Experimental Investigation and Thermodynamic Modeling of the Ag-Cu-Ge System

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DUE to the low vapor pressure, perfect wet ability, higher strength of welded joints, and excellent corrosion resistance, the Au- and Ag-based brazing filler alloys are applied more and more widely in electronic industry, in which the eutectic Ag-Cu-Ge alloy is one of the most important middle temperature solders which are working at 773 K to 873 K (500 C to 600 C).[1–4] Ge is also an important alloying element in the alloys including Ag component. Sn-Ag-Cu alloys are one of the most promising Pb-free solders. However, Sn-Ag-Cu alloys have several problems in manufacture. One of them is that Sn-Ag-Cu alloys are oxidized easily after multiple reflows or aging at a high-temperature 423 K (150 C).[5] The addition of Ge makes Sn-Ag-Cu alloys show better characteristics for thermal oxidation, so the study on the state of existence of Ge in the Sn-Ag-Cu alloys is very important to investigate the mechanism of thermal oxidation resistance. Additionally, the addition of germanium to sterling silver improves corrosion resistance and mechanical properties.[6] The corrosion resistance and mechanical properties depend on phase constitution of the Ag-Cu-Ge system, so the study of the

CUIPING GUO, LEI ZOU, CHANGRONG LI, and ZHENMIN DU are with the School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People s Republic of China. Contact e-mail: [email protected] Manuscript submitted March 5, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

phase diagram of the Ag-Cu-Ge system was necessary for the development of sterling silver alloys. In the present work, the Ag-Cu-Ge system was studied by means of experimental methods and CALPHAD technique in order to construct the complete phase diagram.

II.

LITERATURE INFORMATION

A. Ag-Cu System The Ag-Cu system was optimized by many researchers.[7–12] Because the mixing enthalpies of liquid changed with temperature were considered in Reference 12, the thermodynamic parameters of the Ag-Cu system from Reference 12 were adopted in the present work. The calculated Ag-Cu phase diagram was shown in Figure 1. B. Ag-Ge System The Ag-Ge system was optimized by Wang et al.,[13] which reproduced experimental thermodynamic properties and phase relations. The thermodynamic parameters from Wang et al.[13] were adopted in the present work, and the calculated Ag-Ge phase diagram was shown in Figure 2. C. Cu-Ge System The Cu-Ge system was optimized by Wang et al.[14] The experimental mixing enthalpies, activities, and

Fig. 1—Calculated Ag-Cu phase diagram using the thermodynamic parameters from Ref. 12.

Fig. 2—Calculated Ag-Ge phase diagram using the thermodynamic parameters from Ref. 13.

phase diagram were well reproduced. In the present work, the thermodynamic parameters from Wang et al.[14] were adopted in the present work. The calculated Cu-Ge phase diagram is shown in Figure 3. D. Ag-Cu-Ge System The liquidus surface projection of the Ag-Cu-Ge system was studied by many researchers.[6,15–18] Nowotny and Bachmayer,[15] Dutkiewicz and

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