Properties Of High-entropy Alloys Research Articles

First-principles calculation of lattice distortions in four single phase high entropy alloys with experimental validation

Exceptional properties of high entropy alloys (HEAs) are attributed to the disordered random solid solution of multiple alloying elements. Despite numerous studies, the fundamental understanding at the electronic and atomic levels is still missing. We report a comparative study of four fcc HEAs of NiFeCoCr and NiFeCoCrX (X = Mn, Cu, or Pd) based on ab initio calculations using large supercells with 500 atoms in equal composition. After fully optimizing their structures using the VASP package, their electronic structure, interatomic bonding, partial charge distribution, and mechanical properties are calculated and compared, revealing the intricate interdependence among them. A novel parameter based on the quantum mechanical metric for internal cohesion, the total bond order density (TBOD), is used to interpret the calculated properties. The highest TBOD is found in Cantor alloy NiFeCoCrMn but lower in NiFeCoCrPd. The atomic radii vary depending on their local chemical environment. The resulting lattice distortions is validated experimentally in NiFeCoCrMn and NiFeCoCrPd. Moreover, modeling of Cu and Pd clustering in the supercell shows they have lower total energy in agreement with the observation of Cu-enhanced nano-participates in NiFeCoCrCu and Pd-induced concentration wave in NiFeCoCrPd HEAs.

Effect of CoCrFeMnNi transition cladding layer on crack resistance of CoCrFeMnNi + x(TiC) composite cladding layer

Ceramic particles can be used as reinforcements to improve the mechanical properties of high entropy alloy (HEA) coatings. However, brittle and hard ceramic particles cause high dislocation density and defects in the coating, which are the potential sites for crack initiation. In this study, in order to improve the crack resistance of CoCrFeMnNi + x(TiC) composite coating (CC), CoCrFeMnNi cladding layer was pre-coated on the substrate before fabricating the composite cladding layer. The results showed that the CoCrFeMnNi transition cladding layer with good plasticity could effectively suppresses crack propagation in the composite cladding layer under compression, and no cracks were found in the substrate. This paper provides a new idea for enhancing the properties of CC.

Effect of nitrogen on microstructure and mechanical properties of CrMnFeVTi6 high entropy alloy

In order to evaluate interstitial strengthening effect on the properties of high entropy alloy (HEA), a nitrogen-doped CrMnFeVTi6 HEA was fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). XRD, SEM, TEM and FIB were used to characterize the phase composition and microstructure of this material. The sintered bulk HEA exhibits a microstructure comprising TiNx, BCC, Laves and B2 phases. The HEA exhibits high yield strength (>2729 MPa) and hardness in lower temperature range of <380 °C. Quantitative calculations of the contributions from each strengthening mechanism in the BCC phase indicate that the interstitial strengthening by nitrogen is the dominant mechanism. Nitrogen additions in the BCC phase can produce a yield strength increase of ∼634 MPa/at.%, which is much higher than the strengthening effects of carbon or boron additions in other alloys. This demonstrates that adding nitrogen is a viable approach for enhancing the strength of HEAs.

Influence of Refractory Elements on Mechanical Properties of High Entropy Alloys

Influence of Refractory Elements on Mechanical Properties of High Entropy Alloys

Core-shell structure mediated microstructure and mechanical properties of high entropy alloy CoCrFeNi/Al composites

In this work, CoCrFeNi high entropy alloy (HEA) with FCC structure as reinforcement has been used to add in the effectiveness of HEA/Al composites, which was fabricated by spark plasma sintering, leading to an in-situ formed shell with FCC + BCC structure surrounding HEA core under higher sintering temperature. This study focuses on two aspects: evaluation of the micromechanics properties through nanoindentation creep tests and analysis of the mechanical properties in composites. Experimental results manifested that the formed shell has higher hardness and nanoindentation creep resistance compared with HEA core, which is attributed to a large number interdiffusion of Al and CoCrFeNi elements along with its small grain size. In comparison with HEA/Al composite without core-shell structure, the compressive strength of HEA/Al composite with core-shell structure is substantially improved about 2.6 times because that the formation hard shells can be regard as reinforcement in composite. However, the cracks are easy to nucleate and rapidly propagate at shell under the applied load, giving rise to ductility decrease in the composite.

First-principles-based high-throughput computation for high entropy alloys with short range order

We extend the small set of ordered structures (SSOS) method to calculate several typical materials properties of high entropy alloys (HEAs) with short range order (SRO). Using both equimolar and non-equimolar quinary CoCuFeNiPd HEAs with SRO as an example, we calculate ground state energy, lattice constant, mass density and elastic moduli, and compare them with those obtained from the special quasi-random structure (SQS) method. The calculated properties are in good agreement with those obtained by SQS. Importantly, the computational cost of the SSOS method for achieving the same accuracy is only a small fraction of the computational cost of the SQS method. We further show that the application of machine learning methods can further reduce the computational cost of the SSOS method and maintain the same level of accuracy, making it especially suitable for high-throughput calculations of HEAs with SRO.

