A MOLECULAR-DYNAMICS STUDY OF TRANSFORMATION TOUGHENING IN THE GAMMA-TIAL BETA TI-V SYSTEM/

The materials evolution in a region surrounding the crack tip was carr ied out using molecular dynamics simulations for the case of a crack i n 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 initia l crack and to prescribe the boundary conditions applied to the comput ational atomistic crystal during the molecular dynamics simulation run s. 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 compare d with the one in the corresponding single phase gamma and single phas e beta materials. The simulation results showed that under the same ap plied 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 clea vage 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 stres s conditions, which are significantly affected by the presence of inte rfacial dislocations, the crystal structure of martensite was found to be either close packed hexagonal, body centered orthorhombic and/or f ace centered orthorhombic. Finally, the implications of crack tip mart ensitic transformation on materials toughness are analyzed in quantita tive terms using the concept of the Eshelby's conservation integral, i .e. the energy release rate.