Kinetics of short-range ordering of substitutional solid solutions (according to data on a scattering of kinds of waves). II. Parameters of atomic microdiffusion within FCC-Ni-Mo

As a result of study of the relaxation of intensity of a diffuse scattering of X-rays bound with the short-range order of a binary f.c.c. alloy, the Fourier components of probabilities of jumps of atoms of both components are defined within the framework of kinetic models of the first and second orders (and determined on an example of solid solution Ni-11.8% at. Mo). With use of these values, the different microscopic parameters of the elementary acts of diffusion are computed. As shown, for the adequate description of a microdiffusion in Ni-Mo alloys, it is necessary to take into account that the 'potential' (force) field caused by composition heterogeneities, inherent to short-range order, is distributed on some (not less than 5-6) co-ordination orbs and is nonmonotonous in dependence on radius of an orb. The probabilities of jumps of atoms within striking distance of nearest and next-nearest site neighbourhood are comparable with each other on the value, and consequently by consideration of a diffusion, it is not allowed to limit oneself to the count of jumps of atoms from the nearest neighbourhood only, as it is traditionally implied at analysis of the vacancy mechanism. In spite of the fact that the kinetic models of the first and second orders qualitatively correctly reproduce character of spatial change of microscopic parameters of the acts of diffusion, their essential difference on the calculated magnitudes of physical quantities takes place. Therefore, correct determination of these parameters is possible only in models of higher orders (at least, third order). The probabilities of atomic jumps are used for definition of the semiphenomenological characteristics of diffusion-coefficients of diffusion and self-diffusion of atoms of both components.