Morphology and interfacial structure of gamma precipitates in the beta phase of a Ti-Al-Nb-Zr alloy

The morphology and interfacial structure of γ precipitates in the β phase of a Ti-44%Al-4%Nb-4%Zr alloy have been studied using transmission electron microscopy. The γ precipitates exhibit an acicular morphology bounded by two principal facets and two minor facets. One of the major facets adopts a commensurate singular interface structure, which forms at an orientation close to the Kurdjumov-Sachs relationship. On an atomic scale these facets adopt a 'step-terrace' configuration, and the coherency strain on the terrace is relieved by the dislocation character of the steps. The other major facet adopts a one-dimensionally incommensurate singular interface structure at the Pitsch orientation relationship. The angular incompatibility between these two singular structures is accommodated by dislocations in the incommensurate interface. The configuration of these dislocations suggests that this is accomplished by nucleation at the facet junctions and subsequent climb along the facets.