Effect of Ta and Nb additions in arc-melted Co-Ni-based superalloys: microstructural and mechanical properties

The current work characterizes the microstructure and mechanical properties of two arc melted Co-Ni-based superalloys containing in atomic percent: 1) Co-40Ni-10Al-7.5W-10Cr-3Ta-0.06B-0.6 C and 2) Co-40Ni-10Al-7.5W-10Cr-3Nb-0.06B-0.6 C. These two materials named 3Ta and 3Nb, respectively, were characterized by X-Ray diffraction analysis, differential scanning calorimetry, scanning electron microscopy, energy dispersive spectroscopy, electron backscatter diffraction and transmission electron microscopy. Furthermore, compression tests were performed from room temperature up to 1000 °C. The 3Ta and 3Nb alloys showed different as-cast microstructures that were confirmed by microstructural characterization and Scheil simulations. The phase transformation temperatures of minor phases were measured and calculated using a thermodynamic approach and some discrepancies observed on the calculated γ′-solvus temperatures reflect that the thermodynamic description of γ′ phase does not take into account the strong partition effect of Ta, Nb and Ti into γ′. As shown by DSC results, the γ′ phase was stable for both alloys up to 1000 °C, being the main reason for the 3Ta and 3Nb alloys to keep the yield stress above 500 MPa.