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 regi on 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 be tween a gamma-TiAl phase and a metastable Ti-15V (at %) phase. The cor responding linear anisotropic solutions for the singular stress and di splacement fields were used to both generate the crack in the original crystal and to prescribe the boundary conditions applied to the compu tational crystal during the molecular dynamics simulation runs. The at omic 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 g amma and the one in pure beta crystals. The simulation results showed that under the same applied level of external stress, the crack tip be came blunt and the crack stopped propagating in the gamma-TiAl-beta-Ti -15V bicrystal and in the single beta-phase crystal while the crack ex tended by brittle cleavage in the single-phase gamma crystal. The blun ting process was found to be controlled by the martensitic transformat ion that took place in the beta-phase ahead of the crack tip. Dependin g on the local stress conditions the crystal structure of martensite w as found to be either hexagonal close packed (h.c.p.), body centred or thorhombic (b.c.o.) and/or face centred orthorhombic (f.c.o.). Finally the implications of crack tip martensitic transformation on the tough ness of the materials are analysed in quantitative terms using the con cept of Eshelby's conservation integral, i.e. the energy release rate.