The interactions between chemical phase fields and structural defects play a key role in the properties of alloys. We illustrate the importance of these interactions in driven alloys, where defects are continuously being created, with particular focus on systems where radiation-induced segregation occurs. Specifically, we compare the microstructural evolution in undersaturated Ni-Si and Ni-Ge alloys during both 100 keV He and 2 MeV Ti irradiations. While the equilibrium phase diagrams of these systems are similar, and both systems show strong radiation-induced segregation, the evolving defect structures are remarkably different. Ni-Si reveals a high density of Frank loops, while Ni-Ge shows a complex array of dislocations. Moreover, a Ni3Ge precipitate shell is observed to coat He bubbles, while no segregation of Si is observed at such bubbles. We explain these differences in behaviors to solute drag by interstitial fluxes in Ni-Si vs solute drag by vacancy fluxes in Ni-Ge.

Nanocrystalline thin films of the undersaturated alloy Ni-8.5 at% Si were subjected to 2 MeV Ti irradiation at temperatures ranging from 450 degrees C to 550 degrees C. Correlative microscopy combining transmission electron microscopy (TEM), scanning-TEM and atom probe tomography (APT) revealed that large dose irradiation at 450 degrees C of samples with initial grain sizes below 100 nm stabilized a novel nanostructure which surprisingly contained three co-existing phases, the gamma face-centered-cubic (FCC) matrix, gamma(y) L12 ordered precipitates on intragranular dislocation loops and Ni31Si12 precipitates at triple junctions (TJs). In contrast, irradiation at 550 degrees C and irradiation of larger grain-size samples at 450 degrees C only produced a gamma- gamma(y) two-phase coexistence. Analysis of the threephase nanostructure and phase field simulations indicates that radiation-induced segregation is most pronounced at TJs, thus triggering the formation of Ni31Si12 precipitates. These incoherent precipitates, in turn, appear to have contributed to stabilizing the grain size under irradiation. The results are generalized using the concept of driven defect-phases. It is suggested that the stabilization of driven defect-phases may impart radiation resilience by providing localized relaxation modes to the microstructure evolution during and after temporary perturbations in irradiation conditions.

This research establishes a systematic, high-throughput computational framework for designing radiationresistant, dilute ternary copper-based alloys by addition of solutes that bind to vacancies and reduce their mobility, thus promoting interstitial-vacancy recombination. The first challenge in developing alloys by this method is mitigating the vacancy-mediated solute drag effect, since density functional theory (DFT) calculations show that solutes that bind strongly to vacancies are also rapidly dragged to point-defect sinks, and thus removed from the matrix. To overcome this problem, two types of solutes are added to the Cu matrix: a first solute with a strong vacancy binding energy (B-type species) and another solute that binds to "B" and is a slow diffuser in Cu (C-type species). Using DFT, 21 synergistic solute pairs are screened, with "B"= Zr, Ge, Sn and "C"= Fe, Co, Mo, Ni, Nb, W, Cr. Two promising alloys, Cu(Zr,Co) and Cu(Zr,Fe) are then investigated in detail in the dilute regime. Diffusion and solute drag in these alloys are modeled using the kinetic cluster expansion approach (KineCluE) under irradiation conditions. It is shown that strong Zr-"C" thermodynamic binding, especially between Zr and Co, significantly reduces the mobility of Zr solute and suppresses the vacancy-mediated solute drag. Using an analytical framework for the standard five-jump frequency model for diffusion in binary alloys, it is found that vacancy-Zr-Co triplets disrupt the kinetic circuits that promote solute drag in the binary alloy by raising the dissociation barrier for the vacancy from the solute.

This research establishes a systematic, high-throughput computational framework for designing radiation-resistant, dilute ternary copper-based alloys by addition of solutes that bind to vacancies and reduce their mobility, thus promoting interstitial-vacancy recombination. The first challenge in developing alloys by this method is mitigating the vacancy-mediated solute drag effect, since density functional theory (DFT) calculations show that solutes that bind strongly to vacancies are also rapidly dragged to point-defect sinks, and thus removed from the matrix. To overcome this issue, two types of solutes are added to the Cu matrix: A first solute with a strong vacancy binding energy (B-type species) and another solute that binds to 'B' and is a slow diffuser in Cu (C-type species). Using DFT, 21 synergistic solute pairs are screened, with 'B'=Zr, Ge, Sn and 'C'=Fe, Co, Mo, Ni, Nb, W, Cr. Two promising alloys, Cu(Zr,Co) and Cu(Zr,Fe) are then investigated in detail in the dilute regime. Diffusion and solute drag in these alloys are modeled using the kinetic cluster expansion approach (KineCluE) under irradiation conditions. It is shown that strong Zr-'C' thermodynamic binding, especially between Zr and Co, significantly reduces the mobility of Zr solute and suppresses the vacancy-mediated solute drag. Using an analytical framework for the standard five-jump frequency model for diffusion in binary alloys, it is found that vacancy-Zr-Co triplets disrupt the kinetic circuits that promote solute drag in the binary alloy by raising the dissociation barrier for the vacancy from the solute.

