Thermodynamic properties of the alloys {(1− x)Cr+ xMn}

Abstract

Manganese vapour pressures have been measured over the b.c.c. alloys {(1− x)Cr+ xMn} as well as over the alloys with compositions belonging to the two-phase equilibrium fields in the temperature range 1031 to 1434 K and a values of x from 0 to 0.725. A Knudsen-effusion technique under ultra-high neutral vacuum was employed. Manganese and chromium activities and their partial thermodynamic characteristics were calculated from the values of p Mn found. Manganese and chromium activities in b.c.c. solid solutions are characterized by sign-variable deviations, with predominance of positive ones, from Raoult's law. An asymmetric molefraction function of the excess molar Gibbs energy of formation was obtained for the b.c.c. phase, the maximum being displaced toward the manganese corner. The molar formation enthalpies of b.c.c. (chromium + manganese) solid solutions have negative values. The extremal value of Δ f H m was found at x = 0.4 to be equal to −5.3 kJ·mol −1. Coordinates of the stability-field boundaries of b.c.c. solid solutions were computed on the basis of the measured values of manganese vapour pressures over the alloys belonging to the two-phase regions. It was stated that the maximum manganese solubility in chromium approaches a value x of 0.69 at 1277 K. The computed boundary coordinates of the two-phase field (b.c.c.+σ) are in a good agreement with values recommended in the literature. Partial molar thermodynamic functions of Mn in heterogenous mixtures as well as integral thermodynamic properties for the formation of intermediate phases were calculated from the measured values of p Mn in the two-phase regions. Thermodynamic characteristics of formation of liquid alloys were evaluated with the help of the thermodynamic functions for the b.c.c. phase and the positions of the lines of equilibrium of (b.c.c. solid solution + liquid) on the phase diagram recommended in the literature.