Concentrated Solid Solution Alloys Research Articles

Suzuki segregation behavior and mechanism in a Co–Cr–Fe–Ni–Mo high-entropy alloy

The knowledge of the Suzuki effect in compositionally complex CoNiCrMo and CoCrFeNiMo solid-solution alloys is rather limited. In this study, the enrichment of Co and Mo atoms and depletion of Cr, Fe, and Ni atoms were determined within the region of four atomic layers at stacking faults (SFs) in a typical quinary concentrated solid-solution alloy (Co0.95Cr0.8Fe0.25Ni1.8Mo0.475) by using scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. To better understand the Suzuki effect, two-stage first-principles Monte Carlo (MC) simulations each with up to 3000 trial swap steps were performed. First, the simulation results suggested the strong tendency of Mo segregation at the second nearest neighbor (2NN) sites at SFs. Nonetheless, as a further Mo segregation induces the excessively large lattice distortion and intrinsic strain energy, it would be inhibited. It was deemed one potential reason for the finite Mo segregation. Second, the simulations revealed the origin and formation of co-segregation of Co and Mo atoms. The segregated Mo atoms at the 2NN sites attract Co atoms, forming the local D019-type ordering at SFs. As a consequence, the stacking fault energy can reach a strongly negative value below −500 mJ/m2. Meanwhile, the Co/Mo co-segregated SFs have high stability against phase transitions to intermetallics and martensite at 773 K.

Long-Range Influence of Cr on the Stacking Fault Energy of Cr-Containing Concentrated Solid-Solution Alloys

Single-phase concentrated solid-solution alloys (CSAs) have exhibited excellent mechanical and radiation tolerance properties, making them potential candidate materials for nuclear applications. These excellent properties are closely related to dislocation movements, which depend on the stacking fault energies (SFEs). In CSAs, SFEs show large fluctuations due to variations in the local atomic environments in the vicinity of the stacking faults. In this work, first-principle calculations were performed to investigate the origin of the fluctuations in the SFEs of the widely studied CSA, NiCoCr, which show a very wide distribution from about −200 mJ/m2 to 60 mJ/m2. Compared to the common understanding that only atoms in close proximity to the stacking fault influence the SFEs in pure metals and dilute alloys, charge redistribution can be observed in several nearby planes of the stacking fault in NiCoCr, indicating that atoms several atomic layers away from stacking fault also contribute to the SFEs. Our analysis shows that Cr plays a major role in the large fluctuation in the SFEs of NiCoCr based on both electronic and magnetic responses. The flexible electronic structure of Cr facilitates easier charge transfer with Cr in several nearby atomic planes near the stacking fault, leading to significant changes in the d-electron number, orbital occupation number, and magnetic moments of Cr.

Atomistic-level analysis of nanoindentation-induced plasticity in arc-melted NiFeCrCo alloys: The role of stacking faults

Concentrated solid solution alloys (CSAs) have attracted attention for their promising properties; however, current manufacturing methods face challenges in complexity, high costs, and limited scalability, raising concerns about industrial viability. The prevalent technique, arc melting, yields high-purity samples with complex shapes. In this study, we explore nanoindentation tests at room temperature where arc-melted samples exhibit larger grain sizes, diminishing the effects of grain boundaries on the results. Motivated by these findings, our investigation focuses on the atomistic-level exploration of plasticity mechanisms, specifically dislocation nucleation and propagation during nanoindentation tests. The intricate chemistry of NiFeCrCo CSA influences pile-ups and slip traces, aiming to elucidate plastic deformation by considering both pristine and pre-existing stacking fault tetrahedra. Our analysis scrutinizes dynamic deformation processes, defect nucleation, and evolution, complemented by stress–strain and dislocation densities–strain curves illustrating the hardening mechanism of defective materials. Additionally, we examine surface morphology and plastic deformation through atomic shear strain and displacement mappings. This integrated approach provides insights into the complex interplay between the material structure and mechanical behavior, paving the way for an enhanced understanding and potential advancements in CSA applications.