Vibrational properties of High Entropy Alloy based metal hydrides probed by inelastic neutron scattering

The vibrational properties of several High Entropy Alloy (HEA) based metal hydrides are investigated by inelastic neutron scattering (INS). HEAs have recently emerged as a new type of materials with a wide range of intriguing properties and potential applications such as hydrogen storage. The special properties of HEAs are believed to originate from the disordered lattice and internal strain that is introduced from the differences in atomic radii. This makes HEA hydrides provide an intriguing situation for the local H coordination, of several different transition metals. INS spectra were collected on a series of HEA-based metal hydrides starting with TiVNbHx and subsequently adding Zr and Hf to increase the atomic size mismatch. A general feature of the spectra are the optical peaks centered around an energy loss of 150 meV that can be attributed to hydrogen vibrations in a tetrahedral environment. Upon the addition of Zr and Hf, a shoulder appears on the optical peak at lower energy transfers that after comparison with in silico calculated INS spectra is indicative of hydrogen also occupying octahedral sites in the structure.

A first-principles study of hydrogen storage of high entropy alloy TiZrVMoNb

In this study, density functional theory calculations have been carried out to study the hydrogen storage properties of high entropy alloy (HEA) TiZrVMoNb. It reveals that a BCC→FCC phase transformation occurs when the hydrogen content reaches 1.5 wt% during hydrogenation process, and octahedral and tetrahedral interstitial sites are preferable for hydrogen occupation before and after phase transformation, respectively. Further energetic analyses show that different hydrogen occupations in HEAs play an important role in the thermal stability of hydrides. The maximum hydrogen storage capacity for TiZrVMoNb is predicted to be 2.65 wt%, which is comparable to the largest value of 2.7 wt% for TiZrVHfNb and larger than that of other reported HEA hydrogen storage materials reported in the literature. As compared with the previously reported HEA TiZrHfMoNb with the change of only one principal element, the TiZrVMoNb not only has much higher hydrogen storage capacity, but also has more moderate hydrogen desorption temperature. The difference in hydrogen storage properties between these two HEAs is mainly attributed to the atomic weight, site occupation, lattice distortion and chemical effect of metal elements. The present study thus suggests that the TiZrVMoNb HEA has great potential as hydrogen storage materials and proposes a strategy to enhance the hydrogen storage properties of HEAs.

Impact of N on the Stacking Fault Energy and Phase Stability of FCC CrMnFeCoNi: An Ab Initio Study

Interstitial alloying has become an important pillar in tuning and improving the materials properties of high-entropy alloys, e.g., enabling interstitial solid-solution hardening and for tuning the stacking fault energies. In this work we performed ab initio calculations to evaluate the impact of interstitial alloying with nitrogen on the fcc–hcp phase stability for the prototypical CrMnFeCoNi alloy. The N solution energies are broadly distributed and reveal a clear correlation with the local environments. We show that N addition stabilizes the fcc phase of CrMnFeCoNi and increases the stacking fault energy.

Effect of Ni content on the phase formation, tensile properties and deformation mechanisms of the Ni-rich AlCoCrFeNix (x = 2, 3, 4) high entropy alloys

Understanding the microstructure evolution and deformation mechanisms are crucial for designing and tailoring the mechanical properties of high entropy alloys. In this work, the microstructure, tensile properties and deformation mechanisms of as-cast Ni-rich AlCoCrFeNix (x = 2, 3, 4) high entropy alloys were investigated and revealed. Based on the CALPHAD simulates and multi-scale microstructure characterization, it was found that increasing Ni content was not only facilitated the formation of FCC/L12 phases, but also caused the microstructure transformation evolved from near-eutectic structure to the divorced eutectic structure. Besides, Ni elements had the capacity to induce a slip mode transformation of FCC phases from planner slip to wavy slip, as well as activation of micro-bands. Because of the high volume fraction of FCC/L12 phases and activation of micro-bands, as well as inhomogeneous element distribution, the AlCoCrFeNi4 alloy exhibited an excellent plasticity with moderate strength in the as-cast state. The findings are beneficial for the study of achieving the high entropy alloys with a better strength-plasticity synergy.

