Phase Diagram of the Ni–Cr–S System at 873 K between 10−4 and 10−8 Pa

Abstract

The phase diagram of the Ni–Cr–S system was determined at 873 Kby sulfidation of thin Ni-Cr alloy sheets (22, 50, 66.6, and 80 at.% Cr) at10^−4.0, 10^−5.5, 10^−7.0,and 10^−8.0 Pa in H₂S–H₂ gasmixtures. X-ray diffraction analysis identified two sulfides,NixCr_3−xS₄ (0<x<1.0) and (Ni,Cr)₃S₂,while concentration profiles were measured by electron-probe microanalysis(EPMA). The compositions of the terminal sulfides and alloys in equilibriumwere determined at sulfur pressures below 10^−5.5 Pa, whereasat 10^−4.0 Pa they were obtained from the Cr₃S₄and Ni₃S₂ phases at their interface. The proposedphase diagram shows that the solubility limit of Cr into Ni₃S₂is only a few atomic percent Cr, while the Ni content in Cr₃S₄decreases rapidly with decreasing sulfur pressures from 14 at.% Ni, a metalratio N_Ni/(N_Ni+N_Cr) of 0.32, at 10^−4.0Pa to a negligible value at sulfur pressures below 10^−8.0 Pa.