Concentrated Solid Solution Alloys Research Articles

The interactions between dislocations and displacement cascades in FeCoCrNi concentrated solid-solution alloy and pure Ni

Concentrated solid-solution alloys (CSAs) have been proposed as structural materials for advanced nuclear reactors due to their great irradiation tolerance. We studied and compared the interaction processes of displacement cascades and edge dislocations in FeCoCrNi CSA and pure Ni by molecular dynamics to explore the influence of dislocations on the irradiation resistances of CSAs. The edge dislocations preferentially absorb interstitials in both materials, however, the differences between the numbers of absorbed interstitials and vacancies in FeCoCrNi are smaller than in Ni, which is attributed to the smaller difference of mobilities of interstitials and vacancies in FeCoCrNi. When the centers of displacement cascades coincide with edge dislocations, the numbers of absorbed interstitials and vacancies in edge dislocations of FeCoCrNi are essentially the same, which is independent with PKA energies. The amounts of survived interstitials and vacancies in the matrix of FeCoCrNi are closer than that in pure Ni, which would avoid the formation of numerous large vacancy clusters induced by excessive vacancies. Furthermore, the edge dislocations are enriched with Ni, Co and depleted with Fe, Cr after interactions with displacement cascades, which is consistent with experimental results. The molecular dynamics simulation results of current study reveal that the edge dislocations in CSAs can probably serve as weak biased sinks for the annihilation of irradiation-induced defects and enhancing the irradiation resistances of CSAs.

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.

An atomic scale study on self-interstitial formation and diffusion behaviors in TiVTa and TiVTaNb concentrated solid-solution alloys

The formation and diffusion behaviors of self-interstitial defects in TiVTa and TiVTaNb concentrated solid-solution alloys (CSAs) are studied systematically by first-principles calculations and molecular dynamics simulations. It is found that [111] VV and TiV dumbbells are the most stable configurations in TiVTa-based CSAs, the formation energies of self-interstitials in TiVTaNb are higher than that in TiVTa CSAs. Local chemical environment can affect the formation of interstitials, where interstitials prefer V-rich environment. Larger local lattice distortion in TiVTa can lead to an irregular energy landscape and make it easier to form interstitials than in TiVTaNb. Compared with pure V, the diffusivity of interstitial is much lower than in TiVTa-based CSAs, which is related to the specific stability of interstitial configurations and lattice distortion. The preferential elemental diffusion is also observed which can be demonstrated by the interstitial formation preferences. Significant lattice distortion caused by atomic size mismatch in TiVTa-based CSAs can hinder the formation of interstitials, resulting in chemically biased diffusion and preferential diffusion pathways of interstitials.

Atomistic simulations of dislocation plasticity in concentrated VCoNi medium entropy alloys: Effects of lattice distortion and short range order

Face-centered cubic (fcc) high and medium entropy alloys (H/MEAs) have been shown to display superior mechanical properties at low temperatures, but significant improvement of their strength at high temperatures is required for industrial applications at extreme conditions. Recently, it has been shown that the breakthrough of the MEAs from equiatomic/near-equiatomic to non-equiatomic ratios leads to strong MEAs with good ductility. To design new H/MEAs, we consider two important factors that may influence strength: the chemical composition and chemical short range order (CSRO). In this study, we investigate the depinning stress (σc) as a criterion of strength of several compositions of VCoNi concentrated solid solution alloys (CSSAs) including V0.33Co0.33Ni0.33, V0.35Co0.2Ni0.45, V0.33Co0.17Ni0.5, and V0.17Co0.33Ni0.5 at 5 K and 300 K, using atomistic simulations. The chosen interatomic potential is shown to be reliable by comparing experimental/ab initio values and calculated parameters such as lattice constant, shear modulus, depinning stress, and temperature variation of stacking fault width for equimolar VCoNi. We find a good agreement between experimental friction stress and the depinning stress extracted from our results for equimolar VCoNi. Also, we find that Vclusters are the main pinning points of dislocations, and With a random distribution of atoms, we find that the alloy composition V0.33Co0.17Ni0.5 displays the largest depinning stress at both 5 and 300 K. Furthermore, to investigate how CSRO affects the strength of these alloys, we design CSRO into the microstructure using two different methods: In the first method, hybrid Molecular-dynamics/Monte-Carlo simulations were employed to simulate annealing at various temperatures. We observe that such simulations create CSRO so that it increases with decreasing annealing temperature. Recently, the CSRO motif and its concentration in an equimolar VCoNi have been determined by experiment. By modeling this experiment, we also implemented the CSRO into microstructure as the second method. By using both methods, the effect of CSRO on the magnitude of the depinning stress is discussed. It was shown that in both methods, CSRO significantly influences the strength of non-equimolar VCoNi alloys.

