Kinetic Analysis of Uphill Diffusion of Carbon in Austenite Phase of Low-Carbon Steels

To better understand the kinetics associated with the uphill diffusion of carbon (C) in the austenite (gamma) phase of low-carbon steels, the effect of substitutional component M on the intrinsic diffusion coefficient, D-C(gamma), of C in the y phase of the ternary Fe-C-M system was quantitatively analyzed based on thermodyuamics. here, M corresponds to various metals, including Mn and Si. When the concentration, C-M(gamma) of M in the y phase is homogeneous, this effect was found to be negligible. In contrast, the effect was not negligible in the case of C diffusion trapped by M. If C-M(gamma) is inhomogeneous, D. varies depending on the concentration, C-i(gamma) (1 = C and M), in the y phase and the ratio of the gradient of C-C(gamma) to that of C-M(gamma). In this case, D-C(gamma), takes positive or negative values, and the negative value results in the uphill diffusion of C. Using the (Fe C)/(Fe C Si) diffusion couple as an example, the diffusional flux, continuously varies with the distance, r, along the direction normal to the original interface, although the dependence of C-C(gamma) on r is irregular across the original interface. The extent of uphill diffusion can he estimated from the diagonal and off-diagonal diffusion coefficients for the intrinsic diffusion of C and from the value of Ci(gamma)