Kinetic and thermodynamic description of intermediary phases formation in Ti-Al system during reactive sintering

Abstract

Reactive sintering is currently considered as a promising production route for titanium aluminides in many research works. However, the published descriptions of the reaction mechanism are contradictory or lacking, especially at the temperatures below the melting point of aluminium. This work aims to fill this gap, providing the description of the reactive sintering process at the temperatures between 400 and 900 °C. The phases’ formation sequence and reaction kinetics were studied and explained using experimental model (Ti/Al diffusion couple) and real reactively sintered samples of equiatomic Ti-Al compressed powder blend. Moreover, phase formation was thermodynamically assessed. It was revealed that Ti2Al5 phase formed preferentially. This phase has not been reported previously as a starting phase in reactive sintering. According to results obtained by experimental model, its formation is controlled by diffusion at 700 °C. This phase reacted with aluminium forming pure TiAl3 phase or with titanium, resulting in TiAl phase. Subsequently, TiAl phase reacted with titanium, leading to the Ti3Al phase, or with already present Ti2Al5 phase yielding TiAl2 intermetallic compound. Titanium-rich Ti3Al phase could form only at the temperature of 600 °C or above.