A novel approach for imparting radiation resistance to dilute alloys is proposed whereby two synergistic solute species are employed, a first one, solute B, that binds strongly to vacancies and a second one, solute C, that binds to solute B and is also a slow diffuser in solvent A. This combination results in B-C solute clusters that are immobile traps for vacancies. These traps promote point-defect recombination over irradiation doses far beyond that achievable in binary alloys, where solutes that strongly bind to vacancies are typically fast diffusers and thus quickly removed from grain interiors by radiation-induced segregation. A parametric study, performed using atomistic kinetic Monte Carlo simulations with realistic metallic solute properties in Cu, reveals that alloy stability under irradiation derives largely from the formation of mixed B-C solute clusters comprised of 10 or more atoms. The solute loss at sinks, moreover, is found to follow stretched exponentials, with the most promising alloys corresponding to values of the stretch exponent beta approaching 0.5. The effects of irradiation dose rate and grain size are discussed using simple scaling relationships. Lastly, the approach is illustrated by identifying promising solute combinations in Cu, Ni and Al alloys.

The effect of grain boundary character on radiation-induced segregation (RIS) is investigated in a 316L austenitic stainless steel irradiated with 2 MeV protons at 360 degrees C. Orientation imaging microscopy is employed to select specific grain boundaries (GBs), including Sigma 3{111} coherent twin boundaries, fully characterized by a five- degree of freedom analysis, as well as high angle GBs. Chemical maps along these GBs below the irradiated surface, at depths corresponding to damage levels ranging from 2.3 dpa to 4.2 dpa, are acquired using energy- dispersive spectrometry in a scanning transmission electron microscope (STEM-EDS). RIS levels are defined as elemental GB excess quantities and are used to compare RIS at twin boundaries and high-angle GBs. These measurements are complemented by the analysis of void distributions near GBs and by characterizing the structure of coherent twin GBs prior to and after irradiation using high-resolution STEM imaging. In light of the results obtained in this work, the evolution of the efficiency for point defect elimination of coherent twin GBs with the irradiation dose is discussed.

Most materials are brought into nonequilibrium states during processing and during their service life. Materials for nuclear and space applications, for instance, are continuously exposed to energetic particle irradiation, which is often detrimental to materials' performance. Here we demonstrate, however, that sustained irradiation can induce self-organization of the microstructure of polycrystalline alloys into steady-state patterns and, in turn, improve their radiation resistance. Using an Al -1.5 at.% Sb alloy as a model system, we show using transmission electron microscopy and atom probe tomography that, for nanocrystalline thin films irradiated at 75 degrees C with 2 MeV Ti ions to large doses, the microstructure consists of finite-size, self-organized AlSb nanoprecipitates inside the grains and along the grain boundaries. Furthermore, this steady state is independent of the initial microstructure, thus self-healing. Phase field modeling is employed to construct a steady-state phase diagram and extend the experimental results to other alloy systems and microstructures.

The evolution of segregation and precipitation at grain boundaries (GBs) in dilute alloys subjected to ion irradiation is investigated by combining phase field (PF) modeling and experiments. The irradiation conditions and alloy parameters promoting the self-organization of finite-size precipitate structures at GBs are specifically investigated. PF simulations indicate that, in two-phase systems, irradiation may induce segregation and precipitation at GBs when point-defect solute drag exceeds back thermal diffusion and irradiation atomic mixing. Fast solute diffusion along GBs promotes GB precipitate coarsening, and it is thus detrimental to the dimensional stability of GB precipitates. Experimental evaluation was performed on Al-1.5 at

Light-weight, high-strength, aluminum (Al) alloys have widespread industrial applications. However, most commercially available high-strength Al alloys, like AA 7075, are not suitable for additive manufacturing due to their high susceptibility to solidification cracking. In this work, a custom Al alloy Al92Ti2Fe2Co2Ni2 is fabricated by selective laser melting. Heterogeneous nanoscale medium-entropy intermetallic lamella form in the as-printed Al alloy. Macroscale compression tests reveal a combination of high strength, over 700 MPa, and prominent plastic deformability. Micropillar compression tests display significant back stress in all regions, and certain regions have flow stresses exceeding 900 MPa. Post-deformation analyses reveal that, in addition to abundant dislocation activities in Al matrix, complex dislocation structures and stacking faults form in monoclinic Al9Co2 type brittle intermetallics. This study shows that proper introduction of heterogeneous microstructures and nanoscale medium entropy intermetallics offer an alternative solution to the design of ultrastrong, deformable Al alloys via additive manufacturing. Commercial high-strength Al alloys often suffer from cracking during additive manufacturing. Here, the authors present an additively manufactured, a strong and plastically deformable Al alloy with heterogeneous nanoscale intermetallics.

