New Insights into the β_o → γ Phase Transformation in an Intermetallic Ti-44Al-7Mo Alloy

Intermetallic γ-TiAl based alloys are innovative structural materials for lightweight high-temperature applications. While previous generations of these alloys were notoriously difficult to deform, some of the latest generation β-stabilized γ-TiAl based alloys have demonstrated significantly improved hot workability, which nowadays even enables conventional forging. In this work, ternary Ti-Al-Mo model alloys are investigated with regard to the influence of β-stabilizing elements, such as Mo, on the phase transformation behavior of this group of γ-TiAl based alloys. Here, in particular, the transformation of a strongly supersaturated, ordered body-centered cubic β_o phase into the ordered tetragonal γ phase is addressed.
Previous studies on the β_o → γ phase transformation in a Ti-44Al-7Mo (at.%) alloy have combined in-situ high-energy X-ray diffraction (HEXRD), high-energy small-angle X-ray scattering (SAXS), and atom probe tomography as a direct imaging technique to study early stages of the γ growth sequence. Specimens were homogenized in the β single phase region at 1450 °C, water-quenched, and subsequently continuously re-heated in a dilatometer setup at beamline P07 at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany. Tracing the diffusional processes and growth kinetics of the γ particles within the supersaturated β_o matrix during these in-situ HEXRD/SAXS experiments, it was found that the growth sequence is controlled by elemental redistribution. Based on the diffraction data, it was suggested that coherent γ precipitates are formed initially, which may lose coherency upon further heating.
Here, we provide first experimental proof of this previous assumption as to the coherency of the precipitates formed. Transmission electron microscopy (TEM) was used to characterize γ precipitates in selected heat-treated conditions in terms of their relationship with the β_o matrix. Combining the TEM results with those gained by means of differential scanning calorimetry, it is shown that, in fact, two competing populations of γ precipitates are involved in the phase transformation. Finally, all experimental results are summarized in a consistent and comprehensive description of the β_o → γ phase transformation.