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Title: Novel mechanism for order patterning in alloys driven by irradiation

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Journal Article: Publisher's Accepted Manuscript
Journal Name:
Physical Review B
Additional Journal Information:
Journal Volume: 96; Journal Issue: 10; Related Information: CHORUS Timestamp: 2017-09-18 14:13:11; Journal ID: ISSN 2469-9950
American Physical Society
Country of Publication:
United States

Citation Formats

Lear, C. R., Bellon, P., and Averback, R. S. Novel mechanism for order patterning in alloys driven by irradiation. United States: N. p., 2017. Web. doi:10.1103/PhysRevB.96.104108.
Lear, C. R., Bellon, P., & Averback, R. S. Novel mechanism for order patterning in alloys driven by irradiation. United States. doi:10.1103/PhysRevB.96.104108.
Lear, C. R., Bellon, P., and Averback, R. S. 2017. "Novel mechanism for order patterning in alloys driven by irradiation". United States. doi:10.1103/PhysRevB.96.104108.
title = {Novel mechanism for order patterning in alloys driven by irradiation},
author = {Lear, C. R. and Bellon, P. and Averback, R. S.},
abstractNote = {},
doi = {10.1103/PhysRevB.96.104108},
journal = {Physical Review B},
number = 10,
volume = 96,
place = {United States},
year = 2017,
month = 9

Journal Article:
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This content will become publicly available on September 18, 2018
Publisher's Accepted Manuscript

