In situ observations of lattice expansion and transformation rates of α and β phases in Ti-6Al-4V

In situ X-ray diffraction experiments using synchrotron radiation were performed on Ti-6Al-4V samples to directly observe the alpha --> beta phase transformation during heating. These experiments were conducted at the Advanced Photon Source (APS) using a 30 keV synchrotron X-ray beam to monitor changes in the alpha and beta phases as a function of heating time under different heating rates. The results were compared to computational thermodynamic predictions of the phase fractions versus temperature, providing information about the kinetics of the alpha --> beta transformation in Ti-6Al-4V. The measured transformation rates were shown to be consistent with a diffusion-controlled growth mechanism, whereby diffusion of V in the beta-Ti phase controls the rate. Based on the X-ray diffraction data, real time measurements of the alpha and beta phase lattice parameters were made. Dramatic differences were observed in the changes of the lattice parameters of the two phases during the transformation. These changes are believed to be due to the partitioning of V and its strong effect on the lattice parameter of the beta phase. An unexpected contraction of the lattice parameter of the beta phase was further observed during heating in the temperature range between 500 and 600degreesC. The origin of this contraction is most likely related to the annealing of residual stresses created by the different thermal expansion behaviors of the two phases. Published by Elsevier B.V.