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 degreesC) have been investigated. Microstructure of the alloy fabricated at 1150 degreesC consists of fine grains of gamma and alpha(2) phases in association with coarse grains of B2 (ordered bee) phase. Metastable omega 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 degreesC under a strain rate of 2 x 10(-5) s(-1). Microstructure of the alloy fabricated at 1250 degreesC consists of coarse grains Of alpha(2) phase and fine grains of gamma and alpha(2) phases. This alloy (containing no 132 phase) on the other hand displayed no low-temperature superplasticity, as recorded from the alloy fabricated at 1150 degreesC. A refined fully lamellar (FL) microstructure was successfully developed within the alloys fabricated at 1400 degreesC. The widths of gamma lamellae are in a range between 100 and 350 run and the widths Of alpha(2) lamellae are in a range between 10 and 50 run. Solute effects on the creep resistance of refined FL-TiAl alloy were critically investigated. (C) 2002 Elsevier B.V. All rights reserved.