Effect of tetragonal distortion introduced by anti-site defect configuration on additional hardening in off-stoichiometric Al-rich Ni3Al alloys

A theoretical approach to the hardening mechanism of L12–Ni3Al compound with Al-rich off-stoichiometric composition is attempted based on the classical elastic interaction model between an edge dislocation and a unit cell with an equi-atomic composition embedded in Ni3Al. A tetragonal distortion is expected to arise from a local anisotropic atomic configuration in the unit cell which is recognized as a face-centered tetragonal-L10 or distorted B2 NiAl structure around an anti-site Al atom. According to the study by Cochardt et al. (Acta Metall 1955;3:533), it is found that the maximum point force Fm on an edge dislocation from the local L10 unit cell increases with decreasing the ci/ai ratio of the L10 structure toward the stable B2 structure, which is expected from the authors’ recent electronic structure calculations on the stability of L10–NiAl relative to B2-NiAl. This increment of Fm by decreasing the ci/ai ratio accounts for a larger hardening in Al-rich Ni3Al than that in Ni-rich Ni3Al at 77 K.