Mechanical Properties Of High-entropy Alloys Research Articles

Impact of edge dislocation and grain boundaries on mechanical properties in CoCrCuFeNi high entropy alloy

This study uses molecular dynamics simulations to explore the mechanical behavior of a CoCrCuFeNi high-entropy alloy (HEA) with Σ5 and Σ13 grain boundaries (GBs) as well as without GBs and dislocation. The analysis focused on understanding the influence mechanisms of these grain boundaries on the mechanical behavior of the HEA. Our findings reveal that the atomic size disparity among the constituent elements induces lattice distortion, leading to deformation in HEAs. The determined elastic constants met Born stability requirements, ensuring mechanical stability across both the examined GBs. Higher elastic moduli were associated with increased strength and stiffness, particularly evident in HEAs with Σ5 GB, surpassing those of non-GB structures. Notably, GB Σ5 demonstrated enhanced strength and hardness, indicated by larger elastic moduli compared with those of non-GB structures. Conversely, GB Σ13 exhibited increased Cauchy pressure and Poisson and Pugh's ratios. The ductility of face-centered cubic HEAs was found to be significantly influenced by the GBs, affecting mechanical properties. The Kleinman parameter highlighted a bending-type bonding with reduced strength at the GBs. Machinability indices indicated high machinability of the CoCrCuFeNi alloy, further enhanced by the presence of the GBs. Direction-dependent parameters underscored the anisotropic nature of the HEA, mitigated by the GBs. Overall, this study elucidates the nuanced influence of different GBs on the mechanical properties of HEAs, offering valuable insights for materials design and applications. The results of this investigation shed light on HEAs with improved mechanical properties via GB engineering.

Improving Mechanical Properties of Co-Cr-Fe-Ni High Entropy Alloy via C and Mo Microalloying.

The as-cast [Co40Cr25(FeNi)35-yMoy]100-xCx (x = 0, 0.5, y = 3, 4, 5 at.%) HEAs (high-entropy alloys) were prepared by a vacuum arc melting furnace and were then hot rolled. The effect of C and Mo elements on the microstructure evolution and mechanical properties of HEAs was systematically analyzed. The results showed that when no C atoms were added, the HEAs consisted of FCC + HCP dual-phase structure. In addition, as the Mo content increased, the grain size of the alloy increased from 17 μm to 47 μm. However, only the FCC phase appeared after adding 0.5 at.% carbon in Mo microalloyed HEAs, and the grain size of the Mo4C0.5 HEA decreased significantly. Due to the Mo atom content exceeding the solid solution limit, the carbides of Mo combined with the C element appeared in the Mo5C0.5 HEA. The strength of C and Mo microalloyed HEAs significantly increased compared to HEAs with no C added. However, the Mo4C0.5 HEA exhibited excellent comprehensive mechanical properties, which was superior to a majority of reported HEAs and conventional metal alloys. Its yield strength, tensile strength, and elongation were 757 MPa, 1186 MPa, and 69%, respectively. The strengthening mechanism was a combination of fine grain strengthening, TWIP effect, and solid solution strengthening.

Exploring the effects of temperature on the mechanical properties of high-entropy alloy (CoCrFeNiAl0.1) based on molecular dynamics simulation

In order to further explore the influence of temperature on the face-centered cubic (FCC) single-phase crystal CoCrFeNiAl0.1, we conducted a series of Nano-indentation experiments on CoCrFeNiAl0.1 at different temperatures. At room temperature, the effects of indentation can convert a portion of CoCrFeNiAl0.1’s FCC phase into a funnel-shaped hexagonal close-packed (HCP) phase, resulting less deformation on the sides of the indenter. What we analyzed shows that CoCrFeNiAl0.1’s HCP phase has excellent heat resistance and mechanics, allowing CoCrFeNiAl0.1 to maintain great properties in high-temperature environments. However, if T ⩾ 1500 K, high temperature will decrease the number of the HCP phases and dislocation density, leading to an accelerated decline in material strength. This research can provide a theoretical relationship between temperature and microstructural evolution for the research and application of CoCrFeNiAl0.1 in high-temperature environments.

