Nanocrystalline High-entropy Alloy Research Articles

Microstructural evolution of CrCuFeNiZn nanocrystalline high entropy alloy prepared by mechanical alloying

High Entropy Alloys (HEAs) have become the most researched structural materials in the scientific community during the last decade due to their attributes like excellent strength and wear resistance. In this research, CrCuFeNiZn HEA was prepared using mechanical alloying technique. Further, X-ray diffraction (XRD) and Scanning electron microscope (SEM) analysis of prepared HEA were carried out at various milling time intervals (10 min, 5 h, 10 h, 15 h, 20 h, and 25 h) to determine solid solution formation. Results manifest that CrCuFeNiZn HEA exhibited BCC + FCC (dual phase) structure after 25 h of milling. Moreover, the crystallite size as measured from the XRD analysis for the 25 h milled powder was found to be 32.8 nm and lattice strain at the end of milling is calculated to be 0.428%. SEM-EDS analysis further confirms the homogeneous distribution of the constituting elements in the as-fabricated alloy aggregate.

Crack growth and fracture mechanics of CuCrFeNiCo high-entropy alloy during tension testing

In this work, the crack growth and fracture mechanics of CuCrFeNiCo high-entropy alloy (HEA) during tension process are studied through molecular dynamics simulation method. The single-crystalline, nanocrystalline, and twinned-nanocrystalline CuCrFeNiCo HEA samples with an initial crack are prepared. The influences of boundary conditions, crack length and crystallographic orientation are considered in detail. The results indicate that the phase transition from face-centered cubic (FCC) structure into hexagonal close-packed (HCP) structure and the appearance of Shockley dislocations are the majority in all samples. The dislocations appear most densely in the twinned-nanocrstalline sample and most sparsely in the single-crystalline sample. The growth of the initial crack combined with the formation and expansion of new cracks along the grain boundaries (GBs) is the determining factor in the fracture mechanics of the CuCrFeNiCo HEA samples. The deformation capacity of the samples with free boundary conditions along the y-axis is better and the plastic deformation process is longer than the samples with periodic boundary conditions along the y-axis. The tensile strength values of the CuCrFeNiCo HEA samples change significantly in the range from 2.61 GPa to 7.75 GPa when changing the simulation conditions. The von Mises stress in the grains is markedly lower than that in the GBs.

Rapid preparation of nanocrystalline high-entropy alloy coating with extremely low dilution rate and excellent corrosion resistance via ultra-high-speed laser cladding

Ultra-high-speed laser cladding (UHSLC) not only offers an efficient solution to the limitations of conventional laser cladding but also aligns with the contemporary objectives of material conservation and efficiency enhancement in metal protective coatings. This study investigates the fabrication of CrFeCoNiMo0.2 high-entropy alloy (HEA) coatings using UHSLC and traditional laser cladding (TLC) methods. Key aspects such as macroscopic forming quality, element distribution, dilution rate, grain morphology, hardness, and electrochemical corrosion properties of both UHSLC and TLC coatings are thoroughly examined. Notably, UHSLC-produced coatings, with their significantly reduced thickness ranging from 100 to 300 μm, demonstrate an impressively low substrate dilution rate compared to the 2–3 mm thickness of TLC coatings. The rapid solidification inherent in UHSLC leads to smaller grain sizes in the coating, resulting in enhanced hardness. Moreover, the UHSLC coating exhibits superior corrosion resistance, characterized by a lower maintaining-passivity current density in 1 mol/L H2SO4 and a higher self-corrosion potential relative to TLC coatings. This work presents a novel approach, employing high-efficiency and low-cost laser-cladding technology, to minimize dilution in the matrix and improve the coating properties of HEAs.

