Copper-beryllium (<1wt% Be) binary alloys pro­vide a good combination of strength and conductivity. The precipitation of Cu-Be binary alloys occurs in both continuous and discontinuous modes. Continu­ous precipitation creates uniformly distributed fine particles in the copper matrix, as a result of the following precipitation process19:

a0 (supersaturated) ! GP zones! y" ! g! y(CuBe)

The sequence and morphology of precipitation depends mainly on aging temperature. The first phase to nucleate from a supersaturated Cu-Be solid solution is coherent Cu-rich GP zones. Follow­ing the GP zones formation is the precipitation of so-called transition phases, y00and y0. The equilibrium phase, y, forms after the transition phases, and its appearance indicates overaging of the alloy. Discon­tinuous precipitation in Cu-Be binary alloys leads to nonuniform precipitation of long, lamellar precipi­tates, resulting in cell structure at grain boundaries, which increases the tendency to intergranular frac­ture in the alloy.

High-conductivity Cu-Be alloys generally con­tain a third element. The addition of a small amount of nickel to Cu-Be binary alloys further increases the strength of the alloys without degrading elec­trical and thermal conductivities. The addition of nickel increases the precipitate solvus temperatures of Cu-Be binary alloys.2 A higher solute super­saturation condition can be reached in the solution treatment which provides a larger driving force for precipitation during the aging treatment. The strength of ternary Cu-Ni-Be alloys, therefore, is significantly increased from enhanced precipitation hardening. The electrical and thermal conductivities of Cu-Ni-Be alloys are also increased because of the depletion of the alloying elements from the solid solution during aging, resulting in high strength and high conductivity. CuNiBe exhibits very high strength with respect to other PH copper alloys. The drawback ofthis alloy is its very low ductility and low fracture toughness after low-dose irradiation.

4.20.2.2.1 CuNiSi

CuNiSi is another PH copper alloy that has been considered for fusion applications. CuNiSi has a nominal composition of 2.5% Ni and 0.6% Si. When heat treated properly, CuNiSi can have a much higher yield strength and higher electrical resistivity than CuCrZr. It has been extensively used for the Joint European Torus (JET) compo­nents, for example, the divertor cryopump, the water-cooled baffles, and the Lower Current Hybrid

Drive cryopump.21