Phase transition and ordering behavior of ternary Ti – Al – Mo alloys using in - situ neutron diffraction

Neutron diffraction has been used for in-situ investigations to elucidate the phase transformation behavior of two Mo-containing TiAl alloys with compositions of Ti-44Al3Mo and Ti-44Al-7Mo (in at.%). Five different phases are present in these alloys. These include three ordered phases at room temperature, namely a2, b0 and c and two disordered phases, a and b, which occur at higher temperatures. The sequence of the three phase transformations in each alloy has been determined. The phase transformation and disordering/ordering temperatures were determined on heating and cooling from the diffracted peak intensities. The neutron experiments are particularly sensitive to the order – disorder transitions in TiAl alloys, which are compared with the overall phase fractions obtained from previous high energy X-ray diffraction. Hysteresis and undercooling effects are observed for the various phase transformations and depend on the nature of atomic rearrangements.