Unraveling the Hall-Petch to inverse Hall-Petch transition in nanocrystalline high entropy alloys under shock loading

The transition from Hall-Petch (HP) to inverse Hall-Petch (IHP) behaviors associated with grain size reduction has been recognized for over two decades. However, the underlying mechanisms for such transition in high entropy alloys (HEAs) under dynamic loading, in which abundant deformation mechanisms could be activated either sequentially or simultaneously, remain unclear. Here, we investigate the HP to IHP transition in nanocrystalline CoCrFeMnNi HEAs under shock loading by examining their deformation mechanisms and flow stresses using large-scale molecular dynamics (MD) simulations. It is found that this transition is strongly dependent on the shock pressure as a result of the complex interplay among multiple competing deformation mechanisms, including the hardening mechanisms such as dislocations interactions and grain boundary (GB) blocking, as well as the softening mechanisms like phase formation, amorphization, GB thickening, and grain rotation. Moreover, there exists a critical shock pressure, which corresponds to the largest critical grain size for the HP-IHP transition. Below the critical shock pressure, the critical grain size increases with pressure due to a stronger hardening effect in grain interior (GIs), while above the critical pressure, the critical grain size first decreases and then undergoes a pressure-insensitive plateau before further decrease due to softening effects in GIs. A theoretical model that includes different deformation mechanisms is proposed for the first time to capture the shock pressure-dependent HP-IHP transition. Our work provides valuable guidance for optimizing the grain size of nanocrystalline HEAs for applications involving dynamic loadings.

Theoretical study on the influence of grain size on the strength, toughness and plastic deformation mechanism of pre-cracked polycrystalline high entropy alloys

High entropy alloys (HEAs) is a kind of structural material with excellent mechanical properties. The material exhibits excellent damage tolerance at low temperature, which is closely related to its potential crack tip plastic mechanism and overall fracture mode. In the research, the plastic deformation and fracture mechanical properties of polycrystalline CrMnFeCoNi HEAs with pre-crack under type I tensile load were investigated by molecular dynamics (MD) simulation. The effects of grain size and crack length on the strength, toughness and microstructural deformation of the alloy were analyzed. It is found that with the increase of grain size, the strength of the material increases and the toughness decreases. With the increase of crack length, the strength and toughness decrease, and the influence of grain size on the strength is weakened. The plastic mechanism of crack initiation in nanocrystalline HEAs is ductile cracking under the joint action of dislocations, amorphization and grain boundary plasticity, which provides high damage tolerance. As the grain size increases, additional cavities appear near the crack tip or at the grain boundary, resulting in brittle fracture mode along the grain boundary or at the end of the cavity, which greatly affects the plastic properties of the material. The initial crack length has a significant effect on the transformation of the fracture mode. With the increase of the initial crack size, the critical grain size of the brittle fracture mode increases, and its proportion in the whole fracture mode decreases. This study provides a theoretical reference for the design and application of nanocrystalline HEAs.

Thermal stability of electrodeposited nanostructured high-entropy alloys

Over the past decade, the study of nanostructured high-entropy alloys (HEAs) has attracted great attention due to their high strength (grain boundary hardening) and improved thermal stability over conventional nanostructured metals and alloys. However, a lingering issue yet to be resolved by these studies is that of commercial viability; the synthesis processes used to date are costly, not easily scalable, and energy intensive. This work examines the thermal stability and mechanical properties of electrodeposited nanostructured HEAs, towards establishing a cost-effective and versatile synthesis route to commercialize this underutilized material class. Nanocrystalline, amorphous, and nanoglass alloys of NiFeCoW, NiFeCoMo, and NiFeCoMoW were systematically characterized to assess the stability of their structure and mechanical properties, compared against conventional nanocrystalline materials and HEA counterparts. The structural stability of these electrodeposited HEAs was shown to improve with increasing number of elements, from a peak temperature for grain growth of ∼270 °C in pure nanocrystalline Ni, to ∼500 °C in electrodeposited HEAs, corresponding to a near doubling of the activation energy for grain growth from 1.3 eV (Ni) to a maximum of 3.1 eV (NiFeCoW). This improved stability was matched with a 60 % increase in hardness after annealing to the point of nanograin nucleation (∼500 °C), to a maximum hardness of ∼8.4 GPa. Such hardening trends followed an extrinsic inverse-to-regular Hall-Petch relationship, where the inverse regime was largely dominated by grain boundary relaxation and nanoglass structural breakdown. With these results, we have established electrodeposition as a promising candidate route for fabricating nanostructured HEAs for applications at elevated temperatures.

