Primary crystallization in amorphous Al-based alloys

An important characteristic of the new marginal glass forming alloys, that include amorphous Al-based alloys, is the development of a high number density of Al nanocrystals during initial devitrification. The observed nanocrystal densities can range from 1021 to 1023 m−3 and display a remarkable thermal stability that is reflected in a wide separation of 75 °C or more between the primary and final crystallization reactions. Isothermal crystallization studies based upon nanocrystal size distribution and microcalorimetry heat flow measurements in an Al–8at.% Sm alloy confirm that the nanocrystal dispersions can develop by a heterogeneous mechanism that appears to be related to a site density retained from melt quenching. Annealing experiments at temperatures spanning the glass transition as well as the use of incorporated nucleation catalysts and deformation treatments have revealed new aspects of the crystallization kinetics and strategies for the control of the nanocrystal density during microstructure evolution.