Concentrated Solid Solutions Research Articles

Multiple origins of extra electron diffractions in fcc metals.

Diffuse intensities in the electron diffraction patterns of concentrated face-centered cubic solid solutions have been widely attributed to chemical short-range order, although this connection has been recently questioned. This article explores the many nonordering origins of commonly reported features using a combination of experimental electron microscopy and multislice diffraction simulations, which suggest that diffuse intensities largely represent thermal and static displacement scattering. A number of observations may reflect additional contributions from planar defects, surface terminations incommensurate with bulk periodicity, or weaker dynamical effects.

The effects of refractory elements on mechanical properties in CoCrNiFe concentrated solid solution alloys across different temperatures

Mo, W and Re are all important strengthening elements in either single phase solid solution (SS) alloys or the SS phases in precipitation-strengthened Ni/Co-based superalloys, of which the compositions are usually complicated. To directly compare the effects of Mo, W and Re on the mechanical properties of concentrated SS alloys, a model CoCrNiFe concentrated SS alloy as the base and three alloys with 3 at.% Mo, W and Re additions are fabricated and their mechanical properties are examined in both quasi-static tensile and creep tests at room and two high temperatures. Experimental and modeling results demonstrate that at room and low temperatures, the order of strengthening effect is W > Mo > Re, in accordance with solid solution strengthening model and thermal activation analysis. On the other hand, Re can significantly decrease the minimum creep rate at 1273 K, primarily due to the reduced diffusivity as confirmed from the diffusion couple analysis.

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.

Solute clustering and its role in a titanium alloy made by laser powder bed fusion

In recent years, atomic clusters and composition undulation have been observed in a number of concentrated solid-solution alloys and were found to be beneficial for strength-ductility synergy. Some reports are also available on the observation of atomic clusters in body-centred cubic metastable β titanium alloys but the role of these atomic clusters in mechanical property development is unclear. In this study, we report the formation of solute atomic clusters in an additively manufactured and solution-treated metastable titanium alloy and investigate their role in deformation and strengthening mechanisms. It was found that the atomic clusters together with ultrafine ω precipitates present in the material effectively pin dislocation motion and contribute to an ultrahigh yield strength. The interaction between dislocations and solute atomic clusters leads to an increased shear stress necessary for dislocation motion. The atomic clusters and the resulting chemical undulation generate pronounced atomic strain fluctuations and distortions with alternating tensile and compressive strain fields, which lead to large local internal stresses and thus resistance to dislocation glide. Cross slip has also been frequently observed in the additively manufactured and solution-treated metastable titanium alloys, which is beneficial for strain hardening and ductility. We also reveal that thermal cycling in the alloy can lead to a significant influence on the microstructural evolution. During solidification and cooling upon laser powder bed fusion, only β and athermal ω precipitates form. With repetitive thermal cycling, the microstructure transforms into α + β grains together with isothermal ω particles. The present findings suggest that solute atom clustering can be introduced through proper selection of solute elements, providing a new strengthening mechanism for titanium alloys.

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.

Theory-guided design of duplex-phase multi-principal-element alloys

Density-functional theory (DFT) is used to identify phase-equilibria in multi-principal-element and high-entropy alloys (MPEAs/HEAs), including duplex-phase and eutectic microstructures. A combination of composition-dependent formation energy and electronic-structure-based ordering parameters were used to identify a transition from FCC to BCC favoring mixtures, and these predictions experimentally validated in the Al-Co-Cr-Cu-Fe-Ni system. A sharp crossover in lattice structure and dual-phase stability as a function of composition were predicted via DFT and validated experimentally. The impact of solidification kinetics and thermodynamic stability was explored experimentally using a range of techniques, from slow (castings) to rapid (laser remelting), which showed a decoupling of phase fraction from thermal history, i.e., phase fraction was found to be solidification rate-independent, enabling tuning of a multi-modal cell and grain size ranging from nanoscale through macroscale. Strength and ductility tradeoffs for select processing parameters were investigated via uniaxial tension and small-punch testing on specimens manufactured via powder-based additive manufacturing (directed-energy deposition). This work establishes a pathway for design and optimization of next-generation multiphase superalloys via tailoring of structural and chemical ordering in concentrated solid solutions.

Grain boundary segregation in iron doped strontium titanate: From dilute to concentrated solid solutions

Strontium titanate, a perovskite oxide, is a frequently studied material for a large variety of applications. When acceptor-doped (with Fe, for example), the material is useful for its mixed oxygen and electronic conductivity, with potential use in oxygen transport membranes or as a cathode for solid oxide fuel cells. A barrier to conductivity in perovskites is the presence of space charge regions at the grain boundaries, which form due to the segregation of charged point defects. Typically, space charge theory assumes bulk dopant concentrations beneath the dilute limit, however concentrated solid-solutions are often utilized in applications. The current work aims to address this disparity: grain boundary segregation in strontium ferrite-strontium titanate solid-solutions is analyzed at three compositions, with Fe contents ranging from near the dilute limit to well above the dilute limit (Fe contents of 2 %, 5 %, and 25 % on the B-site of the perovskite). Electrochemical impedance spectroscopy shows an increase in material conductivity as Fe is added. High-resolution STEM imaging and spectral mapping is utilized, showing that Fe segregates to the grain boundary core, contrary to what is expected from space charge theory. As the Fe content is increased, the amount of Fe segregated to the boundary increases significantly, but the segregation width of Fe at the boundary remains consistent.

