Strength Of High-entropy Alloy Research Articles

Regulation of chemical microenvironment to overcome strength-ductility trade-off in FeCrVTiSix high-entropy alloys coating

Developing metallic materials with both high strength and ductility is an enormous challenge in the field of materials science and engineering due to commonly known “strength-ductility trade-off”. In this work, we proposed a regulation strategy of the chemical microenvironment through changing valence electron concentration (VEC) to achieve simultaneous improvement of the strength and ductility of metallic materials. Taking FeCrVTiSix system as a proof-of-concept model, we first predicted the influence of Si on the chemical microenvironment through density functional theory calculations. These calculations unveiled that Si atoms effectively modify the bonding state among alloy elements, resulting in an enhancement of the strength within the FeCrVTi alloy system while maintaining its ductility. Building upon this insight, we successfully synthesized FeCrVTiSix alloys coating using laser cladding techniques. Subsequent tests, including nanoindentation, Vickers hardness, and tensile assessments, unequivocally demonstrated that the introduction of Si significantly increases the strength of the alloy while maintaining its ductility. Si atoms play a pivotal role in modulating the slip behavior of high-entropy alloys (HEAs) by reshaping the chemical microenvironment, thus enabling the elevation of strength in HEAs while preserving ductility. This work provides a novel approach for designing HEAs that effectively circumvent the “strength-ductility trade-off”, potentially broadening the horizons of engineering applications for HEAs coating.

Cryo-pre-straining contributes to achieving high yield strength in high-entropy alloys

In this study, we propose an effective and feasible strategy to significantly improve the room-temperature yield strength of the Co35Cr20Fe15Mn15Ni15 HEA by 338%, from 281 MPa (as-recrystallized, before CR) to 1231 MPa (30% CR) through cryo-pre-strain-inducing defects such as dislocations, stacking-faults (SFs), nano-twins, Lomer-Cottrell (L-C) locks, and HCP martensite, while maintaining good ductility (16.4% elongation in 30% CR). Additionally, the contribution of these defects to the yield strength enhancement was quantified. This study offers a new perspective for the preparation of advanced FCC high-entropy alloys.

Atomic-scale analysis of deformation behavior of face-centered cubic nanocrystalline high-entropy alloys with different grain sizes at high strain rates

In the present study, the microscopic deformation mechanism and tensile properties of Al0.1CoCrFeNi nanocrystalline high-entropy alloys (HEAs) during uniaxial tension were studied through molecular dynamics simulation, and the dependence of mechanical properties on grain size and strain rate has been explored. As the tensile strain increases from 0 to 0.1, the atoms undergo a phase transformation from the face-centered cubic (FCC) structure into the hexagonal close-packed (HCP), body-centered cubic (BCC) and amorphous structure, and the dislocation density increases gradually due to dislocation annihilation and interaction. HEA grains undergo the transformation of the inverse Hall-Petch (H–P) relation into the H–P relation after exceeding one critical size of 14.77 nm at a strain rate of 5 × 107/s, and this critical size increases with increasing strain rate. The microscopic deformation mechanism of the H–P relation is that the accumulation of dislocations at the grain boundaries leads to an increase in the strength of HEAs. In contrast, the inverse H–P relation is caused by the migration of grain boundaries, and the rotation and merging of HEA grains. The increase of strain rate positively affects the tensile properties of nanocrystalline HEAs, and the flow stress and yield strength exhibit sensitive dependence on the extremely high strain rates. In addition, the amorphization of atoms has become an important way for HEAs to deform plastically at high strain rates.

Enhanced strength without sacrificing ductility in FeCrMnVSix high entropy alloys via controlling the ratio of metallic to covalent bonding

It has been the pursuit of materials science to enhance strength without sacrificing the ductility of a material. By introducing the semi-metallic Si into the FeCrMnV high-entropy alloy (HEA) to control the ratio of metallic and covalent bonding, we obtained a FeCrMnVSix HEAs with high strength and favorable ductility from the perspective of interatomic bonding. First-principles calculation was employed to calculate the structural model, electronic environment, and mechanical properties. The theoretical calculations demonstrated that the covalent bonding had been obtained in the FeCrMnV HEA consisting of metallic bonding due to the addition of Si. Under the guidance of theoretical calculation, FeCrMnVSix HEAs were successfully prepared by laser cladding on 1Cr13 steel substrate. The nanoindentation, Vickers microhardness, and tensile tests were performed, and the results indicated that the strength of HEA was enhanced without sacrificing ductility. In addition, the as-prepared Si-containing HEA coatings exhibited excellent tribological properties. The mechanism of simultaneous formation of metallic and covalent bonds on the mechanical properties was analyzed. The enhanced strength without sacrificing ductility in FeCrMnVSix HEAs is attributed to the proper combination of metallic and covalent bonding, the metallic bonding facilitates favorable ductility while the covalent bonding contributes to excellent strength.

