Abstract
The age-hardening characteristics of a dental low carat gold alloy with a dual hardener system of indium (In) and Cu (33.9 Au–26.2 Ag–20.28 Cu–9.8 Pd–7.8 In–2 Zn (at%)) were examined by observing the age hardenability and related phase transformation, microstructural changes and elemental distribution during the aging process at 400°C. The dual hardener system by the use of both In and Cu provided more powerful hardening effect compared to a single-hardener system of In or Cu, without the formation of a AuCu type ordered phase. The alloy showed apparent initial hardening, which was attributed to the pre-precipitation or zone formation by the help of quenched in excess vacancies. During the constant increase in hardness, the single parent phase separated into three phases, Au–Ag-based phase, Au–Cu-based phase containing Pd and In, and InPd-based phase, through a metastable state. Indium which was added as one of the hardeners induced initial grain boundary precipitation, followed by an expansion of the lamellar structure, which was responsible for softening. The alternative lamellar structure was composed of a Cu-rich layer (Au–Cu-based phase containing Pd and In) and an Ag-rich layer (Au–Ag-based phase) replaced partly by the InPd-based phase. Separation of the Ag-rich layer from the Cu-rich layer is based on the miscibility limit of Ag and Cu due to their eutectic property.
Highlights
The recent increase in gold price has increased the demand for alternative dental alloys with lower Au contents
The alloy composed of only Au, Ag, and Pd do not exhibit age hardenability, which is essential for dental crowns and bridges to withstand the occlusial force [1]
It was reported that the addition of In to AuCu alloy reduced the age hardenability at low temperatures, which is controlled by the number of free vacancies and diffusion [11]
Summary
The recent increase in gold price has increased the demand for alternative dental alloys with lower Au contents. The well-known hardener, Cu, is added to dental low-carat gold alloys to achieve the age hardenability by forming a AuCu type ordered phase or by precipitating a Cu-based phase [2,3,4,5,6] In such cases, if there is insufficient Au content for the formation of the AuCu phase or if Cu preferentially forms a stable phase with other elements in the as-cast state, the hardening effect of Cu will be unsatisfactory. Indium (In) has been replacing Cu in an alternative Ag–Pd alloy because it exhibits age hardenability by the precipitation of an In-based phase from the Ag-rich matrix with sufficient tarnish and corrosion resistance [7, 8].
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