Microstructure and phase relationships in Ti-Al-Si system

The phase equilibria between beta and alpha titanium, Ti3Al (alpha(2)), and Ti5Si3 have been investigated in the temperature range 700-1200 degrees C, Sot binary and ternary titanium alloys containing 14-22 nt,-% aluminium and 1-3.5 at,-% silicon. These results have been used to construct sections of the ternary phase diagram. Silicon reduces the solubility of aluminium ill both alpha and alpha(2) phases and increases the solubility of aluminium in the beta phase. Extrapolation of the experimental data indicates the existence of eutectoid reactions given by beta --> Ti5Si3 + alpha(2) and alpha --> Ti5Si3 + alpha(2). The existence of a beta + alpha(2) phase field at 1100 degrees C suggests the existence of a beta + alpha + alpha(2) phase field in the Ti-Al-Si system and two possibilities arise to represent a peritectoid reaction involving these phases: beta + alpha(2) --> alpha and beta + alpha --> alpha(2), depending on the alloy composition and on the effect temperature on the solubility of silicon in the a phase. Finally, results indicate that the microstructure resulting from the allotropic decomposition of the beta phase controls the morphology of alpha + alpha(2) and alpha(2) alloys. MST/3972.