Interactions between irradiation-induced defects and dislocations in concentrated solid solution alloys

Dislocation is a common structural feature in metallic alloys. Under irradiation, interactions between dislocations and irradiation-induced defects play a crucial role in governing various microstructural changes and the irradiation response of materials. Though such interaction has been intensively studied in elemental metals, little is known about it in concentrated solid solution alloys (CSAs). In this work, we study defect-dislocation interactions in NiFe CSAs through both elastic theory and atomistic simulations. Unexpectedly, we find that dislocations interact more strongly with defects in CSAs than in pure metals due to their higher elastic moduli, which is adverse to their irradiation performance based on conventional understanding. By comparing random models (Rand) and average atom models (Ave) for NiFe, we show that the random occupancy of atoms in the Rand model compared to the Ave model can reduce the interaction between dislocations and defects, thereby diminishing the dislocation bias factor. We further perform defect kinetic simulations and find that the complex energy landscape for defect motion in NiFe tends to restrict the movement of defects and weaken the ability of dislocations to absorb interstitials. Therefore, the chemical disorder in CSAs can partly reduce the defect-dislocation interaction strength thermodynamically and, more importantly, limit the movement of defects kinetically, both of which suppress the absorption ability of dislocations toward defects. Our integrated efforts shed light on the mechanisms governing defect-dislocation interactions in CSAs, providing insight into the reasons for the superior irradiation resistance of CSAs and offering possible directions to tune their irradiation performance.

Atomistic Study on Defect–Grain Boundary Interactions in TiVTa Concentrated Solid–Solution Alloys

The elemental segregation behaviors and interactions between point defects and symmetrical tilt grain boundaries (GBs) in TiVTa concentrated solid–solution alloys (CSAs) have been studied through hybrid Monte Carlo/molecular dynamics (MC/MD) simulations. A pure V model, a random TiVTa CSA with randomly distributed elements, and an equilibrated TiVTa CSA with Ti segregation were constructed to investigate the influence of chemical disorder and local elemental segregation on defect–GB interactions. For defect–GB interactions, GBs interact more strongly with interstitials than with vacancies. Compared with the pure V, the vacancy absorption length scale of GBs is greater, whereas the interstitial absorption length scale of GBs is shorter in TiVTa CSAs due to the chemical fluctuation and local lattice distortion. This means a higher recombination efficiency of point defects in TiVTa CSAs. The elemental (Ti) segregation in TiVTa CSAs can further enhance the sink strength of GBs towards interstitials, while simultaneously reducing their sink strength towards vacancies. Consequently, the preference effects of GBs towards interstitials and vacancies are amplified in the equilibrated CSA due to local ordering, thereby reducing efficient defect annihilation around GBs. These results provide fundamental insights into the irradiation defect dynamics of CSAs with body-centered cubic (bcc) structure.

Harmonic transition state theory applied to vacancy diffusion pre-exponential factors in a concentrated solid-solution alloy

Harmonic transition state theory applied to vacancy diffusion pre-exponential factors in a concentrated solid-solution alloy

Molecular dynamics study of primary radiation damage in TiVTa concentrated solid-solution alloy

The primary radiation damage in pure V and TiVTa concentrated solid-solution alloy (CSA) was studied using a molecular dynamics method. We have performed displacement cascade simulations to explore the generation and evolution behavior of irradiation defects. The results demonstrate that the defect accumulation and agglomeration in TiVTa CSA are significantly suppressed compared to pure V. The peak value of Frenkel pairs during cascade collisions in TiVTa CSA is much higher than that in pure V due to the lower formation energy of point defects. Meanwhile, the longer lifetime of the thermal spike relaxation and slow energy dissipation capability of TiVTa CSA can facilitate the recombination of point defects. The defect agglomeration rate in TiVTa CSA is much lower due to the lower binding energy of interstitial clusters and reduced interstitial diffusivity. Furthermore, the occurrence probability of dislocation loops in TiVTa CSA is lower than that in pure V. The reduction in primary radiation damage may enhance the radiation resistance of TiVTa CSA, and the improved radiation tolerance is primarily attributed to the relaxation stage and long-term defect evolution rather than the ballistic stage. These results can provide fundamental insights into irradiation-induced defects evolution in refractory CSAs.

