Stress-induced and atomic ordering in 18-carat Au-Cu-Ag alloys

Atomic ordering and stress-induced ordering (Zener relaxation) were studied in a series of 18-carat (75 wt%) yellow gold alloys (Au-Cu-Ag) by means of isothermal mechanical spectroscopy with a forced torsion pendulum at low frequencies. In the high-temperature solid solution, the Zener peak increases critically according to a Curie-Weiss type law when approaching the transition temperature to the ordered phase from above, then breaks down and disappears as the alloy orders. The two critical temperatures as well as the activation energy for the Zener relaxation are reported for each alloy of the series. The composition dependence of the two ordering phenomena is discussed, including the aspect of phase separation into a copper-rich and silver-rich phase as predicted by the ternary phase diagram.