（F＋M＋A）三相钢低周疲劳的扫描电镜动态研究SCIEI

用扫描电子显微镜疲劳加载台进行动态原位观察，研究了奥氏体（Ａ）在（Ｆ＋Ｍ＋Ａ）三相钢低周疲劳裂纹萌生和扩展过程中的作用．结果表明，在低周疲劳裂纹的萌生和早期扩展阶段，三相钢中的奥氏体会降低疲劳缺口敏感性，延长裂纹萌生期，使裂纹路径弯折并降低扩展速率．在高应变疲劳阶段，因奥氏体应变诱发相变生成马氏体，其边界是裂纹扩展的低能通道，从而加速裂纹的扩展．并提出了一个能量判据：当裂尖扩展功小于Ａ→Ｍ转变的能量时，奥氏体会提高材料的疲劳抗力，反之奥氏体会加速疲劳裂纹的萌生和扩展．

IN SITU OBSERVATION ON LOW CYCLIC FATIGUE IN TRIPHASE STEEL UNDER SEM

The behaviour of austenite in the crack initiation and propagation of low cyclic fatigue for triphase steel (F-M-A) was observed in situ under SEM with fatigue loading stage. Results showed that the austenite in triphase steel during the crack initiation and early propasation of low cyclic fatigue may decrease the fatigue notch sensitivity and prolong the cyclic number of fatigue crack initiation, so as to flex the propagation path and to reduce the propagation rate of the cracks. At high strain fatigue stage, the strain-induced transformation of austenite may form the martensite, of which has the boundaries of low energy path for cracks propagation, the cracks may propagate more rapidly. Furthermore, an energy criterion was proposed, i, e., when the propagation work at crack tip is less than energy transformed from austenite into martensite, the fatigue resistance of steel may be improved by austenite, on the contrary, the crack initiation and propagation accelerated