Published: 2001
Total Pages: 0
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When a liquid is subdivided into a fine droplet dispersion in order to isolate nucleation catalysts, substantial undercooling may be observed before solidification as demonstrated by thermal analysis in the current work. At high undercooling, alloy solidification is rapid and can result in the suppression of the usual reactions to yield amorphous phases and nonequilibrium crystalline phases with distinct and novel microstructures. In a complimentary approach, the intense deformation of an elemental layered array drives an atomic scale mixing at the layer interfaces to yield alloying and in some systems an amorphization reaction. In both cases attention to the relevant metastable phase equilibria and reaction kinetics is necessary for the interpretation of the precessing pathway and microstructure that can guide alloy design and control structure synthesis. In studies on Al-base amorphous alloys an enhanced control has been achieved for primary crystallization. This microstructure is characterized by an ultrahigh number density (10(exp 21)/cu m of Al nanocrystals (20nm in diameter) in an amorphous matrix with a high thermal stability (250 deg C) as reflected by a relatively high glass transition temperature, T(sub g). From complimentary rapid solidification and deformation induced amorphization, a critical factor in controlling primary crystallization has been identified as the quenched-in nuclei that are generated during melt quenching. A novel strategy to control and enhance the nanocrystal density has been discovered based upon the introduction of nucleants to catalyze nanocrystalline Al and increase the number density to 10(exp 21)/cu m. Alternatively, by avoiding quenched-in nuclei through deformation processing, bulk glass formation may be achieved in Al-base alloys The basic information that the structure synthesis studies yield also has a broad application to many aspects of solidification and deformation processing of ultrafine microstructures.
