Phase equilibria and solid state transformations in niobium-rich niobium-titanium-aluminum intermetallic alloys

The phase equilibria and solid state transformations within seven Nb-rich Nb-Ti-Al alloys were investigated. The alloys ranged in composition between 15 and 40 at.% Al with Nb:Ti ratios of 1.5:1 to 4:1. Examination of the as-cast microstructures revealed that all alloys solidified from the β phase field, with subsequent solid-state transformations occurring within four of the alloys during cooling. The range of primary β phase solidification was determined to extend beyond the limits of previous liquidus projections. The high temperature β phase field was verified in each alloy through quenching experiments. The β phase exhibited B2 ordering at room temperature with the order-disorder transition temperatures increasing with Ti concentration. The site occupancy preferences within the β phase were evaluated through the ALCHEMI technique, which determined that Ti substitution occurred for Nb on Nb sublattice sites with the degree of sublattice partitioning found to depend upon alloy composition.; The phase equilibria and transformations that occurred within the alloys were explored over a complete range of temperatures, with experimental isotherms produced at 1400, 1100, 900 and 700°C. Formation of metastable O-Ti ₂AlNb plates occurred within the 15 at.% Al alloys at 700°C, instead of the δ-Nb₃Al phase due to the slow diffusion growth kinetics of the δ phase below 900°C. A new phase with a body centered tetragonal structure belonging to the I4₁/amd space group was discovered in the Nb-24Ti-40Al alloy below 1100°C. This phase formed as lamella with the γ-TiAl phase from the β solid solution at 900 and 700°C, and was also present following treatments at 1100 + 900, 1100 + 700°C and within material oil quenched from elevated temperatures. No correlation existed between this phase and either the r-TiAl₂ phase, which also has the I4₁/amd structure, or the γ1 phase, which was not observed within this work. The formation of a metastable phase with the D8₈ structure through a ω-type transformations from the β phase was also observed for the first time in the Nb-Ti-Al system. In addition, the β→δ massive transformation that was observed within cast Nb-25Ti-25Al was reproduced within furnace and air cooled samples following alloy homogenization.