Components of the Thermodynamic Functions of Iron

Abstract

The thermodynamic functions enthalpy, entropy, and free energy of alpha and gamma iron are determined from existing data. These functions are resolved into their magnetic, lattice, and electronic components on the basis of additivity of the respective specific heat components. The total magnetic entropy at the melting point approaches $R\ln \mathrm{}(2s+1)\mathrm{}$, where $s$ is the unpaired spin per atom, indicating the validity of the method of separating specific heat components. A comparison plot of magnetic entropy versus temperature and saturation magnetization versus temperature qualitatively distinguishes the long-and short-range magnetic order. It is shown that the magnetic enthalpy at the melting point is of order $k{T}_c$, where $T_c$ is the Curie temperature. It is further shown that in the absence of magnetic effects the $\gamma$ lattice at absolute zero is more stable than the $\alpha$ lattice by approximately 130 cal/mole. Finally, the components of the free energy are appropriately modified for the iron-rich FeMn alloy in order to determine the phase boundaries.