Solidification behavior of René 77 nickel-based superalloy produced via vacuum lost-wax casting and its resultant microstructure, strength and oxidation resistance

The as-cast microstructure, physico-chemical properties, mechanical characteristics, and oxidation resistance of the René 77 nickel-based superalloy, produced via lost-wax casting, were analyzed. The experiments included thermodynamic simulations, dilatometry, differential scanning calorimetry (DSC), light and scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), scanning-transmission electron microscopy (STEM), hardness and tensile tests, as well as steam oxidation tests. According to DSC analysis, the superalloy began to solidify at 1355 °C forming γ matrix dendrites and, during subsequent cooling, the first strengthening phase, namely MC carbides, precipitated directly from the liquid at 1307 °C. Dilatometry results indicated that the onset of γ' dissolution led to a significant increase in the coefficient of thermal expansion above 730 °C. The dendritic regions of the casting exhibited a relatively uniform microstructure, characterized by near cube-like secondary γ' precipitates, with a volume fraction of 27.7% (± 1.9%) and an average size of 0.278 μm. In the interdendritic spaces, the secondary γ' precipitates displayed both a higher volume fraction (42.2% ± 2.1%) and larger mean size (0.493 μm). Additionally, eutectic γ - γ' islands, (Ti, Mo)C carbides, and (Mo, Cr)₃B₂ borides were detected. The ultimate tensile strength, yield strength and elongation were in the range of 935-1050 MPa, 737-762 MPa and 3.6-9.3%, respectively.