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Platinum as a Metallic Material

Platinum-iridium Alloys

Even though the platinum-iridium alloys are not used in the same quantities as the platinum-rhodium alloys, they play an important role in very many applications. Small additions of iridium (typically 0.3 or 1 %) improve the microstructural stability of the platinum at elevated temperatures, without significantly affecting the mechanical strength and the forming characteristics. Typical alloy compositions are platinum with 10, 20 or 30 % iridium. As the range of phase separation between iridium and platinum extends to a temperature of about 1,000 °C exposure of alloys with 20 or 30 % iridium to temperatures around 800-900 °C can lead to the precipitation of iridium and thus cause embrittlement of the material.

Fig.1
Phase diagram of the alloy system platinum-iridium
© "Binary Alloy Phase Diagrams plus Updates", ASM International 1996

With increasing iridium contents, the strength increases more rapidly than with the platinum-rhodium alloys. However, a disadvantage in the application of the platinum-iridium alloys is their significantly poorer oxidation resistance in comparison with platinum and the platinum-rhodium alloys because the iridium content forms an oxide that evaporates more rapidly than it forms at temperatures > 1,000 °C (the surface remains metallically clean). Platinum-iridium alloys are even more corrosion resistant than platinum in aqueous media and many corrosive media and demonstrate outstanding tissue compatibility with the human body.

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