Spinodal decomposition related to age-hardening and cuboidal structures in a dental low-carat gold alloy with relatively high Cu/Ag content ratio

A dental Au–Ag–Cu–Pd alloy with a relatively low Au content and a high Cu/Ag content ratio was examined to determine the correlation between the microstructural changes by the spinodal decomposition and age-hardening behaviour using a hardness test, X-ray diffraction study, field emission scanning electron microscopy and energy-dispersive X-ray spectrometry. Separation of the parent α0 phase occurred by spinodal decomposition during aging at 350 °C after the solution treatment at 750 °C, and not by a nucleation and growth mechanism, resulting in the formation of the stable Ag-rich α1 and AuCu I phases through a metastable state. Hardening resulted from the coherency lattice strain which occurred along the a-axis between the metastable Ag-rich α1′ and AuCu I′ phases. In addition, lattice distortion occurred along the c-axis between the stable Ag-rich α1 and AuCu I phases due to the tetragonality of the AuCu I ordered phase. The transformation of the stable Ag-rich α1 and AuCu I phases from the metastable state introduced the formation of the fine and uniform cuboidal structures, which compensated for the increased gap in the lattice parameters through the phase transformation. Replacement of the fine cuboidal structures with the coarser lamellar structures occurred without a phase transformation, and resulted in softening by reducing the interfaces between the stable Ag-rich α1 and AuCu I phases.