Atomistic Simulations of Dislocation-Void Interactions in Concentrated Solid Solution Alloys

This paper investigates the interaction of edge dislocations with voids in concentrated solid solution alloys (CSAs) using atomistic simulations. The simulation setup consists of edge dislocations with different periodicity lengths and a periodic array of voids as obstacles to dislocation motion. The critical resolved shear stress (CRSS) for dislocation motion is determined by static simulations bracketing the applied shear stress. The results show that shorter dislocation lengths and the presence of voids increase the CRSS for dislocation motion. The dislocation–void interaction is found to follow an Orowan-like mechanism, where partial dislocation arms mutually annihilate each other to overcome the void. Solute strengthening produces a ‘friction stress’ that adds to the Orowan stress. At variance with classical theories of solute pinning, this stress must be considered a function of the dislocation line length, in line with the idea that geometrical constraints synergetically enhance the pinning action of solutes. Modifying the equation by Bacon, Kocks and Scattergood for void strengthening to account for the solute hardening in CSAs allows one to quantitatively predict the CRSS in the presence of voids and its dependency on void spacing. The predictions show good agreement with the simulation data without invoking any fit parameters.

Chemical inhomogeneity–induced profuse nanotwinning and phase transformation in AuCu nanowires

Nanosized metals usually exhibit ultrahigh strength but suffer from low homogeneous plasticity. The origin of a strength–ductility trade-off has been well studied for pure metals, but not for random solid solution (RSS) alloys. How RSS alloys accommodate plasticity and whether they can achieve synergy between high strength and superplasticity has remained unresolved. Here, we show that face-centered cubic (FCC) RSS AuCu alloy nanowires (NWs) exhibit superplasticity of ~260% and ultrahigh strength of ~6 GPa, overcoming the trade-off between strength and ductility. These excellent properties originate from profuse hexagonal close-packed (HCP) phase generation (2H and 4H phases), recurrence of reversible FCC-HCP phase transition, and zigzag-like nanotwin generation, which has rarely been reported before. Such a mechanism stems from the inherent chemical inhomogeneity, which leads to widely distributed and overlapping energy barriers for the concurrent activation of multiple plasticity mechanisms. This naturally implies a similar deformation behavior for other highly concentrated solid-solution alloys with multiple principal elements, such as high/medium-entropy alloys. Our findings shed light on the effect of chemical inhomogeneity on the plastic deformation mechanism of solid-solution alloys.

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.

The impact of vacancy formation energies on the nucleation and growth of helium (He) bubbles in low-activation multicomponent vanadium-based alloys

Generally, the complexity of concentrated solid solution alloys (CSAs) can effectively enhance their radiation tolerance. We studied the effect of helium (He) ion irradiation on body centered cubic (BCC) V, VCr, VCrFe and VCrFeMn alloys with the increase of complexity. The comparison shows the void swelling in terms of the size and density of bubbles in V, VCrFe and VCrFeMn decreases with the increase of the complexity after the same He ion irradiation. However, binary VCr alloy has the smallest void swelling although it has only two types of elements, that is less complexity, which is significantly different from VCrFe and VCrFeMn subjected to the same He ion irradiation. Meanwhile, the irradiated VCr lattice exhibits almost no shrinkage. First-principles calculations reveal that the high vacancy formation energy (VFE) of VCr could prevent He-vacancy clusters from absorbing vacancies to grow. The results suggest that the VFE could play a more important role in enhancing the radiation tolerance under He ion irradiation in addition to the compositional complexity.

Vacuum diffusion bonding between equiatomic CoCrNi-based concentrated solid solution alloys system and 316 stainless steel by spark plasma sintering

Spark plasma sintering (SPS) is a new method for diffusion bonding of dissimilar metals. In this study, equiatomic CoCrNi-based concentrated solid solution alloys (CSSAs) system (CoCrFeNiMn, CoCrFeNi, and CoCrNi) were successfully joined to 316 stainless steels by SPS diffusion bonding, and the effect of bonding temperature and concentration on diffusion behavior and tensile property of bonded joints were investigated. The tensile strength of the CSSAs/316SS joints were gradually increased with increasing bonding temperature. In addition, the tensile strength of the joints bonded at 0.8Tm were all exceeded 80% of that in 316SS. Meanwhile, the bonding at 0.8Tm seemed to have sufficient diffusion and grain growth at the interface, resulted in the disappearance of voids and the increase in strength of joint. In CoCrNi/316SS joint, large element concentration variation would amplitude in the diffusion zone, could result in rapid diffusion of atoms and large diffusion distance. The diffusion distance of diffusion zone in the CoCrNi/SS joint was increased by nearly two times compared to traditional diffusion bonding. Furthermore, it suggests that the continuous FCC solid solution structure was generated in the diffusion zone, which was beneficial to the increase of tensile property in joints. The tensile strength and elongation of diffusion bonded CSSAs/316SS joints were increased compared to laser beam welded joints.

