超高强度钢强化层的微观组织变化和疲劳裂纹扩展行为的透射电镜观察

OBSERVATION ON FINE STRUCTURE OF WORKHARDENED LAYERS AND FATIGUE CRACK PROPAGATION BEHAVIOR IN ULTRA-HIGH STRENGTH STEELS BY TEM

用透射电镜对超高强度钢喷丸和挤压强化层中的精细组织、疲劳裂纹尖端塑性区内的位错结构、微观组织和裂纹扩展行为问的联系进行了研究。结果表明,不同试验条件所产生的拉应力和周期应力均可引起显微组织中残余奥氏体的应变相变和位错胞状组织。喷丸所产生的周期应力可导致下贝氏体内ε-碳化物的退化和显微组织中的亚晶。在塑性区内存在高位错密度的主位错带。就马氏体板条晶而论,裂纹扩展多为穿品,裂纹扩展遇到束界时方向发生较大变化。

The fine structure change in shot peening and extrusion materials and relationship between the dislocation structure in plastic zone ahead of fatigue crack tip and the crack propagation behaviour has been investigated by means of TEM. The results show that three test conditions may cause strain-induced and phaseinduced tansformations in retained austenite and cell dislocation structure in microstructure. The cyclic stress-induced by shot peening may result in ε-carbides in lower bainite decay and may change cell dislocation structure in microstucture to subgrains. A main dislocation band with high dislocation density in the plastic zone was found. The martensite laths were crosscut by the propagation cracks. If the direction of crack propagation is inter-lath, the deviation between laths appears to be small. When cracks cross the covariant packet boundaries, the direction deviation becomes larger.