Phase Formation and Microstructural Development During Solid-State Reactions in Ti-Al Multilayer Films

Abstract

ABSTRACTThe phase selection which is generally observed in the early stages of solid-state reactions was studied using Ti-Al multilayer films as a model system.Although all Ti-Al intermetallic phases have similar driving forces of about 30 kJ/g-atom, TiAl3 is the only phase which is formed as long as the reactants are not consumed. The critical thickness beyond which a second phase is formed is larger than 100 μm. We found that the formation of TiAl3 takes place by nucleation and growth, demonstrating that the driving force available for first-phase formation is considerably reduced by a preceding formation of solid solutions. Furthermore, we observed that nucleation continues at later stages, indicating that non-equilibrium conditions are maintained which are possibly influenced by grain-boundary diffusion. However, the phase selection is determined by the different growth velocities, being much higher for TiAl3 than for all other phases.