Phase relations in an Fe-Cr-S system at temperatures of 1073 and 1173 K in the sulfur pressure range from 100 to 10?5 Pa

Abstract

Phase relations and stability fields in the Fe-Cr-S system were investigated at 1073 and 1173 K in the sulfur pressure range 100–10−5 Pa. The sulfides, produced by the sulfidation-annealing process of Fe-Cr alloys followed by rapid quenching, were characterized using X-ray diffraction powder analysis at room temperature. The Cr3S4 in the Cr-S system extends beyond the FeCr2S4 stoichiometry in the Fe-Cr-S ternary system at intermediate sulfur pressures. The spinel-monoclinic transition of the FeCr2S4 was observed at sulfur pressures of 10−3–10−3.5Pa at 1073 K and at 100–10−0.5 Pa at 1173 K. The free energy change for formation of the spinel, FeCr2S4, from a monoclinic (Cr, Fe)3S4−y, hexagonal (Fe, Cr)1−xS, and sulfur vapor is given by the relation ΔG = −1523 + 1.09 T (kJ/mol). The phase-transition mechanism of FeCr2S4 is discussed on the basis of an enhancement of the cation coordination numbers from 4-6-6 for the spinel to 6-6-6 for the monoclinic, when the sulfur partial pressure decreases.