FATIGUE CRACK INITIATION OF Nb MONOCRYSTALS AND ITS MECHANISM DURING CYCLIC DEFORMATION
单晶Nb循环变形过程中疲劳裂纹的萌生及其机制

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 .