Microstructure Evolution in Cu-Pd-Ag Alloy Wires During Heat Treatment
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RODUCTION
CU-PD-AG alloys are used to fabricate terminals or components of electronic devices because of their relatively low electrical resistance and high strength.[1] This class of alloys is also studied actively for medical applications owing to their excellent corrosion resistance and mechanical properties.[2–7] In dental application, many kinds of hardening process have been studied because of the requirement of a wide variety of mechanical properties.[8–11] The age hardening of this alloys by ordered-phase precipitation in the a phase has been studied widely.[12,13] For the Ag-Pd-Cu-Au alloy, the effect of the microstructure variation of b-phase precipitates on the mechanical properties of the alloy has been investigated systematically.[14–18] The main phase in this alloy is the a phase, which is a solid solution with a face-centered-cubic (fcc) structure. The b phase is a CHIHIRO IWAMOTO is with the Department of Materials Science and Engineering, College of Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan. Contact email: [email protected] NAOKI ADACHI is with the Graduate School of Science and Engineering, Ibaraki University, and also with the Nippon Steel & Sumikin Technology, 3 Hikari, Kashima-shi, Ibaraki 314-0014, Japan. FUMIO WATANABE and RISEI KOITABASHI are with the Yokowo Co., Ltd., 1112 Kanohara, Tomioka-shi, Gunma 370-2495, Japan. Manuscript submitted January 21, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
B2-type ordered phase. An increase in volume fraction of b precipitates increased the strength of this alloy and decreased its ductility.[14] The fracture toughness increased with an increase in diameter of the precipitated b phase.[17] The fatigue and fretting-fatigue strength were also studied.[2,19] A hardening by the lamellar structure of a and an ordered phase by discontinuous precipitation has been reported in several systems.[20–23] Aging of solution-treated specimens resulted in alloy transformation to the lamellar structure. A coarsening of the fine lamellar structure resulted in alloy softening. Precipitation hardening by fine coherent b¢-phase precipitation in the a phase during aging treatment has been studied.[12,24] The b¢ phase is a metastable phase with an L10-type ordered structure. Recently, it has been shown that good mechanical properties of Cu-PdAg-Au alloys are achieved only by solution treatment, followed by water quenching without aging treatment. This phenomenon is related to the precipitation of a b¢ phase in the a phase.[25–31] The hardness increases with an increase in quenching cooling rate. The size of the b¢ phase decreased and the amount of b¢ phase increased with an increase in cooling rate.[29] The effect of the constitution phase on the hardness has also been studied and the contribution of the coarse b phase to the hardening behavior has been found to be much smaller than that of the fine b¢ phase.[27,32]
For the Ag-Cu-Pd system used in this study, the relationship between the composition and the mech
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