Philippe Maugis

Thermodynamic and kinetic properties of carbon (C) atoms in body-centered tetragonal iron (Fe) are investigated by using Monte Carlo (MC) simulations. Pairwise interactions between carbon atoms are obtained by combining the linear elasticity theory and the state-of-the-art results from density functional theory (DFT). This energy database is applied to MC simulations to predict the equilibrium carbon configuration and the collective kinetic motion of carbon atoms in an as-quenched martensite. From the metropolis MC simulation, we obtain a novel equilibrium phase of Fe 6 C 2 structure. However, according to the kinetic MC simulations, it is difficult to reach this equilibrium phase during the martensite ageing because, at room temperature or below, the carbon diffusivity is so slow that it will take an unrealistically long time for the system to achieve the equilibrium; and at higher temperature, even though the kinetics are accelerated, the carbon concentration of the predicted equilibrium phase is so high that other metastable carbides can be formed before such equilibrium is reached. Moreover, the effects of the temperature, the applied stress, and the initial state on the ageing kinetics are highlighted. The evolution of the carbon cluster concentration and the time scale of the ageing kinetics are in good agreement with some existing experimental results.

The pairwise interactions of substitutional solute atom X = Al, Si with interstitial carbon at stable (octahedral) and saddle-point (tetrahedral) positions in body-centered-cubic iron (α-Fe) are computed using density-functional theory. These pairwise interactions are used in atomistic kinetic Monte Carlo approach to simulate carbon internal friction and tracer diffusion measurements in Fe-Si, Fe-Al, and Fe-Al-Si ferritic alloys without any adjusting parameters. The good agreement between the simulated and experimental Snoek relaxation profiles validates the pair interaction model for kinetic simulations. The predicted effect of Al on slowing down carbon diffusion is consistent with previous studies. We highlight a super-cell size effect on the Si-carbon interactions obtained from first principles. Using a carefully tested database, it is shown that the introduction of Si into ferrite only decreases the carbon diffusivity below a critical temperature.