Microstructural size effects in high-strength high-conductivity Cu-Cr-Nb alloys

Microstructural refinement to further improve the strength and stability of high-strength high-conductivity Cu-Cr-Nb alloys was attained by mechanical milling (MM). Mechanically milled Cu-4Cr-2Nb and Cu-8Cr-2Nb exhibited an increase in hot-pressed Vickers hardness of 122 and 96 pct, respectively. Mechanical milling produced a corresponding decrease in electrical conductivity of similar to 33 pct for both alloys. The increase in hardness was more due to Cu grain-size refinement than to second-phase particle-size refinement. The drop in conductivity was due to second-phase particle-size refinement, which both increased particle/matrix interfacial area and solute solubility. Mechanically processed Cu-4Cr-2Nb displayed an enhanced thermal stability. Hot-pressed 4-hour milled Cu-4Cr-2Nb experienced a 30 pct increase in conductivity with only a 22 pct drop in hardness when annealed at 1273 K for 50 hours. Such changes were largely due to an increase in dispersed-particle size (i.e., a decrease in solute and interfacial electron scattering) and Cu pain size (reduced Hall-Fetch effect), respectively. The optimum hardness and conductivity combination was found in 4-hour milled and hot-pressed Cu-4Cr-2Nb material.