Critical Stresses for Slip, Twinning, and Cleavage in Single Crystals of Iron

An X-Ray, optical and metallographic study of the stresses for--and mechanisms of--slip, twinning and fracture in single crystals of iron prepared from decarburized mild steel has been made. The selection of the glide ellipse has been found to be a function of the relative difficulty of atom motions at various positions along a (111); this function being affected by temperature and distribution of stress. A critical shear stress criterion applies. Twinning has been found to occur on (112) type planes and in (111) directions below -170 deg C. as predicted by a critical shear stress law, mutatis mutandis. The twinning stress is greatly affected by slip. Brittle fracture could be accounted for by a critical normal stress theory within wide limits of error. Ductile fractures predominated for specimens where a cleavage plane, (001), was more than 65 degrees axis. Various effects of prestraining were observed.