Phase equilibria and solid state transformations in Nb-rich Nb–Ti–Al intermetallic alloys

The solidification pathways and phase equilibria at 1100, 900 and 700°C in seven Nb-rich alloys of the Nb–Ti–Al system were examined. The as-cast microstructures revealed that all alloys had undergone β solidification, extending the range of the β phase field further into the ternary system than previously reported. Changes in the atomic site occupancy preferences with alloy composition were examined for the B2 ordered β0 phase through atom location by channeling-enhanced microanalysis (ALCHEMI). A β to δ massive-like transformation observed in cast material, was reproduced in continuously cooled homogenized samples. The formation of a new phase, θ, based on the I41/amd body centered tetragonal structure was observed below 1100°C. In addition, a metastable hP18 structure, produced through an ω-type transformation of the β0 phase, was also observed.