内部疲劳微裂纹退火愈合过程中纯铁的密度变化

CHANGE IN DENSITY OF α-Fe DURING HEALING OF INTERNAL FATIGUE MICROCRACKS UNDER ANNEALING

在室温下对工业纯铁试样进行了控制应变幅的等幅、拉-压、低周疲劳加载,随后在1173K温度下从1h延续至7h进行真空退火处理,并分别对疲劳试样和退火试样中的疲劳裂纹形态进行了SEM观察,用电子分析天平对各组试样进行了密度检测,结果表明:同原始试样相比,随疲劳周次增加,疲劳试样密度逐步减小,在随后的1h到3h退火期间,疲劳试样密度值无明显变化;当从3h至7h退火时,试样的密度逐渐增大,并且在7h退火时接近了原始密度值,分析表明,内部疲劳微裂纹的萌生是试样密度值减小的原因;在早期的退火处理阶段,表面扩散机制支配裂纹的形态演变,因而试样密度无明显增大;而在退火处理后期,体扩散及晶界扩散作用缩小了裂纹演变形成的空洞,使得试样密度增大并逐渐恢复至原始密度值。

The plastic-strain-controlled fatigues with uniaxial symmetric and tension-compression loadings at room temperature were performed on pure iron specimens. The as-fatigued specimens were then annealed in vacuum at 1173 K from 1 h to 7 h. The morphologies of internal cracks were observed by SEM upon the as-fatigued and as-annealed specimens, respectively. The change in the density of specimens was measured by the electronically analytical balance. The results show that, compared with the initial density, the density for the as-fatigued specimens becomes smaller as increasing the fractional fatigue life, and is nearly constant when the specimens were annealed from 1 h to 3 h, but increases gradually when annealed after 3 h, and approximates eventually to the initial density at the point of 7 h annealing. It indicates that the decrease in density of as-fatigued specimens is due to crack initiation within the specimens. At the early stage of annealing, the specimen density is nearly constant because the crack morphological change is controlled by surface diffusion; at the later stage, it increases and finally comes to the initial density because the voids evolved from the parent crack are reduced by volume diffusion coupled with grain boundary diffusion.