Enhancement of Strength and Plasticity by Nanoprecipitation Strengthening and Stacking Fault Deformation in a High Entropy Alloy

Precipitation strengthening can effectively improve the strength of high entropy alloys (HEAs), but usually severely reduces the ductility. In this study, a new interstitial carbon-doped HEA with a nominal composition of Ni33.5(CoFe)55V5Nb5C1.5 at.% was developed. The HEA exhibits a high yield strength of 1244 MPa, ultimate tensile strength of 1553 MPa and excellent elongation of ~ 30%. The stacking fault energy of the matrix is affected by the precipitation of L12-Ni3Nb, which enhances the stacking fault (SF)-induced deformation. Since the Taylor lattice domains are subdivided by the SF networks, the mean free path of dislocation is reduced. The enhanced strain-hardening ability leads to a good combination of strength and ductility.

Hierarchical Bayesian approach to experimental data fusion: Application to strength prediction of high entropy alloys from hardness measurements

The discovery of materials with improved properties can be accelerated by models with the ability to combine data from multiple experimental information sources. A recurring task in the toolbox of practitioners is to map input physical descriptors to output properties of interest. Typically, both the outputs and many of the inputs are experimentally measured and, thus, noisy. Probabilistic regression methods, e.g., Gaussian process regression, can easily deal with noisy outputs, even if the noise is input-dependent. However, most regression methods cannot process noisy inputs. Ignoring input uncertainty leads to inaccurate predictive uncertainty, a crucial ingredient for the sequential design of experiments. The objective of this paper is to develop a regression methodology that can deal with input uncertainty when one wishes to correlate an inexpensive experimental measurement (e.g., hardness) to an expensive one (e.g., yield strength). Our hierarchical Bayesian approach uses two Gaussian processes. The first one maps noiseless physical descriptors to the inexpensive experimental measurement. The second Gaussian process maps noiseless physical descriptors and the inexpensive experimental measurement to the expensive experimental measurement. The two Gaussian processes form a nested model that is not analytically tractable. To overcome this issue, we propose semi-analytical approximations to both the marginal likelihood and the posterior predictive distribution. The result is a model that is practical to train and use. We demonstrate the merits of the proposed method through a synthetic dataset in which we control all the uncertainties. The statistical tests clearly show that standard Gaussian process regression cannot cope with input uncertainty whereas our proposed method consistently yields better predictive distributions. Finally, we apply the method to the task of predicting the yield strength of high entropy alloys from hardness on an exhaustive dataset compiled from the available literature.

Integrating machine learning with mechanistic models for predicting the yield strength of high entropy alloys

Accelerating the design of materials with targeted properties is one of the key materials informatics tasks. The most common approach takes a data-driven motivation, where the underlying knowledge is incorporated in the form of domain-inspired input features. Machine learning (ML) models are then built to establish the input–output relationships. An alternative approach involves leveraging mechanistic models, where the domain knowledge is incorporated in a predefined functional form. These mechanistic models are meticulously formulated through observations to validate specific hypotheses and incorporate elements of causality missing from data-driven ML approaches. In this work, we demonstrate a computational approach that integrates mechanistic models with phenomenological and ML models to rapidly predict the temperature-dependent yield strength of high entropy alloys (HEAs) that form in the single-phase face-centered cubic (FCC) structure. Our main contribution is in establishing a quantitative relationship between the HEA compositions and temperature-dependent elastic constants. This allows us to improve the treatment of elastic constant mismatch to the solid solution strengthening effect in the mechanistic model, which is important for reliable prediction of yield strength. We accomplish this by combining Bayesian inference with ensemble ML methods. The outcome is a probability distribution of elastic constants which, when propagated through the mechanistic model, yields a prediction of temperature-dependent yield strength along with the uncertainties. The predicted yield strength shows good agreement with published experimental data, giving us confidence in applying the developed approach for the rapid search of novel FCC-based HEAs with excellent yield strength at various temperatures.

