Evolution of dislocation structure and fatigue crack behavior in Fe-Si alloys during cyclic bending test

Abstract

The evolution of dislocation structures was investigated by TEM in Fe-Si alloys with 0, 0.5 and 1.0mass% Si during a cyclic bending test in conjunction with fatigue crack behavior. The addition of Si increased the fatigue strength. The evolution of dislocation structures was significantly influenced by the Si addition. Namely, in the steel without Si the dislocation cell structure develops, whereas in the steel with 1 mass% Si the vein structure develops, which is considered to lead to increased fatigue strength. The dislocation cell structure observed in the steel without Si is postulated to be caused by the easy cross slip of dislocations during cyclic deformation, whereas the vein structure that developed in the steels with Si is inferred to be caused by the difficulty in cross slip due to the decrease in stacking fault energy. Furthermore, the Si added steel shows a characteristic structure in a manner such that the dislocations are free in approximately 0.5 μm zones along grain boundaries. The examinations of the fatigue fracture surface revealed that trans-granular fracture takes place in steel without Si, whereas in steel with 1 mass% Si many intergranular cracks were observed just beneath the top surface. The intergranular cracks in the 1 mass% Si steel were thought to be caused by the fact that a) strains are dispersed within grains owing to the vein structure and b) micro cracks are initiated and propagated along grain boundaries due to the dislocation free zones. © 2009 ISIJ.