Atomistic simulation of point defects and diffusion in B2 NiAl .2. Diffusion mechanisms

In part I of this work we studied point defect energetics in the ordered B2 compound NiAl by means of computer simulations using 'molecular statics' and the embedded atom method. In the present paper we apply the computation technique and results of part I to study atomic mechanisms of tracer self-diffusion in NiAl. We calculate the activation energy of Ni and Al self-diffusion in perfectly stoichiometric NiAl for three atomic mechanisms: the mechanism of next-nearest-neighbour (NNN) vacancy jumps, the 6-jump vacancy mechanism and the 4-ring mechanism. The results of our simulations indicate that self-diffusion in stoichiometric NiAl is dominated by the mechanism of next-nearest-neighbour vacancy jumps. Diffusion of Al by this mechanism is likely to occur more slowly and with a higher activation energy than diffusion of Ni. The mechanism of 6-jump cycles is less favourable but still highly competitive to the NNN vacancy mechanism. The 4-ring mechanism is the least effective for both Ni and Al diffusion. The effect of off-stoichiometry on diffusion in NiAl is briefly discussed.