Isothermal sections of the Co-Ni-Ta phase diagram at 1200 and 1375Â K

Isothermal sections of the Co-Ni-Ta system have been investigated at 1375 K and 1200 K by characterization of the equilibrated alloys with the help of electron probe microanalysis (EPMA), scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), differential thermal analysis (DTA) techniques. It has been determined by the method of X-ray diffraction on specimen powder that the region of α-phase ((Ni,Co)3Ta) existence observed in previous investigations consists of three phases: α (structural type TiAl3), and two ternary phases - α′ (structural type Mg3Cd) and α″ (structural type BaPb3). Phases form eight three-phase equilibria in the Co-Ni-Ta system at 1375 K: α+α′+γ, α+α′+Ni2Ta, α′+α″+γ, α′+α″+λ, α′+λ+μ, α′+μ+Ni2Ta, α″+γ+λ and γ+λ+λ′. For the first time existence of six three-phase equilibria has been established in the Co-Ni-Ta system at 1200 K: α+α′+γ, α+α′+Ni2Ta, α′+α″+γ, α′+α″+λ, α′+λ+μ and α′+μ+Ni2Ta.