Structure and glide of Lomer and Lomer-Cottrell dislocations: Atomistic simulations for model concentrated alloy solid solutions

Lomer (L) and Lomer-Cottrell (LC) dislocations have long been considered to be central to work hardening in face-centered cubic (FCC) metals and alloys. These dislocations act as barriers of motion for other dislocations, and can serve as sites for twin nucleation. Recent focus on multicomponent concentrated FCC solid solution alloys has resulted in many reported observations of LC dislocations. While these and L dislocations are expected to have a role in the mechanical behavior of these alloys, little is understood about how variations in composition and associated fault energies change the response of these dislocations under stress. We present atomistic simulations of L and LC dislocations in a model Cu-Ni system and find that changes in composition and applied stress conditions result in a wide variety of responses, including changes in core configuration and (100) glide. The results are compared to and extend previous literature related to the nature of L/LC core structures and how they vary with respect to intrinsic materials properties and stress states. This study also provides insights into mechanisms such as twin nucleation that could have important implications for work hardening in FCC solid-solution alloys.

First-principles study of vacancy defects in TiVTa and TiVTaNb concentrated solid-solution alloys

The formation and migration energies of vacancy defects are studied systematically by first-principal calculations method in three concentrated solid-solution alloys (CSAs), namely equimolar TiVTa alloy, TiVTa50 alloy with Ta content increased to 50%, and equimolar TiVTaNb alloy. It is found that the vacancy formation and migration in TiVTaNb alloy are more difficult than that in TiVTa and TiVTa50 alloys. The TiVTa alloy has the larger local lattice distortion than TiVTa50 and TiVTaNb alloys, which would lead to an irregular energy landscape and make it easier to form vacancies. Vacancies are easily formed in Ti-rich environments and the trend reverses for in V-rich environments in these three alloys, originating from the large local lattice distortion in Ti-rich environments and strong binding interactions between Ti atoms and vacancies. Ti atoms with the largest size tend to migrate to neighboring vacancies in these three alloys. The addition of Nb enhances the electron interactions between atoms in TiVTaNb alloy. Compared to TiVTa and TiVTa50 alloys, the migration barrier of atoms diffusing into vacancies in TiVTaNb alloy is improved due to the combination of strong electron interactions between atoms and lattice distortion. It is implied that the influence of alloy element combination on vacancy formation and migration energies may be greater than that of alloy element content in Ti-V-Ta and Ti-V-Ta-Nb alloys. These results provide fundamental insights into the defects evolution of CSAs with body-centered cubic (bcc) structure and supply the scientific basis for the composition design of bcc CSAs.

Effect of atomic types and concentration on the mechanical properties of CoNi-based concentrated solid-solution alloys

Concentrated solid-solution alloys (CSAs) are a new type of alloy material with high strength, extreme hardness, remarkable corrosion resistance, and excellent radiation resistance. The mechanical properties of CSAs are strongly determined by the types and concentration of component elements, which provide a large space for design. In this work, molecular dynamics simulations were implemented to investigate the mechanical properties of CoNi-based CSAs. Our study showed that there is a significant effect of constituent types on the yield stress and Young's modulus of CoNi-based CSAs. The yield stress and Young's modulus of CoNiFeAlCu CSAs with equal atomic ratios were found to be 3.02 and 70.94 GPa, respectively. With the increase in the Fe concentration, the yield stress and Young's modulus of the CSAs decrease gradually; on the contrary, these properties increase gradually with an increase in the Al concentration. The CoNiFeCuAl6 CSA was found to exhibit high strength and good plastic deformation ability, which originates from the significant increase in the dislocation density. Hence, the large design space of CSAs presents a great opportunity to select CSAs with excellent mechanical properties for structural applications.

