On the Temperature-Induced Equilibration of Phase Distribution and Microstructure in a Gas-Atomized Titanium Aluminide Powder

Powder production by gas atomization of gamma-TiAl based alloys typically yields a highly nonequilibrium material regarding the occurring phases and their microstructural appearance. In particular, the equilibration of the powder and the associated phase transformations during heating are of great importance for the subsequently applied densification techniques. The present work employs in situ high-energy X-ray diffraction to investigate how this thermodynamic equilibration manifests itself in the resulting phase distribution, the ordering behavior of the disordered alpha and beta phase, both evidenced in the powder, and the change of the gamma lattice parameters during heating of a Ti-46.3Al-2.2W-0.2B (at%) powder up to 850 degrees C. Complementary microstructural characterization of the gas-atomized powder and the heat-treated material condition reveals that the temperature exposure predominately affects the dendritic parts of the microstructure, especially when the a phase is transformed into y phase with small embedded grains of alpha(2) and beta(0).