A 4-frequency model for diffusion via vacancies in AB alloys with the B2 structure was investigated by means of computer simulation. The ratio of the tracer diffusion coefficients of A and B was of particular interest since this ratio was frequently used to justify the 6-jump cycle mechanism. It was shown that this ratio was confined to the well-known limits of about 0.5 and 2 while the long-range order parameter tended to unity and diffusion occurred mainly via the 6-jump cycle mechanism. At lower degrees of order, the possible range broadened greatly and thereby allowed much greater flexibility in the values of the ratio. The behavior was similar for A-rich compositions, but the possible range widened significantly. On the other hand, the range shrank for A-deficient material. It was concluded that, when deducing the diffusion mechanism from the ratio of the diffusion coefficients, it was essential to know the composition and the degree of order.

Limits of the Tracer Diffusion Coefficient Ratio in Highly Ordered Alloys and Intermetallic Compounds having the B2 Structure. I.V.Belova, G.E.Murch: Philosophical Magazine A, 1999, 79[1], 193-202