Phase tranformations and microstructure evolution in multicomponent gamma titanium aluminides

Phase transformations and microstructure evolution involving the high-temperature beta, alpha or both phases during continuous cooling, as well as during step-quenching + isothermal holding in multicomponent gamma alloys (viz., Ti-(45-47)Al-2Cr/Mn-2Nb-(0-I)Mo-(0-0.3)B) were studied, and the competition in temperature and time space between the various transformation modes, namely, lamellar, cellular, feathery and massive gamma were ascertained. The structure and chemistry of phases present in alloys were determined, and these results were useful for understanding the change in decomposition processes with alloying and temperature. Continuous cooling transformation diagrams were established and detailed microscopy was performed to unravel transformation mechanisms associated with the various non-lamellar transformation modes (i.e. massive, feathery and cellular) and the influence of chemistry. Decomposition of the beta phase is found to take two forms: precipitation of coarse gamma platelets within grain boundary films and as a cellular reaction leading to alternate gamma and beta/B2 lamellae. This reaction is driven by the reduced solubility of Al and increased Cr and Mo solubilities in the beta phase with decreasing temperature. Both the Al level and the concentration of transition elements can significantly alter both the kinetic windows for the various constituents and the ability to form perfect lamellar structures. Reducing the levels of both and additions of boron are beneficial for minimizing unfavorable cellular reactions and promoting the lamellar transformation.