Precipitation hardening in gold-cobalt alloys

The precipitation hardening in gold-cobalt alloys containing less than 5 wt.%Co was studied by means of resistivity, hardness, X-ray diffraction, and optical microscopic examination. The hardness curves showed two hardness peaks after isothermal ageing below the temperature of 250 or 300 degrees C. It was considered that the first peak was due to the formation of G.P. zone which was confirmed by reversion phenomenon. The second peak was due to the growth of metastable cobalt-rich precipitate, and this stage was found to make a major contribution to the age-hardening in the present alloys. The nodular products at grain boundary do not play an important role in age-hardening. Acitvation energies, order of the reaction involved, and the rate constant for the growth process were established. The difference in growth mechanism of the metastable precipitate was deduced from the kinetics data.