Thermodynamics of the structural modifications of AgZn

Abstract

Heats of formation of the three structural modifications of AgZn, the equilibrium b.c.c. β and hexagonal ζ phases and the quenched metastable ordered b.c.c. β′ phase, have been measured by means of liquid tin solution calorimetry. The bonding energies of the ζ and β′ phases were found to be approximately equal, the heat of formation of β′ being actually 50 calories more exothermic than that of the stable ζ phase at 324°K. The relative stability of the ζ phase is ascribed to its higher entropy resulting from the fact that it is only partially ordered. From an estimate of the entropy difference between the two phases the free energy of the ζ phase was calculated to be only 190 cal/g atom less than that of the β′ phase at 324°K. The results of the thermodynamic considerations have been correlated with previously reported structural data for the β and ζ phases. Differences in the nature of the ordering in the β and ζ phases have been analyzed, leading to conclusions which appear to contradict previously expressed views regarding the mechanism of the ζ ζ β transformation.