EFFECTS OF TRACE-ELEMENTS ON TRANSITION-METAL COHESION

First-principles atomic cluster calculations of the effects of trace e lements on transition metal cohesion have identified different mechani sms of cohesive enhancement and decohesion. This study of Li-row inter stitial atoms in a Ni6 octahedral host shows that elements which form covalent bonds with host atoms without leading to much strain (e.g. B and C) increase the cohesive strength of the host. Decohesion, a lower ing of the cohesive strength of the host, is promoted by trace element s which favor the formation of ionic bonds (e.g. Li and F). In this ca se, decohesion occurs by mechanisms involving metal-metal bond density reduction (by charge transfer and/or Pauli exclusion) and/or by incom plete screening of the impurity nucleus. These mechanisms are identifi ed through directly calculated restoring forces in the cluster and ana lysis of deformation densities which detail the charge density rearran gements in the host in the presence of Li-row interstitial atoms.