Nanocrystalline High Entropy Alloys with Ultrafast Kinetics and High Storage Capacity for Large-Scale Room-Temperature-Applicable Hydrogen Storage

Nanocrystalline High Entropy Alloys with Ultrafast Kinetics and High Storage Capacity for Large-Scale Room-Temperature-Applicable Hydrogen Storage

Design and Development of Stable Nanocrystalline High-Entropy Alloy: Coupling Self-Stabilization and Solute Grain Boundary Segregation Effects.

Grain growth is prevalent in nanocrystalline (NC) materials at low homologous temperatures. Solute element addition is used to offset excess energy that drives coarsening at grain boundaries (GBs), albeit mostly for simple binary alloys. This thermodynamic approach is considered complicated in multi-component alloy systems due to complex pairwise interactions among alloying elements. Guided by empirical and GB-segregation enthalpy considerations for binary-alloy systems, a novel alloy design strategy, the "pseudo-binary thermodynamic" approach, for stabilizing NC-high entropy alloys (HEAs) and other multi-component-alloy variants is proposed. Using Al25Co25Cr25Fe25 as a model-HEA to validate this approach, Zr, Sc, and Hf, are identified as the preferred solutes that would segregate to HEA-GBs to stabilize it against growth. Using Zr, NC-Al25Co25Cr25Fe25 HEAs with minor additions of Zr are synthesized, followed by annealing up to 1123K. Using advanced characterization techniques- in situ X-ray diffraction (XRD), scanning/transmission electron microscopy (S/TEM), and atom probe tomography, nanograin stability due to coupling self-stabilization and solute-GB segregation effects is reported in HEAs up to substantially high temperatures. The self-stabilization effect originates from the preferential GB-segregation of constituent HEA-elements that stabilizes NC-Al25Co25Cr25Fe25 up to 0.5Tm (Tm-melting temperature). Meanwhile, solute-GB segregation originates from Zr segregation to NC-Al25Co25Cr25Fe25 GBs; this results in further stabilization of the phase and grain-size (≈14nm) up to ≈0.58 and ≈0.64Tm, respectively.

High-entropy alloy reinforcement in aerospace-grade aluminum 7075: A study of yield strength and elastic modulus

This research aims to identify an appropriate theoretical model for assessing the yield strength and elastic modulus of a composite material consisting of aerospace-grade aluminum 7075 base matrix reinforced with nanocrystalline high-entropy alloy particle (HEAp) composed of CrCuFeMnNi. To fabricate the aluminum composite with a high-entropy alloy base, an innovative casting method was utilized. This method involved the use of a modified bottom-pouring stir casting furnace, integrated with a mechanical supersonic vibrator and a squeeze infiltration setup. The measured theoretical density was higher than the actual density, while the cast specimen exhibited porosity below 6.6%. Scanning electron microscopy (SEM) images revealed the presence of strengthening precipitates and a uniform dispersion of HEAp in the composite. Comparing the developed HEAp composites (CH2, CH3, and CH4) with the base AA7075 cast CH1 (AA7075 + 0 wt.% CrCuFeMnNi-HEAp), the ultimate tensile strength increased by 4% for CH2 (5 wt.% CrCuFeMnNi-HEAp), 21% for CH3 (10 wt.% CrCuFeMnNi-HEAp), and decreased by 9% for CH4 (15 wt.% CrCuFeMnNi-HEAp). Similarly, the yield strength increased by 10% for CH2, 24% for CH3, and decreased by 5% for CH4. The elongation showed an increasing trend of 5% for CH2, 12% for CH3, and 2% for CH4. The flexural strength of the HEAp composites (CH2, CH3, and CH4) increased by 2%, 5%, and 10%, respectively. The proposed model yielded similar yield strength values that closely aligned and were consistent with the experimental value within a deviation of 3.32%. The modified Halpin-Tsai models agreed with experimental value up to a reinforcement volume fraction of 4.89% when calculating the elastic modulus. The proposed model and the Hashin-Shtrikman upper bound also agreed on values up to a 3.32% reinforcement volume fraction.

