工业纯铁内部穿晶疲劳微裂纹的扩散愈合过程

DIFFUSIVE HEALING OF TRANSGRANULAR FATIGUE MICROCRACKS WITHIN a-IRON

采用对称拉压低周疲劳方法在轴对称工业纯铁试样中引入了临界长度约为30μm、穿越铁素体晶界的内部微裂纹,随后在1173K温度下对试样分别进行2和5h的真空加热处理.结果表明,随保温时间的延长,穿晶疲劳微裂纹扁椭圆型的二维纵剖面首先在表面扩散与晶界扩散的耦合作用下在晶界处分隔为两部分,随后在表面扩散单独作用下演化为由多个球形空洞组成的球洞串截面形态.由于晶界在退火过程中的迁移运动,晶界上裂纹演变形成的空洞最终被遗留在晶粒内部.建立了体扩散控制下的晶粒内部空洞的收缩模型,与实验观察结果对比表明,该模型能够很好地预测晶内空洞的收缩愈合过程.

Internal transgranular microcracks were introduced into rounded bars of pure iron via symmetric tension compression low-cycle fatigue. The microcracks were found to have critical length of about 30 μm. After vacuum annealing of fatigued samples for 2 and 5 h at 1173 K, the two-dimensional longitudinal sectioning of transgranular microcracks shows that the crack morphology has disconnected into two sections at the point of intersection with grain boundary controlled by surface diffusion and interface diffusion, and then evolved into arrays of spherical voids dominated only by surface diffusion. The voids, evolved from original cracks on grain boundaries, have been left within grains by migrating grain boundary during annealing. A model for the shrinkage of voids within grains is proposed and well agreed with experimental observations.