单相fcc金属晶内裂纹萌生与扩展的TEM原位分析

IN SITU TEM STUDY OF CRACKS INITIATION AND PROPAGATION IN A GRAIN OF FCC METAL

用TEM拉伸方法，对具有fcc结构的304L钢、进行了晶内裂纹萌生与裂纹连续扩展的原位观察．结果表明：晶内开裂时首先形成无位错区．裂纹在无位错区中形成．裂尖扩展和裂尖前晶面变形行为由原位观察和原位选区衍射的结果进行分析．裂纹沿｛100｝晶面扩展时．裂尖前｛100｝晶面发生倾转，｛111｝晶面局部弹性弯曲，裂尖钝化；｛100｝倾转增大以致开裂，钝化裂尖扩展，｛111｝局部弯曲有所恢复．

The initiation and the continuous propagation of micro--racks were studied by in site TEM tension performed on fcc 304L steel. The in situ observation of a micro-crack initiation indicated that a dislocation free zone (DFZ) firstly formed inside the region of stress concentration and then a micro-crack initiated in the DFZ when a TEM sample without pre-crack was tensioned. The continuous propagation of a pre-crack and the corresponding behaviors of the lattice planes near the crack tip in DFZ were in situ observed and analysed by selected area diffraction (SAD). It was revealed that as the {100} lattice planes tilted and the {111} lattice planes elasticity bent, the crack tip was blunted. And then, the tilting of {100} planes were increased up to the {100} planes cleaved, the blunted crack tip split and the elastic bending of {111} disappeared partly, the crack finished a step of propagation.