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.

Electron-irradiation induced creep rates in amorphous alloys, a-SiO2, Fe79B16Si5, Cu60Ta40, and Cu50Ti50, were measured at room temperature using a miniaturized beam-bending apparatus within a transmission electron microscope operated at 200 keV. The creep rates of these amorphous samples increased nearly linearly with both e-beam current density and applied stress, while a reference crystalline (c-)SiO2 sample failed to creep under the same conditions. The irradiation-induced creep compliance of a-SiO2 was '15 times larger than that of Fe79B16Si5 and over 1,000 times larger than that of the two Cu alloys. Molecular dynamics computer simulations were employed to simulate electron irradiation induced creep using interatomic potentials representing amorphous Cu75Zr25, Ni85P15, and SiO2 as model systems. The irradiation induced creep compliances calculated for Cu75Zr25 during 200 keV electron irradiation provided good quantitative agreement with the two Cu-based alloys, but that for a-SiO2 was '180 times too small. These results indicate that unlike neutron or ion-beam induced creep in a-SiO2, creep under electron irradiation is dominated by the effects of ionization, owing largely to the far higher ratio of electronic stopping to nuclear stopping for electrons than for ions.

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.

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

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.

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.

Radiation enhanced diffusion in the intermetallic alloy Cu3Au was measured directly for the first time. Ni and Pd tracer impurities were used to probe the diffusion of atoms occupying Cu and Au sublattice site's, respectively. Measurements were performed temperatures both above and below the ordering temperature, of TC =390°C The ability to grow high quality Cu3Au films by MBE with buried marker layers in the form of either (Cu2Ni)Au or Cu3(AuPd), made these measurements possible. Under irradiation with 0.75MeV Ar+ ions, the diffusion of Ni is approximately equal to that of Pd above Tc, but smaller below Tc. In the absence of irradiation, the thermal diffusion coefficients reveal a similar trend. Activation energies for RED for the two species are similar both above and below Tc whereas, in absence of irradiation, the activation enthalpies below Tc are different. These results are interpreted on the basis of lattice order.

Atomic transport in irradiated solids has been investigated in both the prompt and delayed regimes. Prompt effects are revealed on an atomic level through molecular dynamics computer simulations. It is demonstrated that for metals like gold, which have high atomic numbers and low melting points, thermal spikes play a primary role in the cascade dynamics and that concepts like melting and rapid quenching are useful descriptions. Surface effects in these metals are also discussed. For metals with higher melting points and lower atomic numbers, the cascade dynamics are determined almost exclusively by energetic collisions far above thermal energies. This is illustrated by simulations of cascades in NiAl. The effect of the high ordering energy in this intermetallic compound on the radiation-induced defect structure has also been studied. Atomic transport in the delayed regime is illustrated by two examples: an order-disorder alloy, Cu3Au, and an amorphous alloy, NiZr. The first example is used to illustrate various aspects of radiation enhanced diffusion (RED): ion beam mixing, diffusion kinetics, the effects of primary recoil spectrum, and the importance of chemical order. The second example illustrates that the basic theory of RED, which was developed to describe crystalline materials, appears to work adequately for amorphous metal alloys, suggesting that similar mechanisms may be operating. It is shown, however, that the kinetics of RED observed in amorphous alloys are not unique to point defect models.

The collision dynamics of Cu, Ni or Al clusters containing 92 atoms with substrates of these same metals were studied using molecular dynamics computer simulations. For energies of 1.0 keV or 326 eV, diverse behavior was observed, depending sensitively on the relative properties of the cluster and substrate. For Cu clusters impacting Cu substrates, the cluster plastically deforms the substrate and creates a deep crater with substrate atoms forming a ridge at the periphery of the crater. In contrast, Al clusters do not much deform Ni substrates, but rather tend to spread epitaxially over the surface. Several Al atoms dissociate from the cluster, and either reflect into the vacuum or scatter over the surface. 326 eV Ni clusters embed themselves almost completely within Al substrates and form localized amorphous zones.

The sintering and deformation kinetics of nanocrystalline (n-) TiO2 were studied. It was found that pressureless sintering yields densities above 95 % with modest increase in grain size whereas pressure assisted sintering (1GPa) results in high density samples without grain growth. In addition compression tests were performed. Large true strains up to 0.6 and strain rates as high as 8 × 10−5 s−1 were observed at 810°C without fracturing. The sintering and creep results are discussed in terms of the microstructure and diffusion coefficients in n-TiO2.

Microstructural evolution of Cu–Nb oxide nanocomposite alloys during ball milling is investigated using a two-step ball-milling approach. In the first step, Cu and Nb powders are milled to create a two-phase alloy comprising a Cu-rich matrix containing a high density of 20- to 30-nm Nb precipitates. In the second step, this nanocomposite is co-milled with CuO, resulting in the reduction of CuO and the oxidation of the Nb nanoprecipitates. Transmission electron microscopy characterization shows that three distinct types of Nb oxide precipitates evolve at different levels of strain. First, nanocrystalline NbO particles ( ∼ 10 nm) are formed by dissolved Nb in Cu reacting with oxygen evolved from the CuO. Next, the Nb nanoprecipitates in Cu further reduce CuO to form Nb/Nb oxide and NbO/Nb oxide core–shell inclusions (20–30 nm). These inclusions coalesce during additional milling to form amorphous Nb oxide agglomerates (>700 nm after 50 h). The growth of Nb precipitates during step-one milling, the initial growth of NbO nanoparticles, and the formation of core–shell Nb oxide precipitates during step-two milling are attributed to the convective transport of atoms and clusters combined with shear-induced agglomeration.

Irradiation induced creep (IIC) compliance in NiCoFeCrMn high entropy alloys is measured as a function of grain size (30 < x < 80 nm) and temperature (23-500 degrees C). For 2.6 MeV Ag3+ irradiation at a dose rate of 1.5x10(-3) dpa(-1) s(-1) the transition from the recombination to sink limited regimes occurs at similar to 100 degrees C. In the sink-limited regime, the IIC compliance scales inversely with grain size, consistent with a recently proposed model for grain boundary IIC. The thermal creep rate is also measured: it does not become comparable to the IIC rate, however, until similar to 650 degrees C. The results are discussed in context of defect kinetics in irradiated HEA systems. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.