Phase separation in ternary Co-Gd-Ti liquids

The phase equilibria and the solidification behavior of ternary Co-Gd-Ti (Co ≤35 at.%) alloys have been investigated. The phase transformation and equilibria in the liquid phase were studied in situ for two alloys, Co ₃₀Gd₃₅Ti₃₅ and Co₃₀Gd ₅₀Ti₂₀, by combining electrostatic levitation of the samples with high-energy synchrotron x-ray diffraction (XRD) at elevated temperature. The XRD patterns with two diffuse maxima for molten Co ₃₀Gd₃₅Ti₃₅ give direct evidence for liquid-liquid phase separation in this composition. In contrast, no indication for phase separation in the Co₃₀Gd₅₀Ti₂₀ alloy is detected. Coarsened microstructures, typical for the phase-separating systems, are observed for the Co₃₀Gd₃₅Ti₃₅, Co₂₅Gd_37.5Ti_37.5, Co₁₀Gd ₄₅Ti₄₅ and Co₃₀Gd₂₀Ti₅₀ cast alloys. Our findings suggest that the stable miscibility gap of binary Gd-Ti extends into the ternary Co-Gd-Ti system (up to about 30 at.% Co). Thermodynamic calculations of the ternary Co-Gd-Ti system by the CALPHAD method are in good agreement with the experimental results.