STRATEGIES FOR THE DEVELOPMENT OF NANOCRYSTALLINE MATERIALS THROUGH DEVITRIFICATION

The crystallization of metallic glass through devitrification reaction s can yield ultrafine nanocrystalline product structures. The recent d iscovery of AI-rich glasses containing approximate to 85 at.% Al and a combination of transition and rare earth element additions has yielde d microstructures of AI nanocrystals in an amorphous matrix with nanoc rystal volume fractions approaching 20% and excellent mechanical prope rties. Similar behavior is reported for Fe-based alloy glass systems s uch as Fe-Nb-B, and Fe-Si-B. This characteristic microstructure is syn thesized by a primary crystallization reaction that yields a density o f > 10(20) m(-3) nanocrystals and limited growth. The systematic contr ol 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 h as been identified to arise from diffusion field impingement. This con trol is enhanced for compositions that yield the highest initial parti cle densities for a given nanocrystal volume fraction. For example, th e addition of transition elements to the Al-based glasses initially en hances the range of glass-forming conditions as the solute level incre ases, but excess levels diminish the driving force for fee nanocrystal formation while increasing the driving force for intermetallic format ion. The kinetic strategies also indicate processing directions to pro mote and retain a high density of nanocrystal dispersions including th e possible utilization of sequential intermetallic crystallization rea ctions to modify the nanocrystal-amorphous matrix stability. The alloy ing strategies to promote and retain a high density of nanocrystal dis persions are quite general and may be applied to any system exhibiting primary nanocrystal formation through devitrification, including the Fe-based FINEMET alloys. (C) 1997 Elsevier Science S.A.