Abstract
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.
Highlights
Dental prostheses made with dental alloys must have sufficient mechanical properties to bear the stress from mastication
Hamasaki et al [2] reported that the precipitation of the metastable Cu3Au ordered phase and spinodal decomposition were represented by two types of solution treatment temperatures, which resulted in different aging behaviours in the alloy composed of 45 Au–24.5 Ag–24.5 Cu–5 Pd–1 Pt
This study examined the age-hardening behaviour of an Au–Ag–Cu–Pd alloy with a relatively low Au content and a high Cu/Ag content ratio, which has the possibility of agehardening by spinodal decomposition
Summary
Dental prostheses made with dental alloys must have sufficient mechanical properties to bear the stress from mastication. In Au–Ag–Cu alloys, hardening occurs through the combined effects of ordering by the Au–Cu system and phase decomposition by the Ag–Cu system Such a hardening process can occur by spinodal decomposition in Au–Ag–Cu alloys under a specific composition and temperature range. Hamasaki et al [2] reported that the precipitation of the metastable Cu3Au ordered phase and spinodal decomposition were represented by two types of solution treatment temperatures, which resulted in different aging behaviours in the alloy composed of 45 Au–24.5 Ag–24.5 Cu–5 Pd–1 Pt (wt.%). Nakagawa and Yasuda [3] reported three types of age-hardening behaviours depending on aging temperatures in the alloy composed of 27.4 Au–17.4 Ag–55.2 Cu (at.%): (1) a dual mechanism of spinodal decomposition and Cu3Au ordering, (2) a single mechanism of spinodal decomposition, and (3) a single mechanism of nucleation and growth of silver-rich precipitates
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