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

While many binary systems exhibit a positive heat mixing that precludes int ermixing in conventional bulk diffusion couples, it is possible to alloy in solid state some of these bulk immiscible elements in nanostructures. Mole cular dynamics simulations demonstrate that in low-dimensional systems such as surfaces and in sub-nanometer layered superlattice structures, excess e nthalpic 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 mecha nism such as severe mechanical deformation. In addition to single-phase all oys, we demonstrate, using X-ray absorption near-edge structure (XANES) ana lysis, a novel two-phase coexistence controlled by kinetically imposed poly morphic constraints. Using a phenomenological model, possible mechanisms re sponsible for driven alloying are discussed with reference to several previ ous proposals in the literature. (C) 2000 Elsevier Science S.A. All rights reserved.