Rapidly solidified Nb-Al powder: Production and characterization

This work consisted of design, construction, and operation of an ultrasonic gas atomization system for production of Nb-Al powder. The target composition, Nb3Al, has an extremely high (∼2,150C) liquidus temperature which required special design considerations, including severe melt-crucible reactions. Initial feasibility of the design was tested with processing of eutectic composition Nb-Al (54 at% Nb) alloy, which has a melting point of 1,590C. Successful pressurized melt ejection and subsequent atomization of eutectic Nb-Al was achieved. The powder was characterized by various methods. Supersaturated bcc of 49.7 at% Al composition (over twice the equilibrium solubility of Al in Nb) was found in minus 38 μm powder produced using a C crucible. This material has a fine (2-5 μm) equiaxed cellular/dendritic microstructure and an average microhardness (368 kg/mm2) typical of supersaturated bcc Nb. Powder particles having a diameter greater than 38 μm contained the equilibrium phases Al3Nb and σ phase. Reproducible atomization of eutectic Nb-Al was achieved with Ta crucibles. Particles consisting solely of metastable phases were found in powder under 75 μm size