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

Phase relations in the ternary systems Ti–{Pd,Pt}–Al have been experimentally established for the partial isothermal sections at 950°C in the Pd/Pt-poor region (<25 at.% Pd/Pt). The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA techniques on about 45 alloys, which were prepared by various methods employing arc melting, levitation melting under argon or by powder reaction sintering in closed crucibles. Three ternary compounds were observed at 950°C in the Ti–Pd–Al system: τ₃-(Ti,Pd)(Ti,Pd,Al)₂ with Laves-MgZn₂-type, τ₂-(Ti,Al)₆(Ti,Pd,Al)₂₃₊₁ with a filled Th₆Mn₂₃₊₁-type and τ₁-(Ti,Pd,Al)(Ti,Pd,Al)₃ with AuCu₃-type. Due to the wide extension of the Laves phase field, there is no compatibility among γTiAl and τ₂-(Ti,Al)₆(Ti,Pd,Al)₂₃₊₁. The Ti–Pt–Al system at 950°C contains three ternary compounds: τ₃-(Ti,Al)(Ti,Pt,Al)₂ with Laves-MgZn₂-type, τ₂-(Ti,Al)₆(Ti,Pt,Al)₂₃₊₁ with the filled Th₆Mn₂₃₊₁-type and τ₁-(Ti,Pt,Al) with Cu-type. Compatibility exists for Al-rich γTiAl and τ₂-(Ti,Al)₆(Ti,Pt,Al)₂₃₊₁. The typical feature for both alloy systems studied is the three-phase equilibrium: ₂Ti₃Al+γTiAl+τ₃-(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 X-ray data, is rather small and at 950°C accounts to about 2.5 at.% Pd and 2.0 at.% Pt. Two new oxide compounds Ti₃PdAl₂O_x and Ti₃PtAl₂O_x with a filled Ti₂Ni-type are observed in both quaternary systems.