The Enthalpy Effect on Forming Diluted Solid Solutions of Two 4d and 5d Transition Metals

Abstract

AbstractPredictions of the enthalpies of solution in the solid state will be presented for arbitrary combinations of 4d and 5d transition metals. The approach is similar to the one used in a previous paper on combinations of two 4d metals. In addition to the contribution stemming from the chemical interaction, which is equal to the enthalpy of solution in the liquid state, the enthalpy of solution in the solid state contains two other terms. The first of these accounts for the elastic energy arising from the atomic size mismatch, where this size mismatch has been corrected for volume changes that accompany the alloy formation. The second term takes into account the variation of the structure dependent energy in relation to the number of d‐electrons per atom. – The predictions are generally in good agreement with the solubilities found in the respective phase diagrams. For two systems, Zr in Pt and Pt in Nb, that on first sight present exceptions, preliminary experiments indicate that parts of the respective phase diagrams have to be revised. The structure‐dependent contribution manifests itself not only in the limiting solubilities in the main crystal structures of the pure metals, but also in the existence of disordered intermediate phases and may also be responsible for the anomalously large entropy of fusion recently observed for some transition metals.