Improving mechanical properties of (Co1.5FeNi)88.5Ti6Al4R1.5 (R = Hf, W, Nb, Ta, Mo, V) multi-component high-entropy alloys via multi-stage strain hardening strengthening

In this study, (Co1.5FeNi)88.5Ti6Al4R1.5 (R = Hf, W, Nb, Ta, Mo, V) high entropy alloys were fabricated by vacuum arc melting, followed by cold rolling and aging treatment. The effects of refractory elements on the phase structure, microstructure and mechanical properties of high entropy alloys (HEAs) were systematically studied by means of XRD, SEM and TEM. The results show that the phase composition of all alloys is the face-centered-cubic (FCC) + L12 dual-phase structure, and the addition of refractory elements has not changed the phase structure of the alloys. The tensile results show that the yield strength and elongation of the matrix HEA are 750 MPa and 18 %, respectively. The strength of HEAs is improved variously by adding refractory elements. The HEAs with Ta element have the best comprehensive mechanical properties, exhibit excellent strength-ductility combination, with the yield strength and elongation of 950 MPa and 20 %, respectively, and the fracture mechanism changes from complete ductile fracture to mixed brittle-ductile fracture mode. In addition, the HEAs have multi-stage strain-hardening behavior, which has higher strength when the strain-hardening curve has larger fluctuations, which indicates that the HEAs can improve the strength-plasticity matching relationship by a multi-stage strain hardening behavior.

Chemical short-range ordering regulated dislocation cross slip in high-entropy alloys

High-entropy alloys (HEAs) have received extensive interest owing to their unusual mechanical properties as a result of the extreme compositional complexity. Understanding the dislocation behavior under the influence of chemical disorder, especially chemical short-range ordering (SRO), which is commonly found in HEAs, is fundamental to the rational design of high-performance alloy systems. Here, we systematically investigate the effect of disorder and SRO on the regulation mechanism of dislocation nucleation and cross-slip in HEAs through a combination of Monte Carlo methods, molecular dynamic (MD) simulations, and theoretical analysis. Our results demonstrate that the incipient plasticity of HEAs under nanoindentation is mainly controlled by the nucleation and cross-slip of Shockley partial dislocation loops, which are found to be highly dependent on SRO. In contrast to traditional strengthening, SRO leads to a significant increase in the stable nucleation radius of the dislocation loop and cross-slip resistance. Strengthening is then achieved due to the extension of the dislocation nucleation period and the high resistance for dislocation cross-slip. Aided by the transition path analysis and theoretical model, we show that the local compositional changes originating from SRO, specifically for strong Co-Cr pairs, are the dominant factors governing the dislocation properties and accordingly, the strengthening effects. Finally, the SRO strengthen effect is further validated against different alloy systems with different SRO types. These findings suggest that the elasticity and strength of HEAs can be improved through the prolonging of the nucleation period of dislocations resulting from SRO, dictating an avenue for enhancing the mechanical properties of HEAs. • The incipient plasticity of HEAs under nanoindentation is controlled by the nucleation and cross-slip of partial dislocation loops. • Strong SRO increases the nucleation energy barriers and the stable nucleation length is longer simultaneously. • Dislocation cross-slip is inhibited by SRO. • SRO increase the critical transition radius from dislocation slip to cross slip. • Strength originating from the interaction between solute and dislocation is enhanced by strong SRO.

Anomalous size effect on yield strength enabled by compositional heterogeneity in high-entropy alloy nanoparticles

High-entropy alloys (HEAs), although often presumed to be random solid solutions, have recently been shown to display nanometer-scale variations in the arrangements of their multiple chemical elements. Here, we study the effects of this compositional heterogeneity in HEAs on their mechanical properties using in situ compression testing in the transmission electron microscope (TEM), combined with molecular dynamics simulations. We report an anomalous size effect on the yield strength in HEAs, arising from such compositional heterogeneity. By progressively reducing the sample size, HEAs initially display the classical “smaller-is-stronger” phenomenon, similar to pure metals and conventional alloys. However, as the sample size is decreased below a critical characteristic length (~180 nm), influenced by the size-scale of compositional heterogeneity, a transition from homogeneous deformation to a heterogeneous distribution of planar slip is observed, coupled with an anomalous “smaller-is-weaker” size effect. Atomic-scale computational modeling shows these observations arise due to compositional fluctuations over a few nanometers. These results demonstrate the efficacy of influencing mechanical properties in HEAs through control of local compositional variations at the nanoscale.

Predicting temperature-dependent ultimate strengths of body-centered-cubic (BCC) high-entropy alloys

This paper presents a bilinear log model, for predicting temperature-dependent ultimate strength of high-entropy alloys (HEAs) based on 21 HEA compositions. We consider the break temperature, Tbreak, introduced in the model, an important parameter for design of materials with attractive high-temperature properties, one warranting inclusion in alloy specifications. For reliable operation, the operating temperature of alloys may need to stay below Tbreak. We introduce a technique of global optimization, one enabling concurrent optimization of model parameters over low-temperature and high-temperature regimes. Furthermore, we suggest a general framework for joint optimization of alloy properties, capable of accounting for physics-based dependencies, and show how a special case can be formulated to address the identification of HEAs offering attractive ultimate strength. We advocate for the selection of an optimization technique suitable for the problem at hand and the data available, and for properly accounting for the underlying sources of variations.

