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

COMPUTER SIMULATION OF <101> TILT GRAIN BOUNDARY STRUCTURES IN B.C.C. METALS

Les structures des joints de grains de flexion symétrique ont été étudiés systématiquement pour le cristaux c.c. par de modèles atomiques. Le potentiel interatomique de forme polynômiale utilisé donne les structures c.c. ainsi que les structures c.f.c. ou h.c. stables. Les configurations atomiques des joints de flexion symétriques peuvent se décrire comme une séquence de différents types d'"unités structurales" qui représentent la structure de joints particuliers au "favorisés" selon le schéma de classification [17]. Néanmoins, la structure du joint de flexion {323} imprévue a été découverte. L'énergie faible de la structure AB' de ce joint est reliée à la transformation c.c. - h.c. qui commence dans le joint et répand dans les grains. Il est démontré que les prévisions de ces propriétés de joint de flexion {323} peuvent être vérifié par les résultats expérimentaux pour des métaux et des alliages c.c. ayant l'anisotropie élastiques élevée.The atomic structures of symmetrical tilt grain boundaries in b.c.c. crystals were systematically studied. The polynomial interatomic potential leading to stable b.c.c. as well as close packed structures was used. The symmetrical atomic boundary configurations can be described as combinations of structural units of some special boundaries which are derived from the classification scheme [17]. However, an unexpected behaviour of the Σ11 {323} grain boundary was found. The low energy of the AB' structure of this boundary is related to the b.c.c.-h.c.p. transformation that initiates at the boundary and penetrates into both grains. It is shown that such properties of the {323} grain boundary could be verified experimentally in b.c.c. metals and alloys with high elastic anisotropy

Iron Aluminides and Petr Kratochvíl

Professor Petr Kratochvíl initiated a modern investigation of this class of materials in our country. He has been interested in various methods how to improve iron aluminides by alloying, precipitation and insertion of hard particles. He also restored the results obtained already in fifties of the last century in the development of so called PyroFerAl that was produced in Czechoslovakia and used, for example, for heat treatment installations. Professor Petr Kratochvíl has been cooperating with leading institutions for material research, for instance, with the Institute of Physics of Materials AS CR in Brno on creep studies or with the Technical University in Ostrava on material formation processes. Since aluminium is near the nonmetals on the periodic table, it can bond with metals differently than do other metals and hence the properties of iron aluminides, in particular, are different from the other intermetallics. These alloys can be also used as functional materials due to their magnetic properties. Iron aluminides are being developed for use as structural materials and/or cladding alloys in fossil energy systems. They have good high temperature mechanical properties and excellent corrosion resistance. These alloys offer relatively low material cost, conservation of strategic elements and a lower density than stainless steels, and thus they have a great potential for substituting steels at elevated temperatures. However, a wider use of these materials is partly hampered by their moderate ductility at ambient temperatures

The effect of grain boundaries on plastic deformation of TiAl

The phenomena involving the grain boundaries of two types are discussed for polysynthetically twinned TiAl. The interfaces separating parallel lamellae are special twist boundaries and their influence on plastic deformation is treated on the mesoscopic level. The boundaries between differently oriented lamellae colonies are grain boundaries in a common sense. Their effect on plastic deformation is studied in a model for calculation of compatibility stresses in elastic continuum. Deformation behaviour is examined for crystals composed of parallel lamellae prepared by directional solidification in the direction (so called Al orientation). A chosen bicrystal has the grain boundary parallel to the growth axis and the misorientation of 90°. The effects caused by the stress redistribution and those derived from the properties of crystal defects are compared

Mechanisms of austenite-martensite transition

Martensitic transformations originate from the interaction of the short-wave (atomic plane shuffling) and long-wave (homogeneous deformation) processes. The mutual role of these movements is examined with the aim to elucidate formation mechanisms of the structures with lower symmetry at the atomic level. Generic many-body potentials differing in the stability of basic simple structures is employed in the calculations of the system energies in the course of phase transition

Grain rolling as a mechanism of superplastic deformation

Various deformation processes taking part in superplastic behaviour of fine-grain metallic as well as ceramic materials are critically reviewed. An essential feature of stable large plastic deformations seems to be grain boundary sliding accompanied by stress concentrations at the triple junctions. Grain rolling induced by the shear deformation of grain layers is discussed in detail as the deformation mode which meets the experimental observations of equiaxed grain shape and of strain rate sensitivity close to 0.5. In the limit case of small stresses, the model based on grain rolling turns into diffusional creep.On fait un résumé critique des différents processus qui interviennent au cours de la déformation superplastique des polycristaux à grains fins dans des métaux ou des matériaux céramiques. Il apparaît que le glissement intergranulaire et la concentration des contraintes aux points triples jouent un rôle important dans la déformation superplastique. Le mécanisme de la rotation des grains qui s'effectue par cisaillement des couches granulaires est discuté en détail et correspond à la présence d'une structure équiaxe et à une sensibilité de la contrainte à la vitesse de déformation proche de 0,5. Dans le cas limite des contraintes petites, le mécanisme de la rotation des grains se transforme en fluage diffusion