A molecular dynamics study of transformation toughening in the gamma TiAl/beta Ti [sbnd]V system

Abstract

The materials evolution in a region surrounding the crack tip was carried out using molecular dynamics simulations for the case of a crack in the gamma TiAl phase impinging at the right angle onto the interface between a gamma TiAl phase and a metastable Ti 15V (at.%) phase. The corresponding linear anisotropic continuum solutions for the singular stress and displacement fields were developed using an enriched finite element method. These solutions were used to both generate the initial crack and to prescribe the boundary conditions applied to the computational atomistic crystal during the molecular dynamics simulation runs. The atomic interactions were described in terms of the appropriated embedded atom method (EAM) type interatomic potentials. The crack-tip behavior for the two-phase gamma/beta material was ultimately compared with the one in the corresponding single phase gamma and single phase beta materials. The simulation results showed that under the same applied level of external stress, the crack tip becomes blunted and the crack stops propagating in the gamma TiAl/beta Ti 15V bicrystal and in the single beta-phase crystal while the crack extends by brittle cleavage in the single-phase gamma crystal. The blunting process was found to be controlled by the martensitic transformation which takes place in the beta phase ahead of the crack tip. Depending on the local stress conditions, which are significantly affected by the presence of interracial dislocations, the crystal structure of martensite was found to be either close packed hexagonal, body centered orthorhombic and/or face centered orthorhombic. Finally, the implications of crack tip martensitic transformation on materials toughness are analyzed in quantitative terms using the concept of the Eshelby's conservation integral, i.e. the energy release rate.