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

Abstract

Novel low aluminium, high niobium containing γ-TiAl based alloys, such as Ti-42Al-8.5Nb (at.%), can exhibit an excellent combination of high-temperature strength and ductility. Nano scaled modulated microstructures consisting of lamellae with a tweed substructure are believed to contribute to the balanced properties. These tweed-like appearing lamellae are an arrangement of multiple stable or metastable phases with at least one orthorhombic constituent. Due to the crystallographic similarity of these orthorhombic phases to the hexagonal α2-Ti3Al and cubic B2 ordered βo-TiAl phases an unambiguous identification of their crystal structure and formation path is difficult. By using in situ high-energy synchrotron X-ray diffraction (HEXRD) measurements, the nature of the orthorhombic phases involved as well as their formation paths were directly observed and identified in the temperature range between 550 °C and 700 °C. Calculations and Rietveld analysis based on the in situ data showed that the crystal structure of the newly formed orthorhombic phase is comparable to that of the O phase but deviates from the ideal stoichiometric composition (Ti2AlNb). The formation of the orthorhombic phase out of the α2 phase was monitored in situ.