Electro-discharge consolidation of nanocrystalline Nb—Al powders produced by mechanical alloying

Nanocrystalline powders of a 77 at% Nb—Al system prepared by mechanical alloying were subjected to external pressures of up to 450 MPa in a ceramic die, producing a green density between 75% and 85% of theoretical. These powders were consolidated into a solid bulk with a relative density of up to 99% by discharging a high-voltage, high-density current pulse (discharge time <500 μs and input energy of 0.5–1.0 kJ g-1). The consolidated bulk was still a mixture of two nanocrystalline intermetallic phases of Nb2Al and Nb3Al. The resultant grain sizes ranged from 13–33 nm. A negative Hall–Petch relation between Vickers microhardness and grain size was clearly observed, indicating that grain-size softening occurs.