High-entropy Alloys Research Articles

Insight into annealing-induced hardening and softening behaviors in a laser powder-bed fusion printed in-situ composite eutectic high-entropy alloy

Laser powder-bed fusion (LPBF) printed AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) typically exhibits an in-situ composite structure with an out-of-equilibrium character. Its distinctive hierarchical microstructure implies the potential challenges and opportunities in engineering advanced materials with high performances. Here, an informative overview of the microstructure evolution across various scales and its effect on tensile properties in LPBFed AlCoCrFeNi2.1 subjected to annealing treatments over a wide temperature range is discussed. Microstructural observations indicate that spinodal decomposition within the B2 lamellae becomes evident after annealing at 500 °C, marked by a pronounced compositional fluctuation of Cr. Annealing the sample at 600 °C leads to the precipitation of BCC nanoparticles through structural disordering of Cr-rich regions within the modulated nanostructure. Tensile tests and strengthening models reveal that the annealing-induced hardening responses are largely associated with precipitation strengthening resulting from the ordering strengthening of L12 nanoprecipitates and the coherency and modulus strengthening of BCC nanoprecipitates, as well as the spinodal hardening from modulated nanostructure. Notably, BCC precipitation strengthening plays a crucial role in enhancing the strength of the annealed alloys. A strong precipitation strengthening effect is realized due to the coarsening of the BCC and L12 nanoprecipitates at 600 °C, and the strengthening effect from BCC precipitates gradually weakens as their dissolution at 650 °C, resulting in abnormal hardening behavior. Upon annealing at higher temperatures (700–1000 °C), phase decomposition, precipitation dissolution, even recrystallization occur sequentially, resulting in varying degrees of softening behaviors.

Additively manufactured low-gradient interfacial heterostructured medium-entropy alloy multilayers with superior strength and ductility synergy

Metallic additive manufacturing (AM) offers near net-shape fabrication but often results in insufficient strength and/or ductility due to voids or cracks, coarse initial crystalline microstructure, insufficient volume fraction of precipitates and yet post-AM strengthening/toughening methods non-destructive to shape are generally lacking. Here, we report a laser-directed energy deposited (L-DED) medium entropy alloy (MEA) with a heterostructure (HS) comprising alternate layers of solid-solution and intermetallic-dispersed MEA that possess ultimate tensile strength of 1132.8 MPa and elongation of 50.6 %, corresponding to strength-ductility synergy higher than other medium- or high-entropy alloys reported. The multilayers were produced from a dual source of CoCrNi MEA powder and a powder mixture of the same MEA, Al and Ti with stoichiometry (CoCrNi)86Al7Ti7, and the remarkable strength-ductility synergy is achieved only after post-L-DED heat treatment, which causes Al and Ti to diffuse across the layers to form a low-gradient HS. The significant back stress due to the HS contributes to the high strength, while the high ductility results from a high strain-hardening rate suppressing necking. This study demonstrates the concept of using AM not just as a near net-shape fabrication method but also a unique tool to produce low-gradient HSs with superb mechanical properties, via the use of multiple powder sources combined with suitable shape-preserving post-fabrication heat treatment.

The Prognosis of Simple Solid Solution for Lightweight High-Entropy Alloys Based on the Al–Mg–Li System with Additions

The Prognosis of Simple Solid Solution for Lightweight High-Entropy Alloys Based on the Al–Mg–Li System with Additions

Microstructural and Mechanical Investigations in the Solution Annealing Heat Treatment of AlMo0.5NbTa0.5TiZr Refractory High-Entropy Alloy

Microstructural and Mechanical Investigations in the Solution Annealing Heat Treatment of AlMo0.5NbTa0.5TiZr Refractory High-Entropy Alloy

Investigation of microfriction properties of graphene/AlCoCrFeNi high entropy alloy

