Single-phase Concentrated Solid Solution Alloys Research Articles

Effects of composition complexity of single-phase concentrated solid solution alloys on He bubbles behavior

Three types of single-phase concentrated solid solution alloys (SP-CSAs) with compositions of NiCoFe, NiCoFeCr and NiCoFeCrMn were prepared using the vacuum arc melting method. The bulk samples were implanted by 100 keV helium ions to a fluence of 5 × 1016 ions/cm2 at 673 K. The transmission electron microscopy (TEM) is employed to systematically characterize helium bubbles along the depth of implantation. The observation results revealed that the He bubble density followed the sequence NiCoFeCr > NiCoFeCrMn > NiCoFe, and the bubble sizes exhibited the trend NiCoFe > NiCoFeCrMn > NiCoFeCr. For NiCoFe and NiCoFeCr, the higher the number complexity of components, the higher the density and the smaller the size of He bubbles. However, the introduction of Mn as a larger-sized atom appeared to increase the vacancy diffusivity and lower the melting temperature, consequently promoting helium bubble growth.

MD simulation of primary radiation damage in fcc multi-principal element alloys: Effect of compositional undulation

Fcc single phase alloys with highly concentrated solid solution are competitive candidates for the next-generate radiation-tolerant materials. The present study uses molecular dynamics simulations to study the displacement cascades of NiCoFeCr fcc single-phase concentrated solid-solution alloys (SP-CSAs). The effects of compositional undulation on the generation and evolution of the defects, produced by primary knock-on atoms, are quantitatively analysed. Our simulation results show that the undulation renders considerable compositionally graded interfaces (CGIs) with high threshold energy for displacement, and these CGIs can act as pre-existing defect sinks and confine interstitial defects motion. This increases the recombination rate of residual defects, thus leading to the suppression of Frenkel pairs and large-sized interstitial clusters. In addition, such effect of compositional undulation is correlated with the specific chemical elements, i.e., the Ni-Fe and Co-Fe undulated SP-CSA yield slower radiation damage accumulation. Our findings suggest a new degree of freedom to tailor radiation-tolerance for concentrated solid-solution alloys.

Interactions between displacement cascades and grain boundaries in NiFe single-phase concentrated solid solution alloys

Single-phase concentrated solid solution alloys (SP-CSAs) are promising structural materials with excellent irradiation resistance. It is very important for their application in nuclear reactor structures to understand the interactions between their grain boundaries (GBs) and irradiation-induced defects. With molecular dynamics (MD) simulations, the displacement cascades in NiFe SP-CSAs with Σ5(210) symmetrical tilt GB were studied. Compared with pure Ni, NiFe has higher sink strength of GB for vacancies but has little effect on the interstitial absorption of GB. The formation energies of interstitials and vacancies near the GB are more dispersed, so that the point defects are more difficult to aggregate, which is beneficial for the recombination of interstitials and vacancies near the GB. Both the formation energies of point defects and the sink strengths of GB for point defects could be changed with the SP-CSA compositions. When the Fe atom concentration is higher than 30 at.%, the point defects are easier to annihilate. Both the effects of the PKA energy and the PKA-GB distance on the displacement cascades damage can be explained with the size of overlapping region between the cascade region and GB. In addition, the formation of SP-CSAs helps to annihilate the vacancies in NiFe with Σ37(750) symmetrical tilt GB and Σ5 twist GB.

Irradiation performance of concentrated solid-solution alloys: Insight into defect behaviors

Single-phase concentrated solid-solution alloys (SP-CSAs) intensively attracted researchers for their promising performances and tailorable constituents. In this work, molecular dynamic simulations were employed to investigate the radiation effects in equiatomic FeCrNi and FeNi, compared with an elemental Ni, mainly based on threshold displacement energy (Ed), individual recoil and overlapping cascades. Anisotropy of Ed distribution is a common feature in the three materials. However, defect behaviors concerning production in a single cascade and accumulation in multi-cascade process all exhibit distinctive features, which are particularly distinguished between CSAs and the elemental Ni. The discrepancy starts from the defect creation in the colliding phase, escalates in the thermal annealing phase of the single cascade, and is further amplified in the overlapping cascades. Corresponding physical processes involve atomic collision, heat dissipation and atomic diffusion. We discussed the channeling effect of atomic collision, thermal conductivity and sluggish diffusion in the CSAs to discover the origin causing the difference in defect morphology. Concentrated distribution of defects created in the colliding phase and low thermal dissipation contributes to higher defect recombination rate in the CSAs. Sluggish effect suppressing defect mobility further balances the production and annihilation of defects during the multi-cascades. We interpreted why the number of displacement defects is prone to be saturated under multi-cascades and only small-size clusters are left in the CSAs, unlike the pure Ni in which an oversized cluster was observed. The results are beneficial to designing radiation-tolerant materials by shedding light on the genesis of defect morphology in CSAs under irradiations.