Nitrogen-induced hardening of refractory high entropy alloys containing laminar ordered phases

Most attempts to improve the properties of high entropy alloys (HEAs) involve the exploration of non-equimolar compositions, addition of alloying elements, and/or manipulation of the microstructure. Alternatively, this work reports on intentionally doping HEAs with non-metallic species to precipitate coherent, ordered phases in order to maximize hardness. A refractory MoNbTaW HEA was synthesized via cryogenic mechanical alloying and doped with nitrogen by using liquid nitrogen as the cryogen. Overall, the doping strategy was successful as multiple nitrogen-rich secondary phases were observed. In particular, a unique ordered laminar phase was developed and identified as tetragonal (Mo,W)(Nb,Ta)N nitride via aberration-corrected scanning transmission electron microscopy (STEM). Another complex (Nb,Ta)2CN carbonitride was also identified. The growth behaviors of the ceramic phases were studied using long term aging treatments of up to 100 hours at 1200°C. A second MoNbTaW alloy was also prepared via high-energy ball milling, without nitrogen, by using liquid Ar as the cryogen. A comparison of the two alloys’ microstructures and properties confirm that intentional formation of complex nitride phases greatly enhanced the hardness of mechanically alloyed MoNbTaW by up to 3-4 GPa.

Decelerated grain growth kinetic and effectiveness of Hall-Petch relationship in a cold-rolled non-equiatomic high entropy alloy

The present work deals with the grain growth behavior and mechanical properties of a cold-rolled non-equiatomic metastable Fe50Mn30Co10Cr10 high entropy alloy. The bimodal grain size distribution which was developed during isothermal annealing at the temperature range of 750–1050 °C, was gradually annihilated through increasing the annealing time. This was due to the higher capability of fine grains to grow up compared with coarse grains. The obtained grain growth exponents and grain growth activation energies were well higher than those reported for solid solution alloys and also reported for five component CoCrFeMnNi high entropy alloy. The extremely low grain growth kinetics of the experimented alloy even at high homologous temperature was discussed regarding the solute drag like effect of whole-solute matrix in addition to the sluggish diffusion as an intrinsic properties of high entropy alloys. The variation in mechanical properties of the experimented alloy was mainly discussed relying on the effectiveness of Hall-Petch relationship and the stability of the matrix. It was found that the grain boundaries acted as an effective barrier against dislocation motion, and caused postponement of the strain induced transformation leading to an acceptable strength/ductility balance.

Friction Stir Welding/Processing of High Entropy Alloys (HEAs)

Design and development of high entropy alloys is very important to overcome the shortcomings of conventionally used alloys in applications where operating conditions of temperature and loading are extreme. In this recent paper, the application of Friction Stir Welding for joining the high entropy alloys and also using Friction Stir Processing for improving the mechanical and microstructure properties of high entropy alloys are discussed.

Selected Properties of High Entropy Alloys Based on the AlFeMnNbNiTi System

The aim of this work was to study the impact of various fabrication methods used to prepare high entropy alloys based on the AlFeMnNbNiTi system. Chemical composition was customized to ensure a solid solution structure with precipitation of the Laves phase. The three manufactured alloys were prepared by melting, but with the use of various input materials and different furnaces in protective atmospheres. After the melting process, heat treatment was carried out. Structures of obtained materials were analyzed by means of a Scanning Electron Microscope (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) mapping. Mechanical properties were represented by Vickers hardness. In this paper, impact of the use of low purity input materials is shown, as well as differences in structure resulting from the utilization of different melting furnaces.

Lattice distortion as an estimator of solid solution strengthening in high-entropy alloys

Over the past decade, select high-entropy alloys (HEAs) have exhibited excellent structural properties, even at high temperatures, outperforming conventional alloys in some cases. Intriguingly, some reports in the literature suggest that HEA properties may be enhanced by increasing the number of elements, while another school of thought negates this notion and suggests that there is no clear dependence of mechanical properties on number of elements. We further examine this question in the context of a quinary refractory alloy system (MoTaTiWZr) and scrutinize whether number of elements in a HEA positively impact its mechanical properties. The present work showcases that certain equiatomic low- and medium- entropy alloys can exhibit superior structural properties (hardness, Young's modulus) relative to their higher-entropy counterparts composed of the same family of elements. Evidently, incorporating a higher number of constituent elements does not guarantee enhanced structural properties. Using a synergy of experimental measurements, complementary microscopic characterization and materials theory, we conclusively demonstrate that the intrinsic lattice distortion and cohesive energies are the predominant strengthening mechanisms that are reflected as high hardness and Young’s moduli of single-phase multicomponent alloys investigated in this work. Severe lattice distortion is one of the core effects of HEAs which imparts excellent room temperature structural properties and is generated by mixing multiple atom types. Likewise, a higher cohesive energy between the atoms in a lattice requires greater shear stresses to break the metallic bonds that increases the stiffness. An alloy with lower number of elements may intrinsically possess a higher cohesive energy than one with a higher number of elements within the same series, thereby outperforming the higher-entropy alloy on the structural properties.

