Effects of small silver addition to AuCu on the AuCu I ordering process and age-hardening behaviours

The effects of small silver additions to an equiatomic AuCu on the AuCu I ordering process and age-hardening behaviours at 300°C were investigated by hardness testing, X-ray powder diffraction, optical microscopy, and transmission electron microscopy. The twinned structure developed at 300°C in all of the experimental alloys containing silver of up to 6.0 at,%. However, the widening rate of the inter-twin spacing and the growth rate of the antiphase domain size markedly slowed down with the addition of silver to AuCu. The softening rate in the overageing stage also markedly slowed down with increasing the silver content. The strong tendency of retaining a high hardness value in the silver-containing alloys was attributed to the high densities of both twin boundary and antiphase boundary in these alloys. The small silver addition to an equiatomic AuCu tends to stabilize the mechanical properties of the aged alloy. This effect is beneficial in designing and producing a high-duty dental casting gold alloy.