Effect of stress concentrations on crack nucleation in 3% silicon-iron single crystals

Abstract

Single crystals of 3% silicon-iron were deformed in tension in the [110]direction at temperatures from 77° to 293°K under conditions equivalent to plane strain. The crystals behaved in a ductile manner, but the mode of fracture depended on the testing temperature. In unnotched crystals tested at 77° or 123°K, a shallow neck formed, and fracture occurred by longitudinal cleavage nucleated in the center of the necked region. The effect of stress concentrations on crack nucleation was studied at several temperatures using crystals with deep or shallow notches, or a central hole. Longitudinal cleavage cracks were found to nucleate a small distance below the notch root on each side of the specimen. Crystals containing a central hole deformed in a manner, similar to unnotched crystals. Cleavage was always initiated at a small, spherical non-metallic inclusion. The nucleation of cleavage cracks (longitudinal and transverse) in the center of the necked region in unnotched crystals, below notch roots in notched crystals, and in the vicinity of the hole in crystals containing a central hole, suggest the existence of a critical tensile stress criterion for crack nucleation in 3 % Si-Fe single crystals. Metallographic evidence to support such a criterion is presented.