The influence of chemical short-range order on the nanoindentation properties of high-entropy alloys prepared via laser powder bed fusion

Chemical short-range order (CSRO) plays an instrumental role in determining the mechanical properties of high-entropy alloys (HEAs). Current methods for controlling CSRO mainly focus on cast HEAs, which can not solve the problem of grain coarsening caused by homogenization treatments. Laser Powder Bed Fusion (LPBF), which has extremely high cooling rates, can suppress compositional segregation, refine grain structures, and achieve low CSRO. This offers a promising avenue for quantitatively controlling CSRO without the need for homogenization treatments. In this study, we investigate the CSRO of HEAs fabricated via LPBF and elucidate their influence on nanoindentation behavior. The results show that, at the low cooling rate, the average size of CSRO is 0.66 nm and they occupy 2.8% of the cross-sectional area, while at high cooling rate, the corresponding values are 0.92 nm and 9.9%, respectively, both of which are significantly smaller than those of cast HEAs. By analyzing samples with different CSROs, we observe that the hardness increases and then decreases with the increase of CSRO, primarily due to the preferential formation of dislocations in regions enriched with Cr-Mn-Ni. With the improvement of CSRO, both the size and quantity of Cr-Mn-Ni rich region gradually increased; subsequently, as multiple Cr-Mn-Ni rich regions tend to merge into one during the CSRO improvement, the size of the Cr-Mn-Ni rich region continued to increase while the quantity gradually decreased; when the size of the Cr-Mn-Ni rich region is moderate and widely dispersed, the dislocation density is higher with much more uniform distributions, which favorably improves nanoindentation hardness.

Significant strength enhancement of high-entropy alloy via phase engineering and lattice distortion

The extraordinary mechanical properties of high-entropy alloys (HEAs) hold great promise for their applications under extreme conditions (high pressure, high temperature, etc.). However, investigations on the mechanical properties of HEAs under extreme conditions (especially at high pressures) are extremely scarce, preventing their future applications. In this study, significant strength enhancement of Cantor (CoCrFeMnNi) HEA is achieved via phase engineering by using high-pressure techniques. The pressure-induced fcc-hcp phase transition of Cantor HEA leads to a strength increase of 65 % across the phase transition. Consequently, the hcp-Cantor HEA behaves as strongly as typical superhard ceramic B4C and surpasses corundum at moderate pressures. Moreover, the hcp-Cantor HEA quenched to ambient pressure is 47 % harder than the pristine fcc phase. In addition, by substituting the Mn atom with the Pd atom to introduce a severe lattice distortion in Cantor HEA, the CoCrFePdNi HEA turns out to be almost twice harder than Cantor HEAs at ambient pressure. The excellent mechanical properties of HEAs promise their potential applications at high pressures. These results also demonstrate that phase engineering and the introduction of severe lattice distortion represent two promising methods of developing high-strength HEAs in the future.

Microstructure refinement and enhanced mechanical properties in rapid-quenched MnCrFeCoNi high-entropy alloy

High-entropy alloys (HEAs) have gained significant attentions in recent years, due to their unique properties derived from the combination of multiple elements in equimolar or near-equimolar ratios. The mechanical properties of HEAs are influenced by microstructural characteristics. In this study, MnCrFeCoNi HEA ribbons were produced using a technique called melt spinning, for which the wheel speed was adjusted to control the undercooling levels. The rapid solidification process under undercooling condition resulted in refined grain sizes to micrometers in the ribbons. One notable feature was the appearance of twin boundaries, which especially accounted for approximately 7.36 % of the microstructure for the ribbons produced at a wheel speed of 10 m/s. For the ribbons with thickness of micrometer scale, the mechanical properties (ultimate tensile strength up to 2.5 GPa and hardness up to 300 MPa) were analyzed by microstructure (grain boundaries and homogeneity) and exterior factors (e.g. thickness). Overall, this study provides a new approach for tailoring the microstructures and mechanical properties of HEAs via melt spinning technique. The HEA ribbons present a novel form that could potentially broaden the scope of applications for these materials.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of the Al0.6CoCrFeNi High-Entropy Alloy.

