Strategies for the development of nanocrystalline materials through devitrification

Abstract

The crystallization of metallic glass through devitrification reactions can yield ultrafine nanocrystalline product structures. The recent discovery of Al-rich glasses containing85 at.% Al and a combination of transition and rare earth element additions has yielded microstructures of Al nanocrystals in an amorphous matrix with nanocrystal volume fractions approaching 20% and excellent mechanical properties. Similar behavior is reported for Fe-based alloy glass systems such as Fe-Nb-B, and Fe-Si-B. This characteristic microstructure is synthesized by a primary crystallization reaction that yields a density of > 10 20 m −3 nanocrystals and limited growth. The systematic control of the nanocrystal development can be addressed by strategies based on thermodynamic and kinetic factors. The first issue is the initial glass formation which may be approached on the basis of the Egami size factor criteria. However, this must be applied to the amorphous phase that coexists with the nanocrystals following primary crystallization based on metastable phase boundaries. The growth control limitation has been identified to arise from diffusion field impingement. This control is enhanced for compositions that yield the highest initial particle densities for a given nanocrystal volume fraction. For example, the addition of transition elements to the Al-based glasses initially enhances the range of glass-forming conditions as the solute level increases, but excess levels diminish the driving force for fcc nanocrystal formation while increasing the driving force for intermetallic formation. The kinetic strategies also indicate processing directions to promote and retain a high density of nanocrystal dispersions including the possible utilization of sequential intermetallic crystallization reactions to modify the nanocrystal-amorphous matrix stability. The alloying strategies to promote and retain a high density of nanocrystal dispersions are quite general and may be applied to any system exhibiting primary nanocrystal formation through devitrification, including the Fe-based FINEMET alloys.