Design of self-stable nanocrystalline high-entropy alloy

Nanocrystalline (NC) materials are prone to grain-coarsening at low temperatures—requiring extra solute-element addition for stability. While this approach is established mainly in simple-binary-alloys, it is adjudged “complex” for multicomponent-alloys due to complex-interactions among constituent-elements. We report for the first time that nanograins in multicomponent-high entropy alloy (HEA) stabilize themselves without requiring additional solute if constituent-HEA-elements with highest mixing enthalpy and melting point preferentially segregate to grain boundaries (GBs); a process we term self-stabilization effect in HEAs. Using in-situ X-ray diffraction, scanning/transmission electron microscopy, and atom-probe-tomography (APT), we show that Cr and Fe in NC-AlCoCrFe-HEA (9 nm grain-size) segregate at GBs by site-competition to stabilize it at 0.5Tm (Tm–melting temperature). At 0.6Tm, GB-desegregation is established to be precursor to phase decomposition, and it competes with nanograin stability; this culminates in the onset of grain coarsening at this temperature. Compared with the literature (e.g., NC-AlCoCrFeNi), NC-AlCoCrFe HEA shows exceptional nanograin-stability at high homologous-temperatures; this suggests possible breakdown of the cocktail and sluggish-effects in HEAs, since more elements do not necessarily improve nanograin stability. To the authors’ knowledge, this is the finest stable-NC-HEA produced—it paves a new way of engineering NC-HEAs without coarsening in scalable solid-state processes that require substantially-high temperatures.

Phase decomposition of AlCrFeCoNiCu0.5 HEA thin films by vacuum annealing

The most attractive advantages of high entropy alloy (HEA) thin films are their excellent properties, such as high strength and high corrosion and oxidation resistance, with a lower material cost. However, the thermal stability of the HEA thin films is always controversial. This is critical since the thermal stability of metals and alloys is closely related to the material's performance at high temperatures and also determines the application field of the material. The question of how will the nanocrystalline high entropy alloy thin film perform in terms of thermal stability needs to be clarified urgently. In this research, the thermal stability of AlCrFeCoNiCu0.5 HEA thin film fabricated by cathodic arc deposition was investigated at 500 °C as a function of time during vacuum annealing. X-Ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HRTEM) and Transmission Kikuchi Diffraction (TKD) were applied to analyze the chemistry and microstructure of HEA thin films in detail. The results showed that there was no obvious change in the elemental compositions of the films after annealing, but the elemental distribution of the annealed films was different from the as-deposited film, especially at the film surface and film-substrate interface for the 24 h-annealed sample. The existence of the FeCo phase with B2 structure was a significant sign to demonstrate that the spinodal decomposition occurred in the single face-centered-cubic (FCC) phase matrix of the film during annealing. Simultaneously, the segregation of Cu and Cr near the film-substrate interface and the penetration of Ni and Cu towards the Si substrate were clearly observed. These unusual phenomena indicated that the thermal stability of HEA was not always excellent. Also, it was noticed that a few Kirkendall voids forming at the film-substrate interface after 24 h of annealing would affect the adhesion of the film. Additionally, heat recovery and recrystallization were achieved by 500 °C annealing, which were confirmed by the decrease of lattice parameter and the increase of grain size in the 24 h-annealed thin film. Furthermore, the hardness and elastic modulus of the thin films before and after annealing were measured by nanoindentation. The results showed all the thin films exhibited excellent mechanical properties, and the optimal hardness and elastic modulus of 7.7 ± 0.3 GPa and 183.9 ± 4.4 GPa were obtained after 3 h-annealing.

