Alloying and strengthening of gold via rare earth metal additions

Abstract

A sketch map of simplified phase diagrams for Au-RE systems has been plotted and the interactions and the alloying behaviour between gold and the rare earth (RE) metals are summarized in this paper. The solid solubihties of the rare earth metals in gold increase as their atomic number increases. The soUd solubihties and alloying behaviour of yttrium and scandium are close to and similar to those of dysprosium and lutetium, respectively. The factors influencing the solid solubility are discussed. Hume-Rothery and Darken-Gurry proposed that the relationship between atomic size and electronegativity was a criterion for soUd solubifity. This can be revised for the alloying behaviour between gold and RE metals. The maximum solubility of RE metals in gold is greater than 1.0 at% RE when the difference in atomic size is less than 25% and that of the electronegativity is less than 0.7 imits. The relationship between the way in which the intermediate phases that exist in the various Au-RE systems are formed and their respective crystal structures are also summarized. The changes in transformation temperatures of eutectic reactions and the various intermediate phases, expressed as ratios with the melting points of gold or the RE metals, have been studied in relation to the atomic sizes and atomic numbers of the RE metals. These temperature ratios decrease as the atomic numbers of the RE metals increase although those for cerium and ytterbium have anomaloxxsly high values. The trends in the distribution and strengthening effect of RE metals present as trace or dilute solute in gold are discussed.