Structure Synthesis and Kinetics During Undercooled Liquid Solidification and Intense Plastic Deformation

摘要

At high undercooling, solidification is rapid and can result in the suppression of the usual reactions to yield amorphous phases and nonequilibrium crystalline phases with novel microstructures. In a complimentary approach, the intense deformation of an elemental layered array drives an atomic scale mixing at the interfaces to yield alloying and in some systems an amorphization reaction. Similarly, deformation of amorphous ribbons can drive instabilities that result in the development of nanocrystal dispersions without annealing. In studies on Al-base amorphous alloys an enhanced control has been achieved for the nanometer-scale microstructure formation processes that operate during primary crystallization. This microstructure is characterized by an ultrahigh number density (10EXP 21 - 10EXP 22 cubic m) of Al nanocrystals (20nm in diameter) in an amorphous matrix with a high thermal stability as reflected by a relatively high glass transition temperature, T(sub g). In order to elucidate the nature of the quenched-in atomic configurations, a novel application of nuclear magnetic resonance and fluctuation microscopy has allowed for the identification of medium range ordered regions that will be analyzed further. A novel strategy to control and enhance the nanocrystal density has been discovered based upon the introduction of nucleants to catalyze nanocrystalline Al. Alternatively, by avoiding quenched-in nuclei through deformation processing, bulk glass formation may be achieved in Al-base alloys. The combination of microcalorimetry and careful size distribution analysis has established the non-steady state nature of primary crystallization. The analysis and modeling of the kinetics is central to devising strategies for reproducible control of primary crystallization including the modification of the crystallization path by exploiting multicomponent diffusion behavior in systems with large differences in component diffusivities.