A Study on Mechanisms of Environmental Brittle Behavior by Single-and Bicrystal Experiment.

研究了环境对铜和铜铝致密拉伸（CT）单晶和双晶疲劳裂纹扩展的影响。采用Bridgman技术培养具有SIGMA41（338）、SIGMA9（221）和随机[110]倾斜边界的单晶和双晶，并将其用于疲劳试验。分析了初始晶界（GB）结构在双晶中的作用。疲劳试验在296 (]SY.+-下进行。[)在扫描电子显微镜（SEM）室中，在空气中或在1.3*10<@D1-3@>D1 Pa的真空中，2K和湿度为（50 (]SY+-[)5）%。结果表明，在真空条件下，裂纹萌生和扩展的优先位置不一定是GB。在空气中引入的整晶裂纹在真空中可能改变其扩展模式为穿晶。研究了真空条件下GB裂纹与PSB裂纹的相对重要性。主要的断裂机制是由两个因素决定的：岩石结构和相对于岩石取向的滑移几何。提出并修正的循环变形PSB-GB裂纹起裂模型能较好地解释疲劳裂纹扩展的大部分实验结果。

The environmental effect on fatigue crack propagation in copper and copper-aluminium compact tensio (CT) single and bicrystals was examined. Single and bicrystals with either SIGMA41 (338), SIGMA9 (221) and random [110] tilt boundaries were grown by Bridgman technique and used to the fatigue test. The role of the initial grain boundary (GB) structure is anlyzed for the bicrystals. Fatigue tests were done at 296 (]SY.+-。[)2K and humidity of (50 (]SY+-[)5)% in air or in vacuum of 1.3*10<@D1-3@>D1 Pa in a chamber of a scanning electron microscope (SEM). It is shown that the GB is not necessary a preferential site for crack initiation and propagation in vacuum. A crack introduced integranulary in air may change its mode of growth to transgranular in vacuum. The relative importance of GB cracking compared with persistent slip bands (PSB) cracking in vacuum is examined. The dominant mechanism of fracture is determined by two factors : The GB structure and the geometry of slip with respect to the GB orientation. The model of PSB-GB crack initiation proposed and modified for cyclic deformation explains satisfacctorily most of the experimental results of fatigue crack propagation.