FCC High Entropy Alloys Research Articles

Design of Nickel-Cobalt-Ruthenium multi-principal element alloys

The possibility of designing multi-principal element alloys in the Ni-Co-Ru system is investigated by first-principles and experimental means. A high-throughput first-principles approach is employed to probe the effect of composition on planar fault energies. Of most interest, a FCC alloy, Ni40Co40Ru20, exhibits a high room temperature strength compared to existing nickel-containing solid solution FCC high entropy alloys. The presence of Ru imparts the alloy with unique properties. First, the low stacking fault energy provides a low critical resolved shear stress for twinning, allowing a TWIP effect. Second, the high lattice friction and the associated high yield strength enables the critical resolved shear stress for twinning to be reached at low strains at room temperature, resulting in the formation of a dense network of planar defects for increased work hardening. A TRIP effect is also observed at higher strains at room temperature.

Revisit the VEC rule in high entropy alloys (HEAs) with high-throughput CALPHAD approach and its applications for material design-A case study with Al–Co–Cr–Fe–Ni system

Valence electron concentration (VEC) was treated as a useful parameter to predict solid solution phases, and the VEC rule was proposed for high entropy alloys (HEAs). However, this empirical rule has its limitations, which restricts its applications for phase predictions in HEAs. In this paper, we revisited the empirical VEC rule with the HT-CALPHAD approach in the Al–Co–Cr–Fe–Ni system. Our investigation showed that more than 90% compositions are observed to have BCC structures when 5.7≤VEC≤7.2 and we got 100% FCC structures when VEC≥8.4. Meanwhile, we proposed a data screening procedure to classify and discover the new HEAs. The concepts of average density (ρHEA), highest BCC/FCC temperatures (TBCC/FCCmax), and temperature ranges (ΔTBCC/FCC) were introduced as useful data screening criteria to down-screen the candidate alloy compositions for specific engineering applications. In the current work, we have identified three HEA categories (refractory BCC HEAs, light-weight BCC HEA and refractory FCC HEAs) in the Al–Co–Cr–Fe–Ni system and proposed the best candidate in each group. More importantly, our investigation showed the HEA non-equiatomic composition space provides ample opportunities for the discovery of the next-generation HEAs.

An Atomistic Modeling Study of the Relationship between Critical Resolved Shear Stress and Atomic Structure Distortion in FCC High Entropy Alloys — Relationship in Random Solid Solution and Chemical-Short-Range-Order Alloys —

An Atomistic Modeling Study of the Relationship between Critical Resolved Shear Stress and Atomic Structure Distortion in FCC High Entropy Alloys — Relationship in Random Solid Solution and Chemical-Short-Range-Order Alloys —

Chemical short range order strengthening in a model FCC high entropy alloy

In order to understand the role of chemical short-range order on deformation mechanisms in FCC compositionally complex alloys, a random model alloy (Co30-Fe16.67-Ni36.67-Ti16.67) is annealed at various temperatures using Hybrid Molecular-dynamics/Monte-Carlo simulations. The simulations produce significant chemical short-range order (CSRO) that increases with decreasing annealing temperature. Annealing tends to homogenize regions of high enthalpy due to: (1) chemical species redistributing into more compact configurations, and (2) pairs of atoms forming chemical bonds that lower the overall energy of the system; the composition explored here shows significant amount of ordering in Ti-Fe pairs with respect to random distributions as described by pairwise (EAM) potentials due to Johnson and Zhou. An energy topology approach is used to assess the local strengthening behavior in random solid solutions and annealed systems, where an interesting interplay is observed between misfit components and chemical short-range order affecting the overall critical resolved shear stress. The role of short-range order on the critical yield stress is quantified and compared with current solid solution models. Finally, we propose and validate an extension to the Labusch-Varvenne class of high-concentration solid-solution analytic models that incorporates the effects of chemical short range order.

A new strategy of tailoring strength and ductility of CoCrFeNi based high-entropy alloy

In this paper, a new strategy of high entropy alloy is presented, high strength and promising ductility are realized by precipitation strengthening mechanism. The structure of nano-L12 precipitates on the FCC high entropy alloy (HEA) matrix is formed, by adding Ni and Al with a fixed stoichiometric ratio (keeping Ni: Al = 3:1, Ni3Al) to CoCrFeNi matrix. The study found that the mechanical properties of the alloy can be effectively controlled by optimizing the addition of Ni3Al content. Through tension test and theoretical analysis, we found that, when the addition of Ni3Al reaches 0.75, the HEA exhibits the excellent comprehensive strength and ductility. With the tensile fracture strength, yield strength and elongation are 1200 MPa, 910 MPa, and 14%, respectively. Compared with the CoCrFeNi high entropy alloy without precipitates, the yield strength is increased by three times. TEM analysis and theoretical calculation show that the strengthening mechanism of the second phase is the main factor to improve the properties.

