Experimental study and computer modelling of the β → α + β phase transformation in Ti15-3 alloy under isothermal conditions

The kinetics of the β → α + β phase transformation in Ti-15V-3Cr-3Al-3Sn (Ti15-3) alloy was studied at 500 °C. A method is proposed and used to estimate the volume fraction of α phase as a function of aging time based on X-ray diffraction (XRD). The method uses the results of calculations of phase equilibria at 500 °C performed using Thermo-Calc. The present work also reports results of calculations of phase equilibria over the temperature range 200-600 °C. In order to validate XRD based approach, select measurements of the quantity of α phase precipitation were done by the use of Field Emission Scanning Electron Microscopy. Results obtained were in reasonable agreement with those obtained using XRD. The kinetics of the β → α + β phase transformation was modelled in the theoretical framework of the Johnson-Mehl-Avrami (JMA) theory and a mechanism based on slow diffusion controlled growth of very fine α plates is proposed. Using the derived JMA kinetic parameters, fraction of transformed α was calculated as a function of aging time; the calculated and experimentally measured values were found to be in good agreement.