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

Abstract

To better understand the kinetics associated with the uphill diffusion of carbon (C) in the austenite (γ) phase of low-carbon steels, the effect of substitutional component M on the intrinsic diffusion coefficient, D_C^γ, of C in the γ phase of the ternary Fe–C–M system was quantitatively analyzed based on thermodynamics. Here, M corresponds to various metals, including Mn and Si. When the concentration, c_M^γ, of M in the γ 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^γ is inhomogeneous, D_C^γ varies depending on the concentration, c_i^γ (i = C and M), in the γ phase and the ratio of the gradient of c_C^γ to that of c_M^γ. In this case, D_C^γ 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, J_C^γ, continuously varies with the distance, r, along the direction normal to the original interface, although the dependence of c_C^γ on r is irregular across the original interface. The extent of uphill diffusion can be estimated from the diagonal and off-diagonal diffusion coefficients for the intrinsic diffusion of C and from the value of c_i^γ.

Fig. 13 Ratio D_C^γ/D_CC^γ versus ratio K for M = Co, Cr, Mn, and Mo, shown as various lines. The corresponding results for M = Si in Fig. 11 are also included as a solid line.