Deformation mechanisms in single crystal Ni-based concentrated solid solution alloys by nanoindentation

Nanoindentation is a critical technique to probe mechanical properties at the micrometer and sub-micrometer scales, accompanied by challenges from indentation size effect, pile-up/sink-in effect, and strain rate sensitivity. In this study, different nanoindentation techniques have been employed to explore Ni-based concentrated solid solution alloys (CSAs) with the addition of 3d transition metal elements including Co, Cr, Mn, and Fe, including unique single-crystal Ni, NiCo, NiFe, Ni80Cr20, and NiCoFeCr samples with (100) surfaces. A procedure of nanoindentation tests and data analysis/correction have been developed, and a data set of hardness, elastic modulus, strain rate sensitivity, and activation volume for Ni-based CSAs are provided, including the less explored binary alloys such as Ni80Cr20 and Ni80Mn20. The results show that the type of alloying elements is more critical than the number of elements in strengthening: Co does not provide strengthening in NiCo, while Cr, Mn, and Fe are effective strengthening elements. Cr is the most effective among all the 3d transition metal elements. Furthermore, atomic-level lattice distortion is responsible for the strengthening and the role of stacking fault energy is insignificant in Ni-based CSAs at room temperature. In summary, nanoindentation shows increasing promise as a reliable and fast tool to provide comprehensive mechanical information for new alloy design and development.

A “local” stacking fault energy model for concentrated alloys

We propose a simple bond energy model that can accurately predict the average stacking fault energy (SFE) in concentrated solid-solution alloys. The SFE is approximated by the energy difference of relevant bulk phases for which bond energies are determined. These bond energies are then used to determine the SFE distribution across the compositional space within a given alloy system, without performing additional calculations or refit parameters, thereby avoiding the computational cost and other limitations of stacking fault supercell calculations. We demonstrate that the model can be applied to random solid solutions and alloys with short-range ordering.

Unexpected role of prefactors in defects diffusion: The case of vacancies in the 55Fe-28Ni-17Cr concentrated solid-solution alloys

Variations in diffusion rates of simple point defects, such as vacancies and interstitials, are generally assumed to be dominated by energy barriers, as demonstrated for example by the large number of kinetic Monte Carlo studies that rest on constant rate prefactors. While this is mostly correct when energy barriers are well separated from each other, typically in crystals, entropic variations between the local minima and associated activated states become increasingly important as energy barriers of diffusion-mediating-mechanisms get closer, typically in disordered environments. The unexpected slower defect diffusion observed in high entropy alloys, which are characterized by the presence of a large number of different elements in roughly equal proportions, has brought us to revisit the role of prefactors. Combining the Activation-Relaxation Technique nouveau (ART nouveau) and the harmonic approximation for computing diffusion prefactors, we find that vacancy diffusion prefactors in a 55Fe-28Ni-17Cr concentrated solution alloy modeled with EAM empirical potentials can vary by up to six orders of magnitude, at almost constant energy barrier. This variation, mostly associated with changes in local pressure, suggests that prefactor could play a much more important role than previously thought in the defect kinetics of high entropy alloys and of disordered systems in general.

Understanding effects of chemical complexity on helium bubble formation in Ni-based concentrated solid solution alloys based on elemental segregation measurements

Helium bubble formation and swelling were systematically studied in Ni-based concentrated solid solution alloys containing different numbers and types of elements. Our microscopy analysis showed that although increasing the alloy chemical complexity helps suppress bubble formation in general, there is no monotonic relationship between the bubble growth rate and the number of alloying elements. Certain elements (e.g., Fe and Pd) are more effective in suppressing bubble growth than others (e.g., Cr and Mn). Atom probe tomography was applied to accurately measure elemental segregation around bubbles, revealing unique effects of certain alloying elements on vacancy migration towards bubbles. More specifically, the high vacancy mobility via Cr sites leads to a large vacancy flux and an increased bubble size, while the high degree of atomic size mismatch introduced by Pd helps deflect vacancy flow away from bubbles and decrease the amount of swelling. The effects identified in this study provide new strategies to design concentrated solid solutions with superior resistance to swelling.

