Thermodynamic Activities in Iron-Cobalt Solid Solutions

概要

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The thermodynamic properties of γ, α and superlattice phases of iron-cobalt alloys were studied from the equilibrium among H_2O, H_2 and the alloys of various compositions. In the temperature region of γ phase, the entropy of mixing and the heat of mixing showed almost ideal behavior, and the activity obeyed Raoult's law. In the α phase, the activity showed negative deviation from Raoult's law ; the heat of mixing was negative (〓H_<max>=-1.8kcal/g atom at 820℃), and the excess entropy was of a large negative value. This anomalous entropy might be considered to show the inapplicability of Neumann-Kopp's law to this alloy system, which is consistent with the fact that the magnetic transformation temperatures of the alloys are different from that of pure iron. The effect of superlattice formation of FeCo was seen in RTln α_<Fe> vs. temperature diagram, that is, the signs of the partial molar configurational entropies of 49.7 atom. per cent Fe and 51.9 atom. per cent Fe alloys were opposite to each other, and the numerical values coincided with the theoretical ones. The heat of α-γ transformation at 50 atom. per cent was obtained from the difference between the heats of mixing above and below the transition temperature.
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