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

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 alpha+beta 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 + alpha structure when heat treated in the temperature range of 100-1100 degrees C showed the gradual transformation of fcc to parent alpha phase and the maximum reversion was observed at temperatures >600 degrees C. The alpha phase was nucleated within the fcc as well as on the fcc/alpha interfaces. Orientation relationship between fcc and hcp phases has been derived using standard [(1) over bar 12](fcc) and [0001](alpha) 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. (C) 2016 Elsevier Inc. All rights reserved.