Calculation of the solute diffusion enhancement factor in BCC metals

The solute enhancement factor B was calculated for b.c.c. alloys containing small concentrations of solute (< 2at%). Only the second term in the equation D-2(c) = D-2(0) [1 + B(1)c + B(2)c(2) + ...] was taken into consideration. D-2(c) and D-2(0) are the solution diffusion coefficients in the alloy and in the pure solvent, respectively. It is shown that if the absolute value of B-1 > 5 the other terms in the above equation cannot be neglected if the convergence of the series is to be maintained. Experimental values for B are considered and are compared with theoretical estimations. The derived equation for B is given by B = (3 omega(12)/omega(2) exp (-Delta g(p2)/kT) + 4 omega(24)/omega(2)) exp (-Delta g(1)/kT) + (3 omega(21)/omega(2) exp (-Delta g(p1)/kT) + 3 omega'(24)/omega(2) x exp(-Delta g(2)/kT) + 4 (omega(23)/omega(2) exp (-Delta g(p1)/kT) + 3 omega'(23)/omega(2) exp (-Delta g(p2)/kT) - 20, where omega are the jump frequencies, Delta g are the additional free energies to form a vacancy at a specific neighboring site and the Delta g(p) are the negative of the binding energies between two solute atoms.