MICROSTRUCTURAL EVOLUTION AND THERMAL-STABILITY ASSOCIATED WITH A GAS-ATOMIZED CU-NB ALLOY

High-pressure inert gas atomization (HPGA) has been used to produce rapidly solidified Cu-21Nb-2Mo (weight percent) powders with a range of particle sizes and microstructures. The associated microstructures depend on particle size. Specifically, fine-scale particles (<15-mu-m) are characterized by a predominance of multiphase spheroids and a small population of Nb-based dendrites in an almost pure matrix of Cu. In contrast, large particles (45 to 75-mu-m) contain only Nb-based dendrites in a Cu matrix. The volume fraction of the dispersed constituent is much higher in the former instance than in the latter. The change in microstructure with particle size is analyzed in terms of both the amount of undercooling and cooling rate of the liquid droplets prior to and during solidification. In particular, the large undercoolings in the fine particles are believed to induce a nonequilibrium liquid phase separation which results in a high volume fraction of spheroidal, multiphase Nb-Cu particles within a Cu-rich matrix containing Nb-rich dendrites. High-temperature (900-degrees-C) isothermal annealing treatments have also been performed on consolidated material to determine the inherent thermal stability of these microstructures.