Platinum-rhodium Alloys

The platinum materials used in the largest quantities – in particular for applications at high temperatures – are the alloys of platinum with rhodium, and the most commonly used alloys have rhodium contents of 10 or 20 %. Compositions with up to 30 % rhodium are also used. The phase diagram of the platinum-rhodium alloy system shows a continuous solid solution with no tendency to phase separation above approx. 760 $\text{°C}$. Because of the relatively low temperature of the phase separation, it does not normally arise under practical conditions. Whereas platinum has a Young’s modulus of 165 $\text{GPa}$, the Young’s modulus of rhodium is 372 $\text{GPa}$¹⁾. As one would expect on the basis of these values, platinum-rhodium alloys have substantially higher mechanical strength at all temperatures than chemically pure platinum. Even at temperatures immediately below the melting temperature, the platinum-rhodium alloys demonstrate oxidation resistance that is similar to that of platinum.