Evaluation of off-diagonal diffusion coefficient from phase diagram information

Abstract

A theoretical investigation in order to clarify the physico-chemical meaning involved in off-diagonal multicomponent diffusion coefficients has been carried out. From a mathematical expression for the diffusion coefficients of interstitial elements in Fe-Metal-C or Fe-Metal-N ternary solid solutions, it was found that the off-diagonal diffusion coefficient of the interstitial element (C) could be evaluated from phase diagram information on isopotential curves of that element resulting in the following relation: $$D_{CM} /D_{CC} = - \left( {\frac{{dy_C }}{{dy_M }}} \right)_{\mu _C } $$ where DCC and DCM are the diagonal and off-diagonal diffusion coefficient of C, respectively, in an Fe-M-C ternary system. This relation has been applied to the Fe-Si-C and Fe-Mn-C ternary fcc alloys in order to evaluate DCSi and DCMn from known values of DCC and experimentally reported isoactivity data of carbon. A computer simulation of well-known Darken’s uphill diffusion in the fcc Fe-Si-C and Fe-Si-Mn-C alloys has been carried out. Based on the good agreement between simulation and experimental data, it is concluded that the above relation can be used as an alternative way to evaluate the off-diagonal diffusion coefficients of interstitial elements in multicomponent diffusion simulation works.