Multiscale nanoindentation modelling of concentrated solid solutions: A continuum plasticity model

Recently developed single-phase concentrated solid-solution alloys (CSAs) contain multiple elemental species in high concentrations with different elements randomly arranged on a crystalline lattice. These chemically disordered materials present excellent physical properties, including high-temperature thermal stability and hardness, with promising applications to industries at extreme operating environments. The aim of this paper is to present a continuum plasticity model accounting for the first time for the behaviour of a equiatomic five-element CSA, that forms a face-centred cubic lattice. The inherent disorder associated with the lattice distortions caused by an almost equiatomic distribution of atoms, is captured by a single parameter α that quantifies the relative importance of an isotropic plastic contribution to the model. This results in multiple plasticity mechanisms that go beyond crystallographic symmetry-based ones, common in the case of conventional single element metals. We perform molecular dynamics simulations of equiatomic CSAs: NiFe, NiFeCr, NiFeCrCo, and Cantor alloys to validate the proposed continuum model which is implemented in the finite element method and applied to model nanoindentation tests for three different crystallographic orientations. We obtain the representative volume element model by tracking the combined model yield surface.

Nanoprecipitates to Enhance Radiation Tolerance in High-Entropy Alloys.

The growth of advanced energy technologies for power generation is enabled by the design, development, and integration of structural materials that can withstand extreme environments, such as high temperatures, radiation damage, and corrosion. High-entropy alloys (HEAs) are a class of structural materials in which suitable chemical elements in four or more numbers are mixed to typically produce single-phase concentrated solid solution alloys (CSAs). Many of these alloys exhibit good radiation tolerance like limited void swelling and hardening up to relatively medium radiation doses (tens of displacements per atom (dpa)); however, at higher radiation damage levels (>50 dpa), some HEAs suffer from considerable void swelling limiting their near-term acceptance for advanced nuclear reactor concepts. In this study, we developed a HEA containing a high density of Cu-rich nanoprecipitates distributed in the HEA matrix. The Cu-added HEA, NiCoFeCrCu0.12, shows excellent void swelling resistance and negligible radiation-induced hardening upon irradiation up to high radiation doses (i.e., higher than 100 dpa). The void swelling resistance of the alloy is measured to be significantly better than NiCoFeCr CSA and austenitic stainless steels. Density functional theory simulations predict lower vacancy and interstitial formation energies at the coherent interfaces between Cu-rich nanoprecipitates and the HEA matrix. The alloy maintained a high sink strength achieved via nanoprecipitates and the coherent interface with the matrix at a high radiation dose (∼50 dpa). From our experiments and simulations, the effective recombination of radiation-produced vacancies and interstitials at the coherent interfaces of the nanoprecipitates is suggested to be the critical mechanism responsible for the radiation tolerance of the alloy. The materials design strategy based on incorporating a high density of interfaces can be applied to high-entropy alloy systems to improve their radiation tolerance.

Effects of local chemical ordering on defect evolution in NiFe concentrated solid solution alloy

Single-phase concentrated solid solution alloys (SP-CSAs), including high-entropy alloys (HEAs), are promising candidates for structural materials in nuclear reactors. Recent studies have demonstrated that local chemical ordering (LCO) is an important structural feature in these alloys, which can significantly affect defect evolution. The presence of LCO leads to elemental rearrangement in CSAs and results in distinct interfaces separating the ordered and disordered phases. In this work, we study the role of LCO and order/disorder interface on defect evolution in NiFe CSAs. Our results indicate that the interface can act as defect sinks in the defect recombination stage. At the diffusion stage, defect evolution is strongly affected by LCO due to chemically biased diffusion in CSAs and preferential diffusion pathways of defect clusters. We further show that the degree of LCO and the dimensions of the LCO domains play significant roles in defect clustering and annihilation behavior. These results provide fundamental insight into the influence of LCO on defect evolution in CSAs and pave ways for designing highly irradiation-tolerant structural materials for nuclear applications.