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

Elucidating the roles of chemistry, compositional complexity, and short-range order in the dislocation energetics of body-centered-cubic concentrated solid solutions

Elucidating the roles of chemistry, compositional complexity, and short-range order in the dislocation energetics of body-centered-cubic concentrated solid solutions

Which Interfaces Matter Most? Variability in Grain Boundary Defect Chemistry and Conductivity in a Concentrated Solid Electrolyte

Inorganic solid electrolytes with fast ionic conductivity are critical components of fuel/electrolysis cells, all-solid-state batteries, membrane and separation reactors, sensors and other electrochemical devices. For example, solid oxygen ion (O2-) conductors are used in solid oxide fuel/electrolysis cells (SOFCs/SOECs) for high-efficiency conversion between chemical and electrical energy and chemical production from CO2. One major challenge in engineering such materials is the diminished grain boundary (GB) conductivity in polycrystalline oxide ceramics. This phenomenon, which was initially accredited to highly resistive silica-based glassy impurity phases, is attributed to the formation of intrinsic space charge layers (SCLs) directly adjacent to GB cores. The positive potential at the GB core (due to large concentrations of oxygen vacancies) results in a depletion of oxygen vacancies in the space charge layer as well as the segregation of charge-compensating acceptor solutes from grain interiors which ultimately reduces GB conductivity. Several strategies have been explored to improve GB conductivity by understanding and mitigating the SCL effects. While the standard Poisson–Boltzmann approach models the near-grain boundary defect behavior in dilute solid solutions and recent models by Mebane et al [1] and Virkant et al [2] incorporate the effects of defect-defect interactions (in concentrated solid solutions) and chemomechanical stress, respectively, there is yet to be a model developed that can predict individual GB conductivities in concentrated solid solutions based on direct experimental measurements of near-GB defect chemistry.Here, we present an experimental-computational framework that predicts which GBs in a polycrystalline electrolyte likely facilitate ionic conductivity. We developed a thermodynamic phase-field modeling framework and applied it to a model oxygen electrolyte Gd0.25Ce0.75O2-δ, wherein the GB-to-GB variability in defect concentrations (Gd3+ solutes, electrons localized at Ce3+, and oxygen vacancies) was measured microscopically using scanning transmission electron microscopy electron energy-loss spectroscopy (STEM EELS). These data were used to predict the GB-to-GB variability in cross-GB ionic conduction using a unique model, which prioritizes reproducing microscopically observed GB defect concentrations and considers defect-defect interactions in highly concentrated solid solutions, making the framework applicable to other technologically relevant solid electrolytes. Across the GBs studied, we revealed a non-monotonic relationship between depletion and GB conductivity, with the highest conductivity predicted for intermediate depletion amounts. This work lays the foundation for future experimental-computational research and advanced data-driven design of solid electrolytes needed for functional and energy storage/conversion devices.[1] D. S. Mebane, R. A. D. Souza, Energy & Environmental Science, 2015.[2] K. S. N. Vikrant, R. Edwin García, Energy & Environmental Science, 2018.[3] H. Vahidi, W.J. Bowman (Submitted). Acknowledgments HV and WJB acknowledge primary support from the UCI new faculty startup funding. This research was partially supported by the National Science Foundation Materials Research Science and Engineering Center program through the UC Irvine Center for Complex and Active Materials (DMR-2011967). We acknowledge the use of facilities and instrumentation at the UC Irvine Materials Research Institute (IMRI) supported in part by the same NSF MRSEC (DMR-2011967). AM, DM, and AC acknowledge funding from the NSF [WB1] [hv2] and the WVU MAE research department for the use of the Thorny Flat HPC Cluster for the numerical modeling work and computational resources.

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.

Entropy-stabilized binary alloys

Alloying W with Re is an effective method for improving its intrinsic ductility. The solubility limit of Re in W is ∼27% at 851 K. However, this alloy possesses a high enthalpy of formation, which can result in phase-instability. Configurational entropy alone cannot stabilize this alloy as a single-phase solid solution. Recent research questions the importance of configurational entropy in determining the single-phase stability of “complex, concentrated solid solutions” and suggests that vibrational entropy may play a critical role. The question arises: Can vibrational entropy contribute significantly enough to ensure single-phase stability in a binary alloy? Our first-principles phonon calculations using density functional theory demonstrate that vibrational entropy is equally essential as the configurational entropy in promoting the formation of a single-phase solid solution in WRe binary alloys. This finding necessitates revisiting the role of vibrational entropy in the single-phase stability of binary alloys.

