Dental Ag-Pd-Cu alloys exhibit the characteristics of age-hardenability. Palladium represses the sulphurization of silver, and the age-hardenability is caused by the addition of copper. In order to improve the corrosion resistance, a small amount of gold is added to the commercial alloys. In dentistry, the gold content has increased year by year so that 20% is contained in the present commercial alloys. The specimens used were three Ag-Pd-Cu ternary alloy and three Ag-Pd-Cu-Au alloy. These alloys were melted and casted by induction electric furnace and centrifugal casting machine in Ar atmosphere. They were rolled and drawn into plate and wire shape. These specimens were solution treated for 2 hr at 800℃ and were then quenched into iced water, and aged at 350~550℃. Age-hardening characteristics of the small Au-containing Ag-Pd-Cu dental alloys were investigated by means of hardness testing, electrical resistance, differential scanning calorimetric, and electron microscope observations. Principal results are as follows : Hardening occured in two stages, i. e., stage Ⅰin low temperature and state Ⅱ in high temperature regions, during continuous aging. The case of hardening in stage I was due to the formation of the L10 type face-centered tetragonal PdCu-ordered phase in the grain interior and hardening in stage I was affected by the Cu concentration. In stage Ⅱ, decomposition of the α solid solution to a PdCu ordered phase (L10 type) and an Ag-rich α2 phase occurred and a discontinuous precipitation occurred at the grain boundary. From the electron microscope study, it was concluded that the cause of age-hardening in this alloy is the precipitation of the PdCu ordered phase, which has AuCu I type face-centered tetragonal structure. Precipetation procedure was α → α ＋ α2 ＋ PdCu → α1 ＋ α2 ＋ PdCu at Pd/Cu = 1.7. The formation of the lamellar structure developed from grain boundaries by discontinuous precipitation. Lamella structure will contribute more to hardenability.
Ag-Pd-Cu alloy is more effective dental alloy as ageing treatment and is suitable to isothermal ageing at 450℃.
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