Crystallographic aspects of the βo → γ decomposition in an intermetallic Ti-44Al-7Mo alloy

Essential to many intermetallic systems, the ordered β o (B2) phase is also gaining increasing interest in γ-TiAl based alloys for lightweight high-temperature applications. This study focuses on the crystallographic aspects of β o decomposition during continuous heating of a supersaturated, β-homogenised Ti-44Al-7Mo (at.%) alloy. Using differential scanning calorimetry, transmission electron microscopy (TEM), and in-situ high-energy synchrotron X-ray diffraction (HEXRD), we demonstrate that the β o → γ phase transformation occurs in two stages, involving the precipitation and growth of two distinct γ populations. New experimental evidence on the interfaces and orientation relationships between particles and matrix is provided, including a description of coherent γ particles obeying a Bain-type orientation relationship. Diffuse streaks in reciprocal space (TEM, HEXRD) suggest a displacive instability of the quenched B2 phase, while vectorial streaks during heating (in-situ HEXRD) evidence the appearance of the Kurdjumov-Sachs orientation relationship in the final transformation stage.