Unveiling anneal hardening in dilute Al-doped AlxCoCrFeMnNi (x = 0, 0.1) high-entropy alloys

Anneal hardening has been one of the approaches to improve mechanical properties of solid solution alloys with the face-centered cubic (FCC) structure, whereby a considerable strengthening can be attained by annealing of cold-worked alloys below the recrystallization temperature (Trx). Microscopically, this hardening effect has been ascribed to several mechanisms, i.e. solute segregation to defects (dislocation and stacking fault) and short-range chemical ordering, etc. However, none of these mechanisms can well explain the anneal hardening recently observed in phase-pure and coarse-grained FCC-structured high-entropy alloys (HEAs). Here we report the observations, using high-resolution electron channeling contrast imaging and transmission electron microscopy, of profuse and stable dislocation substructures in a cold-rolled CoCrFeMnNi HEA subject to an annealing below Trx. The dislocation substructures are observed to be thermally stable up to Trx, which could arise from the chemical complexity of the high-entropy system where certain elemental diffusion retardation occurs. The microstructure feature is markedly different from that of conventional dilute solid solution alloys, in which dislocation substructures gradually vanish by recovery during annealing, leading to a strength drop. Furthermore, dilute addition of 2 at.% Al leads to a reduction in both microhardness and yield strength of the cold-rolled and subsequently annealed (≤500 °C) HEA. This Al induced softening effect, could be associated with the anisotropic formation of dislocation substructure, resulting from enhanced dislocation planar slip due to glide plane softening effect. These findings suggest that the strength of HEAs can be tailored through the anneal hardening effect from dislocation substructure strengthening.

Prediction of the strength of high entropy alloys of the Al-Cr-Nb-Ti-V-Zr system by the solid-solution hardening model

An approach based on the phase prediction of both the phase composition and the solid solution hardening was investigated. This approach showed good agreement between the calculated and experimental values of the yield strength at 20 and 600 °C.

On the real-time atomistic deformation of nano twinned CrCoFeNi high entropy alloy

High entropy alloys (HEAs) holding several principal elements in high concentration have unprecedented combination properties. The design of strong and highly ductile HEAs has attracted extensive attention from researchers in the last decade, such as in mechanisms for inducing different types of phase and nano-sized precipitates. Since some HEAs have low stacking fault energy, nanotwins can form during the plastic deformation process or magnetron sputtering, resulting in enhanced mechanical properties due to the existence of twin boundaries. The addition of twin boundaries is implied to be a promising method in engineering HEAs. Understanding how these twin boundaries affect the mechanical properties of nanotwinned HEAs is the key to designing strong and ductile examples. In this study, we performed a large-scale molecular dynamic simulation to investigate the mechanical properties of HEAs with different twin boundary spacings at various temperatures. The results show that the strength of HEAs at all tested temperatures increases with decreasing twin boundary spacing until a lower critical value of 1.83 nm is reached, which is close to the experimental value (2 nm). The strength of the HEAs at all tested temperatures decreases as the twin boundary spacing is decreased further. The dislocation motion transitions at the critical twin boundary spacing. In the sample with a twin boundary spacing bigger than 1.83 nm, Shockley dislocations tend to intersect the twin boundaries and glide in the hardening modes; on the other hand, Shockley dislocations travel along the direction parallel to the twin boundaries in samples with a twin boundary spacing smaller than 1.83 nm, leading to detwinning and softening in the HEAs. The dislocation motion and entanglement at 1 K are respectively slower and stronger than those at 300 K; the grain boundary activity is more obvious at a higher temperature. A mechanistic theoretical model together with a Hall–Petch relationship is then proposed to consider the coupled twin boundary and temperature effect on the deformation of nanotwinned HEAs.

