Self-Diffusion and Interdiffusion in γ Solid Solutions of the Iron-Manganese System

Abstract

The self-diffusivities of iron and manganese, DFe* and DMn*, in Fe-Mn alloys containing from 1.04 to 33.98 at% Mn were determined in the γ phase (FCC) region at 990∼1240°C by the residual activity method. At the same time, the interdiffusion coefficients, \Tilde{D}, were determined by the Matano method in the temperature range between 850 and 1300°C, using diffusion couples of pure iron and 33.98 at% Mn alloy.It was shown that DFe* is smaller than DMn* in the experimental concentration and temperature ranges. The Kirkendall markers in the interdiffusion couple were found to move towards the higher Mn-concentration side. The intrinsic diffusion coefficients, DFe and DMn, for the composition in the vicinity of 24 at% Mn were calculated, and it was also shown that DFe is smaller than DMn.The values of \Tilde{D} for 1177°C calculated using the experimental values of DFe*, DMn* and the activity coefficients applying Darken’s equation were shown to be in fairy good agreement with the experimental values of \Tilde{D} in the composition range lower than 20 at% Mn. Moreover, for \Tilde{D} in the composition range higher than 20 at% Mn, a better agreement between them was also obtained taking into account the effect of vacancy flow.For each concentration, the activation energy for self-diffusion of iron, QFe* was shown to be larger than that of manganese, QMn*, and it was found that QMn*<\Tilde{Q}<QFe* in the whole concentration, where \Tilde{Q} is the apparent activation energy for interdiffusion. QFe*, QMn* and \Tilde{Q} showed the minimum value at the respective compositions lower than 10 at% Mn. The calculated values of QFe* and QMn* using the empirical equation proposed by Toth et al. were in good agreement with the observed values. The apparent activation energy for interdiffusion \Tilde{Q}, was considered as the total of three components: temperature dependence of thermodynamic factor, temperature dependence of the effect of vacancy flow, and temperature dependence of DFe* and DMn*. It was shown that the vacancy flow produced an effect which decreased the calculated value of \Tilde{D} in the whole range of composition.The activation entropy ΔS* for self-diffusion which was calculated from the experimental values of the frequency factor D0*, was shown to be expressed by ΔS*=α(Q*⁄Tm)+ΔS0, where Tm is the melting temperature, ΔS0 is a negative term of the activation entropy which arises from the altered vibrational frequencies of the atoms constituting the saddle point configration, and α is a constant.