RETRACTED ARTICLE: A study on the microstructure of precipitation strengthened B2-ordered NiAl

Abstract

Microstructural control to produce a multiphase structure and there by improve the high temperature strength as well as low temperature ductility of intermetallics has received much attention. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Cr in B2-ordered β-NiAl with different stoichiometry. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and β-NiAl increases appreciably by the fine precipitation of (Ni,Co)₂Al and α-Cr, and overage softening occurs after prolonged aging. In the case of B2-ordered (Ni,Co)Al, the (Ni,Co)₂Al phase has a hexagonal structure and takes a rod-like 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)₂Al phase. The orientation relationship between the (Ni,Co)₂Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)_p//(111)B2 and [$\bar 1$2$\bar 1$0]_p//[$\bar 1$10]_B2, where the suffixes p and B2 denote the (Ni,Co)₂Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by fine precipitation of the (Ni,Co)₂Al phase. On the other hand, in the case of B2-NiAl, perfect lattice coherency is retained at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, a loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface followed by climbing around the particles.