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 an d 1-3.5 at,-% silicon. These results have been used to construct sections o f the ternary phase diagram. Silicon reduces the solubility of aluminium il l both alpha and alpha(2) phases and increases the solubility of aluminium in the beta phase. Extrapolation of the experimental data indicates the exi stence of eutectoid reactions given by beta --> Ti5Si3 + alpha(2) and alpha --> Ti5Si3 + alpha(2). The existence of a beta + alpha(2) phase field at 1 100 degrees C suggests the existence of a beta + alpha + alpha(2) phase fie ld in the Ti-Al-Si system and two possibilities arise to represent a perite ctoid reaction involving these phases: beta + alpha(2) --> alpha and beta alpha --> alpha(2), depending on the alloy composition and on the effect t emperature on the solubility of silicon in the a phase. Finally, results in dicate 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.