Extreme thermal stability in crystal-amorphous nanocrystalline CoCrFeMnNi high entropy alloy

This work investigates the crystal-amorphous CoCrFeMnNi dual-phase nanocrystalline high entropy alloy (HEA), focusing on how the amorphous layer affects the abnormal crystal growth. The result showed that the wide amorphous layer and high entropy effects in grain boundary, which provide lower driving force for grain growth, the average grain size of the crystal-amorphous HEA only increases to 8 nm after annealing at 500 ℃ for 1 h, indicating the exceptional thermal stability. Therefore, the crystal-amorphous dual-phase HEA offers a new model for developing advanced nanocrystalline materials with outstanding thermal stability.

Utilization of structural high entropy alloy for CO oxidation to CO2

The nanoengineered high entropy alloy (HEAs) catalysts have attracted the attention of the scientific community due to their exceptional characteristics, wide range of compositional tunability and the utilization of low-cost transition metals. During various electrochemical reactions, the oxidation of carbon-mono-oxide (CO) is an intermediate and it acts as a poison to reduce the efficiency of the reactions. Also, CO is a poisonous gas which is generated during gasoline and diesel combustion in the automobiles. A nanocrystalline HEA catalyst (CoFeNiGaZn) is prepared by easily scalable casting-cum-comminution method, providing pristine catalyst surfaces. It is capable of catalyzing the CO-oxidation to CO2 with high conversion efficiency (∼98%). Density functional theory calculations show that the high activity of the HEA can be attributed to the presence of a considerable amount of filled states of dxz and dyz orbital near the Fermi level for Ni atoms over the surface. Due to the favorable transfer of electrons from this orbitals to the lowest unoccupied molecular orbital of O2 and adsorption of gaseous CO directly on O∗ to form CO2, the endothermicity of the rate-determining step is 0.43 eV in the Eley-Rideal pathway.

Compositional modulation through galvanic displacement in electrochemically deposited FeCoNiCuZn high entropy alloy thin films

After completing the aqueous electrodeposition of FeCoNiCuZn nanocrystalline high entropy (HEA) alloy system consisting of elements with significant differences in melting points and reduction potentials, unwanted galvanic displacement reactions (GDR) were observed on the alloy system when the film was held back in the electrolyte. This phenomenon, observed in alloys electroplated from aqueous baths containing a noble metal (currently Cu), leads to preferential deposition of the noble elements and changes the composition progressively at open circuit potential. A change in composition occurs in the electrodeposited alloy where Cu (from ∼13 to 58 at. %) was deposited on the alloy due to its nobler reduction potential causing displacement of Zn > Fe ∼ Co > Ni (order of displacement) from the deposited alloy with the Cu2+ ions present in the electrolyte. This work is the first observational report of galvanic displacement in HEAs composed of five elements. We categorically state that this GDR is crucial when designing/synthesizing HEAs alloys through electrodeposition.

Shock-induced deformation and spallation in CoCrFeMnNi high-entropy alloys at high strain-rates

Shock-induced deformation and fracture in single-crystalline and nanocrystalline CoCrFeMnNi high-entropy alloys (HEAs) under different shock intensities are investigated systematically using large-scale molecular dynamics simulations. The strong anisotropy in the single-crystalline HEA and how the reversibility of HCP-structured atoms influences the void nucleation and growth are revealed. Specifically, shock-induced FCC to HCP structural transformations are largely reversible in the case of the [001] loading direction, leading to homogeneous void nucleation; while they are largely irreversible in the cases of the [110] and [111] loading directions, resulting in void nucleation in the intersection of HCP layers. In nanocrystalline HEA, the deformations in the grain interior are similar to those in the single crystals, while the grain boundaries serve as void nucleation sites and thus weaken the spall strength. The SRO effects on the shock response in single-crystalline and nanocrystalline CoCrFeMnNi HEA are revealed, and the chemical heterogeneity is analyzed. Our results show that SRO can only slightly increase the shear stress and the spall strength, but it can cause a reduction of ductility during the spallation. The differences in SRO effects between single-crystalline and nanocrystalline models are attributed to the grain boundaries, where Cr and Mn are favorable, while Ni is favored inside the grains in the SRO model. Moreover, the comparison between CoCrFeMnNi HEA and its subsystem as well as single-element metal indicates Mn seems to be the most influential elements in CoCrFeMnNi HEA, resulting in significant changes in strength. A relation between the spall strength and strain rate is proposed to describe the simulation results and reported experimental data. Our comprehensive and systematical studies provide deep insights into the deformation and fracture in CoCrFeMnNi HEA subjected to shock loading, which deepens the understanding on the dynamic deformations of CoCrFeMnNi HEA and benefits the rational design of new HEAs/MEAs with enhanced performance.

