Mass-spectrometric determination of the thermodynamic mixing behavior of liquid ternary Fe–Ni–Cr alloys

Abstract

Thermodynamic investigations on liquid ternary Fe–Ni–Cr alloys have been performed by means of the computer-aided Knudsen cell mass spectrometry. Ternary thermodynamically adapted power (TAP) series are used for the algebraic representation of the thermodynamic excess properties. At 1950 K, the molar excess Gibbs energy G E is negative in the predominant part of Gibbs triangle (minimum value: −2980 J/mol at x Ni=0.55, x Cr=0.45), and positive only near the liquid binary Cr–Fe alloys (maximum value: 940 J/mol at x Cr=0.375, x Fe=0.625). The molar heat of mixing H E is exothermic for nearly all Fe–Ni–Cr melts (minimum value: −4770 J/mol at x Fe=0.38, x Ni=0.62). Only the Cr-richest ternary melts near the Ni–Cr binary alloys are slight endothermic (maximum value: 80 J/mol at x Ni=0.125, x Cr=0.87). The molar excess entropy S E is slight negative for ternary Fe–Ni–Cr melts near the two binary boundary systems Fe–Ni and Cr–Fe with a minimum S E value of −1.27 J/mol K at x Cr=0.40 and x Fe=0.60, and slight positive at the side of binary Ni–Cr melts (maximum S E value: 1.35 J/mol K at x Ni=0.50, x Cr=0.50). The Fe-activities of liquid ternary Fe–Ni–Cr alloys as determined, in this work, show slight negative deviations from the ideal behavior in the range of composition below the section of constant mole fraction x Cr/ x Ni=1, and slight positive deviations from Raoult's law above this section line. The Ni-activities of all ternary Fe–Ni–Cr melts show negative deviation from the ideal behavior. The Cr-activities show slight positive deviations from Raoult's law in the range of composition on the left-hand side of the section of constant mole fraction x Ni/ x Fe=1, and slight negative deviations from the ideal behavior on the right-hand side of this section line. The results of this work can be used successfully for phase-diagram calculations.