Lattice-distortion dependent yield strength in high entropy alloys

High entropy alloys (HEAs) have attracted great attention due to their impressive properties induced by the severe lattice distortion in comparison to the conventional alloys. However, the effect of severe lattice distortion on the mechanical properties in face-centered-cubic (FCC) and body-centered-cubic (BCC) structured HEAs is still not fully understood, which are critically important to the fundamental studies as well as the industrial applications. Herein, an analytical model for predicting the solid-solution strengthening and the yield stress in FCC and BCC HEAs accounting for the lattice distortion is presented. Both the calculated solid-solution strengthening and the yield strength are compared to the experimental results, to verify the rationality of the built theoretical model. The numerical predictions considering the severe lattice-distortion effect agree well with the experimental results for both FCC and BCC HEAs, in terms of the yield strength and the solid-solution strengthening. Based on theoretical model, the constructed contour line of solid-solution strengthening can be used to evaluate the effect of elemental type on yield strength of HEAs, which provides guideline for discovering and screening the advanced HEAs. Furthermore, it has been identified the atomic-radius mismatch and solid-solution strengthening do not increase directly as the number of components increases in HEAs based on the theoretical analysis. In the Alx-Cr-Co-Fe-Ni-Mn HEA system, the atomic-radius mismatch and shear-modulus mismatch induced by the added Al element govern the solid-solution strengthening, but this situation disappears in the Alx-Hf-Nb-Ta-Ti-Zr HEA system. It is further confirmed that the effect of the atomic-radius mismatch on the solid-solution strengthening is obviously higher than effect of the shear-modulus mismatch, dominating the yield strength. These results provide an insight into the effect of severe lattice distortion on the yield strength, and demonstrate a theoretical framework for identifying the desired compositions to create the excellent HEAs.

The effect of randomness on the strength of high-entropy alloys

High-entropy alloys (HEAs), i.e., single-phase, (nearly) equiatomic multicomponent, metallic materials, are associated with novel mechanical properties, such as high strength, fracture resistance etc. In this paper, a stochastic Peierls-Nabarro (PN) model is proposed to understand how random site occupancy affects intrinsic strength. The stochastic PN model accounts for the randomness in the composition, characterized by both the standard deviation of the perturbation in the interplanar potential and the correlation length within the spatial compositional distribution. The model presented includes the effects of non-uniform compositional distribution both in the direction of dislocation glide and along a dislocation line to predict overall dislocation glide resistance. The model predicts the intrinsic strength of HEAs as a function of the standard deviation and the correlation length of the randomness. We find that, in most of the parameter space, the compositional randomness in an HEA gives rise to an intrinsic strength that far exceeds that of any of the pure metals from which the HEA is composed. This approach provides a fundamental explanation to the origin of the high strength of HEAs.

Effects of the stacking fault energy fluctuations on the strengthening of alloys

In alloys and high entropy alloys the stacking fault energy varies with the local composition. The effects of these energy fluctuations on the strengthening are studied using dislocation dynamics simulations that track the evolution of partial dislocations in FCC metals at zero temperature. Different values of the intrinsic stacking fault energy are assigned to regions with size in the range of 0.5 nm–12 nm in the slip plane, while the other mechanical properties are left constant. A theoretical model is derived and compared to the simulation results. In the model and the simulations the predicted value of the yield stress grows with larger fluctuations of the stacking fault energy. Furthermore, a strong size dependency is observed, with a maximum in the strength attained when the mean region size approaches the average equilibrium stacking fault width. In summary, the strength of high entropy alloys can be improved by introducing disorder in the chemical misfit with a characteristic length scale of the order of the average stacking fault width.

Correlation between lattice distortion and friction stress in Ni-based equiatomic alloys

Many recent efforts have been made to apply traditional theories for solid solution strengthening to explain the strength increase in concentrated equiatomic alloys (or high-entropy alloys), but always faced the challenge of differentiating solvent from solute atoms. In this report, we conducted a systematical analysis of Ni-based equiatomic alloys with a face-centered cubic structure and found that the lattice distortion in this alloy system could be simply described by the parameter of atomic size mismatch. It was found that lattice friction stresses of these alloys were well correlated with the lattice distortion. Dislocation core width in this Ni-based alloy system was also estimated and compared with that in the pure nickel. The intrinsically high strength in high-entropy alloys was probably resulted from a high lattice friction stress.

Processing and properties of nanocrystalline CuNiCoZnAlTi high entropy alloys by mechanical alloying

The present study describes the synthesis of nanocrystalline equiatomic CuNiCoZnAlTi high entropy alloy by mechanical alloying and characterized by XRD, SEM and TEM. The CuNiCoZnAlTi high entropy alloy is mainly composed of BCC solid solution with crystallite size less than 10nm in as milled condition. The alloy is thermally stable at elevated temperature about 800°C as it retained its nanostructure. This alloy powder was consolidated using vacuum hot press at 800°C with 30MPa pressure to a density of 99.95%. The hardness and compressive strength of the high entropy alloy are found to be 7.55 and 2.36GPa, respectively. Superior strength of high entropy alloy is attributed to solid solution strengthening and its nanocrystalline nature.