Microstructural Evolution and Element Partitioning in the Phase Transformation of Ti-17 Alloy under Continuous Heating and Cooling Conditions

The microstructural evolution and alloying element partitioning in the α + β ↔ β phase transformation of Ti-17 alloy were explored under continuous heating and cooling conditions using the dilatometric method. Scanning electron microscopy and transmission electron microscopy were used to evaluate microstructural characteristics and trace alloying element partitioning behaviors occurring at different temperatures during heating and cooling. Results showed that the finer needle-like α phase first dissolved into the β phase in the matrix with increasing temperature, while the grain boundary α phase first coarsened and then transformed gradually into β phase during continuous heating. The dissolution of α phase of the alloy with the alloying element partitioning during continuous heating was observed. On the contrary, αGB formed at the prior β grain of the alloy during continuous cooling, which might be the nuclei of α colony, thus resulting in the formation of α colony in the matrix. As the temperature decreased, the elements’ concentrations in the α and β phases became increasingly varied due to element partition. Moreover, Al and Cr, which had higher diffusion coefficients than Mo, easily reached the concentration equilibrium of alloying elements in the α and β phases, respectively. The shrinkage of dilatometric curves during heating in the Ti-17 alloy are mainly attributed to the change of α-HCP (hexagonal close-packed) lattice to β-BCC (body-centered cubic) lattice; while the element partitioning during the β → α + β transformation plays an important role in the shrinkage of the dilatometric curves of the Ti-17 alloy during cooling.