Study of the order disorder transitions and the mechanical properties in the metallic alloys (AuCu and AuCuAg).

Abstract

This work is motivated by a scientific interest in gaining a better understanding of the phase transformations that govern the mechanical properties of gold alloys due to their huge industrial importance such as their common use in jewelry applications, luxury watch making, catalyses, electronic industries ...etc... The selected alloys fall in one of two categories of yellow gold, based on the binary system of GoldCopper and the ternary system Gold–Copper–Silver In this thesis, selected alloys from the two categories are studied, using two principal compositions of 12 and 18-carat. The study starts primarily by the thermal identification of the possible anomalies that could happen during the heating from the room temperature up to 973K (temperature near to the industrial use -1023K). Un expectable anomaly at low temperature has been noticed by the thermal analyses while the primary state before the heating process was the disorder. The fact that has orientate the study to the use of surface techniques in order to determine the relationship of the thermally detected anomalies with the surface aspect of the alloys. The study has lead to highlight new kind of metastable phase transition that precede the ordering reaction in gold alloys at low temperature presented in dendrite shape of precipitated phase on the surface. The participation of the silver in the alloy composition leads to accelerate the ordering reaction. 18-carat binary composition, has been treated in the thorough way in terms of structural study. Using the in-situ temperature XRD measurement with data acquisition frequency of 1 pattern/1.1min. Phase transition sequences and temperatures have been determined over the fitting of the obtained patterns and through the following of the behavior evolution of the intensities of the typical peak of every phase as well as the atomic volume occupancy. The analysis and interpretation of the experimental data stemming from the mechanical spectroscopy reveal three important anelastic relaxations (internal dissipation processes), which dominate the mechanical loss spectrum of the binary 18-carat alloy above room temperature. A Zener relaxation, due to directional ordering of atoms in the substitutional solid solution, occurs nearby the phase transition AuCuI - AuCuII upon heating and the classic Zener relaxation peak accompanies the phase transition A1 - AuCuI + AuCuII upon cooling. An antiphase boundaries relaxation peak has been highlighted as typical peak of the AuCuII. Grain boundaries sliding relaxation peak has been attributed to the pure disordered phase