PHASE-STABILITY AND DEFECT STRUCTURE OF THE C15 LAVES PHASE NB(CR,V)(2)

Vanadium alloyed C15 Laves phase based on NbCr2 and phase equilibria i n the Nb-Cr-V system have been studied, focusing on the physical metal lurgy and defect structures of the C15 phase. Based on the Nb-Cr-V pha se diagram established in this work, the defect structure/mechanism in the ternary C15 Laves phase is investigated using a combination of me tallography, SEM/EDS, TEM/ALCHEMI, and X-ray powder diffraction. It is found that the C15 phase field of NbCr2 is extended by V alloying up to more than 30 at.% V and V substitutes exclusively on the Cr site in stead of the Nb site, although the atomic sizes of the constituent ele ments indicate r(Cr)<r(V)<r(Nb). The lattice constants of the ternary C15 phase Nb(Cr,V)2 increase linearly with increasing V content. The e lectronic structure and total energy of the C15 intermetallic phase Nb Cr2 have been calculated using the linear muffin tin orbital (LMTO) me thod with the atomic sphere approximation (ASA). The electronic band s tructure and density of electronic states were obtained. The formation and stability of the ternary C15 Laves phase Nb(Cr,V)(2), the defect structure/mechanism, and the compositional dependence of the lattice c onstants are elucidated using a combination of geometric and electroni c considerations. These results indicate that the electronic factor ma y also play an important role, in addition to the atomic size rule, in determining the alloying behavior and the defect structure/mechanism of C15 Laves phases based on NbCr2. (C) 1998 Acta Metallurgica Inc.