Microstructural evolution and tensile properties of titanium-rich TiAl alloy

Abstract

Several different heat treatments using α-Ti/γ-TiAl phase transformation were employed to a polycrystalline Ti–48at.%Al alloy. The as-cast alloy exhibits a nearly lamellar structure, and the heat treatments in the α + γ two-phase field result in four types of microstructure. (1) Nearly-γ structure, (2) duplex structure with an equal amount of γ grains and lamellar grains, (3) subduplex structure consisting mainly of lamellar grains, with a small amount of γ grains, and (4) a Widmannstätten-type structure in which the α phase precipitates along four {111} habit planes in each γ grain. These microstructures exhibit a variation of grain size in the range 300–50 μm and a volume fraction of γ single-phase grain from 0.07 to 0.83. The room temperature ductility is a strong function of the volume fraction of γ grain, rather than grain size, and the ductility increases from nearly 0 to 1.2% with increasing volume fraction of y grain. A further increase in ductility was obtained in the specimen having the Widmannstätten-type structure.