Limits of the tracer diffusion coefficient ratio in highly ordered alloys and intermetallic compounds taking the B2 structure

In this paper a four frequency model for diffusion via vacancies in AB alloys taking the B2 structure in investigated by computer simulation. Of particular interest is the ratio of the tracer diffusion coefficients D-A./D-B. since this ratio is frequently used among other things to justify the six jump cycle mechanism. It is shown that this ratio is confined to the well known limits of about 0.5 and 2 when the long range order parameter tends to unity and diffusion occurs principally by the six jump cycle mechanism. At lower levels of order however the allowable rang broadens greatly thereby providing much greater flexibility in the values of the ratio. For A-rich compositions the behaviour is similar bur the allowable range widens significantly. Conversely, for A deficient material the range shrinks. For conclusions to be made about the mechanism of diffusion from the ratio of the diffusion coefficients it is essential that the composition and degree of order are known.