Transformations in CrFeCoNiCu High Entropy Alloy Thin Films during In-Situ Annealing in TEM

In-situ TEM-heating study of the microstructural evolution of CrFeCoNiCu high entropy alloy (HEA) thin films was carried out and morphological and phase changes were recorded. Post annealing investigation of the samples was carried out by high resolution electron microscopy and EDS measurements. The film is structurally and morphologically stable single phase FCC HEA up to 400 °C. At 450 °C the formation of a BCC phase was observed, however, the morphology of the film remained unchanged. This type of transformation is attributed to diffusionless processes (martensitic or massive). From 550 °C fast morphological and structural changes occur, controlled by volume diffusion processes. Fast growing of a new intermetallic phase is observed which contains mainly Cr and has large unit cell due to chemical ordering of components in <100> direction. The surface of the films gets covered with a CrO-type layer, possibly contributing to corrosion resistance of these.

Defect properties in a VTaCrW equiatomic high entropy alloy (HEA) with the body centered cubic (bcc) structure

We report first-principles results of the point defect properties in a V-Ta-Cr-W high-entropy alloy (HEA) with the body-centered cubic (bcc) structure. Different from the widely-investigated face-centered cubic (fcc) HEAs, the local lattice distortion is more pronounced in bcc ones, which has a strong influence on the defect properties and defect evolution under irradiation. Due to the large size of Ta, the exchange between vacancies and Ta exhibits lower energy barriers. On the other hand, interstitial dumbbells containing V and Cr possess lower formation energies. These defect energetics predicts an enrichment of V and Cr and a depletion of Ta and W in the vicinity of defect sinks. Besides, we find that interstitial dumbbells favor the [110] orientation in the HEA, instead of [111] direction in most nonmagnetic bcc metals, which helps to slow down interstitial diffusion significantly. Consequently, the distribution of migration energies for vacancies and interstitials exhibit much larger overlap regions in the bcc HEA compared to fcc HEAs, leading to the good irradiation resistance by enhancing defect recombination. Our results suggest that HEAs with the bcc structure may bear excellent irradiation tolerance due to the particular defect properties.

Revisit the VEC Rule in High Entropy Alloys (HEAs) With High-Throughput CALPHAD Approach and Its Applications for Material Design-A Case Study With Al-Co-Cr-Fe-Ni System

Valence electron concentration (VEC) was treated as a useful parameter to predict solid solution phases, and the VEC rule was proposed for high entropy alloys (HEAs). However, this empirical rule has its limitations, which restricts its applications for phase predictions in HEAs. In this paper, we revisited the empirical VEC rule with the HT-CALPHAD approach in the Al-Co-Cr-Fe-Ni system. Our investigation showed that more than 90% compositions are observed to have BCC structures when 5.7≤VEC≤7.2 and we got 100% FCC structures when VEC≥8.4. Meanwhile, we proposed a data screening procedure to classify and discover the new HEAs. The concepts of average density (ρHEA), highest BCC/FCC temperatures (TBCC/FCCmax), and temperature ranges (∆TBCC/FCC) were introduced as useful data screening criteria to down-screen the candidate alloy compositions for specific engineering applications. In the current work, we have identified three HEA categories (refractory BCC HEAs, light-weight BCC HEA and refractory FCC HEAs) in the Al-Co-Cr-Fe-Ni system and proposed the best candidate in each group. More importantly, our investigation showed the HEA non-equiatomic composition space provides ample opportunities for the discovery of the next-generation HEAs.

Grain boundary structure and mobility in high-entropy alloys: A comparative molecular dynamics study on a Σ11 symmetrical tilt grain boundary in face-centered cubic CuNiCoFe

We employ atomistic computer simulations to study the structure and migration behavior of a Σ11 symmetrical tilt grain boundary in a 4-component model FCC high-entropy alloy (HEA) (CuNiCoFe). The results are compared to grain boundaries in elemental metals and a so-called ‘average-atom’ sample. We find that the repeating structural units characterizing the static grain boundary structure show the same repeating structural units for all samples, while the high temperature equilibrium grain boundary structure is most strongly influenced by presence of stacking faults. Under an applied synthetic driving force, this GB migrates by a mechanism assisted by partial dislocations in all materials. For this reason the grain boundary mobilities and stacking fault energies are directly related. Moreover, the HEA sample and the average-atom sample show almost identical mobilities suggesting that local chemical fluctuations play a minor role. Solute segregation to the GB in the HEA suppresses GB migration up to very high temperatures and might be the main cause for reduced grain growth in FCC HEAs.

