Microstructural evolution in rapidly solidified Cu-Nb powders

High pressure inert gas atomization has been used to produce rapid solidification Cu-21.2wt percent Nb powders with a range of particle sizes and microstructures. The associated microstructures depend on particle size. Specifically, fine-scale particles are characterized by a predominance of Nb-rich, multiphase spheroids and a small population of nearly pure Nb dendrites in an almost pure matrix of Cu. In contrast, large particles contain only Nb dendrites in a Cu matrix. The volume fraction of second phase is much lower in the latter instance than in the former. The change in microstructure with particle size is believed to be a result 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 non-equilibrium liquid phase separation which results in a high volume fraction of spheroidal, multiphase Nb-Cu particles within a Cu-rich matrix containing Nb dendrites.