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We consider the stability of precipitates formed at grain boundaries (GBs) by radiation-induced segregation in dilute alloys subjected to irradiation. The effects of grain size and misorientation of symmetric-tilt GBs are quantified using phase field modeling. A novel regime is identified where, at long times, GBs are decorated by precipitate patterns that resist coarsening. Maps of the chemical Péclet number indicate that arrested coarsening takes place when solute advection dominates over thermal diffusion right up to the precipitate-matrix interface, preventing interfacial local equilibrium and overriding capillary effects. This contrasts with liquid-solid mixtures where convection always accelerates coarsening.

Strengthening of nanocrystalline Al by grain boundary solute additions was investigated for a series of dilute aluminum alloys, Al-Sc, Al-Sb, Al-Cr, and Al-W with grain sizes in the range of 50-200 nm. Thermal annealing of the alloys at low temperatures led to alloy softening, but with negligible change in the grain size. The re-duction in strength can be attributed to the loss of solute in the grain boundaries arising from grain boundary diffusion and precipitation. Annealing at higher temperatures led to grain growth, but with little additional loss of strength, a result of precipitation hardening. The Al-Sc and Al-Sb alloys were additionally subjected to ion irradiation at various temperatures. These studies revealed that annealed samples regained their hardness due to solute redistribution by ion beam mixing. Alloy strength was independent of grain size between 50 and 150 nms. Irradiation-induced segregation of Sb to grain boundaries in Al-Sb further enhanced strengthening.

The role of interfaces on phase formation during severe plastic deformation (SPD) was investigated using molecular dynamics simulations. It is found that dilute solute additions that segregate to interphase boundaries, and strengthen them, can greatly extend the solubility limits of the bounding phases at steady state, providing a novel approach to processing nanocomposite alloys by SPD. The results are rationalized in terms of a modified effective temperature model, whereby increasing the interface strength leads to higher effective temperatures and correspondingly to higher solubilities. Comparison of the segregation coefficient at the interphase boundary with solubilities in the bounding phases illustrates the effective temperature model is self-consistent and comprehensive. Past experimental studies on severe plastic deformation are found consistent with these observations.

The original Cahn-Hilliard derivation of the contribution of compositional inhomogeneity to the free energy of a binary alloy with pairwise interactions is extended to include higher-order inhomogeneity terms. For alloys on a cubic lattice, the coefficient of the first inhomogeneity is a second-rank tensor and reduces to a scalar, but it is shown that the second order and the third order inhomogeneity terms are weighted by fourth-rank and sixth-rank tensors, thus resulting in anisotropic contributions. Furthermore, each interaction shell generates a unique set of inhomogeneity coefficients that is determined by the set of vectors connecting an atom to its neighbors on that shell. These coefficients are calculated for fcc and bcc alloys with interactions up fourth nearest neighbors. Phase field simulations based on these extended Cahn-Hilliard free energies are performed to measure interface free energies along specific crystallographic directions as a function of temperature, and to obtain the equilibrium shape of precipitates. Interface free energies, and the resulting anisotropies, are compared to those obtained by discrete models and Monte Carlo simulations.

Compositional patterning (CP) in binary alloys during energetic particle irradiation is studied using a kinetic model that considers two competing kinetic processes, a thermally activated one promoting macroscopic phase separation (MPS) of the concentration field c(r, t), and a forced one resulting in finite-range random atomic mixing. The forced mixing is modeled by a Gaussian relocation distribution with a characteristic distance R. A series of approximate kinetic models are introduced by expanding the mixing function into a series of n terms, thus replacing the nonlocal evaluations of the concentration field c(r' - r, t) by local derivatives of c(r, t). This approach makes it possible to obtain exact effective potentials and build steady-state diagrams for each order-n model. Phase-field (PF) simulations using these order-n models reveal that near the onset of patterning, phase evolution is accurately described using an order-3 model, which changes smoothly from an extended Cahn-Hilliard free energy in the MPS regime to a one-mode Swift-Hohenberg functional in the CP regime. Deeper into the patterning regime, higher-order models are required to achieve convergence, yielding squarelike concentration profiles characteristic of a strong segregation regime. These higher-order effective free energies are analogous to multimodal Swift-Hohenberg functionals. An alternative definition for the effective interfacial energy is proposed in the CP regime, since the interfacial area is no longer an excess quantity in that regime, precluding the use of the standard thermodynamic definition of interfacial energy.