Molecular dynamics simulation of radiation defect evolution mechanism of NiFe-graphene nanocomposite

Atomistic simulations were applied to investigate the coupling effect of the chemical complexity of single-phase concentrated solid solution alloys (SP-CSAs) and the defect-sink effects of graphene/CSA interfaces. The radiation-induced defect evolution and radiation resistance of NiFe–graphene (NiFeGr) nanocomposite were studied via energy calculation, cascade simulation, and defect insertion method. Results show that the interface acts as a defect trap to absorb interstitials and promote the migration of point defects toward the interface. The inherent effect of the CSAs promotes defect recombination, which significantly reduces the number of residual defects. At the same time, the concentrated solid solution alloy as matrix can effectively promote the strengthening of the interface. The combination of the two effects causes no obvious temperature effect of the radiation damage, but the material still shows good radiation resistance at high temperature. Therefore, the NiFeGr nanocomposite with synergy of concentrated solid solution alloy as a matrix and defect-sink effects from graphene interfaces shows good radiation resistance and broad application prospect in nuclear engineering materials.

Effects of temperature on helium cavity evolution in single-phase concentrated solid-solution alloys

Single-phase concentrated solid-solution alloys (SP-CSSAs) have shown great potential as structural materials in advanced reactor systems. Up to date, nevertheless, the synergistic effect of compositional complexity and irradiation temperature on helium (He) irradiation-induced cavity formation has not been systematically studied yet. In this work, pure Ni and three prototype face-centered-cubic SP-CSSAs, i.e., NiCo, NiCoCr and NiCoFeCrMn, were irradiated with He + ions at 673, 773, 873 and 973 K. The evolution of He-induced cavities were investigated in detail with transmission electron microscopy . It was found that growth of He cavities was suppressed with increasing compositional complexity at 673 and 773 K. Due to the lower melting temperature, NiCoFeCrMn exhibited the strongest temperature susceptibility in terms of He cavity growth with the increasing temperature. Interestingly, the He cavity growth tended to be enhanced with increasing compositional complexity when the radiation temperature increaseed to 973 K, which can be attributed to the prominent chemical-biased vacancy migration at high temperatures.

Temperature-dependent helium induced microstructural evolution in equiatomic NiCo and NiFe concentrated solid solution alloys

Irradiation temperature and the species of elemental constituents are two pivotal factors in determining the helium (He) radiation behaviors. However, their synergistic effect on the He-induced defect evolution in single-phase concentrated solid-solution alloys (SP-CSSAs) has rarely been studied. In this work, the evolution of He cavities and dislocation structural damages in two equiatomic SP-CSSAs, i.e., NiCo and NiFe, were investigated at 673 K, 773 K and 873 K with 275 keV He+ ions. He cavity formation behaviors were diverse at different temperature regimes. NiFe was found to have a stronger suppression effect on He cavity growth at 673 K. However, with the increasing temperature, a more pronounced increment of He cavity size was exhibited in NiFe. Unlike the evolution of He cavities, the growth of dislocation loops tends to be delayed in NiFe over the studied irradiation temperatures. The underlying mechanisms for the temperature-dependent defect evolution in NiCo and NiFe are discussed.

Segregation of Ni at early stages of radiation damage in NiCoFeCr solid solution alloys

Defect evolution under irradiation is investigated in a set of single-phase concentrated solid solution alloys (SP-CSAs) containing Ni with Co, Fe and/or Cr. We show that atomic segregation of Ni takes place already at very early stages of radiation damage in the 2–4 element SP-CSAs containing Fe or Cr, well below 1 dpa. We arrive at this conclusion by following the evolution of positron annihilation signals as a function of irradiation dose in single crystal samples, complemented by molecular dynamics simulations in the same model systems for high entropy alloys (HEAs). This manifestation of short-range order calls attention to composition fluctuations at the atomic level in irradiated HEAs. Ion irradiation may induce short-range order in certain alloys due to chemically biased elemental diffusion. The work highlights the necessity of updating the assumption of a totally random arrangement in the irradiated alloys, even though the alloys before irradiation have random arrangements of different chemical elements.

