Platinum Metals Rev., 1973, 17, (3), 87
Rhodium and Iridium of Improved Ductility
Rhodium and iridium are more difficult to work than any other face-centred cubic metal and no completely satisfying explanation for this anomalous behaviour has yet been advanced. It is now ten years since Dr Reinacher of Degussa first demonstrated the advantages of platinum sheathing, which allowed working temperatures to be reduced to levels which encouraged the formation of a fibrous texture within the wire or sheet being produced.
Platinum, however, tends to alloy with the rhodium or iridium being worked and is very difficult to remove completely by etching, so cheaper and more effective alternatives have been sought. Dr Reinacher has now shown (Metall, 1973, 27, (1), 1–4) that rhodium and iridium of excellent ductility can be obtained by working within a pure nickel envelope which can be readily removed by etching.
Hot rolled rhodium and iridium rods having a diameter of about 3 mm were sealed within nickel tubes having a wall thickness of 1.8 mm. The rhodium assemblies were then rolled at temperatures of 800°C and below into wires approximately 1 mm diameter. The iridium assemblies needed working at 1000°C to avoid cracking on rolling.
Rhodium wires with a fibrous structure and high ductility were then obtained by etching away the nickel sheath and drawing the wire through carbide dies at temperatures down to 350°C. Higher temperatures were needed for drawing the iridium wires and, since sheath removal was found to be essential for the production of round wire of uniform diameter, complete freedom from cracking was only avoided by working at 1000°C. Iridium wire so produced retained its fibrous structure and at a diameter of 0.2 mm could be safely coiled round a 2 mm mandrel.
The effectiveness of this procedure is attributed by Dr Reinacher to the way the nickel isolates the surface of the refractory noble metal from the severe local and unsymmetrical deformations inevitably associated with hot working in grooved rolls.