Produzione e caratterizzazione di una lega di oro a memoria di forma

In this work we present a phase transformations study occurring in an 18 carat alloy similar to the commercial SpanGold. The Au76Cu18Al6 alloy shows, after thermal treatments, the formation of an irregular roughness giving peculiar reflection of the incident light. Jewels prepared using this material show a very interesting sparkling effect, “spangling”, of surfaces from which the name “SpanGold”. Samples as been prepared from pure elements and characterized by Differential Scanning Calorimetry (DSC), X-Ray Diffraction (XRD), Optical and Electron Microscopy (OM and SEM) and nanoindentation tests, to find the exact range of temperature of transformation and identify the phases present. It has been verified that the formation of tiny lamellae is a direct consequence of the thermal treatments applied; for example, a solutioning at high temperature (>600°C) followed by an annealing at low temperature (>100°). The crucial step of the transformation seems to be the ageing at low temperature. Various quantities of parent phase are retained under different conditions so the morphology, and the shape of the irregularities of surfaces are variable. The obtained results confirm this behaviour and also the possibility to reach a one-way memory effect.