Experimental reinvestigation of the Co-Nb phase diagram

The phase diagram of the binary Co-Nb system was redetermined in the whole range of compositions by differential thermal analysis (DTA), electron-probe micro analysis (EPMA), powder X-ray diffraction (XRD), and metallography. Equilibrated alloys of more than 30 different compositions and a series of diffusion couples were used in order to establish the invariant reactions in the system and the homogeneity ranges of all phases. A special feature of the Co-Nb system is the occurrence of three different types of the Laves phase NbCo2, which are a cubic variant (C15) and two different hexagonal variants (C14 and C36). The existence of these three types of the Laves phase was confirmed and their homogeneity ranges were established for the first time. The two hexagonal Laves phase variants are stable only at high temperatures and strongly off-stoichiometric compositions. Two more intermetallic compounds exist in the system: The monoclinic phase Nb2Co7 forms by a peritectoid reaction and has a very small homogeneity range, whereas the μ phase Nb6Co7 in the central part of the system melts congruently and is stable within a broad homogeneity range.