Exploring the Phase Transformation in β-Quenched Ti-55531 Alloy During Continuous Heating via Dilatometric Measurement, Microstructure Characterization, and Diffusion Analysis

Phase transformations of the beta-quenched Ti-55531 alloy during continuous heating were studied by combining dilatometric measurement with phase analysis, microstructure observation, and local composition mapping. The corresponding transformation sequence can be verified as beta -> omega, beta + omega -> beta + alpha, beta -> alpha, and alpha -> beta. By revisiting the individual phase transformations in the aforementioned transformation sequence, the variations of macro volume discovered in dilatometer test were reinterpreted based on three mechanisms, i.e., modification of crystallographic symmetry, thermodynamically partitioning, and diffusion kinetics. The interplay of different mechanisms was discussed for individual transformations, especially for the combinative partitioning and diffusion kinetic mechanisms of governed alpha a dagger" beta transformation. The pileups of Al atoms behind the growing alpha/beta interface during beta -> alpha and of Mo atoms in the beta side of alpha/beta interface during alpha -> beta were observed and explained by incorporating the diffusion kinetic mechanism. All of these discoveries and discussions provided a deeper understanding of the phase transformations in Ti-55531 alloy.