Defect evolution in Ni and solid-solution alloys of NiFe and NiFeCoCr under ion irradiation at 16 and 300 K

Single-phase concentrated solid-solution alloys (SP-CSAs) have shown unique chemical complexity at the levels of electrons and atoms, and their defect evolution is expected to be different from conventional dilute alloys. Single crystals of Ni, NiFe and NiFeCoCr are chosen as model systems to understand the chemical complexity on defect formation and damage accumulation in SP-CSAs under ion irradiation. The high-quality crystals were irradiated at 16 and 300 K to different ion fluences, to form irradiated region with little to heavy damages. The ion-induced damage was determined using Rutherford backscattering spectrometry technique along a channeling direction (RBS/C) and the level of lattice damage in irradiated Ni and SP-CSAs was quantified from Monte Carlo (MC) simulations. The results are interpreted using the Multi Step Damage Accumulation model to reveal material damage accumulation kinetics. Key findings of the study are that in case of room temperature irradiations the damage level measured for complex alloys at the highest irradiation fluence of 2 × 1015 cm−2 (∼3 dpa) is significantly higher than that obtained for pure nickel samples and suggest two-step damage accumulation process with a defect transformation taking place at a fluence of about 1.5 × 1015 cm−2. Moreover, structural and damage kinetic differences clearly imply that, with increasing degree of chemical complexity and high solid-solution strengthening effects from Ni to NiFe and to NiFeCoCr, the enhanced lattice stiffness resists to randomization of atomic configurations and inhibits the growth of extended defects.

Effects of 3d electron configurations on helium bubble formation and void swelling in concentrated solid-solution alloys

Elemental specific chemical complexity is known to play a critical role in microstructure development in single-phase concentrated solid-solution alloys (SP-CSAs), including both He bubble formation and irradiation-induced void swelling. While cavity formation and evolution under ion irradiation at elevated temperature are complex nonequilibrium processes, chemical effects are revealed at the level of electrons and atoms herein in a simplified picture, using Ni and a special set of Ni-based SP-CSAs composed of 3d transition metals as model alloys. Based on Ni and the model alloys with minimized variables (e.g., atomic mass, size, and lattice structure), we discuss the effects of chemically-biased energy dissipation, defect energetics, sluggish diffusion, and atomic transport on cavity formation and evolution under both self-ion Ni irradiation and He implantation. The observed difference in microstructure evolution is attributed to the effects of d electron interactions in their integrated ability to dissipate radiation energy. The demonstrated impact of alloying 3d transition metals with larger differences in the outermost electron counts suggests a simple design strategy for tuning defect properties to improve radiation tolerance in structural alloys.

Dissipation of radiation energy in concentrated solid-solution alloys: Unique defect properties and microstructural evolution

Abstract

Effect of electronic energy dissipation on strain relaxation in irradiated concentrated solid solution alloys

The effect of energy deposition by energetic particles on Ni and two single-phase concentrated solid solution alloys (NiFe and NiCoCrFe) is investigated through combined experimental and modelling efforts. Damage evolution as a function of increasing ion fluence is monitored via elastic strain developed in the irradiated crystals. We show that damage produced from displacement collision cascades is sensitive to subsequent highly ionizing irradiation that the strain generated by elastic nuclear collisions undergoes partial relaxation upon high-energy irradiation. This finding indicates a change in the damage structure upon electronic energy deposition due to both predominant defect annealing and growth of small defect clusters. Strain relaxation, more pronounced in the alloys than in Ni, is ascribed to both higher thermal conductivity and weaker electron-phonon coupling in Ni.

Effects of Fe concentration on helium bubble formation in NiFex single-phase concentrated solid solution alloys

The formation of helium bubbles and the consequential property degradation are critical challenges for materials used in advanced nuclear energy systems. The mechanism of helium bubble formation in a series of NiFex (x = 0, 20, 35, or 50 at.%) single-phase concentrated solid solution alloys (SP-CSAs) are studied in this work. Transmission electron microscopy analysis shows that with increasing Fe concentration, the average bubble size decreases while the bubble number density increases, indicating the growth of large helium bubbles in NiFex SP-CSAs is suppressed by increasing the alloy chemical complexity through increased Fe concentration. Mechanisms for the slower bubble growth rate in NiFex binary SP-CSAs compared Ni are identified. These mechanisms include promotion of Frenkel pair recombination, denser and more homogeneously distributed nucleation sites, and increased helium atom densities inside the bubbles. The results demonstrate that optimizing the chemical complexity in SP-CSAs can be an effective approach to design materials with superior resistance to helium bubble nucleation and growth.