Effect of irradiation on microstructure and hardening of Cr–Fe–Ni–Mn high-entropy alloy and its strengthened version

ABSTRACTA single-phase fcc high-entropy alloy (HEA) of 20%Cr–40%Fe–20%Mn–20%Ni composition and its strength with yttrium and zirconium oxides version was irradiated with 1.4 MeV Ar ions at room temperature and mid-range doses from 0.1 to 10 displacements per atom (dpa). Transmission electron microscopy (TEM), scanning transmission electron microscopy with energy dispersive X-ray spectrometry (STEM/EDS) and X-ray diffraction (XRD) were used to characterise the radiation defects and microstructural changes. Nanoindentation was used to measure the ion irradiation effect on hardening. In order to understand the irradiation effects in HEAs and to demonstrate their potential advantages, a comparison was performed with hardening behaviour of 316 austenitic stainless steel irradiated under an identical condition. It was shown that hardness increases with irradiation dose for all the materials studied, but this increase is lower in high-entropy alloys than in stainless steel.

Atomistic modeling of dislocations in a random quinary high-entropy alloy

Abstract The structure and mobility of dissociated ½〈1 1 0〉 dislocations in a model FCC high entropy alloy is studied using atomistic simulations. The simulations are performed using model embedded atom method (EAM) potentials for a model five component random equiatomic alloy, and a corresponding “average atom” potential. The dislocation line that corresponds to minimum energy in the complex alloy is not straight but wavy and significant variations in dissociation distances are found. This effect is more significant for edge dislocations than for screw dislocations. Calculations also show that both the stable and unstable stacking fault energies vary according to the local composition of the alloy. The range of Peierls stresses computed for the dislocations in the alloy are significantly higher than in the pure components or those computed using an average atom potential.

Nitrogen induced heterogeneous structures overcome strength-ductility trade-off in an additively manufactured high-entropy alloy

The strength-ductility trade-off has been the long-standing barrier in the pursuit of high-performance structural materials. Here, by exploiting the merit of interstitial nitrogen, a novel additive manufacture-based approach is proposed to process the hierarchically heterogeneous structured FeCoNiCrN high-entropy alloy (HEA). Compared to the undoped alloy, the 1.8 at. % nitrogen-doped HEA shows a simultaneous increase in both strength and ductility, thereby producing exceptional combination of tensile strength (853 MPa) and elongation (34 %) that outperforms many other single phase FCC HEAs. The unusual inverse strength-ductility relationship is ascribed to the strong strengthening effect and progressive strain hardening arising from the hierarchically heterogeneous structure, the latter of which is highly beneficial to the ductility. The present approach creates heterogeneous structure avoiding surface-mechanical or thermo-mechanical treatments, and thus, enables to produce readily available components with complex geometry.

Improving mechanical properties of an FCC high-entropy alloy by γ′ and B2 precipitates strengthening

The FCC-structured high entropy alloys (HEAs) possess exceptional ductility and fracture toughness, but they generally exhibit insufficient strength for engineering applications. In this work, a precipitation strengthening non-equimolar (FeCoNi)81Cr9Al8Ti1Nb1 HEA was fabricated via arc melting and subsequent two-step aging treatment. The microstructure, phase constitution and mechanical properties of this alloy during aging were investigated systematically. The results indicate that coherent γ′ and incoherent B2 are major precipitates for the alloy under the two-step aging. An excellent balanced tensile property is achieved at room temperature even with extensive B2 grain boundary coverage. Quantitative calculations of the individual strengthening effects demonstrate that particle (γ′) shearing mechanism is the predominant strengthening mechanism. The high work-hardening capability of the FCC matrix could greatly suppress the propagation of microcracks originated at these brittle B2 phases and promote a retention of ductility. In addition, this alloy exhibits outstanding high-temperature tensile properties up to 700 °C. It is attributed to the high thermal stability of the γ′ precipitates as well as the pinning effect of the grain-boundary B2 phases on the grain boundary. Present work will focus on optimizing of the alloy design of HEAs and the precipitation strengthening of HEAs for high-temperature structural applications.

