Interatomic potentials for atomistic simulations of the Ti-Al system

Semi-empirical interatomic potentials have been developed for Al, alpha-Ti, and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large database of experimental as well as ab-initio data. The ab-initio calculations were performed by the linear augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide reasonable description of the lattice thermal expansion, demonstrating their usefulness in the molecular dynamics or Monte Carlo studies at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in gamma-TiAl calculated with the EAM potential is in a fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in gamma-TiAl are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that gamm-TiAl is an antisite disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press

MIGRATION OF DIFFERENTLY ORIENTED GRAIN BOUNDARIES MEASURED ON Fe-3wt%Si 37° <001> BICRYSTALS

Nous avons étudié l'effet de l'orientation de la normale au joint de flexion (37° ) sur la migration des joints de grains les bicristaux Fe-3%Si en poids. La vitesse de migration est proportionnelle à la force motrice si la températures et les forces motrices imposées sont suffisamment élevées. L'énergie d'activation de la migration des joints symétriques est égale 3.7 eV/at, tandis que pour les joints asymétriques elle est 1 eV/at seulement.The dependence of the grain boundary migration on the orientation of the boundary surface was investigated on Fe-3wt%Si bicrystals with 37° tilt grain boundary. It was found that the migration velocity is proportional to the driving force at sufficiently high temperatures and for large values of the imposed driving force. The emigration activation energy is 3.7 eV/at for symmetrical boundary while it is only about 1eV/at for asymmetrical one

The Significance of Entropy in Grain Boundary Segregation

The role of entropy in materials science is demonstrated in this report in order to establish its importance for the example of solute segregation at the grain boundaries of bcc iron. We show that substantial differences in grain boundary chemistry arise if their composition is calculated with or without consideration of the entropic term. Another example which clearly documents the necessity of implementing the entropic term in materials science is the enthalpy-entropy compensation effect. Entropy also plays a decisive role in the anisotropy of grain boundary segregation and in interface characterization. The consequences of the ambiguous determination of grain boundary segregation on the prediction of materials behavior are also briefly discussed. All the mentioned examples prove the importance of entropy in the quantification of grain boundary segregation and consequently of other materials properties