Phase transformation kinetics in Ti 575 titanium alloy during heat treatment: Role of the initial microstructure during ageing

Phase transformation kinetics were studied of Ti-575 titanium alloy during cooling from solution annealing and subsequent ageing using in-situ high energy X-ray diffraction. Cooling rates of 1.7, 10 and 30 °C·s−1 were applied from Tβ-60 °C therefore generating several microstructures with different size, morphologies or spatial distribution of α precipitates. Isothermal ageing at 500 °C for 1 h on these three microstructures allowed the study of the influence of the initial microstructural state upon phase transformation during ageing. For this purpose, the combined analysis of mass fraction, aβ mean lattice parameter and full width at half maximum is carried out for both cooling and ageing segment and is presented in this study. Displacive and diffusion-controlled transformations were recorded and seem to be cooling rate dependent, particularly with the precipitation of the α″ orthorhombic phase. The chemical and mechanical evolution of these transformation products will influence the phase transformation kinetics during cooling and following ageing hence conditioning the final microstructure and mechanical properties of this alloy. •High cooling rates lead to α″ orthorhombic phase precipitation.•Capability to retain β-metastable phase at room temperature.•Early age microstructure evolution mainly depends on the initial solute partitioning.