Phase Transformations in 18-Carat Gold Alloys Studied by Mechanical Spectroscopy

This work is motivated by an industrial interest in gaining a better understanding of the phase transformations that govern the mechanical properties of 18-carat gold alloys commonly used in jewelry applications and luxury watchmaking. These alloys fall in one of two categories: yellow gold based on the gold-copper-silver system and white gold based on gold-copper-palladium, but may contain further alloying elements that improve color, castability, strength, and wear resistance. In this thesis, selected alloys from the two series are studied, primarily by mechanical spectroscopy. The analysis and interpretation of the experimental data identifies three important anelastic relaxations (internal dissipation processes), which dominate the mechanical loss spectrum of each of these materials above room temperature. A Zener relaxation, due to directional ordering of atoms in the substitutional solid solution, occurs at intermediate temperatures, between 550 K and 700 K depending on the alloy. Near an order-disorder transition, the Zener relaxation increases markedly in strength when approaching the transition temperature from above, and breaks down when the materials forms the long-range ordered phase below it. In a preliminary study on a Au-Cu alloy (close to the equiatomic composition), this behavior is, for the first time, directly documented by measurements of the mechanical loss in isothermal conditions. It is demonstrated that this experimental method provides a precise value of the transition temperature as well as useful data of the transformation kinetics. The Zener relaxation in yellow gold alloys (of sufficiently high copper content) exhibits the same characteristics. These materials harden because they form an ordered phase of tetragonal symmetry like AuCu. Compared to Au-Cu, the addition of silver reduces the transition temperature. Furthermore, it is concluded that no atomic ordering occurs in the white gold alloys. Above 700 K, the mechanical loss spectrum of 18-carat gold features an anelastic relaxation peak that is shown to be caused by the sliding of grain boundaries. The analysis of this part of the spectrum exposes the age-hardening mechanism acting in some of the white gold alloys. Their composition is such that they form a second phase that precipitates as fine particles. Particles segregating on grain boundaries block the sliding and the grain boundary relaxation peak subsides, leaving only the high-temperature background. The background is created by vibration of bulk dislocations. Precipitates forming inside the grains pin these dislocations, which explains the increased resistance to plastic deformation in the age-hardened state.