On the six-jump cycle mechanism of self-diffusion in NiAl

For a detailed interpretation of recent new experimental results on self-diffusion in NiAl (Divinski SV, Frank St, Herzig Chr, Södervall U. Solid State Fenomena 2000;72:203) molecular static calculations were applied to compute the energy barriers corresponding to general six-jump cycles in NiAl. It was found that the [110] six-jump cycle involves the lowest migration barriers among other possible cycles in the B2 structure of NiAl and has the highest probability to be accomplished. The attempt frequencies of different jumps were calculated within the quasiharmonic approximation. The Monte-Carlo approach with residence–time algorithm was then applied to compute the diffusional correlation effects. The temperature dependence of the Ni diffusion coefficient by the six-jump mechanism was found to obey the Arrhenius law D0exp{−QkT} with D0≅1.3×10−5m2s−1 and Q≅3.12eV in the temperature interval from 800 to 1500 K. These values agree well with our experimental results for 63Ni tracer diffusion Do≅3.6±1.11.6×10−5m2s−1,Q≅3.01±0.04eV, in single crystalline NiAl samples of stoichiometric composition. However, the analysis predicts that the six-jump cycles may be easily broken if the vacancy meets some specific configurations. The resulting contribution of the six-jump cycle mechanism to the total Ni diffusivity was estimated to be smaller than about 30% at lower temperatures (⩽1100K) and perfect stoichiometric composition. With deviation from the stoichiometry and/or increase of temperature the effectiveness of the six-jump cycles decreases rapidly.