Calculation of the solute diffusion enhancement factor in B.C.C. 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 D₂(c) = D₂(0) [1 + B₁c + B₂c² + . . .] was taken into consideration. D₂(c) and D₂(0) are the solution diffusion coefficients in the alloy and in the pure solvent, respectively. It is shown that if the absolute value of B₁ > 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 ω₁₂/ω₂ exp (-Δg_p2/kΤ) + 4 ω₂₄/ω₂) exp (-Δg₁/kΤ) + (3 ω₂₁/ω₂ exp (-Δg_p1/kΤ) + 3 ω¹₂₄/ω₂) X exp (-Δg₂/kΤ) + 4 ω₂₃/ω₂ exp (-Δg_p1/kΤ) + 3 ω¹₂₃/ω₂ exp (-Δg_p2/kΤ) - 20, where ω are the jump frequencies, Δg are the additional free energies to form a vacancy at a specific neighboring site and the Ag_p are the negative of the binding energies between two solute atoms.