Abstract
Platinum is one of the most precious metals in the world: 15 times rarer than gold and it is widely used in jewellery for its characteristic of unicity, incorruptibility, and colour neutrality. The standard grade in the jewellery industry is Platinum 950‰. In fact, pure platinum jewels are easily scratched, due to the poor hardness linked with the high purity. Among all the commercial alloys available, the Pt-Ru system is the one characterized by the highest mechanical properties. Aim of this work is to find an ideal softening heat treatment, in term of operating time and temperature. The heat treatment must be able to satisfy two main needs: it must permit to complete recrystallization of the work piece, preparing the material for further severe deformation by restoring high ductility and an equiaxed grain distribution; and at the same time, the final average grain size must be fine, in order to avoid counter side aesthetic effects that occur when grain size go over the micrometric scale. Due to its rarity, in literature there are few information about the alloy under discussion. To find the optimal parameters, several samples had been heat treated at different times and temperatures. Then they had been analysed by optical microscopy and with micro hardness tests, providing microstructure images and hardness data. By using a combination of three different temperatures and three different times, significant differences were found between the various samples both in terms of mechanical properties and of grain size. The comparison of the collected data permit to better understand the behaviour of the alloy under recrystallization conditions. In the end, it was observed that the dynamics of recrystallization changes significantly with relative small changes of temperature and was found that the optimal treatment to obtain a fine recrystallized microstructure with the desired mechanical properties is at 1000°C for 15 minutes.
Highlights
Platinum alloys represent the second most important production filed in the jewelry world, after gold alloys [1].Despite gold, the most widespread standard for platinum jewels is platinum 950 (95 wt.%)
Object of the study is the platinum-5 wt.% ruthenium alloy, a widely used alloy for jewelry manufacture which has good values of hardness compared with other platinum alloys
The samples were heat treated at 950 °C, 1000°C and 1050°C for 10, 15 and 20 minutes and the results in term of microstructure and hardness properly evaluated
Summary
The most widespread standard for platinum jewels is platinum 950 (95 wt.%). This means that the addition of other metals can be done up to 5 wt.%, limiting the possibility of reinforcing the metal by alloying [2]. Fine jewelry products can be manufactured via mechanical processes: a forged bar is cold rolled and annealed, and subsequently machined, until the net shape product is obtained.
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