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  • Ion-beam mixing of immiscible alloys is viewed as a competing dynamic process, where irradiation-induced mixing opposes thermal decomposition. The external perturbation drives the system away from equilibrium, and in the long-time regime the system can exhibit phase and microstructural modifications. Due to the nonequilibrium nature of the process, the steady state depends explicitly on the details of the interplay between irradiation and the internal kinetics of the alloy. In particular, we have recently developed a continuum model that takes into account the finite range of atomic relocations during collision cascades [R. A. Enrique and P. Bellon, Phys. Rev. Lett. 84,more » 2885 (2000)]. Using this model, we have shown that self-organized compositional patterns can spontaneously appear if the range of atomic relocations is large enough, and we have introduced a dynamical phase diagram describing the steady state regimes as a function of the forcing and material parameters. In this paper we follow up with the analysis of the continuum model, and we consider the problem of fluctuations. In order to study the phenomenology and test the predictions, we perform kinetic Monte Carlo simulations of an immiscible binary alloy undergoing finite-range atomic relocations. The simulations show that compositional patterns at the nanometer scale can indeed be stabilized, and that the behavior of those patterns as a function of the control parameters can be suitably described by our continuum model, and previous theory of fluctuations in driven alloys. The results corroborate the idea that irradiation can be used as a processing tool to synthesize nanostructures.« less
  • Atomistic kinetic Monte Carlo simulations are employed to analyze the dynamical stabilization of nanoscale patterning of L1{sub 0} chemical order in a model binary alloy subjected to sustained irradiation. The effect of irradiation-induced displacement cascades on the chemical order is modeled by the introduction at a controlled rate of nearly fully disordered spherical zones, which compete with the reordering promoted by the thermally activated migration of vacancies. When the size of the disordered zones is small, the alloy reaches a steady state that is either long-range ordered at low irradiation-induced ballistic jump frequency, {gamma}{sub b}, or disordered at high {gamma}{submore » b}, with a first-order dynamical transition between these two steady states at {gamma}{sub b}={gamma}{sub b}{sup c}. Furthermore, in the disordered steady state, the intensity of order fluctuations scales with the reduced variable {gamma}{sub b}/{gamma}{sub b}{sup c}, a scaling that is consistent with an effective temperature approach. For larger cascade sizes, however, an additional steady state is stabilized at intermediate ballistic jump frequency, with a microstructure comprised of well-ordered nanoscale domains. In this patterning-of-order steady state, the above rescaling breaks down but we show that, after deconvolution of the structure factor into Gaussian and Lorentzian components, scaling of the Gaussian component is recovered by introducing a new reduced variable, {gamma}{sub b}/{gamma}{sub b}{sup p}, where 1/{gamma}{sub b}{sup p} is interpreted as the characteristic time for new domains to form in a disordered zone. This new scaling relationship provides a rigorous definition of the regime of patterning of order. This regime corresponds to the steady states stabilized by cascade sizes and ballistic jump frequencies satisfying {gamma}{sub b}{sup c}{<=}{gamma}{sub b}{<=}{gamma}{sub b}{sup p}. A dynamical phase diagram based on this new criterion is constructed and it agrees well with direct visualization of atomic configurations. Extensions to nonstoichiometric compositions are investigated. Consequences for the direct synthesis of functional nanocomposite structures comprised of chemically ordered phases are discussed.« less
  • Dense displacement cascades produced by irradiation with energetic particles lead to the formation of disordered zones in chemically ordered alloys. At temperatures below the order-disorder transition, these disordered zones, whose sizes range from a few to several nanometers, are annealed out by thermally activated atomic migration. Under sustained irradiation, the competition between these two dynamics may drive the system into various steady states of order. Kinetic Monte Carlo simulations are employed to identify these steady states in a model binary alloy that forms an L1{sub 2} ordered phase at equilibrium. Besides the expected long-range ordered and disordered steady states, amore » new state is observed, where the microstructure is comprised of well-ordered domains of finite size. This steady-state patterning of order is identified by direct visualizations of the configurations, and by using an effective fluctuation-dissipation formula to analyze the behavior of the fluctuations of order upon approaching the long-range ordered steady state. It is shown that the patterning state becomes stable only when the disordered zones exceed a threshold size. Above this threshold size, reordering of cascade-induced disordered zones proceeds in two stages: new antiphase domains form first, and then shrink to the benefit of the matrix. This two-stage reordering is at the origin of the dynamical stabilization of patterns of order. The present results, which indicate that ion-beam processing could be used to synthesize ordered nanocomposites with tunable sizes, call for specific experimental tests.« less
  • We study analytically the effect of the chemical disorder produced by displacement cascades in a stoichiometric L1{sub 2} alloy under irradiation. A continuum kinetic model is proposed to describe the coupled evolution under irradiation of the specific area covered by antiphase boundaries and of the volume fraction of the domains belonging to the four translation variants of the L1{sub 2} structure. Combining this description with existing mean-field results for the order-disorder transition under irradiation, we construct a steady-state dynamical phase diagram, where the control parameters are the cascade size and the irradiation-induced disordering rate. Three stable steady states are predicted:more » long-range ordered, disordered, and a state of patterning of order. This analytical phase diagram is in good agreement with the one constructed from kinetic Monte Carlo simulations. The present analytical model indicates that patterning of chemical order should be a general phenomenon in irradiated ordered structures, provided that the cascade size is large enough for the annealing of disordered zones to form additional antiphase domains.« less
  • Theoretical predictions indicate that ordered alloys can spontaneously develop a steady-state nanoscale microstructure when irradiated with energetic particles. This behavior derives from a dynamical competition between disordering in cascades and thermally activated reordering, which leads to self-organization of the chemical order parameter. We test this possibility by combining molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations. We first generate realistic distributions of disordered zones for Ni{sub 3}Al irradiated with 70 keV He and 1 MeV Kr ions using MD and then input this data into KMC to obtain predictions of steady state microstructures as a function of the irradiationmore » flux. Nanoscale patterning is observed for Kr ion irradiations but not for He ion irradiations. We illustrate, moreover, using image simulations of these KMC microstructures, that high-resolution transmission electron microscopy can be employed to identify nanoscale patterning. Finally, we indicate how this method could be used to synthesize functional thin films, with potential for magnetic applications.« less