For the metallurgist the availability of phase equilibrium data is an essential requisite if he is to relate the microstructure of alloys with their behaviour. While binary diagrams of the platinum group metals are now widely available, it is unfortunate that there is still relatively little published on ternary and higher order systems. It is, therefore, encouraging to find that new work on ternary systems of platinum has been undertaken. The phase equilibria of the platinum-zirconium-yttrium system at 1000°C has been reported recently by Yu. J. Konobas, M. V. Raevskaya and I. G. Sokolova of Moscow State University (J. Less-Common Met., 1986, 115, (2), L5–L6).

This ternary system is of some interest since both the platinum-zirconium and the platinum-yttrium binary systems exhibit a number of intermetallic compounds, whereas the zirconium-yttrium system is a simple eutectic.

The 1000°C isothermal section published by the authors was based on experiments on arc-melted alloys homogenised at 1000°C for 1200 hours prior to quenching in iced water. They found no new ternary phases, only those reported previously in the binary systems.

The isothermal section is characterised by regions of ternary solid solubility, based on the initial binary compounds, which spread into the ternary system along the corresponding pseudobinary section. The solubility of yttrium in platinum-zirconium intermetallic compounds is less than 5 atomic per cent whereas zirconium has much higher solubility in platinum-yttrium compounds, extending up to 18 atomic per cent in the Pt₃ Y intermetallic which has a AuCu₃ type structure. The Laves phase Pt₂ Y only extends into the ternary system to about 2 atomic per cent zirconium, not interacting with other binary compounds.

Below the solidus line, the Pt₃ Zr-Pt₃ Y, PtZr-PtY and the Pt₃Zr₅-Pt₃ Y₅ sections are pseudobinary, as might be anticipated. At the platinum-rich corner, the platinum solid solution is in equilibrium with two three-phase regions, each containing two intermetallic compounds. This could have significance for new platinum-alloy development, possibly as an alternative approach to the high strength oxide dispersion-strengthened platinum materials which are based, coincidentally, on the oxides of zirconium and yttrium.