Ordering Kinetics and Diffusion in Some L1₂ Alloys

Abstract

A comparison is made of, ordering kinetics for Cu 3 Au, (Tc = 388 0 C), Ni 3 Mn (Tc = 500 0 C) Ni 3 Fe (TC = 517'C) and Ni 3 A1 (Te = 1450 0 C), which all form the L1 2 superlattice, and these kinetics are correlated with diffusion data for the same alloys. Comparing different alloys, it is shown that for a fixed relaxation time for the establishment of order, the lower is the self-diffusivity, Ds, of the slowest-diffusing species, the higher the critical temperature, Tc, for the first appearance of order. Ni 3 A1, in particular, has an estimated D. of about 5 orders of magnitude lower, on average, than the three other alloys. The implications of this inverse relation between Te and Ds are discussed in relation to published models for diffusion in ordered alloys and ordering kinetics. THE RELATION OF DIFFUSIVITIES TO ORDERING KINETICS Formal Kinetic TheoriesThe number of attempts to interpret the kinetics of homogeneous long-range ordering is legion. Bragg and Williams (B * Present address: