Phase transformation pathway and microstructural refinement by feathery transformation of Ru-containing γ-TiAl alloy

The phase transformation sequence and transition temperatures of Ti-48Al-4Nb-2Cr-0.3Ru alloy are studied by using in-situ synchrotron radiation high energy X-ray diffraction, which was confirmed as α → α+γ→ α/α2+β/B2+γ. The α transus and β/B2 solvus temperatures are located at 1390 °C and 1300 °C, respectively. Significantly, the acceleration of Ru element on the precipitation of metastable massive and feathery microstructures was found for the first time. Based on that, a multi-step heat treatment consisting of homogenization treatment, cyclic air-cooling heat treatment, γequiaxed treatment and lamella reprecipitation treatment was designed to optimize the Ti-48Al-4Nb-2Cr-0.3Ru alloy. The coarse microstructure after homogenization was refined by feathery microstructure during cyclic air-cooling heat treatment. Finally, a fine duplex microstructure with γgrain and lamellar colony sized approximately 25 μm and 40 μm, respectively, was achieved. First-principles calculation revealed that the interfacial energies can be decreased when Ru atom occupies the Al sublattice position of true twin and ς5 coincidence site lattice boundaries, which promote the occurrence of metastable transformations. Comparing with the initial microstructure, the tensile properties of the duplex microstructure were highly improved both at room temperature and 800 °C.