INFRARED JOINING OF TIAL INTERMETALLICS USING TI-15CU-15NI FOIL - II - THE MICROSTRUCTURAL EVOLUTION AT HIGH-TEMPERATURE

The microstructural evolution of TiAl joint during infrared joining at 1150 degrees C under different holding times using Ti-15Cu-15Ni foil as brazing filler metal was investigated. Based on the observed micros tructures, a five-step microstructural evolution mechanism at 1150 deg rees C joining temperature is proposed in this study. These time-depen dent evolution steps including (a) beta-Ti layer formation, (b) column ar alpha + beta two-phase zone formation, (c) alpha(2)-phase nucleatio n, (d) high Al% alpha-phase formation and (e) alpha(2)-phase integrati on, are consistent with the multiphase diffusion theories in solid-sta te systems. Since different joining temperatures (T-w) have different corresponding ternary isotherms and stable phases, small variations of T-w can result in significant changes of the microstructural morpholo gies, especially concerning the microstructural evolution of zones of alpha(2)- and the high Al% alpha-phases. The mechanism proposed in thi s study has predicted such evolutions, which agree well with observed microstructures. All the observed microstructures at ambient temperatu res can be clearly elucidated by this proposed mechanism. (C) 1998 Act a Metellurgica Inc.