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, Co30Gd35Ti35 and Co30Gd50Ti20, 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 Co30Gd35Ti35 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, Co25Gd37.5Ti37.5, Co10Gd45Ti45 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. (paper)