COHESIVE PROPERTIES, ELECTRONIC-STRUCTURE, AND BONDING CHARACTERISTICS OF RUAL - A COMPARISON TO NIAL

Recent experiments on promising high-temperature aluminide intermetallic compounds discovered that, in contrast to NiAl, RuAl has critical [111] slip systems that facilitate ductility under compression at room temperature. In order to understand this different mechanical property from a microscopic point of view, the cohesive and electronic properties of NiAl and RuAl have been studied by means of the first principles local density all-electron self-consistent linearized muffin-tin orbital (LMTO) and full-potential linearized augmented plane wave (FLAPW) methods. The ground state cohesive properties calculated by the LMTO method (including equilibrium lattice constant, bulk modulus, and formation energy) are found to be in good agreement with experiment. The analysis of the band structure and density of states shows that the transition metal (Ni or Ru) d-Al p hybridization provides the major contribution to the cohesive energy in both compounds. The anti-phase boundary (APB) energy of RuAl associated with the 1/2 [111] {110} superdislocation (580 mJ/m2) is found to be only 65% that of NiAl. Moreover, the charge density near the Fermi energy reveals that (i) the strong Ni d-Al p hybridization at E(F) for NiAl causes a directional charge distribution along the [111] direction which may affect its dislocation dissociation; (ii) for RuAl, a mostly Ru-d electron charge distribution shows only d-d bonding along the [100] direction between Ru atoms.