Precipitation of α-Cr in B2-ordered NiAl

Abstract

Abstract A transmission electron microscope investigation has been performed on the precipitation of α-Cr in B2-ordered β-NiAl with different stoichiometry. By aging at temperatures around 1073 K after solution annealing at 1563 K, fine spherical particles of α-Cr appear in the NiAl matrix between the α-Cr which rapidly precipitated during quenching. The hardness of NiAl increases appreciably by the fine precipitation of α-Cr and overage softening occurs after prolonged aging. The perfect lattice coherency is kept at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, the loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface 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.