Undercooling and glass formation in Al-based alloys

In most solidification reactions, undercooled melts develop prior to the onset of crystallization. For Al-based systems, the development of undercooling at low levels is often recognized by the formation of metastable intermetallic phases. The control of the type and distribution of the intermetallics is of importance in commercial applications. Examples of such microstructural control in Al–Fe–V–Si alloys are discussed. At increased solidification rates that are facilitated by high undercooling and effective heat flow, other opportunities for microstructure control become available. In systems with eutectic reactions such as Al–Si, Al–Ni and Al–Fe, the asymmetric coupled zone affords the option to alter the primary phase and to produce unique fine-scale distributions of phase assemblies. Similarly, nanoscale microstructures are accessible either through direct formation during rapid solidification or in a more controlled manner through the annealing of precursor products such as supersaturated solid solutions or amorphous phases. The general classes of nanoscale microstructures include nanocrystalline intermetallic phases in an Al matrix and nanocrystalline Al in an amorphous matrix and offer outstanding mechanical property advantages. The increasing diversity of microstructural options presents important opportunities and challenges to the modeling and analysis of rapid solidification processes. In this effort, the fundamental information on undercooling behavior that has been obtained through systematic studies of droplet solidification behavior has provided an essential guidance in modeling the potency and distribution of effective catalytic sites during nucleation. For the amorphous Al-based systems, the high undercooling for glass formation appears to be controlled largely by the suppression of growth of nuclei formed during rapid melt-quenching. A key issue in the controlled synthesis of nanocrystalline Al microstructures is the capability to control the nucleation density which appears to be linked to quenched-in, pre-existing clusters. Recent evidence based upon amorphization during solid state mechanical alloying has provided insight into the origin of the nanocrystalline Al dispersions and has demonstrated new possibilities for the synthesis of fully amorphous bulk Al-based glasses.