Liquidus projection of the Cr–Nb–Ta system

Phase equilibria data for the ternary Cr–Nb–Ta system are essential for the development of high-temperature structural materials, as these three refractory metals are promising alloying elements in various multicomponent systems. Among the numerous AB 2 Laves phases (with C14, C15 or C36 structures), NbCr 2 and TaCr 2 are particularly promising due to their superior corrosion and oxidation resistance, low density, and high microstructural stability. To expand the existing literature on this system, the liquidus projection of the Cr–Nb–Ta system is proposed for the first time in the present work. Experimental investigations were conducted through microstructural characterization of nineteen as-cast alloys using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results revealed four primary solidification regions: the BCC (Cr) phase field, the C15 and C14 Laves phase fields, and the BCC (Nb,Ta) phase field, the latter being dominant. No ternary compounds were detected. Two class II ternary invariant reactions were proposed: L + C 14 ↔ C 15 + (Nb,Ta) (U I ), and L + C 14 ↔ C 15 + (Cr) (U II ).