Role of chemical disorder on radiation-induced defect production and damage evolution in NiFeCoCr

Understanding chemical disorder in many concentrated solid solution alloys (CSAs) at the levels of electrons and atoms has attracted increasing attention as a path forward to reveal and identify underlying mechanisms for extraordinary mechanical properties and improved radiation tolerance. Single-phase NiFeCoCr CSA is a common base for many high-entropy alloys (HEAs) that have shown improved mechanical strength and radiation tolerance. In this study, defect production and damage evolution in NiFeCoCr under ion irradiation at room temperature to dose over 20 dpa are determined using ion channeling technique along both <100> and <110> directions utilizing multiple probing beam energies. The results obtained from the multi-axial and multi-energy channeling analysis are compared with those previously obtained for Ni crystals irradiated under similar conditions. The influence of chemical complexity on defect production and clustering at early-stage under room temperature irradiation up to dose of 1 dpa is discussed based on positron annihilation spectroscopy results. Defect structure evaluation in Ni and NiFeCoCr is also discussed based on transmission electron microscopy results over a prolonged irradiation at both room and elevated temperatures. Compared with chemically complex NiFeCoCr, larger dislocation loops thus less lattice strain are expected to form in pure Ni. Moreover, the role of chemical disorder in this CSA is also investigated based on ab initio calculations using large supercells. To understand the impact of chemical complexity effect on defect structure evolution, this integrated research effort attempts to link the relatively large charge redistribution due to difference in valence electron counts resulting from alloying different 3d transition metal elements, moderate lattice distortion arising from similar adaptable atomic size, and notable suppressed or delayed damage evolution in NiFeCoCr.

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.

Unexpected Role of Prefactors in Defects Diffusion: The Case of Vacancies in the 55fe-28Ni-17Cr Concentrated Solid-Solution Alloys

Unexpected Role of Prefactors in Defects Diffusion: The Case of Vacancies in the 55fe-28Ni-17Cr Concentrated Solid-Solution Alloys

Effect of Atomic Types and Concentration on the Mechanical Properties of Coni-Based Concentrated Solid-Solution Alloys

Effect of Atomic Types and Concentration on the Mechanical Properties of Coni-Based Concentrated Solid-Solution Alloys

The dynamic evolution of swelling in nickel concentrated solid solution alloys through in situ property monitoring

Defects and microstructural features spanning the atomic level to the microscale play deterministic roles in the expressed properties of materials. Yet studies of material evolution in response to environmental stimuli most often correlate resulting performance with one dominant microstructural feature only. Here, the dynamic evolution of swelling in a series of Ni-based concentrated solid solution alloys under high-temperature irradiation exposure is observed using continuous, in situ measurements of thermoelastic properties in bulk specimens. Unlike traditional evaluation techniques which account only for volumetric porosity identified using electron microscopy, direct property evaluation provides an integrated response across all defect length scales. In particular, the evolution in elastic properties during swelling is found to depend significantly on the entire size spectrum of defects, from the nano- to meso-scales, some of which are not resolvable in imaging. Observed changes in thermal transport properties depend sensitively on the partitioning of electronic and lattice thermal conductivity. This emerging class of in situ experiments, which directly measure integrated performance in relevant conditions, provides unique insight into material dynamics otherwise unavailable using traditional methods.

On the origin of internal obstacles to dislocation glide in single-phase NiFe random alloys

Over the past decade, a new class of unconventional alloys, where several constitutive elements are presented in close to equal concentrations, is considered as promising structural materials in many applications. Properties and behavior of many concentrated solid solution alloys (CSSAs) under different conditions, including mechanical loading, differs from that of conventional alloys and has been the subject of intensive studies by different techniques. A key feature of these materials is that there is no clear distinction between solute and solvent species of atoms, which is the conventional approach to considering dislocation motion in alloys. To understand this regime, we have recently reported on the glide of a screw dislocation glide in a simple equiatomic NiFe alloy and reported the two-mode character. At low applied stress, the dislocation motion is similar to a glide through a field of obstacles. In this paper, we report the results of a detailed study on the nature of these internal obstacles. We demonstrate that these are not conventional localized “obstacles” but is the result of significantly different energies of instantaneous dislocation configurations over the alloy bulk. The strong pinning positions are associated with composition changes in the partial dislocation cores, while observing no significant composition changes along the stacking faults. Local energies can vary significantly, as demonstrated using simulations of short-periodicity dislocation lines. Therefore, the critical stress to move a dislocation under such conditions is not a constant Peierls stress but represents a wide spectrum of barriers between different non-straight valleys that the dislocation line accommodates in the bulk at particular stress/strain conditions.

