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

Abstract

Powder production by gas atomization of γ‐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 α and β phase, both evidenced in the powder, and the change of the γ lattice parameters during heating of a Ti–46.3Al–2.2W–0.2B (at%) powder up to 850 °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 α phase is transformed into γ phase with small embedded grains of α2 and βo.

Alloying Elements in Intermetallic γ‐TiAl Based Alloys – A Review on Their Influence on Phase Equilibria and Phase Transformations

“…[175,177] Especially, α 2 phase was still present in the microstructure of an IRIS alloy heat-treated at 750 °C for 8200 h. [178] Similarly, the microstructure of a Ti-46Al-2 W-0.2B (at%) alloy was found to consist of α 2 , β o, and γ phase at room temperature after an in situ high-energy X-ray diffraction experiment adapted to reproduce a spark plasma sintering (SPS) densification. [179,180] Furthermore, the conducted in situ experiment granted insights into the equilibrium phase transformation pathway during cooling from 1325 °C, i.e., [179] Especially, no α single-phase field region was present in this Ti-46Al-2 W-0.2B (at%) alloy, despite the relatively high content of α-stabilizing Al. Interestingly, the isoplethal section presented in Figure 6 predicts an extensive α single-phase field region for the same amount of Al and a similar amount of β-stabilizing elements Mo and Nb, i.e., a Mo-equivalent of 2 at% compared to the 2 at% W. Thus, further evidence for the strong β-stabilizing effect of W in comparison to Mo is given.…”

“…[ 175,177 ] Especially, α 2 phase was still present in the microstructure of an IRIS alloy heat‐treated at 750 °C for 8200 h. [ 178 ] Similarly, the microstructure of a Ti–46Al–2 W–0.2B (at%) alloy was found to consist of α 2 , β o, and γ phase at room temperature after an in situ high‐energy X‐ray diffraction experiment adapted to reproduce a spark plasma sintering (SPS) densification. [ 179,180 ] Furthermore, the conducted in situ experiment granted insights into the equilibrium phase transformation pathway during cooling from 1325 °C, i.e., α + β → α + β/β o + γ → α + α 2 + β o + γ → α 2 + β o + γ. [ 179 ] Especially, no α single‐phase field region was present in this Ti–46Al–2 W–0.2B (at%) alloy, despite the relatively high content of α‐stabilizing Al.…”