Microstructures and properties of powder metallurgy TiAl alloys

The microstructures and properties of powder metallurgy (PM) TiAl alloys fabricated by hot-extrusion of gas-atomized powder at different temperatures (1150, 1250 and 1400 °C) have been investigated. Microstructure of the alloy fabricated at 1150 °C consists of fine grains of γ and α2 phases in association with coarse grains of B2 (ordered bcc) phase. Metastable ω particles of ordered hexagonal phase were observed within some B2 grains. This alloy (containing metastable B2 phase) displayed a low-temperature superplastic behavior, i.e. a tensile elongation of 310% was obtained when the alloy was tested at 800 °C under a strain rate of 2×10−5 s−1. Microstructure of the alloy fabricated at 1250 °C consists of coarse grains of α2 phase and fine grains of γ and α2 phases. This alloy (containing no B2 phase) on the other hand displayed no low-temperature superplasticity, as recorded from the alloy fabricated at 1150 °C. A refined fully lamellar (FL) microstructure was successfully developed within the alloys fabricated at 1400 °C. The widths of γ lamellae are in a range between 100 and 350 nm and the widths of α2 lamellae are in a range between 10 and 50 nm. Solute effects on the creep resistance of refined FL-TiAl alloy were critically investigated.