Characterization of controlled microstructures in a γ-TiAl(Cr, Mo, Si, B) alloy

Abstract

This paper summarizes our recent work on microstructure evolution and phase transformations in a Ti–46Al–2Cr–2Mo–0.25Si–0.3B alloy (composition in at%). The microstructures of the alloy in as-cast, as-extruded and heat-treated conditions were characterized by optical, scanning and transmission electron microscopy. Analysis of the as-cast condition revealed the existence of a very fine lamellar microstructure (the colony grain size is about 120 μm and the lamellar spacing within the colonies is about 200 nm). The microstructure of the cast alloy shows evidence of the following solidification and transformation pathway: L→ L+ α → α+β+ γ → lamellar(α+γ)+β+ γ → lamellar(α 2+γ)+B2+γ . After a heat treatment at 1350 or 1400°C for 2 h (i.e. 45 and 95°C above the α-transus temperature of the alloy), the grain size of the as-cast alloy does not increase dramatically but an extremely fine lamellar spacing is observed (about 50 nm) upon air cooling. Hot extrusion at 1290°C has been proven to be an effective way in further refining the microstructure of the alloy. Furthermore, both in the as-cast and extruded condition the alloy shows only a small tendency for rapid grain coarsening at annealing temperatures above the α-transus temperature, a fact which is attributed to alloying effects of Cr, Mo, Si, and B.