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.

Effect of local chemical order on the irradiation-induced defect evolution in CrCoNi medium-entropy alloy

High- (and medium-) entropy alloys have emerged as potentially suitable structural materials for nuclear applications, particularly as they appear to show promising irradiation resistance. Recent studies have provided evidence of the presence of local chemical order (LCO) as a salient feature of these complex concentrated solid-solution alloys. However, the influence of such LCO on their irradiation response has remained uncertain thus far. In this work, we combine ion irradiation experiments with large-scale atomistic simulations to reveal that the presence of chemical short-range order, developed as an early stage of LCO, slows down the formation and evolution of point defects in the equiatomic medium-entropy alloy CrCoNi during irradiation. In particular, the irradiation-induced vacancies and interstitials exhibit a smaller difference in their mobility, arising from a stronger effect of LCO in localizing interstitial diffusion. This effect promotes their recombination as the LCO serves to tune the migration energy barriers of these point defects, thereby delaying the initiation of damage. These findings imply that local chemical ordering may provide a variable in the design space to enhance the resistance of multi-principal element alloys to irradiation damage.

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.

Charged particles: Unique tools to study irradiation resistance of concentrated solid solution alloys

• Both energetic ions and high-energy primary recoils induce defect processes in complex alloys that are consequences of the spatial and temporal coupling of atomic displacement processes and energy dissipation from ionization. • Chemical disorder does not monotonically increase with more alloying elements but can be tuned by specific alloying elements and the corresponding concentrations. • Differences between ions and neutrons need to be taken when relying on ion beam testing for reactor applications. Many multicomponent concentrated solid solution alloys (CSAs), including high-entropy alloys (HEAs), exhibit improved radiation resistance and enhanced structural stability in harsh environments. To study and assess irradiation resistance of nuclear materials, energetic ion and electron beams are commonly used to create displacement damage. Moreover, charged particles of ions, electrons, and positrons are unique tools to create and characterize radiation effects. Ion beam analysis (e.g., Rutherford backscattering spectrometry, nuclear reaction analysis, and time-of-flight elastic recoil detection analysis), electron microscopy techniques (e.g., transmission or scanning electron microscopy, and electron diffraction), and positron annihilation spectroscopy have been applied to characterize irradiated CSAs or HEAs to understand defect formation and evolution together with chemical and microstructural information. Their distinctive analyzing power and some perspectives in these techniques are reviewed. In developing structural alloys desirable for applications in advanced reactors, neutron exposure is a critical test but the limitation in achievable high damage levels is, however, a bottleneck. Ion irradiation is often used as a surrogate for neutron irradiation, and the associated reduced transmutations and higher displacements per atom (dpa) rates are desirable for materials research. Nevertheless, cautions need to be taken when relying on ion irradiation results for reactor evaluations. Literature on differences between ions and neutrons is briefly reviewed. In addition, the links to bridge the current advances on fundamental understandings to reactor applications are discussed to lay the groundwork between neutrons and ions for radiation effects studies.

First-Principles Calculation and Kink-Dislocation Dynamics Simulation on Dislocation Plasticity in TiZr-Based Concentrated Solid-Solution Alloys

The dislocation plasticity of TiZr-based hexagonal close-packed (HCP) concentrated solid-solution alloys (CSAs) is investigated using a multiscale simulation approach combining the first-principles calculation and Frenkel–Kontonova kink-dislocation model. The first-principles calculation reveals that dislocation-mediated slip is significantly enhanced by the additions of Y and Sc in TiZrHf CSAs. The dislocation kinetics is simulated using the kink-dislocation model at mesoscopic scales, and the predicted mechanical strength of CSA is found to be consistent with experimental results. In addition to predicting the mechanical properties of CSAs accurately, the multiscale simulation approach elucidates the deformation mechanisms in CSAs at atomic scales, suggesting that the approach is robust in modeling the dislocation plasticity of CSAs.

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.