Effect of carbon on cryogenic tensile behavior of CoCrFeMnNi-type high entropy alloys

High entropy alloys (HEAs) with a face-centered cubic (fcc) structure are considered as promising structural materials, in particular due to their impressive ductility and toughness at cryogenic temperature; at the same time strength of these HEAs is often quite low. An addition of interstitial elements like carbon substantially increases the strength of the fcc HEAs at room temperature, however the effect of C on cryogenic properties has not been properly studied. Therefore in this work we examined cryogenic tensile behavior of the fcc high entropy alloys with different carbon content (0–2 at.%). The alloys had non-equiatomic proportions of principal elements, i.e. Co1Cr0.25Fe1Mn1Ni1. The lower Cr concentration in comparison with the equiatomic alloy led to the higher solubility of carbon confirmed by both ThermoCalc calculations and experimental results; only in the alloy with 2 at.% C a small (<1%) fraction of Cr-rich M7C3 carbides was found in the as-cast condition. The microstructure of the alloys was not significantly affected by the carbon content and generally consisted of coarse (250–300 μm) fcc phase grains with dendritic segregations. In turn, the carbon content influenced on mechanical behavior substantially: the strength of the alloys progressively increased with the carbon content along with some reduction in ductility. Solid solution strengthening by carbon at 77 K was much stronger than that at room temperature: 67 MPa/at% and 178 MPa/at%, respectively. The increase in solid solution strengthening agreed well with an anticipated increase in lattice friction at lower temperatures. Plastic deformation was associated with dislocations slip both at 293 K and 77 K; a decrease in temperature and an increase in the carbon concentration increased the inclination to planar slip. The obtained results offer new approaches to increase the cryogenic properties of fcc HEAs.

Deformation microstructures and strength of face-centered cubic high/medium entropy alloys

In this study we report the deformation microstructures and strength of medium entropy alloys (MEAs) and high entropy alloys (HEAs), which are defined as alloys composed of four or less, and five or more principal elements, respectively, with (near-) equi-atomic concentrations. The friction stress (fundamental resistance to dislocation motion in the crystal lattice) and Hall-Petch relationship of various MEAs (CoCrFeNi, CoCrNi, etc.), taken as subsystems of the equi-atomic CoCrFeMnNi HEA, were precisely measured at room temperature. Experimental values of the friction stresses were found to fit with a theoretical model proposed by Toda-Caraballo et al. very well, which indicates that the strength of the alloys is closely related to a heterogeneously distorted crystal lattice. At the same time, values of the average lattice distortion in the alloys were found to be comparable to those in some dilute alloys, contradicting the belief that “severe” lattice distortion is a reason for the higher strength than in dilute systems. Finally, a strengthening mechanism due to element-element interactions was proposed as an additional mechanism in FCC HEAs and MEAs.

A fractal approach to predict the oxidation and corrosion behavior of a grain boundary engineered low SFE high entropy alloy

The current study presents a method to predict the oxidation and corrosion properties in a grain boundary engineered low stacking fault energy (SFE) FCC high entropy alloy (HEA) Ni14Cr21.5Co21.5Fe21.5Mn21.5 through fractal analysis. Two thermo-mechanical processing (TMP) routes namely, unidirectional rolling (UDR) and multistep cross rolling (MSCR) with intermediate annealing, were employed to enhance the percentage of coincident site lattice (CSL) boundaries and break the random boundary network (RBN) in the alloy. Electron backscattered diffraction (EBSD) was utilized to analyze the microstructure and local texture of the samples before and after subjecting the samples to TMP. Fractal analysis (box-method) was carried out to analyze and quantify the RBN in the processed samples. Potentiodynamic polarization experiments were carried out to study the corrosion resistance as a function of CSL boundaries in the processed samples. Thermogravimetric analysis (TGA) was carried out to study the oxidation behavior in the samples. Elemental segregation in the oxide layer was also analyzed using Energy dispersive spectroscopy (EDS) line scan analysis. In the final part of the study, empirical relationships were established between fractal number, oxide layer thickness, and corrosion rate respectively. It was observed that Fractal numbers could be used as a powerful and reliable tool to predict some of the grain boundary character dependent properties like corrosion and oxidation in the alloy system under study.