Helium irradiated cavity formation and defect energetics in Ni-based binary single-phase concentrated solid solution alloys

Binary single-phase concentrated solid solution alloys (SPCSAs), including Ni80Co20, Ni80Fe20, Ni80Cr20, Ni80Pd20, and Ni80Mn20 (in atomic percentage), were irradiated with 200 keV He+ ions at 500 °C. He cavity size and density distribution were systematically investigated using transmission electron microscope. Here we show that alloying elements have a clear impact on He cavity formation. Cavity size is the smallest in Ni80Mn20 but the largest in Ni80Co20. Alloying elements could also substantially affect cavity density profile. In-depth examination of cavities at peak damage region (∼500 nm) and at low damage region (∼300 nm) demonstrates that cavity size is depth (damage) dependent. Competition between consumption and production of vacancies and He atoms could lead to varied cavity size. Density functional theory (DFT) calculations were performed to obtain the formation and migration energies of interstitials and vacancies. Combined experimental and simulation results show that smaller energy gap between interstitial and vacancy migration energies may lead to smaller cavity size and narrower size distribution observed in Ni80Mn20, comparing with Ni80Co20. The results of this study call attention to alloying effects of specific element on cavity formation and defect energetics in SPCSAs, and could provide fundamental understanding to predict radiation effects in more complexed SPCSAs, such as high entropy alloys.

Irradiation responses and defect behavior of single-phase concentrated solid solution alloys

Abstract

Single-Phase Concentrated Solid-Solution Alloys: Bridging Intrinsic Transport Properties and Irradiation Resistance

Single-phase concentrated solid-solution alloys (SP-CSAs), including high entropy alloys, are compositionally complex but structurally simple, and provide a playground of tailoring material properties through modifying their compositional complexity. The recent progress in understanding the compositional effects on the energy and mass transport properties in a series of face-centered-cubic SP-CSAs is the focus of this review. Relatively low electronic and thermal conductivities, as well as small separations between the interstitial and vacancy migration barriers have been generally observed, but largely depend on the alloying constituents. We further discuss the impact of such intrinsic transport properties on their irradiation response; the linkage to the delayed damage accumulation, slow defect aggregation, and suppressed radiation induced swelling and segregation has been presented. We emphasize that the number of alloying elements may not be a critical factor on both transport properties and the defect behaviors under ion irradiations. The recent findings have stimulated novel concepts in the design of new radiation-tolerant materials, but further studies are demanded to enable predictive models that can quantitatively bridge the transport properties to the radiation damage.

Effects of chemical alternation on damage accumulation in concentrated solid-solution alloys

Single-phase concentrated solid-solution alloys (SP-CSAs) have recently gained unprecedented attention due to their promising properties. To understand effects of alloying elements on irradiation-induced defect production, recombination and evolution, an integrated study of ion irradiation, ion beam analysis and atomistic simulations are carried out on a unique set of model crystals with increasing chemical complexity, from pure Ni to Ni80Fe20, Ni50Fe50, and Ni80Cr20 binaries, and to a more complex Ni40Fe40Cr20 alloy. Both experimental and simulation results suggest that the binary and ternary alloys exhibit higher radiation resistance than elemental Ni. The modeling work predicts that Ni40Fe40Cr20 has the best radiation tolerance, with the number of surviving Frenkel pairs being factors of 2.0 and 1.4 lower than pure Ni and the 80:20 binary alloys, respectively. While the reduced defect mobility in SP-CSAs is identified as a general mechanism leading to slower growth of large defect clusters, the effect of specific alloying elements on suppression of damage accumulation is clearly demonstrated. This work suggests that concentrated solid-solution provides an effective way to enhance radiation tolerance by creating elemental alternation at the atomic level. The demonstrated chemical effects on defect dynamics may inspire new design principles of radiation-tolerant structural alloys for advanced energy systems.