The effect of heat treatment on the microstructure and tensile properties of an as-cast Al0.6CoCrFeNi high-entropy alloy (HEA) was investigated in this paper. The results show that the as-cast Al0.6CoCrFeNi HEA presents a typical FCC dendrite morphology with the interdendritic region consisting of BCC/B2 structure and heat treatment can strongly affect the microstructure and mechanical properties of HEA. Microstructure analysis revealed the precipitation of a nano-sized L12 phase in the FCC dendrite and the formation of the FCC and σ phases in the interdendritic region after annealing at 700 °C. The coarse B2 phase was directly precipitated from the FCC dendrite in the 900 °C-annealed sample, with the coexistence of the B2, FCC, and σ phases in the interdendritic region. Then, the interdendritic region converted to a B2 and FCC dual-phase structure caused by the re-decomposition of the σ phase after annealing at 1100 °C. The tensile test results show that the 700 °C-annealed HEA presents the most significant strengthening effect, with increments of corresponding yield strength being about 107%, which can be attributed to the numerous nano-sized L12 precipitates in the FCC dendrite. The mechanical properties of 1100 °C-annealed alloy revert to a level close to that of the as-cast alloy, which can be attributed to the coarsening mechanism of B2 precipitates and the formation of a soft FCC phase in the interdendritic region. The observed variation in mechanical properties during heat treatment follows the traditional trade-off relationship between strength and plasticity.

An atomistic study on grain-size and temperature effects on mechanical properties of polycrystal CoCrFeNi high-entropy alloys

The origin of the outstanding mechanical properties of high-entropy alloys (HEAs) is still elusive. In this paper, we have investigated the influence of grain size and temperature on the mechanical properties of polycrystalline CoCrFeNi HEAs via molecular dynamics simulations. The critical grain size for the inverse Hall-Petch effect in CoCrFeNi HEAs is approximately 11.65 nm. During the stage of the positive Hall-Petch effect (where smaller grain sizes lead to higher tensile strength), there is a positive correlation between the tensile strength and dislocation density in CoCrFeNi HEAs. The higher tensile strength at low temperatures in CoCrFeNi HEAs is also attributed to an increase in dislocation density. At low temperatures, the reduced thermal vibrations of atoms slow down the motion of dislocations, leading to an increased storage time of dislocations within the crystal. In the stage of the inverse Hall-Petch effect, due to a significant reduction in grain size, the storage capacity of dislocations within grains decreases. This leads to the accumulation of more dislocations at grain boundaries, forming defects, and consequently causing a decrease in the tensile strength of CoCrFeNi HEAs as the grain size decreases. Our results might be helpful in material design of polycrystal HEAs.

Element-dependent evolution of chemical short-range ordering tendency of NiCoFeCrMn under irradiation

The evolution of short-range order (SRO) structures under irradiation has a great impact on the mechanical properties of high-entropy alloys. In this study, the atomistic mechanism of the evolution of SRO during and after cascade collisions was investigated in NiCoFeCrMn by multiscale modeling using molecular dynamics and lattice kinetic Monte Carlo simulations. SRO structures could be destructed by cascade collisions in short time and recovered by atomic diffusion in a much longer time. The destruction rate depends on the primary knock-on atom energies in cascade collisions and shows a universal law with respect to the number of replacement-per-atom. The vacancy diffusion simulations reveal that the SRO recovery rates of different element pairs vary significantly due to the distinct diffusion rates. Consequently, the SRO state under irradiation differs from that in thermodynamic equilibrium due to the difference of destruction and recovery rate for each element pair. The evolution of SRO is a result of the competition between the destruction and recovery mechanisms and depends heavily on the irradiation conditions.

