Revealing complex planar defects and their interactions at atomic resolution in a Laves phase containing Co-base superalloy

Abstract

In this study, extensive formation of different planar faults and substantial fault intersections were found and examined at atomic resolution in the cubic ZrCo2 based Laves phase formed in a Co-9Al-9W-2Zr alloy after annealing at 900 ºC. By analysis, it is confirmed that synchro-shear is the mechanism to form twins, extrinsic stacking faults and intrinsic stacking faults in the cubic Laves phase with three equivalent Burgers vectors 1/6<112> Shockley partials available for synchro-shear. But atomic shuffling in the dislocation core region is also required to fit the atomic arrangement in perfect and faulted lattices. Novel types of planar faults featured by a missing single Co-atom layer and a constant gap width are for the first time observed. The missing single layer is believed to be induced by the vacancy segregation on planes, especially on {111} twinning planes. The atoms below the gap shuffle into their new positions with different shuffling vectors to form different types of planar faults. Most intersections in the heat-treated specimen are quite gentle. Only occasionally, incident planar faults pass the barrier planar faults, and W is enriched in the intersection regions. In cases of more complex intersections, synchro-shear and atomic shuffling occur in extrinsic and intrinsic stacking faults as well as in twins, and vacancy diffusion and atom shuffling occur in the novel types of planar faults to form the observed intersection configurations. The extensive formation of planar faults might significantly alter the mechanical behavior and also influence functional properties of Laves phase containing materials.