Microstructure of Co precipitation strengthened B2-ordered NiAl

A transmission electron microscopy investigation on the phase decomposition of B2-ordered (Ni,Co)Al supersaturated with Ni and Co has revealed the precipitation of (Ni,Co)^sub 2^Al which has not been expected from the reported equilibrium phase diagram. The (Ni,Co)^sub 2^Al phase has a hexagonal structure and takes a rodlike shape with the long axis of the rod parallel to the 111 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)^sub 2^Al phase. The orientation relationship between the (Ni,Co)^sub 2^Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)^sub p^//(111)^sub B2^ and [ 1 2 1 0]^sub p^//[ 1 10]^sub B2^, where the suffix p and B2 denote the (Ni,Co)^sub 2^Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by the fine precipitation of the (Ni,Co)^sub 2^Al phase.[PUBLICATION ABSTRACT]