Papers by Author: F. Willaime

Abstract: Carbon atoms are always present in Fe-based materials, either as impurities even in high purity samples or as an intrinsic constituent in steels. Density Functional Theory calculations have been performed to study the interaction between C atoms and vacancies (V) in α-Fe. We find that the formation of VCn complexes is energetically favourable for n ≤ 3, with VC2 being the most stable one. The energy gain corresponding to the clustering reaction VCn-1 + C → VCn depends mainly on the strength of C-C covalent bonds. The vacancy diffusivity is shown to be significantly modified by the formation of vacancy-carbon complexes, exhibiting non-Arrhenius behaviour. Effective vacancy diffusion coefficients in α-Fe are calculated as a function of temperature and carbon content using a simplified thermodynamic model. The results are discussed in detail in the limiting case of excess of C with respect to vacancies.

Abstract: The vacancy properties in group-IV hexagonal close-packed metals (Ti, Zr and Hf) have been investigated by Density Functional Theory (DFT) calculations performed with the SIESTA code. The migration energies are found to be systematically lower by »0.15 eV within the basal plane than out of the basal plane. The electronic origin of this significant contribution to diffusion anisotropy is evidenced by the analysis of the local electronic densities of states and by a comparison with and empirical potential. The average value of the migra- tion energy is in very good agreement with available experimental data in Zr. The activation energies for self-diffusion obtained assuming a vacancy mechanism are in good agreement with experiments in Zr and Hf, although slightly too small, but a significant discrepancy is observed in Ti.

Abstract: We have addressed two issues concerning the relative stabilities of various orienta- tions of interstitial clusters in iron by making a comprehensive comparison between four recent empirical potentials. First, we have investigated the effect of finite temperature on the com- petition between clusters made of a few dumbbells oriented along h111i or h110i. We show by quasi-harmonic calculations that h111i clusters have much larger vibrational formation en- tropies and that they are therefore stabilized with respect to h110i clusters at high temperature. Second, we have compared the formation energies of loops with several hundred atoms with Burgers vector 1 2 h111i or h100i. The 1 2 h111i loops are found to be always more stable, but the energy differences with h100i loops depend strongly on the potential.