Development of novel Ti-based alloys with improved thermal-mechanical capability

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Commercially available high temperature Ti-based alloys possess a very attractive combination of low density, creep and fatigue resistance. These materials can, however, only be used up to a maximum temperature of approximately 550°C because of their limited intrinsic oxidation resistance. In this project, new oxidation-resistant Ti-based alloys with a thermal capability of up to 1000°C will be developed. These new materials should exhibit a favorable balance of (i) room temperature ductility/deformability, (ii) creep and oxidation resistance and (iii) low density. The oxidation resistance will rely on the formation of protective Cr-Ta-based oxide layers. In our previous studies, the minimal concentrations of Cr and Ta required for the formation of a continuous protective scale were determined. In the first step of project, ductile Ti-based alloys of different Mo/Ti ratios and a single-phase disordered bcc(A2) microstructure will be developed. The bcc(A2) crystal structure is stabilized by sufficient amounts of Ta, Mo, Cr and Ti. In the second step, the strength of these ductile alloys will be enhanced by (i) B2 precipitation strengthening of the A2 matrix (mimicking the  – ‘ microstructure of Ni-based alloys) or (ii) oxide dispersion strengthening by additions of Y2O3 dispersoids. Finally, in order to further improve the oxidation behavior and to reduce the alloy density, Ta will partially be substituted by Nb in the alloy exhibiting the best mechanical properties from previous steps. A density of below 7 g/cm3 is envisaged for the Ti-based alloys developed in this project. The alloy development concept is guided by thermodynamic modeling based on the CALPHAD (Calculation of Phase Diagram) approach. The feasibilty of the entire alloy concept will be validated by extensive experimental investigations of microstructure, mechanical properties and oxidation behavior.