Density changes of iron during morphological healing evolution of internal fatigue microcracks

Abstract

Plastic-strain-controlled fatigue was performed on pure iron specimens with uniaxial symmetric tensioncompression loadings at room temperature. The as-fatigued specimens were then annealed in vacuum at 1173 K from 1 to 7 hours. The morphologies of internal fatigue microcracks were observed by scanning electron microscopy (SEM) in the as-fatigued and as-annealed specimens. The density of the specimens was measured with an electronic analytical balance. The density of the as-fatigued specimens decreased continuously as the fractional fatigue life increased, and was nearly constant when the specimens were annealed up to 2 hours at 1173 K, but increased gradually after 2 hours of annealing time. The density of some specimens eventually approximates to the value of 0 , the initial density, at 7 hours of annealing time. This suggests that the initial decrease in density is due to crack initiation and propagation in the as-fatigued 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 stages, the density increases and finally returns to the initial density because the spherical voids evolved from the parent crack are reduced by volume diffusion coupled with grain-boundary diffusion. A combined model is presented to predict the shrinkage of the spherical voids within the specimens, and is in broad agreement with the experimental data.

Research on Crack Healing in Slab during Hot Rolling

“…The heat treatment method was also employed to restore the plasticity margin destroyed in cold plastic deformation and to avoid damage by Kolmogorov et al [13], and experimental data indicated that the micro-damages in metal materials heal. A series of researches on the internal crack healing in pure iron at high temperature were carried out by Zhang et al [14]. SEM observations showed that the micro-cracks changed to an unduloid-like shape at 2 h, then evolved into an array of circular voids at 5 h, with some isolated voids scarcely observed at 7 h. A series of experiments on crack healing in steel through the electric current pulse technique was carried out by Zhou et al [15], the cracks could partly heal through the electric current pulse at the solid state and the healing process was finished within a very short duration, and that the original microstructure in the area without crack did not change.…”