Microstructure evolution of Ti48AlχNb γ intermetallics

The microstructures of Ti48Al alloys containing either 2 at.% or 8 at.% Nb have been studied in as-cast, as-HIPped and in heat-treated samples. The as-cast Ti48Al2Nb and Ti48Al8Nb are heterogeneous and lower Nb content has been detected in the interdentritic γ by EDX analysis. Depending on the heat-treatment temperature and cooling rate, the microstructure obtained vary from martensitic structure in the water quenched samples to duplex structure consisting of γ grains and lamellar α2/γ and to fully lamellar structure in the furnace cooled samples. Nb was found to have a high solubility in both α and γ phases and to expand the γ phase region so that a fully γ structure is obtained in the Ti48Al8Nb sample by annealing at 1200°C. A dense array of planar defects (antiphase boundaries, stacking faults and microtwins) was obtained in the Ti48Al8Nb alloy water quenched from 1400°C.