of the phase interface and the crystal orientations will be analyzed in detail, subsequently, that can be used for exploring the condition to the cleavage initiation. And, the in-situ propagation processes of the cracks under the action of the applied loadings and the residual strain during the stretching of the alloy sample, will be explored. Several key scientific problems on the evolution rule of the residual strain during the crack extension and the effect of the crystal orientation on the crack propagation need to be explored in depth. Then, the extrusion or rolling processing and the heat treatment technique to the alloy to control the distribution of the precipitates and the residual strain will also be explored in order to reduce the initiation of the micro-crack. The research results will have important signification to boosting the carrying capacity, prolonging the service life, improving the reliability and preventing the occurrence of the accidents.

变形铝合金材料在服役过程中所发生的断裂主要包括裂纹的萌生和扩展过程，铝合金中的非共格沉淀颗粒可能诱发裂纹萌生，而沉淀颗粒周围存在的残余应变和由于合金变形加工所引人的残余应变对微裂纹的形成与扩展也有重要影响。本项目拟利用电子背散射衍射技术研究铝合金的残余应变、相界面结构和晶体取向等，重点研究不同晶体学取向下残余应变的空间分布特点；研究铝合金裂纹萌生处的组织与结构特征，分析裂纹萌生与残余应变、沉淀颗粒、相界面结构和晶体取向的内在联系，探究微裂纹的起裂条件；动态研究铝合金微裂纹在外部载荷、残余应变等因素共同作用下的扩展过程,深入研究残余应变在裂纹扩展过程中的变化规律以及晶体取向对裂纹扩展的影响机理等关键科学问题。探索调控沉淀颗粒和残余应变分布的变形加工工艺和热处理工艺以减小微裂纹的萌生。研究结果对于提高铝合金的承载能力、延长其使用寿命、提高其可靠性以预防事故的发生均具有十分重要的意义。

变形铝合金材料在服役过程中所发生的断裂主要包括裂纹的萌生和扩展过程，铝合金中的非共格沉淀颗粒可能诱发裂纹萌生，而沉淀颗粒周围存在的残余应变和由于合金变形加工所引人的残余应变对微裂纹的形成与扩展也有重要影响。本项目利用高分辨电子显微技术基础上的几何相位分析技术定量研究了变形铝合金（Al-12Zn-2.4Mg-1.1Cu）沉淀相周围的残余应变场，同时采用晶体学模拟软件CrystalMaker and JEMS模拟了上述应变场，并与定量测量结果作了对比。残余应变主要集中在沉淀相与基体界面，且随着距离界面的增加逐渐减小。沉淀相周围主要为压应变，最大压应变大约为2.8%。残余应变分布与铝基体的晶体学取向存在明显关系，在铝基体<111>方向上，关于应变的模拟结果与实验结果基本一致。应变集中区宽度大约为1.25 nm；在铝基体<211>方向上，沉淀相周围应变的实验测量值很小，可以忽略，而模拟应变值比实验值高。同时研究了另一沉淀颗粒的应变情况，应变最大值位于距离界面0.25 nm 处，最大值约为2.54%。.微裂纹易于在晶界的沉淀颗粒处萌生与扩展，裂纹穿越晶粒扩展时，晶粒发生偏转，取向发生变化。裂纹周围的晶粒在裂纹萌生和扩展的过程中发生了晶体转动，在晶内也引起大的塑性变形。利用拉伸台和电子背散射衍射技术动态研究了变形铝合金裂纹的扩展过程，裂纹附近是残余应变的集中区域。裂纹附近晶粒中产生了亚晶粒，透射电镜研究表明，亚晶粒主要是由于位错的运动并塞集而引起的。变形铝合金中存在较大的残余应变，晶内存在亚晶、位错胞和位错墙，且在晶界处有明显位错塞积导致的应变集中现象。裂纹易于在残余应变集中处萌生，裂纹宏观扩展方向与拉伸轴垂直，但是在局部区域沿着残余应变较大的区域扩展。裂尖塑性区的残余应变较大，形状大小约为200微米，呈脑干型在裂尖前端左右交替分布。本项目研究结果对于丰富铝合金的断裂理论具有重要的意义。