Age-hardening mechanisms in a commercial dental gold alloy containing platinum and palladium.

The age-hardening mechanism of a commercial dental gold alloy containing platinum and palladium (in wt.%, 15 Cu, 6 Ag, 5 Pt, 3 Pd, 3 Zn, with the balance as gold) was elucidated by means of electrical resistivity, hardness tests, x-ray and electron diffraction and electron microscopy, as well as high-resolution electron microscopy. The sequence of phase transformations during isothermal aging below the critical temperature, Tc = 825 K, was described as follows: disordered solid solution alpha 0 (FCC)----metastable AuCu I' ordered phase (FCT)----metastable alpha 2 disordered phase (FCC) equilibrium AuCu I ordered phase (FCT) + equilibrium alpha 2 disordered phase (FCC). The hardening was due to the introduction of coherency strain at the interface between the AuCu I' platelet and the matrix. These ordered platelets had mutually perpendicular c-axes to compensate for the strain introduced by their tetragonality. A loss of coherency at the interface brought about softening of the alloy, i.e., over-aging.