Density changes of iron during morphological healing evolution of internal fatigue microcracks
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10/30/03
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Density Changes of Iron during Morphological Healing Evolution of Internal Fatigue Microcracks H.L. ZHANG, L. GAO, and J. SUN 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.
I. INTRODUCTION
HEALING of internal fatigue microcracks in pure iron during annealing at elevated temperature has recently been reported.[1] Crack healing has also been observed in other nonmetallic materials, such as ceramics,[2–5] glass,[6] sapphire,[7,8] quartz,[9] or polymers.[10] The morphological evolution of cracks and the evolving mechanisms has been presented in previous studies.[1,3,4,7,11] It was found that, controlled by surface diffusion, the crack first evolves into pore channels, which break up continually into spherical voids,[1,3,7,9] or these spherical voids are formed by the breakup of cylindrical pores that initially exist along the edges of crystals when preparing ceramics.[4] The recovery of strength of cracked materials is achieved by the final shrinkage of these deleterious voids. For example, it is shown that the recovery of creep ductility properties in a 20Cr/25Ni/Nb stainless steel is due to the sintering of grain-boundary cavities.[12] By constructing the constitutive equations for different stages of sintering, researchers have recorded the mechanical response of porous solids during sintering, mainly for powder metallurgical or ceramic materials.[13–16] In this way, the recovery of mechanical strength of the porous materials is obtained as voids shrink during sintering. Unfortunately, this method is less successful at predicting the sintering of the metallic materials within which there are
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