Glass stability and nanocrystal formation in aluminum-based systems

A growing number of new metallic alloy classes have been discovered that can be synthesized as amorphous phases either during rapid melt quenching or by slow cooling of bulk volumes. Aluminum based glasses exhibit high mechanical strength combined with a low density and two aspects of their stability are explored within the current work: 1.) The role of non-dense random packing atomic arrangements on the nanocrystallization kinetics from the glassy state and 2.) The relationship between the glassy arrangements and the energy landscape, in terms of both phase selection and glass formation. The interpretation of these two aspects of glass stability are advanced by experimental evidence from two main system types: Al-Y-Fe and Al-Fe-Si alloys.;These systems give access to relatively unexplored atomic arrangements, and the differences in their formation/phase selection make them ideal candidates to study their behavior. The Al-RE-TM alloys are mainly viewed in terms of their stability relative to crystallization of Al nanoparticles in order to ascertain information on structural heterogeneity within the as spun structure. In many Al rich metallic glasses, including Al-RE-TM systems, crystallization reactions are effective in yielding nanoscale microstructures with crystal densities up to 1022 m-3 or higher. The crystallization kinetics determinations support a transient heterogeneous nucleation mode. Al-Fe-Si alloys, on the other hand, are considered during the transition from glass to liquid in order to gain insight into the energetics of a new aluminum based amorphous phase called q-glass. From both microstructural characterization and rapid heating experiments, evidence suggests that the metastable q-glass phase melts to a liquid by a first order reaction. Data from the rapid heating experiments constitute the first observations of first order melting of a metallic glassy phase.;In order to understand the crystallization kinetics of a system or to discover new systems for future applications, an understanding of the structure-stability relationship is necessary. The glassy phase stability and thermodynamic conditions discovered in the Al-Fe-Si system provide new insight into the novel route to amorphization. These new developments offer exciting possibilities for control of nanoscale microstructures and also challenge the current fundamental understanding of the reaction mechanisms.