Abstract
The deformation and fracture of some alloys possessing the B2 structure when ordered was investigated under conditions of bending fatigue at 293° and 77°K by optical and replica electron microscopy. Alloys included beta'-copper-zinc (42.5 to 48.25 at.% zinc), beta'-gold-zinc, ordered and disordered equiatomic iron-cobalt (containing 2% vanadium) and ordered and disordered iron-23.5 at.% aluminium. Cross slip was found to be more frequent with increasing zinc content in binary beta'-copper-zinc alloys, and on addition of gold, but to be reduced by the addition of manganese or by reducing the test temperature. A cross slip mechanism not previously considered for alloys of the B2 structure is proposed. The localisation of fatigue damage near grain boundaries in beta'-copper-zinc is attributed to the stress concentration resulting from the deformation of a polycrystal of strongly elastically anisotropic grains, for which a simple model is presented. The nature of fatigue crack initiation and propagation is rationalised in terms of the changing ease of cross slip with zinc content or temperature. The formation of intense fatigue slip bands in beta'-gold-zinc was found to be consistent with a cross slip process on intersecting {110} planes. The distribution of damage is considered in terms of the plastic and elastic anisotropy of the alloy. The nature and distribution of fatigue damage in. ordered and disordered iron-cobalt alloys emphasises the importance of cross slip in fatigue processes. The cleavage fracture plane of beta'-copper-zinc was determined as {110}, that of ordered and disordered iron-cobalt and iron-23.5 at.% aluminium as, {100} and that of iron-49 at.% aluminium as {111}. These results are considered in terms of broken bond energy densities and anisotropy of bond strengths.