Comparison of Dislocation Loop Formation Resistance in Nanocrystalline and Coarse-Grained Refractory High Entropy Alloys

In nanocrystalline high entropy alloys (HEAs), the difference in chemistry and the density of interfaces play a role in the microstructure evolution, although their relative importance is not well understood. Here, we compare data published in the literature regarding dislocation loop damage in pure W under ion irradiation to new results on nanocrystalline and coarse grained HEAs irradiated under single and dual ion beams. Dislocation loop damage evolution has been studied. The results suggest that both grain boundaries and grain matrices play important roles in the microstructure evolution of HEAs with grain matrices playing the dominant role. Irradiation response and dislocation loop damage in coarse-grained (a) and nanocrystallinegrained (b) W-based HEAs as compared to coarse-grained (c) and nanocrystalline-grained (d) pure W material.

Real-time atomic deformation behavior of nano CoCrCuFeNi high-entropy alloy

Tensile tests were performed by molecular dynamics at various strain rates ranging from 2×108 to 1×1010 s−1 and various temperatures ranging from 10 to 1200 K, and the real-time deformation behavior of a nanocrystalline CoCrCuFeNi high-entropy alloy was investigated. The results indicate that the main deformation mechanism is grain boundary slip at high temperatures and low strain rates. Dislocation slip replaces grain boundary slip to control plastic deformation with decreasing temperature and increasing strain rate, correspondingly enhancing the strength of the alloy. Furthermore, the alloys with different grain sizes were simulated to evaluate the effects of grain boundaries on the mechanical behavior. It is found that the strength of the nanocrystalline high-entropy alloys increases with increasing grain sizes when the grain size is too small, exhibiting an inverse Hall—Petch relationship.

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.

Phase engineering in nanocrystalline high-entropy alloy composites to achieve strength-plasticity synergy

In this work, the strength-plasticity synergy of Al/CoCrFeNi nanocrystalline high-entropy alloy composite (nc-HEAC) prepared by laser-inert gas condensation is achieved via nanoscale diffusion induced phase transition (DIPT) process. The yield strength and elongation of the 16%-Al/CoCrFeNi nc-HEAC annealed at 900 °C were 1338 MPa and 12.1%, showing the combination of outstanding strength and considerable ductility. The synergy of multiple strengthening mechanisms were revealed. Driving the phase transition of nanocrystalline dual-phase composites by thermal diffusion establishes a novel phase engineering design strategy for high-performance alloys.

The impact of surface scratches on the corrosion behavior of nanocrystalline high entropy alloy coatings: Electrochemical experiments and first-principles study

To prolong the service life of engineering components used in aggressive environments, a TiZrHfMoW refractory high entropy alloy (RHEA) coating was prepared onto a titanium alloy substrate. Various electrochemical analytical techniques were used to evaluate the corrosion resistance of the scratched RHEA coating in a 3.5 wt.% NaCl solution. The electrochemical corrosion tests indicate that the scratched RHEA coating exhibits a higher electrochemical stability and a lower corrosion rate than the bare titanium alloy. The effects of the localized plastic deformation on the corrosion behavior for the RHEA coating was investigated. A slab model for the RHEA surface was proposed for the first-principles calculation to explored the change of the electron work function (EWF) as a function of external stress. The influence of the constituent elements in the RHEA on the mechanical properties and the electron work function was study to uncover the mechanism underlying the high scratch corrosion resistance of the RHEA coating and provide guidance for the composition design of refractory high entropy alloy.