Coupled effects of cooling rate and composition on phase evolution in PREP-produced Ti–46Al–8Nb powders

The microstructural and phase evolution of PREP-produced Ti-46Al-8Nb powders was investigated over a wide particle size range (15-180 μm) using SEM, high-energy synchrotron radiation diffraction and EBSD. All powder particles solidify as single β grains, indicating that solidification proceeds via heterogeneous nucleation followed by rapid growth that fills the entire droplet. The subsequent β→α transformation exhibits a strong particle-size dependence. With decreasing cooling rate, the transformation evolves from diffusionless martensitic transformation in fine particles (≤45 μm), to a mixed mode in intermediate particles (45-125 μm), and finally to diffusion-controlled nucleation and growth in larger particles (≥125 μm). The formation pathway of the γ phase also varies with particle size. In fine particles, γ forms directly from β via massive transformation, whereas in larger particles, it forms within α grains through α→γ massive transformation. In addition to the cooling rate, particle-to-particle compositional variation plays an important role. Significant compositional variation in fine particles, inherited from segregation in the parent alloy, leads to the coexistence of multiple phase constitutions within the same size range. As particle size increases, this variation decreases, leading to a more uniform microstructure. The lattice parameters of the α phase are governed not only by the average composition but also by its distribution. Overall, the microstructural and phase evolution in PREP-produced Ti-46Al-8Nb powders is controlled by the coupled effects of cooling rate and composition. These results provide insight into optimizing powder production and developing processing strategies for powder metallurgy and additive manufacturing of TiAl alloys.