Combining experiments and modeling to explore the solid solution strengthening of high and medium entropy alloys

The mechanical properties due to solid solution strengthening are explored within the single phase face-centered cubic (fcc) domain of the Co–Cr–Fe–Mn–Ni high entropy alloy (HEA) system. This is achieved by combining an efficient and reproducible metallurgical processing of alloys to X-ray diffraction and nanoindentation characterization techniques, thus enabling to get access to 24 different bulk alloys. Large variations of nanohardness are seen with composition. Experimental results are rationalized in terms of lattice misfit and elastic constant variations with alloy-composition, through the use of an analytical mechanistic theory for the temperature-, composition- and strain-rate-dependence of the initial yield strength of fcc HEAs, with predictions made using only experimental inputs. The good agreement obtained by comparing model predictions to experiments provides the basic framework for mechanical properties optimization within the Co–Cr–Fe–Mn–Ni system; the approach could be systematically applied to all classes of fcc HEAs.

Dislocation core structures and Peierls stresses of the high-entropy alloy NiCoFeCrMn and its subsystems

High entropy alloys (HEAs) have emerged as promising next-generation structural materials. In order to understand the strengthening mechanism in these multicomponent alloys, a theoretical investigation is presented here in the framework of the Peierls-Nabarro (PN) model, which we consider would be appropriate for FCC HEAs due to their homogeneous feature, small lattice distortions and wide dislocation core structures. More importantly, there is no need to differentiate solutes and solvents in this model, which avoids the conceptual difficulties for such alloys. Using PN model, we calculate the dislocation core structures and Peierls stresses of the prototypical NiCoFeCrMn HEA and its six subsystems using the averaged gamma surfaces. The calculated stacking fault widths are in good agreement with available experimental data, and the obtained core structures are important for the future evaluation of the solute-dislocation interaction energies. The Peierls stresses in these multicomponent alloys are found to be much larger than pure FCC metals, and are generally in the same order of magnitude as the critical resolved shear stresses (CRSSs) extrapolated to zero temperature. The results indicate that in contrast to conventional FCC metals, the increased Peierls stresses in MEAs and HEAs could be responsible for their high yield stresses.

Development of a large size FCC high-entropy alloy with excellent mechanical properties

A large size 10 kg Fe40Mn40Co10Cr10 high-entropy alloy (HEA) with 3.3 at. % C-doped was prepared by electromagnetic levitation melting. The hot deformation behavior of the as-cast ingot was investigated to guide the hot forging process. Annealing was further conducted on the as-forged samples to obtain more optimized mechanical properties. The as-forged HEA had yield strength (YS) of 792 MPa, ultimate tensile strength of 1055 MPa and a satisfactory fracture elongation (EL) of 22.6%. The YS of the HEA annealed at 1050 °C increased from 393 MPa to 512 MPa and EL from 26.5% to 55.3% in comparison with the as-cast HEA. It is suggested that the enhanced yield strength of the as annealed HEA was attributed to carbon interstitial solid solution strengthening, grain refining and precipitation of tiny carbides, and the improvement of ductility was due to the elimination of cast defects and twinning-induced plasticity. This work paved the way for the application of HEA as high performance structure materials.

Processing pathway effects in CoCrCuNi+X (Fe, Mn) high-entropy alloys

ABSTRACTA parallel study of mechanical alloying and solidification was carried out on FCC high-entropy alloys (HEAs) CoCrCuNi, CoCrCuFeNi and CoCrCuMnNi to investigate the effects of each processing methods on the resulting microstructure, crystal structure and microhardness. Elemental powders were mechanically alloyed followed by spark plasma sintering (SPS) at 800°C and 900°C to achieve densified discs, while arc melting was carried out from bulk pieces of the elemental metals followed by furnace annealing at 800°C and 900°C for 5 h. Both processing routes resulted in a primary FCC phase with secondary Cu-rich FCC segregation as interdendrites for the solidified alloys and particle boundaries for the SPS alloys, with the exception of a small amount of σ phase present in the SPS processed alloys. The solidification of the CoCrCuNi, CoCrCuFeNi and CoCrCuMnNi HEAs resulted in typical dendritic microstructure, followed by the precipitation of a small Cr-rich phase in the CoCrCuMnNi alloy after annealing. The grain size of the mechanically alloyed powder was approximately 20 nm from the Scherrers equation and the SPS processed HEAs consisted of a Cu-rich phase in the particle boundaries, forming cobblestone-like microstructure. The microhardness was examined in the as-cast, annealed and SPS states. It was found that the SPS processed samples had an increased microhardness by a factor of 2.5.