AN ATOMISTIC SIMULATION STUDY OF THE EFFECT OF CRYSTAL DEFECTS ON THEMARTENSITIC-TRANSFORMATION IN TI-V BCC ALLOYS

An atomic level analysis of the effect of crystal defects such as free surfaces, dislocations and grain boundaries on the martensitic transf ormation in bcc Ti, Ti-15V (atom %) and Ti-25V (atom %) has been carri ed out using the embedded atom method (HAM) interatomic potentials to quantify the atomic interactions and molecular dynamics to study the e volution of atomic positions with time. The presence of crystal defect s has been found to have a profound effect on the martensitic transfor mation in these alloys. The defects can either facilitate the bcc --> hcp martensitic transformation, the transformation which is typically observed in these alloys, or promote formation of the martensitic phas e with a face centred orthorhombic (fco) structure. In the case of fre e surfaces, the atomistic simulation results revealed the role these d efects and their crystallographic characters play in the heterogeneous nucleation of martensite and their effect on the progress of martensi tic transformation and the crystal structure of the martensitic phase. In Ti and Ti-15V, where the hcp phase is thermodynamically more stabl e than the bce phase, the presence of dissociate a/2 [111] edge disloc ations was found to facilitate the bcc --> hcp transformation. In Tr-2 5V where the bcc phase is more stable, these dislocations give rise to the formation of dormant hcp martensitic embryos. These embryos are l ikely to be activated under stress or during cooling giving rise to th e bcc --> hcp transformation. The grain boundaries were found to promo te formation of the fco martensite accompanied by significant reductio n of the mismatch stresses in the grain boundary region.