Microstructure and phase selection in supercooled copper alloys exhibiting metastable liquid miscibility gaps

Abstract

The influence of both bulk supercooling and cooling rate on the microstructure and phase selection during solidification of Cu–Co, Cu–Co–Fe, and Cu–Nb alloys exhibiting metastable liquid miscibility gaps were investigated using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. Containerless electromagnetic levitation was used to achieve large bulk supercoolings in the specimens. Supercooling of these alloys below a certain temperature resulted in metastable separation of the melt into two liquids, a Cu-lean (Co, Co + Fe, or Nb enriched) melt (L1) and a Cu-rich melt (L2). Usually, the microstructure of the phase-separated alloys consisted of spherulites corresponding to one of the phase-separated liquids embedded in a matrix corresponding to the other. The microstructure and phase selection are found to depend on factors such as: alloy composition, supercooling level, whether the material was dropped before or after recalescence, and the cooling rate during solidification. The following results were observed: (1) solidification of metastable e-Cu with enhanced Co (or Co + Fe, or Nb) solubility; (2) partitionless solidification of the L1 and L2 liquids; (3) spinodal decomposition of the supercooled liquid, and (4) secondary melt separation. The results are discussed and related to current solidification theories regarding solidification paths for the different conditions examined. The miscibility gap boundaries for the different alloys were determined and compared with those reported in the literature.