Transmission electron microscopy of boron-doped Ni3Al single crystals, oriented for single slip and cyclically deformed at room temperature, revealed a high density of dislocation dipoles and point defect clusters. Observations of circular perfect dislocation loops, Frank loops, vacancy tetrahedra and spherical voids provide evidence of vacancy condensation during fatigue cycling at room temperature. It was suggested that lattice misfit develops between persistent slip bands and matrix as a result of the generation and coalescence of excess vacancies in persistent slip bands. The misfit strain at persistent slip band/matrix interfaces was considered to increase with increasing cumulative plastic strain. Together with scanning electron microscopic observations of surface topography, it was suggested that fatigue damage in Ni3Al single crystals was initiated by the formation of microvoids (micro-cracks) at persistent slip band/matrix interfaces. The microvoids (micro-cracks) break down the coherency of the persistent slip band/matrix interfaces and thereby relieved the accumulated misfit strain at the interfaces. A model of fatigue crack initiation based upon a surface energy criterion was proposed.

A Point Defect Model for Fatigue Crack Initiation in Ni3Al+B Single Crystals. Hsiung, L.M., Stoloff, N.S.: Acta Metallurgica et Materialia, 1990, 38[6], 1191-200