Effect of Extrusion Temperature on the Microstructural Development of Powder Metallurgy Ti-47A1-2Cr-1Nb-1Ta Alloy

Abstract

Effect of extrusion temperatures on the microstructural development of a powder metallurgy (PM) Ti-47Al-2Cr-1Nb-1Ta (at. %) alloy has been investigated. Microstructure of the PM alloy extruded at 1150 C consists of a fine-grained ({gamma} + {alpha}{sub 2}) two-phase structure in association with coarse grains of metastable B2 (ordered bcc) phase. In addition, fine {omega} (ordered hexagonal) particles are also found within some B2 grains. The PM alloy containing the metastable B2 grains displays a low-temperature superplastic behavior, in which a tensile elongation of 310% is obtained at 800 C under a strain rate of 2 x 10{sup -5} s{sup -1}. It is suggested that the decomposition of metastable B2 phase and microstructural evolution during the deformation play a crucial role in the low-temperature superplasticity of the PM TiAl alloy. A refined fully-lamellar (FL) microstructure with alternating {gamma} and {alpha}{sub 2} lamellae is developed within the PM alloy extruded at 1400 C. The creep resistance of the refined FL-TiAl alloy is found to be superior to those of the TiAl alloys fabricated by conventional processing techniques. Creep mechanisms for the PM alloy with a refined FL microstructure are critically discussed according to TEM examination of deformation substructure