Irradiation of alloys with energetic particles leads to the forced chemical mixing of atoms and the cre-ation of point defects. At elevated temperatures, the point defects are mobile and enhance diffusion. Since forced mixing occurs in energetic displacement processes, alloy constituents tend to flow down gradients in their concentration, while radiation enhanced diffusion is thermally activated and alloy components respond to gradients in their chemical potential. Phase evolution in irradiated alloys thus depends on the competition between these two dynamics. A similar situation occurs during severe plastic deforma-tion, dislocation glide results in forced chemical mixing, while dislocation reactions lead to the creation of vacancies and enhanced thermally activated diffusion. One possible consequence of these competing dynamics is that alloys self-organize into compositional patterns, i.e., concentration variations adopt a fixed length scale determined by the nature of the driving forces and the internal dynamics of the al-loy. The current perspective illustrates how the details of driving forces relate to the length scale of the compositional patterns and their morphology. A surprising finding is that irradiation and severe plastic deformation can both lead to compositional patterning even at low temperatures, i.e., in absence of thermally activated diffusion. While the composi-tional patterning again derives from competing dynamical processes, the characteristic length scales are determined by quite different aspects of the forced mixing. For irradiation, the ballistic recoil distance limits the maximum length scale for patterning at high temperatures, while it is the extent of the ther-mal spike at low temperatures. For severe plastic deformation, the glide distance of dislocations controls the largest length scale for patterning at high temperatures, whereas kinetic roughening of precipitates controls the maximum length at low temperatures. (c) 2021 Elsevier B.V. All rights reserved.

Materials in service and during processing are often subjected to plastic deformation. For multi-phase metallic alloys, simple geometric models and atomistic simulations suggest that two distinctive regimes in these materials' evolution during deformation exist. At low strains, evolutions are often dominated by the kinetic roughening of interfaces, which results from the superdiffusive transport of matter in sheared crystals. At high strains and temperatures where thermal diffusion is sluggish, on the other hand, shearing-induced forced atomic mixing dominates these evolutions, resulting in significantly enhanced solubility. Distinguishing these two regimes is shown to provide a convenient framework for rationalizing and analyzing recent experiments and simulations on wear of layered structures in the low strain-regime and nonequilibrium phase co-existence and self-organization in highly immiscible or reactive alloy systems in the high-strain regime.

Phase evolution in FCC metals with strongly interacting alloy components during severe plastic deformation is investigated using molecular dynamics simulations. Specifically, we study the alloy microstructure in steady state, nucleation and growth of precipitates in supersaturated alloys, and the dissolution of precipitates in undersaturated alloys. The results are compared to a modified effective temperature model, providing a physical understanding for the atomic processes underlying the model and a perspective on its strengths and weaknesses. Key observations in this work are nucleation and growth of precipitates during SPD at a temperature of 100 K; Gibbs-Thomson-like behavior relating steady-state solubility to precipitate size under sustained shearing; a direct relationship between the effective temperature and the shear modulus; and the importance of cluster agglomeration during precipitate growth. The study also reveals that the mechanism of forced chemical mixing depends on precipitate size, adding complications for effective temperature models describing inhomogeneous systems. The simulations are shown to provide good semi-quantitative agreement with experimental findings reported in the literature.

Additions of solute that trap vacancies slow down vacancy diffusion and promote point-defect recombination in alloys subjected to irradiation. Such selective alloying can thus help to minimize the detrimental consequences resulting from point defect fluxes. The current work investigates the effect of solute additions on the recombination rate using kinetic Monte Carlo simulations for a model alloy system, which was parametrized to Cu-Ag in the dilute limit, but with an increased solubility limit, approximate to 0.86 at.% at 300 K. As the solute concentration was increased above 0.1 at.%, solute clustering was observed and led to a strong increase in recombination rate. The beneficial effects of solute clustering on reducing vacancy mobility, and reducing solute drag, were analyzed by calculating relevant transport coefficients using the KineCluE code (Schuler et al., Computational Materials Science (2020) 172,109,191). Moreover, it was observed in the KMC simulations that large recombination rates resulted in a shift of steady-state distributions of solute cluster sizes to smaller clusters compared to equilibrium distributions in the solid solution. This shift is rationalized as resulting from the irreversible character of the interstitial-vacancy recombination reaction. These results suggest a novel irradiation effect on phase stability where a high recombination rate increases the solubility limit of a solute at steady state over its equilibrium value.