Al- and Ta-Modified Alloy 740 Superalloy Castings Produced in Non-vacuum Conditions—Microstructure and Selected Properties

The work focuses on characterizing the microstructure and selected properties of Al- (1.3 or 2.8 wt pct) and Ta-modified (2 or 3 or 4 wt pct) Alloy 740 Ni-based superalloy after heat treatment. The observations and analyses were conducted using thermodynamic simulations, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, steam oxidation, hardness, tensile, and stress rupture tests. The castings exhibit a typical dendritic structure, with the presence of fine γ′ precipitates, Nb-rich Laves phase precipitates, MC carbides, and MN nitrides are found in all modified castings. However, the presence of the η phase, G phase, and M₂₃C₆ is dependent on the superalloy’s chemical composition. At room temperature, the reference cast superalloy had an ultimate tensile strength and yield strength of 655 MPa and 477 MPa, respectively. In modified castings, the ultimate tensile strength and YS are higher, respectively, 713–1046 MPa and 705–903 MPa. After steam oxidation at 760 °C/1000 h, the mass gain of the modified castings was in the range of 0.339–0.429 mg/cm². The addition of tantalum resulted in decreased oxidation resistance and the formation of the TiTaO₄ oxide at the Cr₂O₃/superalloy interface.