Enthalpy of mixing of liquid (gallium-gadolinium-silicon) alloys measured by high-temperature isoperibolic calorimetry

The partial enthalpy of mixing of gadolinium (ΔmixH¯Gd) in liquid ternary (gallium + gadolinium + silicon) alloys has been measured by a high-temperature isoperibolic calorimetry at T = (1750 ± 5) K. The measurements have been performed on alloys of five compositions xGa:xSi = 0.15:0.85, 0.3:0.7, 0.5:0.5, 0.75:0.25 and 0.9:0.1. It has been determined that gadolinium is mixed with (Ga-Gd-Si) alloys with a significant heat evolution. Plots of concentration dependence of {ΔmixH¯Gd/(1-xGd)2} for compositions xGa:xSi = 0.15:0.85, 0.3:0.7, 0.5:0.5 and 0.75:0.25 at 0.24 < xGd < 0.44 show sharp inflections. The inflection points correspond to transition of the liquid alloys into the heterogeneous state and can be used for liquidus surface determination. The integral enthalpy of mixing (ΔmixH) was calculated from the experimental ΔmixH¯Gd by Darken's equation. For the six geometric models applied, the Toop model results are the best when compared with the experimental ΔmixH data. Two different methods were applied to extrapolate experimental ΔmixH into the area of overcooled liquid alloys. The ΔmixH values determined by both methods coincide well for the concentrations studied and for the area of extrapolation. The liquid ternary (gallium-gadolinium-silicon) alloys are characterized by significant negative ΔmixH up to −87.93 kJ · mol−1. The significant negative deviations from the geometric model (up to −13.33 kJ · mol−1) testify about chemical short-range ordering of ternary compound type, possibly associated with the existence of ternary compounds in the (Ga-Gd-Si) system.