Tracking the α" martensite decomposition during continuous heating of a Ti-6A1-6V-2Sn alloy

The effect of heating rate on the decomposition kinetics of alpha '' martensite is continuously studied for a beta-quenched Ti-6A1-6V-2Sn alpha + beta alloy using fast in situ high energy synchrotron X-ray diffraction. The investigations are complemented with ex situ as well as dynamic metallographic analysis to reveal the mechanisms of phase transformation during critical stages of heating. The initial microstructure consists in a homogeneous distribution of fine acicular alpha '' plates subdivided into domains associated with spinodal decomposition of alpha '', formed during beta -> alpha '' transformation upon quenching. Quantitative phase analysis based on the Rietveld method provides the continuous evolution of lattice parameters, revealing the progression of diffusion-driven processes controlling the decomposition of alpha '' into stable alpha and beta phases (alpha '' -> alpha + beta). The onset of this transformation shifts to higher temperatures with increasing heating rate. During the first stage of stabilization, precipitation of fine beta particles occurs along the boundaries of twin related alpha '' plates, while formation of beta and alpha phases in the interior of alpha '' plates is identified at higher temperatures. Early formation of alpha takes place along boundaries of internal (111) alpha '' nano-twins, propagates up the borders of alpha '' plates, and can eventually further extend towards internal twins of neighbour martensite plates. Internal nano-twinning of alpha '' plates takes place during alpha '' decomposition owing to stress concentrations associated with structural heterogeneities within alpha '' plates, i.e. spinodal decomposition of alpha ''. The results obtained point to a relevant role of twin interfaces during phase precipitation. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.