Amorphization and nanostructure synthesis in Al alloys

The recent innovations in metallic glasses have led to new alloy classes that may be vitrified and a re-examination of the commonly used criteria for glass formation and stability. In one case large, bulk volumes may be slowly cooled to the glassy state that signifies a nucleation-controlled synthesis. The other important case is represented by Al and Fe based marginal glass formers that have been synthesized under growth controlled kinetic conditions mainly by rapid solidification processes. With marginal glass forming alloys the termination of the amorphous state upon heating is often characterized by a primary crystallization reaction that yields a high number density in the range of 1021–1022 m−3 of Al nanocrystals (15–20 nm in diameter) dispersed within a residual amorphous matrix. At the same time the results from alternate synthesis strategies involving intense cold rolling reveal that the primary crystallization reaction can be bypassed during deformation alloying of elemental multilayers or enhanced during deformation bonding of amorphous ribbons. The kinetics analysis of the crystallization behavior provides insight into the origin of the dispersed nanocrystal and amorphous matrix microstructures, the important influence of heterogeneous catalysts and an effective assessment of the overall stability that are essential for microstructure control.