单晶Nb循环变形过程中疲劳裂纹的萌生及其机制

FATIGUE CRACK INITIATION OF Nb MONOCRYSTALS AND ITS MECHANISM DURING CYCLIC DEFORMATION

用光学显微镜、扫描电镜和透射电镜观察了Ｎｂ单晶在循环变形后的表面形貌和饱和时的位错组态．发现疲劳裂纹的萌生和表面形变带有关．当滑移线和形变带方向平行时，在双形变带的公共皱折处很容易萌生疲劳裂纹．已萌生裂纹呈交滑移特征，在裂纹萌生处附近，有很多由交滑移引起的滑移台阶．根据我们的实验结果，从理论上对Ｍｏｔｔ的疲劳裂纹萌生模型进行修正。

The optical microscopy, SEM and TEM have been used to observe the morphological changes at the surface of fatigued Nb monocrystals and the saturated dislocation structure. It reveals that fatigue crack initiation is related to the deformation band at the spcimen surface and cracks initiate easily at the common fold area of the double deformation bands if the slip lines and deformation band are paralled to each other. It is also found that the initiated cracks show the behaviour of cross slips and the steps due to cross slips locate mostly in the vicinity of the initiated cracks. Based on our experimental results, a mechanism.which is the modification of the famous Mott model, is given to theoretically explain the initiation of fatigue cracks. In other words. the experimental results confirm the Mott model in general .