EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Production and machining of titanium alloys are difficult as it involves a complex combination of high loading and rapid temperature evolution. In addition, due to the contact between the cutting tool and the material, the temperature can locally reach 800 °C and above. Consequently, understanding the phenomena behind the phase transformation during machining is scientifically crucial toward not only better machining efficiencies but also to improve the quality of the parts produced. Up to now, literature only provides phase transformation kinetics under thermodynamic equilibrium condition and relatively low heating rates, which are insufficient in describing the case of high heating rate.
These considerations motivate the current study to improve the understanding of phase transformation involved under rapid heating in the Ti-5553 titanium alloy. Phase transformation on heating was tracked in situ with high energy synchrotron diffraction (DESY). Rapid heating at 10, 50 and 100 °C/s from room temperature to 1050 °C were considered. Phase transformation during heating was studied by a combined analysis of the microstructural features that are: mass fractions, mean lattice parameters and full width at half maximum (FWHM) of the $\alpha$ and $\beta$ phases, which were determined by Rietveld refinement. The monitoring of mass fractions compared to equilibrium calculations (ThermoCalc) revealed a shift of the transformation domain toward high temperatures when the heating rate increased. Furthermore, a change in the transformation kinetics was evidenced whose origin was discussed in terms of differences between the nodular and lamellar morphologies of the $\alpha$ phase. The combined analysis of mean lattice parameters and FWHM suggested that the $\alpha$ phase dissolution on heating was controlled by the diffusion of molybdenum with the $\beta$ phase inheriting the solute content of the adjacent parent $\alpha$ phase, leading to chemical heterogeneities in the $\beta$ phase field regardless of the considered heating rate.