Synthesis of Solidification Structure in Undercooled Liquids.

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