Deformation attending internal nitridation of Ni-Ti alloys

Abstract

Net volume increase attending precipitation is the source of a number of interesting deformation processes including creep and grain boundary sliding. Grain boundary sliding, a common high temperature deformation process, was observed in Ni-Al but was absent in Ni-Ti and Ag-In. It appears that in Ni-Ti, Ni is preferentially transported outwards along dislocation pipes and some twin boundaries countercurrent to a bulk stress driven vacancy flow (similar to the Nabarro-Herring creep attributed to Ni-Al). Some Ti may be preferentially transported outward by grain boundaries since at compositions near the transition to a superficial scale of TiN outward solute diffusion is significant. A strong element of Ni surface diffusion is involved in redistributing extruded Ni and producing facetted surface features. In respect to Ti diffusion towards the surface the authors have to record a distinction between the process and the classical Wagner models of frontal internal oxidation which applies to Ag-In and Ni-Al.