Structural chemistry and phase relations in intermetallic systems Ti-{Pd,Pt}-Al

Phase relations in the ternary systems Ti-{Pd,Pt}-Al have been experimental ly established for the partial isothermal sections at 950 degreesC in the P d/Pt-poor region (<25 at.% Pd/Pt). The investigation is based on X-ray powd er diffraction, metallography, SEM and EMPA techniques on about 45 alloys, which were prepared by various methods employing are melting, levitation me lting under argon or by powder reaction sintering in closed crucibles. Thre e ternary compounds were observed at 950<degrees>C in the Ti-Pd-Al system: tau (3)-(Ti,Pd)(Ti,Pd,Al)(2) with Laves-MgZn2-type, tau (2)-(Ti,Al)(6)(Ti,P d,Al)(23+1) with a filled Th6Mn23+1-type and tau (1)-(Ti,Pd,Al)(Ti, Pd,Al)( 3) with AuCu3-type. Due to the wide extension of the Laves phase held, ther e,is no compatibility among gamma TiAl and tau (2)-(Ti,Al)(6)(- Ti,Pd,Al)(2 3+1). The Ti Pt Al system at 950 degreesC contains three ternary compounds: tau (3)-(Ti,Al)(Ti,Pt,Al)(2) with Laves-MgZn2-type, tau (2)- (Ti,Al)(6)(Ti ,Pt,Al)(23+1) with the filled Th6Mn23+1-type and tau (1)-(Ti,Pt,Al) with Cu -type. Compatibility exists for Al-rich gamma TiAl and tau (2)(Ti,Al)(6)(Ti ,Pt,Al)(23+1) . The typical feature for both alloy systems studied is the t hree-phase equilibrium: alpha 2Ti3Al + gamma TiAl + tau (3)-(Ti,Pd/ Al)(Ti, Pd/Pt,Al)(2). The solid solubility of palladium and platinum in the binary titanium aluminides, as observed from EMPA and Xray data, is rather small a nd at 950 degreesC accounts to about 2.5 at.% Pd and 2.0 at.% Pt. Two new o xide compounds Ti3PdAl2Ox and Ti3PtAl2Ox with a filled Ti2Ni-type are obser ved in both quaternary systems. (C) 2000 Elsevier Science Ltd. All rights r eserved.