Effects of thermal and mechanical processing on microstructures and desired properties of particle-strengthened copper-chromium-niobium alloys

Ternary Cu-Cr-Nb alloys, particularly Cu-8 Cr-4 Nb (in at.%), have demonstrated good thermal stability as well as high strength and high conductivity at low and high temperatures. This behavior—due to the insoluble and strong Cr₂Nb intermetallic phase that forms from the 2:1 Cr/Nb ratio—has put Cu-Cr-Nb alloys at the forefront as the next-generation particle-strengthened Cu alloys for aerospace applications. The initial powder material, produced by Ar-gas atomization, has a bimodal size distribution of Cr₂Nb precipitates. Primary Cr₂Nb precipitates, formed congruently from the melt, are typically ∼1 μm in size, and secondary Cr₂Nb particles, precipitated from atomized solid solution, are typically 30–200 nm in size. This study provides the first detailed examination of the stability and strengthening effects of these particles in Cu-Cr-Nb alloys. Extruded Cu-8 Cr-4 Nb exposed to temperatures of up to 1323 K for up to 100 hr sustained a drop in strength of only 25–30%.; This investigation also revealed that the primary particles, usually situated at grain boundaries and triple points, provide a direct grain boundary pinning effect, and moreover, an indirect, grain boundary strengthening effect, but virtually no Orowan strengthening effect. The secondary Cr₂Nb particles, typically found within grains (and to a lesser extent, at grain boundaries), do provide Orowan strengthening. For extruded material, it was established that grain-boundary strengthening (Hall-Petch effect) accounts for about two-thirds of the overall strength of material, with Orowan effects essentially contributing the remainder. The proven thermal stability, strengthening effects and, more importantly, strength retention, was the driving force to further improve upon these attributes via microstructural refinement of Cu-Cr-Nb alloys.; Mechanical milling (MM) of Cu-4 Cr-2 Nb and Cu-8 Cr-2 Nb produced an increase in hot pressed Vickers hardness of 122% and 96%, respectively. However, MM also inadvertently produced a corresponding decrease in electrical conductivity of ∼33% 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 increased both particle/matrix interfacial area and solute solubility. This novel detailed study also proved the enhanced stability of mechanically processed Cu-4 Cr-2 Nb. Hot pressed 4 hr-milled Cu-4 Cr-2 Nb experienced a 30% increase in conductivity with only a 22% drop in hardness when annealed at 1273 K for 50 hr. Such changes were largely due to an increase in dispersed-particle size (decrease in solute and interfacial electron scattering) and Cu grain size (reduced Hall-Petch effect), respectively. (Abstract shortened by UMI.)...