Thermodynamic assessment of the titanium aluminum niobium, titanium aluminum chromium, and titanium aluminum molybdenum systems

Two-phase alloys based on a microstructure of disconnected sigma Nb2Al precipitates within a gamma TiAl matrix are promising materials for gas turbine blades because of their expected creep resistance and fracture toughness properties. To optimize alloy microstructures, respective alloys should solidify as single phase beta, and, on aging transform to the two phase microstructures. To extend the high temperature single phase beta field to optimal compositions, beta stabilizers such as Cr and Mo may be used.;The CALculation of PHAse Diagrams (CALPHAD) method is a powerful tool that can be used to guide materials design through the application of computational thermodynamics to the calculation of phase diagrams in multi-component systems. Existing thermodynamic descriptions for the Ti-Al-Nb, Ti-Al-Cr, and Ti-Al-Mo systems could not reproduce experimentally determined phase diagrams; therefore, CALPHAD based re-optimizations of the thermodynamic parameters of the phases in the multi-component system descriptions were required.;The re-optimized description for the Ti-Al-Nb system calculates the experimentally observed extension of the primary crystallization field of the beta phase, the existence of the single phase beta field at sub-solidus temperatures, and the solid state phase transformations and phase transformation temperatures of two experimentally investigated alloys.;Calculations using the new description of the Ti-Al-Cr system are able to reproduce the ternary extension of the Laves phases based on TiCr2, the stoichiometric Tau phase at composition Ti-67 atomic %Al-8 atomic %Cr, the critically evaluated liquidus surface, and isothermal sections at 1073 K and 1273 K.;The continuity of the beta phase to the Al-Mo binary in the Ti-Al-Mo system could only be reproduced through re-optimization of the thermodynamic parameters for the Al-Mo binary sub-system. The new Al-Mo binary description calculates the congruent melting of the beta phase at 50 atomic %Al, and the new Ti-Al-Mo description is in excellent agreement with the extension of the single phase beta field to the Al-Mo binary and the invariant reaction between beta, delta, Al8Mo3, and eta phases at 1540 K.