An overview of microstructure, mechanical properties and processing of high entropy alloys and its future perspectives in aeroengine applications

Modern engineering applications continually strive to develop greater performance mechanical components with good microstructural stability, improved mechanical properties, corrosion resistance and decreased cost of repairing and maintenance. This necessitates the broad use of advanced high performance materials like high entropy alloys (HEAs). These alloys are created by combining five or more alloying elements in equal or substantial amount. About 5 to 35 at. % of the alloying element is present in HEAs. It is characterized primarily by greater entropy, slow diffusion, severe lattice distortion, and cocktail effects. Due to its advanced microstructural stability throughout a larger temperature span and for longer length of time, it demonstrates improved mechanical characteristics at ambient temperature, cryogenic temperature, and elevated temperature. The diversity of elemental contents and significantly higher mixing entropy of HEAs make them mechanically superior to classic metals and alloys. It also shows better strength to weight ratio. Hence, it qualifies as a possible structural and functional material for aeroengine applications. In this work, the studies on the HEAs are briefly reviewed. A basic explanation of the four core effects of HEAs is given. Discussion is held on microstructure and mechanical properties of HEAs. The processing routes for manufacturing of HEAs (arc melting, bridgman solidification, mechanical alloying and vapour deposition) are presented briefly. The influence of heat treatment on mechanical behavior and microstructure of HEAs is presented. The simulation approach of CALPHAD modeling for designing of HEAs is discussed briefly. The future scope for research and development of HEAs in aeroengine applications is briefed.

Microstructure and Properties of Non-Equiatomic Ni10Cr6WFe9TiAlx High-Entropy Alloys Combined with High Strength and Toughness

High-entropy alloys (HEAs) with excellent mechanical properties have broad application scope and application prospects. However, it is difficult to obtain the optimized element composition, based on the traditional equiatomic or near-equiatomic statistical analysis of the phase selection rules. The non-equiatomic HEAs have abundant constituents combination by optimizing the type and content of elements. In this study, Ni10Cr6WFe9TiAlx (x = 0, 1.0 and 1.5, at.%) HEAs were prepared by vacuum arc melting. The effect of Al content x on microstructure and mechanical properties of HEAs was systematically studied. The results show that the HEAs are composed mainly of face-centered cubic (FCC) with hexagonal Al2W phase. The increase of Al content promotes the formation of the hexagonal Al2W phase. When the Al mole content is 1.0, the Ni10Cr6WFe9TiAl HEA material has achieved superior mechanical properties. The alloy exhibited a high ultimate tensile strength of 741 MPa and a large total elongation of 46%. The improvement in the mechanical properties of the Ni10Cr6WFe9TiAl HEA is mainly attributed to the precipitation strengthening of the high-density Al2W phase. This work provides a reference for the future design of Al2W precipitation-strengthened non-equiatomic HEAs with ideal properties.

Thermal and mechanical characterization of under-2-µm-thick AlCrNbSiTi high-entropy thin film

High-entropy alloys (HEAs) exhibit extraordinary physical properties such as superior strength-to-weight ratios and enhanced corrosion and oxidation resistance, making them potentially useful in energy storage and generation industries. However, thermal and mechanical properties of HEAs with various compositions vary significantly. Furthermore, these properties have rarely been investigated simultaneously owing to material or instrumentation limitations. Herein, we synthesize an HEA (AlCrNbSiTi) coating with a thickness of less than 2 μm. We customize a frequency-domain photothermal testing system to characterize the thermal and mechanical properties of the proposed coating with high accuracy. Owing to the large mixing enthalpy of the Al-Ti, Nb-Si, and Ti-Si pairs in the coating, its hardness and elastic modulus are 15.2 and 254.7 GPa, respectively, which are higher than those of previously reported HEAs. The thermal conductivity of the AlCrNbSiTi coating is characterized to be 2.90 W·m−1·K−1, within the expected range and well explained by the free-electron consistency diversity and phonon scattering from the amorphous structure. Additionally, the coating exhibits adequate wear performance, with a wear rate of 5.4 × 10−8 mm3·N−1·m−1. This relatively low thermal conductivity, combined with extraordinary mechanical properties, makes the proposed material an excellent candidate as a protective coating material for nuclear reactor components which require high strength, irradiation resistance, and thermal protection.

