The efficacy of hetero-interfaces as sinks for point defects in Cu was characterized using local measurements of tracer-impurity, radiation-enhanced diffusion (RED). The measurements were performed as a function of irradiation temperature and Cu thickness in multilayer samples, with the results being compared to steady-state kinetic rate equations to determine sink strengths. Cu-Nb Kurdjumov-Sachs (K-S) interfaces are found to be nearly ideal sinks for point defects, whereas Cu-Ni (111) hetero-epitaxial interfaces are poor sinks; Cu-V K-S interfaces are intermediate. Quantitative analysis of the RED data also yields the defect production efficiency for freely migrating defects in Cu, which is on the order of 1% for MeV Kr irradiation.

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What determines precipitate morphologies in co-precipitating alloy systems? We focus on alloys of two precipitating phases, with the fast-precipitating phase acting as heterogeneous nucleation sites for a second phase manifesting slower kinetics. Kinetic lattice Monte Carlo simulations show that the interplay between interfacial and ordering energies, plus active diffusion paths, strongly affect the selection of core-shell verses appendage morphologies. We study a FeCuMnNiSi alloy using the combination of atom probe tomography and simulations, and show that the ordering energy reduction of the MnNiSi phase heterogeneously nucleated on a pre-existing copper-rich precipitate exceeds the energy penalty of a predominantly Fe/Cu interface, leading to initial appendage, rather than core-shell, formation. Diffusion of Mn, Ni and Si around and through the Cu core towards the ordered phase results in subsequent appendage growth. We further show that in cases with higher primary precipitate interface energies and/or suppressed ordering, the coreshell morphology is favored.3

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We investigate the fundamentals of compositional patterning induced by energetic particle irradiation in model A-B substitutional binary alloys using kinetic Monte Carlo simulations. The study focuses on a novel type of nanostructure that was recently observed in dilute Cu-Fe and Cu-V alloys, where precipitates form within precipitates, a morphology that we term "cherry-pit" structures. The simulations show that the domain of stability of these cherry-pit structures depends on the thermodynamic and kinetic asymmetry between the A and B elements. In particular, both lower solubilities and diffusivities of A in B compared to those of B in A, favor the stabilization of these cherry-pit structures for A-rich average compositions. The simulation results are rationalized by extending the analytic model introduced by Frost and Russell for irradiation-induced compositional patterning so as to include the possible formation of pits within precipitates. The simulations indicate also that the pits are dynamical structures that undergo nearly periodic cycles of nucleation, growth, and absorption by the matrix.

a b s t r a c tThe kinetics of precipitation was investigated in the ternary Cu alloy, Cu 83.5 Ag 15 W 1.5 during irradiation with MeV Kr ions at elevated temperatures. The alloy was prepared as a solid solution by physical vapor deposition and then irradiated at room temperature to create a high density of nano-sized W precipitates. These precipitates served as effective sinks for point defects during subsequent elevated-temperature irradiation, suppressing radiation-enhanced diffusion. As a consequence the size of the Ag precipitates formed during elevated-temperature irradiation was stabilized below 20 nm, up to temperatures in excess of 300°C, thus significantly extending the regime for ''compositional patterning'' above 175°C, found for Cu 85 Ag 15 . For higher temperature irradiations (above 400°C), the role of the W precipitates in stabilizing the size of the Ag precipitates switched from simply acting as point-defect sinks to serving as pinning sites for the Ag precipitates. At 500°C, the average Ag precipitate diameter is $30 nm compared to $300 nm in the Cu 85 Ag 15 binary alloy. Rate theory calculations and kinetic Monte Carlo simulations are employed to illustrate how this transition takes place.

Abstract:The dynamical competition between the chemical mixing forced during energetic particle irradiation and thermally activated decomposition can lead to the stabilization of selforganized steady states in alloy systems comprised of immiscible elements. Continuum modeling and atomistic simulations predicted the stabilization of steady-state nanoscale compositional patterns for a well-defined range of ballistic mixing frequencies normalized by the irradiation-enhanced thermal atomic jump frequencies. Irradiationinduced compositional patterning has now indeed been observed experimentally, but a quantitative comparison has been lacking because models and simulations have relied on a simplified treatment with a fixed point defect concentration. We overcome here this limitation by using a kinetic Monte Carlo (KMC) code that includes the production, recombination, and elimination of point defects at sinks, as well as the chemical mixing forced by ballistic replacements. By varying the sink density and their efficiency, the temperature range of stabilization of steady-state compositional patterns is investigated in model binary alloys for point defect regimes dominated by either recombination or elimination on sinks. We find that in the sink regime, compositional patterning can be extended to remarkably high temperatures. The results are discussed by analyzing the relative diffusivities of A and B species, and their dependencies on temperature.