Experimental investigation and thermodynamic optimization of the Sc-Sb binary system

Abstract

The phase relationships of the Sc-Sb binary system were experimentally investigated using the heat-treated alloys as well as the as-cast alloys by scanning electron microscopy with energy dispersive spectrometer (SEM-EDS), powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Four stable intermetallic phases, Sc₂Sb, Sc₄Sb_2.52, Sc₅Sb₃ and ScSb, are confirmed to exist in the Sc-Sb binary system. The phases Sc₅Sb₃, Sc₄Sb_2.52 and ScSb solidify congruently while the phases Sc₂Sb is formed through the peritectic transformation. There are six invariant reactions in the Sc-Sb system. The liquid compositions of the four eutectic isothermal reactions are approximately 16.3 at.% Sb at 1188 °C for L → α-Sc + Sc₂Sb, 38.3 at.% Sb at 1548 °C for L → Sc₅Sb₃ + Sc₄Sb_2.52, 48.4 at.% Sb at 1442 °C for L → Sc₄Sb_2.52 + ScSb and 88.0 at.% Sb at 575 °C for L → ScSb + Rhom-Sb, and a peritectic isothermal reaction is about 23.2 at.% Sb at 1349 °C for L + Sc₅Sb₃ → Sc₂Sb. The liquid composition of the metatectic isothermal reaction β-Sc → α-Sc + L is 14.39 at.% Sb at 1263 °C. The solubilities of Sb in α-Sc are 3.8, 4.8 and 5.5 at.% Sb at 1000, 1100 and 1200 °C, respectively. According to the experimental heat capacity of the Sc₄Sb_2.52 compound, the Gibbs energy of this compound was firstly determined from 0 to 2000 K. Based on the presently obtained experimental data and the data from literatures, the Sc-Sb system was thermodynamically described and critically assessed by means of the CALPHAD approach. A self-consistent set of thermodynamic parameters was obtained. The calculated results show good agreement with the experimental data.