Interpretable hardness prediction of high-entropy alloys through ensemble learning

With the development of artificial intelligence, machine learning has a wide range of applications in the field of materials. The sparsity of data on the mechanical properties of high-entropy alloys makes it difficult to balance between the generalizability and interpretability in data-driven predictive models of material properties. A machine learning model was established based on the HEA hardness data of the Al-Co-Cr-Cu-Fe-Ni system, and several modeling features were screened out through a three-step parallel approach. Model ensemble was performed for RandomForest, XGBoost, LightGBM and CatBoost using the stacking ensemble algorithm, and the coefficient of determination(R2) of the model reached 0.93 after a ten-fold cross-validation. The ensemble learning is stable and accurate for predicting HEA hardness value, and is experimentally verified. The model and selected features can also be applied to different HEA systems as well as low hardness CrFeNi MEA. In addition, we further explained the large prediction deviation of MEA in the high hardness region. Further, the effects of HEA composition and phase formation on the hardness of HEA were qualitatively analyzed based on interpretable tools like SHAP values as well as PDP/ICE plots, respectively. Finally, the model not only has the generalization of ensemble learning, but also has certain interpretability.

Effect of solution annealing temperature on microstructural evolution and mechanical properties of NbC-reinforced CoCrFeNi based high entropy alloys

Recent studies have shown that NbC precipitates in high-entropy alloys (HEAs) are strong obstacles for dislocation motion, which can substantially contribute to the mechanical performance over a wide temperature range. However, there are limited reports discussing the exceptional adjustability of microstructures and mechanical properties using thermomechanical processing (TMP). Therefore, the microstructures and mechanical properties of NbC-reinforced CoCrFeNi based high entropy alloys (HEAs) tuned via TMP were systematically investigated. Homogenized HEAs with ∼0.3 at% C and Nb were solution annealed at 1100, 1200 and 1300 °C, followed by cold-rolling plus annealing. The microstructures and mechanical properties of the HEAs were systematically characterized using scanning electron microscopy, transmission electron microscopy and uniaxial tensile tests. The results indicated that NbC distribution evolved from eutectic-like primary NbC clusters surrounded by disorderly distributed secondary NbC to dispersive nano-sized secondary NbC as the solution annealing temperature increased above the NbC solvus temperature. Two kinds of microstructures were correspondingly produced: 1) samples solution annealed at 1100 and 1200 °C consisted of fine grain lamellae embedded with NbC precipitates and coarse grain lamellae without precipitates, which exhibited a combination of high strength (UTS ∼740 MPa) and good ductility (elongation ∼60%); and 2) samples solution annealed at 1300 °C obtained a homogeneous fine grain microstructure, which demonstrated excellent synergy of high strength (UTS ∼760 MPa) and exceptional ductility (elongation ∼63%). The enhanced strength of samples solution annealed at 1100 and 1200 °C originated primarily from the high back stress strengthening of lamellar microstructure, while precipitation strengthening caused by dispersed nanosized NbC precipitates and grain boundary strengthening provided extra strengthening for the sample annealed at 1300 °C. This work provides a new strategy for tailoring the microstructure and mechanical properties of NbC-reinforced HEAs via adjusting the solution annealing temperature of TMP.

Stacking fault strengthening in CoCrFeMnNi high-entropy alloy

The effects of stacking faults (SFs) on the mechanical properties of high-entropy alloys (HEAs) may not be ignored because of the lower or even negative SF energy. Here, we investigated the SF strengthening of CoCrFeMnNi and clarified its potential mechanism. It showed experimentally that the CoCrFeMnNi with more SFs had higher strength and hardness than that with fewer SFs, revealing significant SF strengthening. To gain an insight into such effects, we performed simulations for the mechanical responses of CoCrFeMnNi samples with different plane defects at the atomic scale. The results showed that the strengths of the CoCrFeMnNi samples containing SFs or twin boundaries were higher than that of single crystal ones. Overall, our results suggested that the effects of SFs on the mechanical properties of FCC HEAs could not be ignored, and the atomistic insights into the strengthening mechanism of HEAs we provided would be beneficial to improve the mechanical properties of FCC HEAs.

