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.<br/>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.<br/>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.