AN EXPERIMENTAL EVALUATION OF THE PHASE-RELATIONSHIPS AND SOLUBILITIES IN THE NB-CR SYSTEM

The phase relationships and solubilities have been determined experimentally in a revised evaluation of the Nb-Cr alloy system. The results from a series of microstructural investigations on arc-cast and equilibrated samples, together with X-ray diffraction and compositional analyses, provide a basis for modifying the invariant reaction phase compositions and phase boundaries. In addition, new evidence based on X-ray diffraction measurements further establishes the existence of a high temperature C14 Laves polytype as well as an intermediate C36 structure for NbCr2. Moreover, lattice parameters have been determined as a function of composition for the b.c.c. and C15 Laves phases. Within the equilibrium solubility limits of the Nb and Cr terminal b.c.c. solid solutions, the lattice parameters exhibit a slight positive deviation from Vegard's rule. For the NbCr2 C15 Laves phase the solubility behavior and the distinct change in the composition dependence of the lattice parameters suggest a different defect structure on either side of the stoichiometric composition in the intermetallic phase field.