SUMMARY Nearly 70 years ago, the first dispersion-strengthened ...

SILVERSilver, with its excellent electrical and thermal properties, is an important material for electrical, electronics,and chemical industry equipment. Because the pure metal is extremely soft, most applicationsrequire silver alloys. In electrical applica~ions particularly, dispersion-strengthened silver alloys havegreat merit since the dispersions accomPI'l'iSh strengthening without the severe degradation of electricalconductivity that always accompanies so id-solution strengthening.The maximum solubility of oxygen in silyer is about 0.01 percent (by mass) at 939 C, but this decreasesto approximately 0.0002 percentat room temperature. Silver oxide, however, is unstable.Thus, other oxides are preferred as the dispersoid. Internal oxidation of alloys and of powders andtheir mixtures are all useful ways for fabricating DS silver alloys.As reported by Harmsen, et al.(1), the first Ag-CdO contact alloys were produced in the 1940's by pressing,sintering, and high-pressure compacting mixtures of silver and CdO powders. These compacts wereporous and brittle because of a coarse dispersion of the oxide. The density of the materials was alsoundesirably low. Some improvements in properties were achieved by using internal oxidation techniqueson Ag-Cd alloys, but these materials were also characterized by coarse CdO crystals. The next processingadvance was to reduce mixtures of Cd(OH)2 and A9N03 and process the resulting powders. (1) Fineparticles of CdO (10 to 15 weight percent) in the silver prevent the growth of the silver crystals evenafter critical deformation and recrystallization annealing for several hours at 800 C.DS Ag-CdO, Ag-ZnO, and Ag-Sn02 alloys were tested by Harmsen,et aI., for arc erosion using different currents as shown in Figure1.The Ag-ZnO and the Ag-Sn02 alloys both had a higher erosionrate than the Ag-CdO alloy. The welding, or sticking, tendencyof these alloys was also determined using the ASTM recommendedweld test fixture.Figure 2 shows the relation between oxide con­In contact materials, the ability to stop arcing on ac currents dependson the thermal stability of the metal oxide present. The presence of metal oxides such as CdO, CuO,ZnO, Sn02, PbO, and NiO, whose decomposition temperatures are below the boiling point of silver,increases the arc extension or reignition voltage, as shown in Figure 2. On the other hand, Zr02, MgO,and AI203 decreases the reignition voltage.Vinaricky and Harmsen(2) investigated possible silver contactmaterials by the addition of up to 25 percent each of CdO, Cuo,ZnO, Sn02, PbO, NiO, MgO, AI203, and Zr02, all having aparticle size between 4 and 8 microns. The mixtures were compactedto bars and sintered at 900 C except for the Ag-PbO,which was sintered at 600 C. The bars were then hot workedinto 12-mm diameter wires. At the 25-percent oxide level, allthe materials were very brittle. The hardness and electricalconductivity of these materials are shown in Table 1.tent and the reignition voltage.u::: 14 ,...------r------r-----..,,...----,