Phase transformation and microstructure evolution of Ti6Al4V-0.55Fe alloy with different initial microstructure during continuous heating

The phase transformation during continuous heating of Ti6Al4V-0.55Fe alloy with various initial microstructure including equiaxed, duplex and lamellar microstructures was investigated by thermal expansion test. The results illuminate that the β→α phase transition curves of the three samples all present the sigmoidal “S" shape, which proves that this process is controlled by nucleation and growth. The activation energies and Avrami indexes of phase transformation were obtained by Kissinger-Achaia-Sunose (KAS) method and Kolmogorov-Johnson-Mehl-Avrami (KJMA) model. The average activation energies of the phase transitions in the three samples with initial equiaxed, duplex and lamellar microstructures are 163.9, 194.4 and 231.5 kJ/mol, respectively. The result implies that the phase transition is more likely to occur in equiaxed microstructure, which is consistent with the results of sigmoidal “S" shaped curves. Besides, the phase transition mechanism of samples with different initial microstructures presents three different stages, including nucleation and diffusion of crystals, diffusion phase transition, and meaningless numerical interval. Furthermore, the microstructure evolutions of the three samples were analyzed by thermal expansion method at a heating rate of 5 K·min⁻¹. The phase transformation is divided into three stages: lamellar α mainly merges into thicker α phase, thick α transforms into β phase and finally equiaxed α transforms into β phase. The layered α occurs obvious coarsening first and then redissolution phenomena during the α phase transition. The difference in the morphology, size and composition of layered α and primary equiaxed α can affect the phase transformation behavior of the alloy during the heating process.