Experimental determination and thermodynamic modelling of phase diagrams for high temperature materials

The phase equilibria of the Ta-Al-Co, and Ti-Fe-Mo systems were investigated and modelled in order to explore their potential as base systems for disordered BCC_A2 (β) matrix - ordered BCC_B2, D03, or L21 (β’) precipitate strengthened alloys. A detailed experimental investigation into the phase equilibria of the Ta-Al-Co system was performed, followed by the production of ternary isothermal sections at temperatures of 1000⁰C and 1150⁰C. This system does not contain a disordered BCC_A2 Ta to ordered BCC intermetallic two phase field. In contrast, recent work has established that the Ti-Fe-Mo system does contain a BCC_A2 + BCC_B2 two phase region, which allows the production of β/β’ precipitate strengthened alloys. CALPHAD assessments were performed, producing thermodynamic databases for these two systems. Models were selected to describe the Gibbs energy of each phase in the systems, which were then fitted to experimental phase diagram, and thermodynamic data, to produce a complete thermodynamic description for each system. Phase diagrams calculated from these databases were in good agreement with those plotted based on experimental data. These ternary CALPHAD databases could potentially be adapted and incorporated into existing alloy databases, in order to improve the accuracy of modelling calculations for alloy design and processing applications