Solid-state alloying in nanostructured binary systems with positive heat of mixing

While many binary systems exhibit a positive heat mixing that precludes intermixing in conventional bulk diffusion couples, it is possible to alloy in solid state some of these bulk immiscible elements in nanostructures. Molecular dynamics simulations demonstrate that in low-dimensional systems such as surfaces and in sub-nanometer layered superlattice structures, excess enthalpic and entropic energy contributions can provide a driving force for spontaneous intermixing to form substitutional solid solution alloys. Such driving forces diminish, however, in coarser nanophase binary mixtures when domain sizes reach beyond approximate to 1 nm. In this case, true alloying on the atomic level can be achieved by employing an external forcing mechanism such as severe mechanical deformation. In addition to single-phase alloys, we demonstrate, using X-ray absorption near-edge structure (XANES) analysis, a novel two-phase coexistence controlled by kinetically imposed polymorphic constraints. Using a phenomenological model, possible mechanisms responsible for driven alloying are discussed with reference to several previous proposals in the literature. (C) 2000 Elsevier Science S.A. All rights reserved.