Dynamic shock response of high-entropy alloy with elemental anomaly distribution

Elemental inhomogeneities formed during the actual preparation or heat treatment of high entropy alloys (HEAs) have a profound effect on the mechanical properties of HEAs. In this work, molecular dynamics (MD) was used to simulate the plastic response of CoCrFeNiMn HEAs with Mn element as the concentration fluctuation variable under shock loading. The results show that the unique physical shock properties, including shock wave propagation, dislocation evolution, defect collapse, and energy dissipation, are related to the anomaly arrangement of Mn elements. Compared to the uniform distribution of Mn elements, the gradient distribution of Mn leads to lattice mismatch between heterogeneous tissues, increasing the energy barriers of the dislocation mediated slip and reducing the probability of generating disordered atoms (from 27.9 to 11.5%), which results in anomalous shock strength in the Hugoniot elastic limit (HEL) (from 69.1 Gpa to about 72.9 Gpa). As the main product of shock energy conversion, energy dissipation is subjected to the complex interaction between the shock compression and the reflection unloading, and the gradient distribution of Mn element can significantly reduce the efficiency of shock energy conversion due to dislocation activity and crystal phase changes caused by lattice mismatch. Finally, when defective structures are introduced, the gradient dislocation nucleation and energy conversion fluctuations of the microstructure were observed during shock loading. This work can provide a new perspective for further understanding the influence of solid solutions with non-uniform element distribution on the plastic deformation mechanism of high entropy alloys subjected to extreme dynamic shock.

Lattice distortion in FCC HEAs and its effect on mechanical properties: Critical analysis and way forward

Lattice distortion is considered to be one of the four core effects in a multicomponent high-entropy alloy. However, their effect is least understood from experiment and theoretical standpoints. The present investigation revealed a unique way to understand this effect by combining experiments with density functional theory (DFT) calculations. A small amount of Al and Si were carefully added to the whole-solute matrix of Cantor alloys. The different-sized atomic species introduces a huge lattice distortion in the matrix, leading to a simultaneous improvement in yield strength (YS), ultimate tensile strength (UTS), and percent elongation. An extensive DFT simulation indicates that a lattice distortion is prominent in an Al-containing alloy, whereas Si does not induce a lattice distortion. However, Si leads to severe interlayer lattice distortion, caused by the displacement of Si, during twinning. This leads to the improvement of YS, UTS, and ductility. Lattice distortion and its variants play significant effects on the mechanical properties of high-entropy alloys (HEAs) in terms of local lattice distortion, providing an uneven energy landscape for the movement of line defects or interlayer distortion. The inherent nature of local lattice distortion in HEAs leads to the wavy or tortuous dislocation, unlike a straight dislocation in conventional alloys. The movement of the wavy type of dislocation through a distorted or defective lattice requires large stress, resulting in a pronounced effect on solid solution strengthening. This local lattice distortion also dictates the degree of the interlayer distance distortion in the vicinity of atoms, leading to an increase or decrease in stable stacking fault energy that decides the deformation mode via slip or twinning.