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 effect of stacking fault energy on the formation of nano twin/HCP during deformation in FCC concentrated solid solution (CSS) alloys

The exceptional mechanical properties of face-centered cubic (FCC) concentrated solid solution (CSS) alloys generally correlate various deformation mechanisms, such as deformation induced nano twins and/or hexagonal close packed (HCP) phases, which is normally determined by stacking fault energy (SFE). To systemically change the SFE but keep the influence of atomic size and modulus mismatches minimum, a series of NiCo-based alloys are designed by adjusting the Ni/Co ratio. All alloys are fabricated to be single phase FCC with grain size carefully controlled to be ∼60 μm. Deformation substructures after tensile tests over temperature ranging from 77 K to 1073 K are carried out to reveal the effect of SFE on mechanical properties and deformation mechanism. SFE has little effect on yield strength (YS) at all test temperature, but do affect the ultimate tensile strength (UTS) and elongation to fracture (EF) of the alloys. At temperature lower than 873 K, it seems that the lower the SFE, the higher the UTS. At 1073 K, there is no obvious trend for UTS and EF, indicating that SFE has little effect on deformation behavior of alloys at high temperature. Formation of deformation nano twin/HCP depends on not only the SFE of the alloys but also the temperature. As testing temperature and/or SFE of the alloys decreases, deformation induced nano twin/HCP are favorable and become dominant deformation modes for the FCC CSS alloys, resulting in a combination of high strength and good plasticity due to the great work hardening capacity.

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.

Температурна залежність границі плинності в термінах двох компонент поля зсувних напружень в площині ковзання в сплаві CrCoNiFeMn

The temperature dependence of the yield strength in the multicomponent CrCoNiFeMn alloy was investigated, using computer modelling and taking into account the short-wave and long-wave components of the shear stress field in the glide plane. The yield strength of a multicomponent alloy in the form of a concentrated solid solution without taking into account grain boundary strengthening is determined by three factors: the periodic lattice potential, the short-wave and long-wave components of the field of stochastic shear stresses in the glide plane, which are created by dissolved atoms. The force barriers resulting from the short-wave component will dominate. Overcoming them by dislocation with the assistance of applied stress and thermal activation will be a critical event to start dislocation movement. Barriers resulting from the periodic lattice potential and the long-wave component will be insignificant against the background of the short-wave component. Their effect on the yield strength can be taken into account by terms that do not depend on temperature. Thermal activation analysis of overcoming barriers resulting from a short-wave component, taking into account the probability of direct and reverse jumps through the barrier, gives the opportunity to describe the temperature dependence of the yield strength of a multicomponent alloy in a wide range of temperatures, including in the region of the high-temperature “plateau”. The dependence of the yield strength calculated in this way for the CrCoNiFeMn alloy correlates well with the corresponding experimental data. Keywords: temperature dependence, multicomponent alloy, glide plane, dislocation.

Concentration anomalies of galvanomagnetic properties of (Bi1-хSbх)2Te3 solid solutions based on Sb2Te3 in the (Bi1-хSbх)2Te3 system

Today, solid solutions based on antimony and bismuth tellurides are among the most widely used materials for the p-legs of thermoelectric converters used at room temperature and below. This paper presents the results of a study of galvanomagnetic properties (electrical conductivity σ, Hall coefficient RH, Hall mobility of charge carriers μH) of thermoelectric solid solutions (Bi1-хSbх)2Te3 in the range of compositions close to pure antimony telluride (x = 1 – 0.96) at temperatures T = 80 K and T = 300 K. The study was carried out on cast polycrystalline samples obtained by the method of crystallization from the melt followed by long-term annealing in vacuum at a temperature of T = 650 K. Galvanomagnetic properties were measured by the standard dc-method, the experimental cell was cooled with liquid nitrogen. The measurement error of RH and σ did not exceed ± 5%. It is shown that the introduction of the first portions of Bi2Te3 leads to a sharp decrease in σ, which is associated with a high degree of disorder of the crystal lattice. In the range of compositions x = 0.99 – 0.9825, the concentration dependences of σ(x) and μH(x) revealed anomalous growth of σ and μH. The presence of concentration anomalies is associated with the transition from dilute to concentrated solid solutions. An assumption is made about the percolation nature of the phase transition. Within the framework of the problem of spheres of percolation theory, within the framework of the task of spheres of the theory of percolation the radius of the deformation sphere of an impurity atom (Bi) is estimated. The obtained value is consistent with the short-range potential of the impurity. It is shown that the position of the anomalies in the σ(x) and μH(x) dependences does not change with the decrease in temperature down to 80K. Thus, the observation of concentration anomalies of the properties for the solid solution (Bi1-xSbx)2Te3 is another confirmation of the hypothesis about the universal nature of the behavior of solid solutions at a low impurity concentration. The detected anomalies must be taken into account when developing methods for increasing the thermoelectric performance of materials by creating solid solutions and doping.

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.