Formation and reversion of metastable fcc phase in a Ti–5Ta–2Nb explosive clad

Abstract

This paper presents the results of an electron microscopy study on the formation of metastable fcc Ti phase under shock loading conditions in an explosive clad of α+β Ti–5Ta–2Nb alloy and the mechanism and kinetics of its reversion during annealing. The fcc Ti phase with a volume fraction of over 0.6 formed in hcp Ti matrix with a characteristic needle shaped morphology. The biphasic fcc+α structure when heat treated in the temperature range of 100–1100 °C showed the gradual transformation of fcc to parent α phase and the maximum reversion was observed at temperatures >600°C. The α phase was nucleated within the fcc as well as on the fcc/α interfaces. Orientation relationship between fcc and hcp phases has been derived using standard 1−12fccand [0001]α stereographic projections. The kinetics of the reversion process followed diffusion controlled nucleation and growth process with an effective activation energy corresponding to the self diffusion of Ti in hcp matrix.