Diffusion of carbon in substitutionally alloyed austenite

where k and h are the Boltzmann and Planck constants, respectively, W is the number of octahedral interstices around a single such interstice, ~F* is an activation free energy, r m is an activity coefficient and), is the distance betwe-en (002) austenite planes. <1'= 1exp(-wy/kT); Wy is the nearest neighbour carbon-carbon interaction energy in austenite. () is the ratio of the number of carbon atoms to the total number of solvent atoms, given by () = xl/(1 Xl)' Bhadeshia [6] found ~F* /k = 21230 K and In {r m/).2} = 31.84. It clearly is possible for the same equation to be used for austenite containing substitutional solutes as well as carbon, if the sole effect of the substitutional solute is to influence the activity of the carbon. The purpose of this work was to demonstrate that this is indeed the case, using published data [7]. These data reveal quite significant changes in the mobility of carbon as a function of the substitutional solute; some of the data are listed in Table I for illustration purposes. For example, nickel and aluminium have a rather small effect when compared with plain carbon steel, whereas chromium and molybdenum tend to reduce the diffusivity. The effects clearly are significant and were analysed using the Siller and McLellan model. The activity coefficient for carbon in alloyed austenite was calculated using the quasichemical thermodynamic model developed in [8]. In this, the activity of carbon depends on the partial molar enthalpy of solution of carbon in austenite (38575 Jmol-l), and on the partial non-configurational entropy of solution of carbon in austenite (13.48 J mol-l K-1) [8]. In addition, there is a carbon-carbon interaction energy which depends on the substitutional solute as well, and was obtained from [9]. This model is restricted to small concentrations, but diffusion data over the following concentrations (wt %) were