Synthesis of a nanocrystalline composite W–25 wt.%Ag powder by high energy milling

W–Ag pseudo-alloys are used in electric contacts of circuit breakers. They are produced by sintering of W–Ag powder mixtures followed by hot pressing or rolling to increase density. High energy milling (HEM) can enhance sintering of systems that present low sinterability because it improves dispersion, promotes refinement of the phases and produces composite particles. This work investigates the effect of HEM upon a W–25 wt.%Ag powder mixture. Phase dispersion, evolution of the shape and size of the particles during milling and the influence of strain on the crystal structure are investigated. The milled powders consist of composite particles that were formed in the first 25 h of milling. Longer milling times improve the distribution of phases inside the composite particles. The formation of the composite particles involves sequential steps of deformation, fragmentation, cold welding, work hardening and piercing of particles of the hard phase in the soft phase. The crystal lattice of W and Cu is damaged, but is not amorphized.

Composite particles of the Ag–W system were produced by high energy milling (HEM). The mechanism of particle formation involves deformation of Ag particles, piercing of tungsten particles in Ag and fragmentation. High phase dispersion can be achieved by HEM. Stacking faults caused by the sever milling was produced by the milling conditions that were not heavy enough to produce amorphization.Figure optionsDownload as PowerPoint slide