Morphological healing evolution of intragranular penny-shaped microcracks by surface diffusion: Part II. Experiments

Micron-sized internal microcracks were introduced into rounded bars of pure iron via symmetric tension-compression low-cycle fatigue. The microcracks were found to have irregular penny-shaped morphology with critical diameter of about 30 μm and thickness of 0.5 to 1.5 μm. The location, geometry, initiation, and propagation of these microcracks were quantitatively investigated. After vacuum annealing of the fatigued samples, two-dimensional (2-D) longitudinal sectioning of the intragranular microcracks showed that the crack morphology had evolved from initially elliptical shape into arrays of spherical voids by surface diffusion. A typical configuration involving a microcrack broken into a center spherical void surrounded by a ring of doughnutlike arranged outer cavities was observed along the transverse section of the specimen. Subsequently, the spherical voids were found to shrink and diminish gradually via bulk diffusion as the annealing time increases. The morphological evolution time is found to be in broad agreement with the corresponding results predicted by our finite-element simulation of an intragranular penny-shaped microcrack evolving under surface diffusion.