Crystal structure and morphology of Co precipitates in B2-ordered (Ni,Co)Al

Transmission electron microscopy and electron diffraction studies were carried out in order to clarify the crystallographic properties of strengthening precipitates in B2-ordered (Ni,Co)Al supersaturated with Ni and Co. The precipitates observed in specimens aged at different aging temperatures are Co phase which has different crystal structure corresponding to different aging temperature. The Co phase has a face centered cubic (f.c.c.) structure at aging temperature higher than 973 K and the shape is rod whose long axis is parallel to the <111> direction of the matrix. The orientation relationship between the f.c.c.-Co precipitates and the B2-(Ni,Co)Al matrix is found to be the Kurdjumow-Sachs (K-S) orientation relation. By aging at temperatures below 873 K, the precipitates of Co phase take a hexagonal close packed (h.c.p.) structure and are rod in shape with the long axis parallel to the <111> direction of the matrix. The orientation relationship between the h.c.p.-Co precipitates and the B2-(Ni,Co)Al matrix is found to be the Burgers orientation relation. (Ni,Co)Al hardens appreciably by the fine precipitation of both the f.c.c.-Co phase and h.c.p.-Co phase. (C) 1997 Elsevier Science Limited.