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Platinum Metals Rev., 1983, 27, (3), 109

Carbide Phase Identified in Platinum

  • C. W. C.

While platinum is known to have considerable solubility for carbon in the molten state, metallographic and X-ray diffraction studies reported previously by G. L. Selman, P. J. Ellison and A. S. Darling have shown that in the solid state carbon solubility is very small, the excess carbon precipitating as graphite in flake form (Platinum Metals Rev., 1970, 14, (1), 14–20). No evidence for the formation of carbide phases was found in this work, nor has it ever been reported in the literature and it is assumed that platinum does not form carbides.

However, recent transmission electron microscopy studies reported from the University of California by M. J. Whitcomb, U. Dahmen and K. H. Westmacott on platinum containing small amounts of carbon retained in supersaturation by quenching has provided direct evidence for the formation of a carbide phase on subsequent ageing (Acta Met., 1983, 31, (5), 743–747). In their studies on material aged in the temperature range 400 to 530°C, very small plate-shaped precipitates were observed to form on {001} crystallographic planes of the platinum matrix. Electron diffraction measurements reveal the precipitates to have a body-centred-tetragonal structure with a c/a ratio of 3/2 and with a composition equating to Pt2C.

The discovery of the formation of a platinum carbide phase is, perhaps, surprising although an orthorhombic carbide has been reported in nickel, which is in the same group in the periodic table as platinum and which had also been thought to form only graphite structures with carbon. The stability of the platinum carbide phase is attributed to the formation of stable octets of valency electrons. The 50 per cent volume difference between Pt2C and the platinum matrix suggests that Pt2C will be more difficult to nucleate than similar metastable eta carbides in ferrous alloys, which also precipitate as platelets on {001} matrix planes and which are transition phases in a sequence ending in graphite formation. The authors believe that Pt2C is also an intermediate phase that provides a more energetically-favourable path to graphite formation.