Diffusive healing of intergranular fatigue microcracks in iron during annealing

Micron-sized internal cracks were introduced into pure iron via plastic-strain-controlled low cycle fatigue (LCF) loadings. The cracks on grain boundaries were found to have a regular shape of penny-like. The length of the cracks is less than 30 μm, and the thickness less than 3 μm. The aspect ratio of diameter-to-thickness is within a range of 5-45. The as-fatigued specimens were annealed for 7 h in vacuum at 1173 K. Scanning electron microscopy (SEM) observations show that the intergranular microcrack first changes as an unduloid-like shape at 2 h of annealing time, then evolves into an array of "circular voids" at 5 h, with some isolated voids scarcely observed at 7 h. Finite element method (FEM) simulations predict that the intergranular penny-shaped microcrack would evolve into a spherical void when its aspect ratio is less than 19.5, and the evolution time is also quantitatively given. The shrinkage time of the spherical voids evolved from the parent microcrack is predicted by a modified Speight-Beere model. The shrinkage rate of the intergranular spherical voids is faster not only than that of a single intergranular spherical void, but also than that of the intragranular spherical voids.