Amorphization and alloy metastability in under-cooled systems

In systems with larger undercoolings, 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. Under nucleation control, crystallization may be bypassed in bulk volumes as the liquid is uniformly undercooled below the glass transition. Alternatively, during interface reactions, nucleation can be suppressed at early times by larger concentration gradients that can expose several forms of metastability and increase the probability of amorphization. For amorphous phase formation during melt processing the kinetic control may be analyzed in terms of nucleation limitations or growth restrictions. Many metallic glasses require quenching for vitrification and often do not have a resolved glass transition upon reheating. 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 nn) crystals during primary crystallization. With alternate synthesis routes based upon solid state alloying resulting from deformation, the kinetic pathways to glass formation can be altered to avoid nanocrystallization reactions in marginal glass-forming alloys. These developments present new opportunities for controlling crystallization in multicomponent glasses. (C) 2000 Elsevier Science B.V. All rights reserved.