Abstract
The use of grain refiners, such as iridium, in 18 kt gold alloys is a common practice in jewelry industrial applications. The use of these elements leads, however, to an increase in the costs of raw materials and greater attention during the solidification phases and during the refining and recycling of alloys is required. This work aims to demonstrate that through the optimization of thermo-mechanical processes, it is possible to obtain a result comparable to that obtained with refiner in terms of workability, mechanical and aesthetic properties and corrosion behavior. The study focused on evaluating the grain growth in annealing processes after plastic deformation, also examining the casting phase and the effect of the different cooling rates. The samples, after the different thermo-mechanical treatments, were characterized in terms of microstructure, grain size and micro-hardness comparing the results with the ones of an iridium-containing alloy. The results showed that with proper optimization of annealing time is possible to obtain, without grain refiners, gold alloys with properties similar to ones obtained with Iridium as a grain refiner.
Highlights
The use of grain refiners in gold alloys in the production of items by cold deformation and casting processes is nowadays a common practice in jewelry industries (Ref 1, 2)
The refining effect given by Iridium in our case is comparable with the one observed in (Ref 9) where a reduction in more than 6 times in the size of the grain is observed with the use of 0.0055% of iridium in an 18 kt gold yellow alloy
The corrosion resistance of the 18 kt gold alloys was tested after the last step studied, where a good homogenized structure is reached in all the samples
Summary
The use of grain refiners in gold alloys in the production of items by cold deformation and casting processes is nowadays a common practice in jewelry industries (Ref 1, 2). These elements can act in the solidification and recrystallization phases. Small quantities of iridium and ruthenium, characterized by high melting temperatures and low solubility in gold alloys, are more often employed. They guarantee a reduction in the crystalline grain, favoring the formation of nucleation centers during cooling and increasing the stability of small radius solidification cores (Ref 4). All the elements mentioned above can act in the recrystallization stage, thanks to their tendency to segregate and precipitate at the grain boundary, slowing down the grain growth during annealing (Ref 5).This thermal treatment is typically employed in the jewelry industry because is an essential step to recover ductility in processes where cold deformation is foreseen, to obtain a more uniform structure and to control the grain size during recrystallization
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