Microstructure study of a γ-TiAl based alloy containing W and Si

The phase transformations and modulated microstructures in the cast alloy Ti-47Al-2W-0.5Si(at.%) have been studied. It has been shown that microstructure is sensitive to heat treatment and composition. The structure in the alloy heat treated at 1300 degrees C mainly consists of coarse lamellar, fine lamellar, equiaxed gamma grains and beta phase. Homogenizing treatment at 1400 degrees C results in nearly fully-lamellar structure which shows colony-boundary gamma phase grains. The observations showed that small increases in W content result in changes in the amount and morphology of beta phase. For the alloy containing 2.1 at.% W, the beta phase has been observed in three types of morphology: blocky beta phase containing gamma needles and fine silicide particles inside; rod-like beta phase and fine needle-like beta phase. For the alloy variant containing 1.5 at.% W, only a very small amount of rod-like beta phase appeared. Different mechanisms are proposed to explain the occurrence of three types of beta phase, in that W may stabilize it by extending the primary beta phase to lower temperature as well as by yielding new phase zones. A few primary silicide particles have been found in the interdendrite region in the alloys containing more than 0.5 at.% Si, while coherent secondary silicides generally nucleated at the interfaces. (C) 1997 Elsevier Science S.A.