Precipitation of alpha-Cr in B2-ordered NiAl

A transmission electron microscope investigation has been performed on the precipitation of alpha-Cr in B2-ordered beta-NiAl with different stoichiome try. By aging at temperatures around 1073 K after solution annealing at 156 3 K, fine spherical particles of alpha-Cr appear in the NiAl matrix between the alpha-Cr which rapidly precipitated during quenching. The hardness of NiAl increases appreciably by the fine precipitation of alpha-Cr and overag e softening occurs after prolonged aging. The perfect lattice coherency is kept at the interfaces between the alpha-Cr particles and the matrix during the initial stage of aging. After prolonged aging, the loss of coherency o ccurs by the attraction of matrix dislocations to the particle/matrix inter face followed by climbing around the particles. The amount of age hardening of stoichiometric NiAl in which equal amounts of Ni and Al are replaced by Cr is smaller than that obtained in off-stoichiometric NiAl. Atom location by channeling enhanced microanalysis technique has been used to determine the site occupancy of Cr in NiAl. (C) 1998 Elsevier Science Ltd. All rights reserved.