Transformation toughening in the gamma-TiAl-beta-Ti-V system .2. A molecular dynamics study

Molecular dynamics simulations of the evolution of materials in a region surrounding a crack tip were carried out for the case of a crack in a gamma-TiAl phase impinging at a rig ht ang le onto the interface between a gamma-TiAl phase and a metastable Ti-15V (at %) phase. The corresponding linear anisotropic solutions for the singular stress and displacement fields were used to both generate the crack in the original crystal and to prescribe the boundary conditions applied to the computational crystal during the molecular dynamics simulation runs. The atomic interactions were accounted for using appropriated embedded atom method (EAM) type interatomic potentials. The crack-tip behaviour for the two-phase gamma-beta material was ultimately compared with the one in the corresponding single-phase material, i.e. to the one in pure gamma and the one in pure beta crystals. The simulation results showed that under the same applied level of external stress, the crack tip became blunt and the crack stopped propagating in the gamma-TiAl-beta-Ti-15V bicrystal and in the single beta-phase crystal while the crack extended by brittle cleavage in the single-phase gamma crystal. The blunting process was found to be controlled by the martensitic transformation that took place in the beta-phase ahead of the crack tip. Depending on the local stress conditions the crystal structure of martensite was found to be either hexagonal close packed (h.c.p.), body centred orthorhombic (b.c.o.) and/or face centred orthorhombic (f.c.o.). Finally the implications of crack tip martensitic transformation on the toughness of the materials are analysed in quantitative terms using the concept of Eshelby's conservation integral, i.e. the energy release rate.