Study on the Microstructure and Mechanical Properties of Non-Equimolar NiCoFeAlTi High Entropy Alloy Doped with Trace Elements

The method of improving the microstructure and thus the properties of alloys by adjusting their composition has been widely used in the study of high entropy alloys (HEAs). However, most studies have focused on improving the properties of HEAs with face-centered cubic (FCC) or body-centered cubic (BCC) structures by adjusting the contents of elements such as Ni, Al, Ti, Cr, Mn and Mo. The doping of B, Mg and Zr also has a certain effect on the mechanical properties of HEAs. In this paper, the phase structure, microstructure, and mechanical properties of Ni45.5Co22Fe22Al5Ti5 HEA doped with B, Mg, and Zr were investigated. The results demonstrated that the three-phase structures of FCC matrix, L12 precipitate, and BCC phase were present in all the as-cast HEAs of Ni45.5Co22Fe22Al5Ti5×0.5 (X = B, Mg, and Zr). The microstructures of the as-cast alloys showed typical dendritic and inter-dendritic architecture. The maximum hardness was found in the alloy doped with B element, with a value of 433 HV. During the compressive test at room temperature, neither the Mg0.5 HEA nor the Zr0.5 HEA cracked until the load limit, but the B0.5 HEA cracked at a compressive strain of about 12%. B0.5 HEA had the highest compressive yield strength of the three alloys, followed by Zr0.5 HEA, while Mg0.5 HEA had the lowest, with values of 1030 MPa, 754 MPa, and 628 MPa, respectively. The work is expected to provide a boost for the research on the optimization of the properties of new HEAs reinforced by precipitation of L12 phase by providing a simple solution-microalloying method.

Spall response of medium-entropy alloy CrCoNi under plate impact

Dynamic mechanical properties of high-entropy alloys (HEAs) or medium-entropy alloys (MEAs) under high strain rate loading are of increasing interest for their promising applications in dynamic extreme environments. Here, spall damage of a coarse-grained CrCoNi MEA with a face-centered-cubic structure is firstly investigated by combining plate impact experiments, and molecular dynamics (MD) simulations. Yield strength, spall strength and spall-induced re-acceleration at different impact velocities are derived from free-surface velocity history measurements. In addition, soft-recovered samples after spallation are characterized with scanning electron microscopy and electron backscatter diffraction. The CrCoNi MEA exhibits the highest spall strength (around 4 GPa) among the MEAs/HEAs ever reported, with similar or better impact ductility compared to the reported MEAs/HEAs. With increasing impact velocity, the dominant damage mode of the CrCoNi MEA undergoes a gradual transition from intergranular damage, to mixed intergranular and intragranular damage, and then to intragranular damage. MD simulations reveal multiple deformation mechanisms (dislocation slip, stacking faults and twinning) for the CrCoNi MEA under impact loading, and show consistent spall damage mode transition with the experimental observations. The excellent combination of spall strength and ductility for the CrCoNi MEA is mainly attributed to the pronounced intragranular void nucleation followed by ductile void growth and coalescence, ensuring broad application prospects of the CrCoNi MEA in impact scenarios.

Influences of alloying elements on microstructure and tribological properties of a medium-weight high-entropy alloy

In this work, individual influences of Al, Fe and Ni on the microstructure, wear resistance and mechanical properties of high-entropy alloys (AlxCrFeyNizTi0.2) were studied. Microstructures and phase constituents of the alloys with different values of x, y and z were characterized via Scanning Electron Microscope, Energy Disperse Spectroscopy, and X-ray diffraction. Mechanical properties and wear resistances of the alloys were evaluated, including hardness, micro-hardness, and resistance to abrasive wear. It was shown that with increasing Al content, the FCC phase was replaced by disordered BCC (A2) and ordered BCC (B2) phases, leading to an increase in micro-hardness by ∼73% and a decrease in wear volume loss by ∼78%. Fe also increased the fraction of A2 phase, but reduced hardness by ∼41% and led to a considerable decrease in wear resistance. In contrast, Ni helped form a FCC phase, resulting in a decrease in hardness by ∼35%, and an increase in wear volume loss by ∼73%. Abrasive wear appeared to be the dominant wear mechanism, while the formed oxide scale played a role in improving the wear resistance. Efforts were made to correlate the changes in hardness and wear resistance with changes in microstructure. Among all specimens, AlCrFeNiTi0.2 and Al0.5CrNiTi0.2 reached the highest wear resistance. The results obtained help shed light on the development of advanced wear-resistant high-entropy alloys via the favorable addition of Al.