Noburnium: Stainless niobium superalloy.

摘要

A systems design process has been used to design a niobium-based superalloy, Noburnium, with mechanical and oxidation resistance properties similar to those of nickel-based superalloys with an increase in use temperature to 1300°C. The alloy design process used first principles calculations to identify a highly stable L2₁ heusler phase, Pd₂HfAl, with lattice parameter mismatch of −4.09% compared to pure niobium. A thermodynamic database was assessed and used to model the stability of the proposed heusler phase with niobium. Prototype alloys successfully demonstrated the stability of Pd₂HfAl, and heat treatments showed the existence of a two-phase field between BCC Nb and Pd₂HfAl. To promote external Al₂ O₃ formation while maintaining the required use temperature, the Wagner equation for critical aluminum content for external alumina formation was applied. From this equation, increasing the ratio D_A1/D _o and decreasing the oxygen solubility were identified as key design parameters. Diffusivity calculations were made using DICTRA software, and identified Pd and Mo as the best elements to increase D_A1/D _o. Where the thermodynamic data was available, the Thermo-Calc database was used to identify Pd and Hf as promising elements to increase the oxygen activity coefficient in Nb and decrease oxygen solubility. Empirical evidence from the literature indicates that increasing the electron/atom ratio (e/a) in the matrix from 5.0 for pure Nb to 5.75 will also minimize oxygen solubility. Using this criterion, the addition of Pd or Ru is recommended to increase e/a. Oxidation resistant prototypes were tested and found to have improved oxidation resistance compared to pure niobium. The data from the prototype analysis was used to refine the existing thermo-chemical database and improve the design models. Values for ΔH and ΔS for Pd ₂HfAl were assessed as −300,000 kJ/mol and −20 kJ/mol, respectively. Using this, another iteration of the design process produced a final alloy composition: Nb-6.9Cr-2.0Mo-18.8Pd-2.5Hf 2.5AI. This alloy includes Cr, Pd and Mo for improved lattice mismatch, Hf for particle strengthening, and Pd and Al for particle strengthening and oxidation resistance. The composition of an alloy containing Ru is discussed qualitatively, and an alloy of composition Nb-6.9Cr-2.0Mo 12.6Pd-2.5Hf-2.5Al-7.6Ru is proposed for further research.