An atomistic simulation study of the effect of crystal defects on the martensitic 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 transformation in bcc Ti, Ti-15V (atom %) and Ti-25V (atom %) has been carried out using the embedded atom method (HAM) interatomic potentials to quantify the atomic interactions and molecular dynamics to study the evolution of atomic positions with time. The presence of crystal defects has been found to have a profound effect on the martensitic transformation 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 phase with a face centred orthorhombic (fco) structure. In the case of free surfaces, the atomistic simulation results revealed the role these defects and their crystallographic characters play in the heterogeneous nucleation of martensite and their effect on the progress of martensitic transformation and the crystal structure of the martensitic phase. In Ti and Ti-15V, where the hcp phase is thermodynamically more stable than the bce phase, the presence of dissociate a/2 [111] edge dislocations was found to facilitate the bcc --> hcp transformation. In Tr-25V where the bcc phase is more stable, these dislocations give rise to the formation of dormant hcp martensitic embryos. These embryos are likely to be activated under stress or during cooling giving rise to the bcc --> hcp transformation. The grain boundaries were found to promote formation of the fco martensite accompanied by significant reduction of the mismatch stresses in the grain boundary region.