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 microstructures, a five-step microstructural evolution mechanism at 1150 degrees C joining temperature is proposed in this study. These time-dependent evolution steps including (a) beta-Ti layer formation, (b) columnar alpha + beta two-phase zone formation, (c) alpha(2)-phase nucleation, (d) high Al% alpha-phase formation and (e) alpha(2)-phase integration, are consistent with the multiphase diffusion theories in solid-state 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 morphologies, especially concerning the microstructural evolution of zones of alpha(2)- and the high Al% alpha-phases. The mechanism proposed in this study has predicted such evolutions, which agree well with observed microstructures. All the observed microstructures at ambient temperatures can be clearly elucidated by this proposed mechanism. (C) 1998 Acta Metellurgica Inc.