Platinum Metals Rev., 1965, 9, (4), 129
Precipitation in Gold-Platinum Alloys
The Effects of Atmospheres
According to some recent observations of Dr Reinacher (Zeitschrift für Metallkunde, 1965, 56, 216), the presence of oxygen has a profound effect upon the mode of precipitation of the gold-rich phase from supersaturated gold-platinum solid solutions. The alloys studied, which contained 8 to 10 per cent by weight of gold, were produced by induction melting under argon in alumina crucibles. When solution treated and aged in hydrogen the test specimens attained a maximum hardness after less than one hour at 600°C. They over-aged rapidly and reverted to the quenched hardness in less than ten hours. Specimens heat treated in air rather than in hydrogen maintained surface hardnesses in excess of 250 HV even after 25 hours at 600°C. This over-ageing effect was caused by the formation of a coarse lamellar precipitate on the surface of the hydrogen-treated specimens. As Reinacher suggests, this may be due to surface hydrogen penetration which, by distorting the lattice, affects the solubility of gold in platinum. An alternative explanation might be that surface oxygen penetration increases the solubility of gold in the lattice and therefore inhibits precipitation. It is certainly true to say, however, that in a system of this type with two phases of very similar thermodynamic stability, effects are likely to make themselves apparent which would be completely masked in other alloys. This characteristic could make the gold-platinum system useful for a variety of fundamental investigations.