An Experimental Assessment of the Effects of Heat Treatment on the Microstructure of Ti-47Al-2Cr-2Nb Powder Compacts

A detailed and systematic microstructural characterization has been carried out on a Ti-47Al-2Cr-2Nb (at. pct) intermetallic alloy processed by powder metallurgy (PM). Heat-treatment parameters such as isothermal temperature, holding time, and cooling rate were varied in order to produce a series of near-γ, duplex, and fully lamellar microstructures. These were then quantitatively analyzed in terms of grain size, surface fraction, lamellar spacing, second-phase spatial distribution, and serrated grain boundary morphology. Owing to these extensive quantitative image analyses, several unusual microstructural features occurring in this well-known TiAl-based alloy were identified and assessed. First, a dissolution of the smallest γ grains was emphasized in subtransus conditions as the isothermal temperature or holding time was increased. Second, the competition that occurs between the α ⇒ α + γ transformation and the direct-ordering α ⇒ α ₂ reaction upon cooling from above the α-transus temperature is mainly governed by the reduction in chemical free energy. Third, new grains were found to nucleate upon cooling, which is presumably induced by a minimization of interfacial energy at prior α grain boundaries. Finally, new γ grains were formed as a result of the coarsening of primary γ lamellae under furnace-cooled (FC) conditions. Manuscript submitted November 12, 2007.