Tunable Chemical Disorder in Concentrated Alloys: Defect Physics and Radiation Performance.

The development of advanced structural alloys with performance meeting the requirements of extreme environments in nuclear reactors has been long pursued. In the long history of alloy development, the search for metallic alloys with improved radiation tolerance or increased structural strength has relied on either incorporating alloying elements at low concentrations to synthesize so-called dilute alloys or incorporating nanoscale features to mitigate defects. In contrast to traditional approaches, recent success in synthesizing multicomponent concentrated solid-solution alloys (CSAs), including medium-entropy and high-entropy alloys, has vastly expanded the compositional space for new alloy discovery. Their wide variety of elemental diversity enables tunable chemical disorder and sets CSAs apart from traditional dilute alloys. The tunable electronic structure critically lowers the effectiveness of energy dissipation via the electronic subsystem. The tunable chemical complexity also modifies the scattering mechanisms in the atomic subsystem that control energy transport through phonons. The level of chemical disorder depends substantively on the specific alloying elements, rather than the number of alloying elements, as the disorder does not monotonically increase with a higher number of alloying elements. To go beyond our knowledge based on conventional alloys and take advantage of property enhancement by tuning chemical disorder, this review highlights synergistic effects involving valence electrons and atomic-level and nanoscale inhomogeneity in CSAs composed of multiple transition metals. Understanding of the energy dissipation pathways, deformation tolerance, and structural stability of CSAs can proceed by exploiting the equilibrium and non-equilibrium defect processes at the electronic and atomic levels, with or without microstructural inhomogeneities at multiple length scales. Knowledge of tunable chemical disorder in CSAs may advance the understanding of the substantial modifications in element-specific alloy properties that effectively mitigate radiation damage and control a material's response in extreme environments, as well as overcome strength-ductility trade-offs and provide overarching design strategies for structural alloys.

Factors affecting the vacancy formation energy in Fe70Ni10Cr20 random concentrated alloy

Due to the randomness in the local atomic environment, the vacancy formation energy in concentrated solid-solution alloys exhibits substantial variations. While the variations have often been attributed to the local composition in the first nearest neighbor (1NN) shell, large variations are seen to persist even when the 1NN compositions are fixed, indicating contributions of other factors. In this study, extensive density functional theory calculations are carried out on two different special quasi-random structures (SQSs) in Fe70Ni10Cr20, each consisting of 256 atoms, to investigate the contributions of various factors that may influence the vacancy formation energy. We observe that the vacancy formation energy is dependent on not only the 1NN composition but also equally the arrangement of 1NN atoms. In particular, we find that the vacancy formation energy can vary up to 0.44 eV even if the composition of the 1NN is fixed and only the relative positions of the twelve 1NN atoms are exchanged. Further, we find that the atomic relaxation and volume of a vacancy before and after vacancy formation can also significantly influence the vacancy formation energy. Finally, we find a negligible influence of second nearest neighbors and their arrangement on the vacancy formation energy.

Temperature effect on irradiation damage in equiatomic multi-component alloys

Multiprincipally designed concentrated solid solution alloys, such as high entropy alloys (HEA) and equiatomic multi-component alloys (EAMC-alloys) have shown much promise for use as structural components in future nuclear energy production concepts. The irradiation tolerance in these novel alloys has been shown to be superior to that in more conventional metals used in current nuclear reactors. However, studies involving irradiation of HEAs and EAMC-alloys have usually been performed at room temperature. Hence, in this study the irradiation damage is investigated computationally in two different Ni-based EAMC-alloys and pure Ni at four different temperatures, ranging from 138 K to 800 K. The irradiation damage was studied by analyzing point defects, defect cluster sizes and dislocation networks in the materials. Dislocation loop mobility calculations were performed to help understanding the formation of different dislocation networks in the irradiated materials. Utilizing the knowledge of the depth distribution of damage, and using simulations of Rutherford backscattering in channeling conditions (RBS/c), we can relate our results to experimental data. The main findings are that the alloys have superior irradiation tolerance at all temperatures as compared to pure Ni, and that the damage is reduced in all materials with an increase in temperature.