Tracking the αʺ martensite decomposition during continuous heating of a Ti-6Al-6V-2Sn alloy

The effect of heating rate on the decomposition kinetics of a¿ martensite is continuously studied for a ß-quenched Ti-6Al-6V-2Sn a + ß 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 a" plates subdivided into domains associated with spinodal decomposition of a", formed during ß ¿ a" 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 a" into stable a and ß phases (a" ¿ a + ß). The onset of this transformation shifts to higher temperatures with increasing heating rate. During the first stage of stabilization, precipitation of fine ß particles occurs along the boundaries of twin related a" plates, while formation of ß and a phases in the interior of a¿ plates is identified at higher temperatures. Early formation of a takes place along boundaries of internal {111} a¿ nano-twins, propagates up the borders of a¿ plates, and can eventually further extend towards internal twins of neighbour martensite plates. Internal nano-twinning of a" plates takes place during a" decomposition owing to stress concentrations associated with structural heterogeneities within a" plates, i.e. spinodal decomposition of a". The results obtained point to a relevant role of twin interfaces during phase precipitation.