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Materials with both high conductivity and high strength are desired in the design of magnets and rotating electrical machineries. Copper-niobium (Cu-Nb) filamentary microcomposites are such a group of materials. Powder metallurgy techniques (P/M) followed by various deformation processes were used to synthesize these composites. A restacking technique was developed to achieve the high areal reduction necessary to reduce the Nb powder particles to ultra-fine fibers within the Cu matrix. The effects of the Nb volume fraction and size on the evolution of the material microstructure, monotonic tensile properties, and fatigue and fracture behaviors were studied. A strengthening model that combines Cu matrix work hardening, fiber strength, and dispersion-type hardening was developed and shown to predict the yield strength of composites at various stages of deformation processing with reasonable accuracy.
The tensile failure of composites with fully deformed Nb fibers was a shear-type failure (a 45° plane to the specimen axis). However, for composites without fully deformed fibers, the fracture surface had a cup and cone appearance. Fatigue properties of a composite with 18 vol.% Nb and fibers at their fully deformed stage (fiber thickness of 5 to 10 nm) were studied. The mechanisms of crack initiation and propagation were investigated. The final composite failure was due to tensile overload rather than attainment of critical crack length.
Cu-Nb microcomposite, powder metallurgy processed, tensile and fatigue behavior, strengthening, damage mechanisms
Professor, Northeastern University, Boston, MA
Research associate, Plasma Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA