Morphological evolution of internal fatigue microcracks in α-iron during diffusive healing

Internal microcracks with critical length of 30 μm and thickness of 0.5 to 1.5 μm were introduced into the round specimens of pure iron by low cycle fatigue with uniaxial symmetric tension-compression loadings. Then the specimens were heated in vacuum at 900°C for 1.5 and 5 h, respectively. The results show that two-dimensional longitudinal section of internal fatigue microcrack has evolved from initial thinly elliptical one into arrays of spherical voids, controlled by surface diffusion. A typical morphology was observed for a broken crack with a center spherical void surrounded by outer doughnut-shaped cavities along the perpendicular cross-section of a specimen. A physical model to describe the morphological evolution for an internal microcrack was discussed, as well as its dependencies upon the morphology and geometry of an original microcrack in materials.