EFFECT OF ELECTRONIC NATURE AND SUBSTITUTION BEHAVIOR OF TERNARY MICROADDITIONS ON THE DUCTILITY OF POLYCRYSTALLINE NICKEL ALUMINIDES

The value of the density of states at the Fermi level N(E(F)) as a fun ction of the energy difference E(d)(i) - E(F)(h) between atoms in the metal-metal bond has been introduced as a criterion in order to estima te the probable microalloying ternary additions capable of improving t he grain boundary ductility of Ni3Al and NiAl compounds, Pd and Cu may be among such additions in the case of substitution for Al sites, for ming Ni-Pd (Ni-Cu) bonds. In turn, Cr, Mo and W, entering the Ni subla ttice, may form bond pairs with Al, all with close to zero energy diff erence. This prevents the splitting of the electron density of states and increases its total value at the Fermi level. To analyse more care fully the site preference of selected metals, a thermodynamic formalis m accounting for the activities of Ni and Al, as well as the lattice p arameter and concentration dependence of atomic interactions, has been applied. It was revealed that in Ni-rich off-stoichiometric Ni3Al (<2 5 at.% Al), even Pd microadditions (<1 at.%) are able to substitute pr eferentially for Al and form Ni-Pd bond pairs. The microalloying of Ni Al with Cu may be favorable for ductility enhancement in the Al-poor o ff-stoichiometric region (<51 at.% Al), where Cu substitution for Al s ites becomes more preferable. Cr, Mo or W may substitute for Ni and co nsequently form pairs with Al in Al-rich off-stoichiometric NiAl (> 52 at.% Al), mainly owing to the formation of a Ni-vacancy structure wit h lower electron density. (C) 1997 Acta Metallurgica Inc.