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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.
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.
At high undercooling, the solidification of alloys can result in the suppression of the usual crystallization reactions and in the formation of nonequilibrium phases with distinct and novel microstructures. When a liquid is subdivided into a fine droplet dispersion in order to isolate nucleation catalysis, substantial undercooling may he observed before the onset of solidification, as demonstrated by the current work, An improved droplet technique has been applied to investigate the phase selection kinetics, nucleation catalysis reactions and thermal history that control microstructural evolution during solidification of highly undercooled melts. New developments involving droplet population and single droplet experiments in the application of nucleation catalysis to control undercooling have been used to Identify specific active nucleants. In studies on Al-base alloys, an enhanced control and reproducibility of fine scale microstructure formation processes has been achieved in elevated temperature alloys and the new class of amorphous Al alloys. A continuing development of droplet methods to treat copper alloys and cast iron has been pursued along with the application of particle incorporated droplets to examine composite solidification processing. Throughout the experimental work, attention will is given to the evaluation of the relevant metastable phase equilibria and reaction kinetics which are quite useful for the interpretation of solidification microstructure and in the identification of alloy design strategies. In addition, processing models have been developed further with the aim to formulate microstructure maps for high undercooling solidification in order to guide the control of microstructure synthesis. An assessment of the undercooling and thermal history of the solidification products is provided by calorimetric measurements, controlled upquenching.
The undercooling of liquid metals is a fairly common observation, but the amount of undercooling is limited usually by the action of heterogeneous catalysts. When a liquid is subdivided into a fine droplet dispersion in order to isolate nucleation catalysis, substantial undercooling may be observed before the onset of solidification, as demonstrated by the increased undercooling limits established in the current work. At high undercooling, the solidification of alloys can result in the suppression of the usual crystallization reactions and in the formation of nonequilibrium phases with distinct and novel microstructures. A number of processing variables have been established to control undercooling including: droplet size. melt superheat, cooling rate and droplet surface chemistry. The application of thermal analysis together with x- ray diffraction and microstructural examination methods has allowed for an evaluation of the kinetic competition during phase selection, for the determination of the decomposition and melting temperatures of metastable phases and for the assessment of metastable phase diagrams. New advances have also been made in the understanding and control of heterogeneous nucleation and in probing the thermal history of rapidly solidifying samples by upquenching studies as well as in the development of new experimental capacity to study composites. The understanding developed from droplet undercooling studies has a direct application to Rapid Solidification Processing (RSP) and allows for an assessment of RSP treatment in terms of the influence of undercooling and metastable phase equilibria on phase selection and novel solidification microstructure development. The basic information that droplet studies yield also has a broad application to many aspects of solidification processing.
Papers from a March 2004 symposium describe recent work in solidification processes and microstructures in the areas of mushy zone dynamics, rapid solidification, and phase field modeling. Some specific topics include kinetics of dendritic mushy zones, anisotropy effects in lamellar eutectic growth, network modeling of liquid metal transport in solidifying aluminum alloys, and the topology of coarsened microstructures. Other topics include diffuse solid-liquid interfaces and solute trapping, phase selection transitions during undercooled melt solidification, dendritic growth in confined spaces, the influence of foreign particles in the formation of polycrystalline solidification patterns, and a cellular automaton for growth of solutal dendrites. Annotation : 2004 Book News, Inc., Portland, OR (booknews.com).
Reflecting the fast pace of research in the field, the Second Edition of Bulk Metallic Glasses has been thoroughly updated and remains essential reading on the subject. It incorporates major advances in glass forming ability, corrosion behavior, and mechanical properties. Several of the newly proposed criteria to predict the glass-forming ability of alloys have been discussed. All other areas covered in this book have been updated, with special emphasis on topics where significant advances have occurred. These include processing of hierarchical surface structures and synthesis of nanophase composites using the chemical behavior of bulk metallic glasses and the development of novel bulk metallic glasses with high-strength and high-ductility and superelastic behavior. New topics such as high-entropy bulk metallic glasses, nanoporous alloys, novel nanocrystalline alloys, and soft magnetic glassy alloys with high saturation magnetization have also been discussed. Novel applications, such as metallic glassy screw bolts, surface coatings, hyperthermia glasses, ultra-thin mirrors and pressure sensors, mobile phone casing, and degradable biomedical materials, are described. Authored by the world’s foremost experts on bulk metallic glasses, this new edition endures as an indispensable reference and continues to be a one-stop resource on all aspects of bulk metallic glasses.
Material processing techniques that employ severe plastic deformation have evolved over the past decade, producing metals, alloys and composites having extraordinary properties. Variants of SPD methods are now capable of creating monolithic materials with submicron and nanocrystalline grain sizes. The resulting novel properties of these materials has led to a growing scientific and commercial interest in them. They offer the promise of bulk nanocrystalline materials for structural; applications, including nanocomposites of lightweight alloys with unprecedented strength. These materials may also enable the use of alternative metal shaping processes, such as high strain rate superplastic forming. Prospective applications for medical, automotive, aerospace and other industries are already under development.