CAREER: Mesoscopic Scale Phase Separation in Alloys under Sustained External Forcing

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9733582 Bellon This CAREER grant addresses research and educational programs which are centered on non-equilibrium processing of materials. The research component examines the new idea that in alloys under sustained external forcing phase separation may take place on a mesoscopic scale, i.e. a scale which is neither atomic, nor macroscopic, and these meso-structures can be stable at steady-state. The objectives of the proposed research are: (a) to perform a detailed characterization of the composition field in alloys with positive heat of mixing at steady-state during by ball milling under controlled conditions; (b) to elucidate the origin of the stabilization of mesoscopic phase separation in alloys under sustained shearing; and (c) to test whether the available experimental results on these mesostructures can be rationalized by a kinetic model with competing dynamics, where dislocation glide would be the dynamics forcing the mixing of immiscible elements. The approach chosen involves the milling of Fe-Cu and Cu-Ag powders under controlled conditions and the use of analytical techniques that can resolve nanometric composition heterogeneity, including for the first time for ball milled materials the use of atom probe and three-dimensional atom probe techniques. Atomistic computer simulations and modeling should assist in understanding of these forced systems. The educational component of the proposal consists in developing a computer-assisted material for a graduate course on materials under irradiation using CyberProf, a human- computer interface developed at the Beckman Institute in Urbana; and in reorganizing an undergraduate materials science laboratory to stimulate initiatives from the students. %%% Mesoscopic phase separations have been recently identified in metal alloys with positive heats of mixing under sustained shearing by atomistic computer simulations. These findings are consistent with the latest published experiments showing complex dy namical phase equilibria and strong chemical heterogeneity in alloys with positive heat of mixing when submitted to ball milling. This research is motivated by the fact that the field of nonequilibrium systems is in rapid development and has received much attention in the last two decades. Indeed in many applications, materials are driven away from equilibrium by some external forcing, either during their preparation (e.g., by ball milling) or during their use (e.g., by irradiation). ***