First phase selection in solid Ti/Al diffusion couple

Ti/Al diffusion couples fabricated by hot pressing were annealed at 525, 550, 575 and 600°C. TiAl₃ was the only observed phase at the Ti/Al interface when the unreacted Al foils remained. TiAl₃ grew towards Al foil side. Few Al atoms were detected in Ti foils. The first formation of TiAl₃ is explained on the basis of solubility limits of terminal solid solution, lattice mismatch among Al, Ti and TiAl₃, and the increasing interfacial energy caused by newly formed interface. The first saturation of Al (Ti) solid solution due to the little solubility of Ti in Al, and the slight misfit among the close-packed planes of Al, Ti and TiAl₃, advance the nucleation of TiAl₃. TiAl₃, rather than other compounds, has the lowest increasing interfacial energy, indicating its preferential formation. The formation of other titanium aluminides is suppressed due to their growth which is kinetically unstable.