Isothermal sections of the Co–Mn–Nb and Co–Mn–Ta phase diagrams at 1200 K

Abstract The isothermal sections of the Co–Mn–Nb and Co–Mn–Ta systems were constructed at 1200 K by investigations of equilibrated alloys and a diffusion couple using electron probe microanalysis (EPMA), scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), and differential thermal analysis (DTA). In the Co–Mn–Nb system at 1200 K, the existence of the (βMn+γСо+λ) three-phase equilibrium has been established, and the existence of three three-phase equilibria (γСо+λ+λ′), (γСо+λ′+Co3Nb) and (λ+λ′+Co3Nb) has been assumed. In the Co–Mn–Ta system at 1200 K, the existence of the (βMn+γСо+λ) three-phase equilibrium has been established, and the existence of five three-phase equilibria (γСо+λ+λ′), (γСо+λ+Co3Ta), (λ+λ′+Co3Ta), (βTa+λ+Ta2Co) and (λ+μ+Ta2Co) has been assumed. The (λ′) Laves phases of the Co–Mn–Nb and Co–Mn–Ta systems at 1200 K are ternary and crystallise with the MgNi2-type of the crystal structure. The (λ) Laves phases in the Co–Mn–Nb and Co–Mn–Ta systems at 1200 K are the single-phase areas consisting of polytypes based on the MgCu2-and the MgZn2-types of the crystal structures.