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

For a detailed interpretation of recent new experimental results on self-di ffusion in NiAl (Divinski SV, Frank St, Herzig Chr, Sodervall U. Solid Stat e Fenomena 2000;72:203) molecular static calculations were applied to compu te the energy barriers corresponding to general six-jump cycles in NiAl. It was found that the [110] six-jump cycle involves the lowest migration barr iers among other possible cycles in the B2 structure of NiAl and has the hi ghest probability to be accomplished. The attempt frequencies of different jumps were calculated within the quasiharmonic approximation. The Monte-Car lo approach with residence-time algorithm was then applied to compute the d iffusional correlation effects. The temperature dependence of the Ni diffus ion coefficient by the six-jump mechanism was found to obey the Arrhenius l aw D-0 exp{-Q/kT} with D-0 congruent to 1.3 x 10(-5)m(2)s(-1) and Q congrue nt to 3.12eV in the temperature interval from 800 to 1500 K. These values a gree well with our experimental results for Ni-63 tracer diffusion [D-o con gruent to (3.6+/-(1.6)(1.1)) x 10(-5)m(2)s(-1), Q congruent to 3.01 +/- 0.0 4eV], in single crystalline NiAl samples of stoichiometric composition. How ever, the analysis predicts that the six-jump cycles may be easily broken i f the vacancy meets some specific configurations. The resulting contributio n of the six-jump cycle mechanism to the total Ni diffusivity was estimated to be; smaller than about 30% at lower temperatures (less than or equal to 1100K) and perfect stoichiometric composition. With deviation from the sto ichiometry and/or increase of temperature the effectiveness of the six-jump cycles decreases rapidly. (C) 2000 Elsevier Science Ltd. All rights reserv ed.