Monte Carlo simulation of correlation effects in a random bcc alloy

Results of high-accuracy Monte Carlo simulations of vacancy and tracer diffusion in a random bcc alloy A-B are reported. The vacancy and tracer correlation factors are calculated using the Einstein equation and the mean square displacements in the absence of driving forces. Additionally, the vacancy correlation factor is calculated from the vacancy drift in the presence of a driving force. The results are compared with the predictions of Manning's random alloy model. Good agreement is observed only for the vacancy correlation factor f v, and the atomic correlation factors f A and f B in the impurity limits. In the range of concentrated alloys the Manning model markedly overestimates f A and f B even for relatively small exchange frequency ratios such as w A/w B = 5. The percolation behaviour of f v and f A when w A ≫ w B is analysed in detail. The percolation threshold and the critical exponents are reasonably consistent with other Monte Carlo simulations. By comparison with the Monte Carlo results we demonstrate that the basic relations of Manning's model (eqns. (10) and (12)) are not strictly obeyed in concentrated alloys.