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

研究了环境对铜和铜铝复合拉伸(CT)单晶和双晶疲劳裂纹扩展的影响。用Bridgman法生长了具有SIGMA41(338)、SIGMA9(221)和随机[110]倾斜晶界的单晶和双晶，并用于疲劳试验。分析了初始晶界(GB)结构对双晶的作用。疲劳试验在296(]sy.+-进行。[)2K，湿度为(50()SY+-5)%，空气或真空1.3*10&lt；d1-3@d1；d1Pa.在扫描电子显微镜(SEM)的腔室中。结果表明，GB不一定是真空中裂纹萌生和扩展的优选点。在空气中引入积分的裂纹会使其在真空中的扩展方式转变为穿晶。研究了GB裂纹与真空中持续滑移带(PSB)裂纹的相对重要性。断裂的主要机制由两个因素决定：GB结构和相对于GB取向的滑移几何。提出并修正的循环变形PSB-GB裂纹萌生模型较好地解释了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.