Nucleation-controlled reactions and metastable structures

In materials systems subjected to large undercooling or high supersaturation conditions, crystal nucleation and growth limitations can expose alloy metastability due either to the suppression of an equilibrium phase or else by the formation of a kinetically favored metastable phase. For the case of amorphous phase formation during melt processing the kinetic control may be analyzed in terms of nucleation limitations or growth restrictions. Under nucleation control, crystallization may be bypassed in bulk volumes as the liquid is uniformly undercooled below the glass transition. Many metallic glasses require quenching for vitrification and often do not exhibit a readily resolved glass transition upon reheating. In these cases the marginal glass formation is related mainly to growth limitations. However, this same kinetic control also provides the foundation for the development of a high density (10(22) m(-3)) of nanometer sized (20 nm) crystals during primary crystallization. Alternatively, during interface reactions, nucleation can be suppressed at early times by large concentration gradients that can promote amorphization and can expose several forms of metastability including conditions that apply to nanostructure size scales. With other synthesis routes based upon solid state alloying resulting from deformation, the kinetic pathways to glass formation can be altered to avoid primary nanocrystallization reactions in marginal glass forming alloys. These developments present intriguing opportunities for controlling microstructural evolution especially at the nanostructure size scale. (C) 2003 Elsevier Ltd. All rights reserved..