Orthorhombic phase formation in a Nb-rich $\gamma$-TiAl based alloy – An in situ synchrotron radiation investigation

Novel low aluminium, high niobium containing g-TiAl based alloys, such as Ti-42Al-8.5Nb (at.%), canexhibit an excellent combination of high-temperature strength and ductility. Nano scaled modulatedmicrostructures consisting of lamellae with a tweed substructure are believed to contribute to thebalanced properties. These tweed-like appearing lamellae are an arrangement of multiple stable ormetastable phases with at least one orthorhombic constituent. Due to the crystallographic similarity ofthese orthorhombic phases to the hexagonal a2-Ti3Al and cubic B2 ordered bo-TiAl phases an unambiguousidentification of their crystal structure and formation path is difficult. By using in situ highenergysynchrotron X-ray diffraction (HEXRD) measurements, the nature of the orthorhombic phasesinvolved as well as their formation paths were directly observed and identified in the temperature rangebetween 550 C and 700 C. Calculations and Rietveld analysis based on the in situ data showed that thecrystal structure of the newly formed orthorhombic phase is comparable to that of the O phase butdeviates from the ideal stoichiometric composition (Ti2AlNb). The formation of the orthorhombic phaseout of the a2 phase was monitored in situ