Microstructure and Elevated Temperature Mechanical Behavior of Ti-15Al-33Nb and Ti-21Al-29Nb Alloys

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This work comprises a study of the physical metallurgy of two titaniumaluminum- niobium (Ti-Al-Nb) alloys, Ti-15Al-33Nb and Ti-21Al-29Nb. In particular, the microstructural evolution, tensile, and creep behavior were characterized in detail in order to determine processing-microstructure-property relationships. Monolithic sheet material was produced through conventional thermomechanical processing. The processing for both alloys was performed at subtransus temperatures, which produced fine-grained microstructures and allowed for the control of microstructural features through post-processing heat treatment. These alloys were found to posses four constituent phases: the intermetallic orthorhombic O phase, the intermetallic HCP α2 phase, the ordered intermetallic BCC B2 phase, and the disordered BCC β phase. Grain size and phase volume fractions were quantified for each microstructure produced and phase field ranges were determined. Widmanstatten precipitation of the α2 and O phases from the parent BCC phase was the only phase transformation mechanism observed. The pseudobinary phase diagram based on TiAl and TiNb, for Ti = 50 at%, was modified according to the findings of this study. The tensile properties of the alloys were determined at room temperature and 650°C. All microstructures displayed a loss in yield strength and a gain in ductility at 650°C. The BCC phase is an essential microstructural requirement for these alloys to possess adequate room temperature ductility. The creep behavior was studied for stresses between 50-275 MPa and temperatures between 650-710°C. Deformation mechanisms were proposed based on calculated creep stress exponents and apparent activation energies. The suggested deformation mechanisms were highly dependent on stress and temperature. Coble creep, grain boundary sliding, and dislocation climb were the deformation mechanisms proposed to be active for the stress and temperature range studied. Grain size was determined to be the most important microstructural feature influencing the secondary creep behavior of these alloys.
Niobium, High temperatures, Titanium, Alloys, Aluminum, Mechanical properties, Microstructures