Nanostructure development during devitrification and deformation

Abstract

The devitrification of amorphous phases can be controlled to yield a nanoscale microstructure based upon either a high density of nanocrystals dispersed in an amorphous matrix or a completely nanocrystalline solid. For both rapidly solidified marginal glass forming alloys and slowly cooled bulk glass forming alloys, the high nanocrystal densities are related to nucleation reactions that are sensitive to the initial as-prepared state of the amorphous phase. For example, nucleation densities of 10 21 m −3 can be enhanced to 10 23 m −3 by selective doping of Al-based glasses and the glass forming ability of bulk amorphous alloys is known to be sensitive to minor impurities. The addition of only 1 at.% of Cu to amorphous Al–Sm–Ni or Al–Y–Fe alloys is an effective microstructure control by narrowing the size distribution of Al nanocrystals and reducing the average size of the nanocrystals from 10 nm to about 7.5 nm while increasing the particle number density. Calorimetry and microstructural analyses of the primary Al crystallization reaction suggest that the addition of Cu modifies the atomic arrangement and induces structural heterogeneities based upon medium range order (MRO) that can act as nucleation sites. The strong composition dependence of the crystallization reactions and resulting microstructures reflect not only underlying thermodynamic constraints, but also indicates a strong composition dependence of the amorphous phase atomic arrangement. These features point to a central role for heterogeneities in the evolution of nanoscale microstructures. During nanostructure synthesis by deformation of multilayers the thermodynamic constraints are also important, but the co-deformation of layers governs the generation of interfacial area that allows for the nanoscale alloying reaction. Moreover, kinetics studies have also revealed that the governing reactions often proceed under transient conditions and can lead to relatively stable and robust nanoscale microstructures.