Tuning Mechanical Properties of High Entropy Alloys by Electro-Pulsing Method

The single-phase face-centered cubic (FCC) high entropy alloys (HEAs) with remarkable ductility but lower strength, limiting the engineering application in a wide range. The ductility-strength trade-off could be addressed by tuning the constituent and chemical composition to lower stacking fault energy (SFE) and introduce additional strain-hardening strategies. In this work, a series of non-equiatomic (Co 40 Fe 25 Cr 20 Ni 15 ) 95 Al 5 HEAs annealed by flash electro-pulsing treatments at different voltages, was prepared with the incorporation of deformation twining during plastic deformation. The 130 V-annealed sample demonstrated a good combination of tensile strength of 0.96 GPa and ductility of 16.5%, while the 150 V-annealed alloy showed dramatically increased ductility of 49.2%. The higher Hall-Petch coefficient k H ( K σy ) value of 341.6 (828.8) and the thinner deformation twins spacing was responsible for the improved tensile strength for 130 V-samples. Moreover, more stacking faults and deformation twins in 150 V-sample accommodated more plastic and delayed the fracture, resulting increased ductility. This work provides a fast-effective method to tune the mechanical properties of non-equiatomic HEAs by adjusting the annealing voltage and achieved in a minute, which might open the avenue for future industrial application.

高熵合金制备与力学性能研究进展

高熵合金制备与力学性能研究进展

Nanoindentation into FeCoNiCrCu high-entropy alloy: an atomistic study

The mechanical behaviours of FeCoNiCrCu high entropy alloys (HEAs) were studied via large-scale molecular dynamics (MD) simulations. A rigid indenter was applied in the indentation to investigate the microstructural evolution and mechanical properties of HEAs in terms of indentation force, Young’s modulus, dislocation behaviours, and shear strain distributions. Effects of Cu precipitation on the mechanical properties of HEAs were discussed. The modulus calculated from simulations were inconsistent with experiments. The precipitation of Cu will increase the elastic modulus of the matrix, while the solution of Cu into the matrix can increase the work strengthening rate. The coherent interface of Cu precipitation can contribute to a high work strengthening rate, while the incoherent interface will lead to a lower deformation resistance.

Microwave sintered porous CoCrFeNiMo high entropy alloy as an efficient electrocatalyst for alkaline oxygen evolution reaction

Developing efficient, stable and economical electrocatalyst for oxygen evolution reaction (OER) is a significant pathway to produce clean energy from water splitting. Herein, the promising and highly efficient porous CoCrFeNiMo high entropy alloys (HEAs) were prepared by microwave sintering using Mg space holder. Owing to unique properties of high entropy alloys and abundant active surface area, the porous CoCrFeNiMo-20 Mg exhibits excellent catalytic performance and prominent electrochemical stability with a low overpotential of 220 mV to reach current density of 10 mA cm−2, a small Tafel slope of 59.0 mV dec-1 and long-term durability for 24 h in 1.0 M KOH. The results of microstructure and element states indicate that crystal defects, porous structure and villous hydroxides are the reasons for high OER performance of the CoCrFeNiMo. This work not only provides a new way to prepare electrocatalysts, but also proves the important application of high entropy alloys in functional materials.

Effects of nickel content on the microstructure, microhardness and corrosion behavior of high-entropy AlCoCrFeNix alloys

In this study the effect of three different nickel concentration on the microstructure, hardness and corrosion properties of high entropy alloys (HEAs) from AlCrFeCoNi system as an alternative material for medical instruments fabrication was investigated. The analyzed HEAs were AlCrFeCoNix obtained by vacuum arc remelting from high purity raw materials and having nickel atomic ratio x = 1.0, 1.4 and 1.8. The microscopy examination revealed the dendritic morphology for the reference alloy (AlCrFeCoNi) and that the extent of the interdendritic areas increased with the concentration of nickel while Cr was more segregated in the interdendritic areas than in dendrites. Hardness values decreased as the percentage of nickel increased due to the dissolution of the precipitates in a nickel-rich matrix and consequently the formation of continuous solid solutions. The corrosion properties of the synthesized HEAs were evaluated using a potentiodynamic polarization method. The alloys were immersed in Simulated Body Fluid during one week and the corrosion parameters were recorded. The low corrosion rates, low corrosion currents and high polarization resistance attest the good stability of these HEAs in simulated biological environment indicating their possible use for surgical and dental instruments.