Nanocrystallization mechanisms of Al-rich glass-forming alloys

The partial devitrification of Al88Y7Fe5 upon controlled thermal treatments results in a microstructure consisting of fine dispersions of Al-nanocrystals (>1021 m−3) in a residual Al–Y–Fe matrix. These nanostructures offer improved performance and exceptional properties. While the origin of the nanocrystals is still uncertain, experiments coupled with kinetics analyses indicate that the nanocrystals originate from quenched-in nuclei. In this respect, the increase of the nanocrystal number density requires the control of the nucleation site density in the molten material. For this purpose, Al–Y–Fe glasses containing 1 at.% Pb or 1 at.% In, respectively, have been produced by rapid melt quenching. Calorimetry experiments show for both alloys a decreased onset temperature of the primary crystallization reaction compared to Al88Y7Fe5. While possible mechanisms have been proposed, careful microstructure analyses by TEM of the as-spun and the thermally-treated In-containing sample give new evidence that the inclusions generate a modification of the local structural arrangements of the amorphous matrix by promoting the retention of precursor nuclei during rapid quenching.