| Aeronautical jet engine applications require turbine blade materials that are light weight and exhibit good high-temperature mechanical properties. Single-crystal Ni-based super alloys are the common turbine blade materials used today. There is an interest to increase the strength-to-weight ratio of turbine blade materials in order to improve the performance of the engines. Ti-Al-Nb alloys based on γ+σ microstructures are light weight and have shown potential for use in high-temperature turbine engines. Research has shown that through the control of morphology and fraction of the σ and γ phases the mechanical properties of these alloys is improved. The design of alloys and heat-treatments requires an accurate understanding of the high-temperature phase equilibrium between these phases and the β-phase, from which they form. In this study the invariant reactions and high-temperature equilibria were investigated experimentally in the Al-rich corner of the Ti-Al-Nb system. Through a collaborative activity, the phase diagram was optimized using the CALPHAD (Calculation of Phase Diagrams) method, and the results were compared with experimental results.;Equilibrium was examined through evaluation of microstructure, transformation temperatures, composition and structure. The areas that were investigated in this study are the extension of the β-phase field, the invariant reactions involving the L, γ, σ, β and L, η, σ, β phases, high-temperature equilibrium among L, γ, σ, β phases and the instability of the γ-phase upon quenching.;The results of high temperature X-ray diffraction (HT-XRD) confirmed the expansion of the primary β-phase field upon solidification to higher Al contents. This finding allows the design of Nb-rich alloys with a high concentration of Al for high-temperature applications. The invariant reaction involving the L, γ, σ and η phases was found to be a ternary eutectic reaction through the evaluation of two alloys which cross though the invariant plane. The L, γ, σ, β invariant reaction cascades into the L, γ, σ, η eutectic reaction. This somewhat elusive reaction was found to be a ternary peritectic reaction through the evaluation of seven alloys. An isothermal section of the phase diagram at 1510C involving the β, γ, σ, η phases revealed that the γ-phase extends to higher Nb contents than previously reported and retracts by lowering the temperature to 1410C. The γ-phase was found to undergo a spinodal decomposition upon quenching from 1510C. |
