Volume 95, Issue 10

In this paper, we make extensive use of various sum-rules relating the phenomenological transport coefficients in randomly mixed systems in order to address cation chemical interdiffusion and tracer diffusion in strongly ionic crystals such as alkali and silver halides and alkaline earth fluorides, oxides and silicates showing Schottky disorder. We derive general expressions for the intrinsic and inter-diffusivities for the case of unbound vacancies. Next, we derive a general expression relating the chemical interdiffusivity and the cation and anion tracer diffusivities. This expression reduces to the well-known Darken-Manning and Nernst-Planck equations according to the relative magnitude of the anion mobility. This analysis is repeated for the vacancy-pair mechanism. It is shown that the resulting expressions have exactly the same formal structure as for the unbound vacancies case, but the internal structure of the transport quantities differs because of the different correlation behaviour of the vacancy-pair mechanism compared with that of unbound vacancies.

The formation volume of an atomic vacancy has been calculated by molecular statics simulation by the method originally proposed by Johnson and Brown (1962) for some transition metals of the body-centred cubic structure using several interatomic potentials. The values obtained are in satisfactory agreement with the analytic approximations proposed by Maysenhölder (1986). The formation volume is found to be correlated with Poisson’s ratio of the host crystal.

“Order-order” relaxation processes in high-temperature intermetallics occur after an abrupt change of temperature and are controlled by atomic migration in the almost perfect superstructure. The related experiments were carried out using systems being of technological interest and representing three common types of superstructures: L1 2 (Ni 3 Albased quasi-binaries), L1 0 (FePd, FePt) and B2 (NiAl, FeAl). The corresponding Monte Carlo (MC) simulations of “Order-order” kinetics involving the Glauber dynamics implemented with vacancy mechanism for atomic jumps were performed. The studies indicate a crucial role of antisite-easy-diffusion channels offered by particular superstructures in determining the character of “Order-order” processes and their relationship to steady-state self-diffusion. Specific mechanisms of the relaxations in triple-defect B2-ordered binaries are discussed.

Cation diffusion in α-alumina has been the subject of recent investigations, self-diffusion by the radiotracer technique and impurity diffusion by the SIMS technique. A review of the data is carried out to clarify the bulk diffusion mechanisms. Aluminium, cobalt, chromium, and yttrium exhibit a “normal” behaviour, which can be accounted for by an extrinsic vacancy diffusion mechanism. Silver, copper, and platinum behave as “fast diffusers”, although their activation energies are close to those of the other elements. From new experiments on magnesium diffusion reported in this paper, it is suggested that fast diffusion can be traced to self-compensating impurities. A diffusion mechanism which reconciles all the observations is proposed for these elements.