Applying graphene (Gr) coatings to high-entropy alloys (HEA) is anticipated to enhance their tribological characteristics. The current understanding of the mechanism by which the Gr/HEA is enhanced at the atomic level is still limited. Molecular dynamics simulations revealed the mechanical behavior and strengthening mechanism of the Gr/AlCoCrFeNi HEA during nanoindentation and nanoscratch. The results demonstrate a substantial increase in the indentation hardness of the Gr/AlCoCrFeNi HEA by about 2.4 times. When Gr changed from a single layer to three layers, it further improved (3.2 times for a double layer and 3.9 times for three layers). At the same time, the friction coefficient is effectively reduced. Furthermore, the elevated in-plane stiffness of the Gr coating leads to an expansion of the effective loading area, resulting in increased Shockley dislocation and stair-rod dislocation density within the Gr/AlCoCrFeNi HEA, thereby amplifying the strain hardening effect and reducing subsurface damage. Qualitative experiments confirmed the excellent wear resistance of the Gr/HEA, and coating Gr increased the width of scratches, effectively confirming our simulation results. These findings provide valuable insights for the development and design of Gr/HEA composite coatings with enhanced mechanical properties.

Clarifying the four core effects of high-entropy materials.

High-entropy materials emerged as a field of research in 2004, when the first research on high-entropy alloys was published. The scope was soon expanded from high-entropy alloys to medium-entropy alloys, as well as to ceramics, polymers and composite materials. A fundamental understanding on high-entropy materials was proposed in 2006 by the 'four core effects' - high-entropy, severe-lattice-distortion, sluggish-diffusion and cocktail effects - which are often used to describe and explain the mechanisms of various peculiar phenomena associated with high-entropy materials. Throughout the years, the effects have been examined rigorously, and their validity has been affirmed. This Perspective discusses the fundamental understanding of the four core effects in high-entropy materials and gives further insights to strengthen the understanding for these effects. All these clarifications are believed to be helpful in understanding low-to-high-entropy materials as well as to aid the design of materials when studying new compositions or pursuing their use in applications.

Direct Mechanocatalysis with Pd Alloys

In this study, we assess the suitability of Pd alloys in direct mechanocatalytic coupling reactions. Copper (Cu) and nickel (Ni) were alloyed with palladium (Pd), alongside testing a high entropy alloy and palladium gold alloy. Compared to pure Pd balls the alloys are promising in terms of mechanical properties and costs. Combining Pd with Cu and Ni improved mechanical resilience by up to 330% while retaining catalytic activity, for the tested Suzuki‐Miyaura and Sonogashira‐Hagihara couplings. Both of which led to good to excellent yields within one hour. Notably, alloys containing 20% Pd showed increased yields of over 80% in the Sonogashira coupling, comparable to pure Pd. XPS and in‐situ PXRD analysis showcased the influence of alloyed metals on catalytic processes, emphasizing the significance of the surface composition of milling balls.

A strong and ductile biocompatible Ti40Zr25Nb25Ta5Mo5 high entropy alloy

A non-equiatomic high entropy alloy (HEA) Ti40Nb25Zr25Ta5Mo5 (at.%) was designed by replacing 5% Ta with 5% Mo in a precursor Ti40Nb25Zr25Ta10 HEA for improved mechanical properties and reduced density. The Ti40Nb25Zr25Ta5Mo5 HEA from copper mould casting exhibited a nearly equiaxed grain structure (grain size: 96 ± 16 μm), but within each equiaxed grain, dendritic structures are discernible in the backscattered electron imaging mode. These dendrites are enriched in Ta, Nb and Mo, while the inter-dendritic regions contain higher Zr and Ti. The as-cast HEA achieved tensile yield strength (σ0.2) of 976 ± 8 MPa and strain to fracture (εf) of 16.80 ± 1.86%, well above the minimum requirements of σ0.2 and εf for medical-grade titanium alloy Ti-6Al-4V (759 MPa and 10%). The formation of microscale and nanoscale shear bands and the presence of significant dislocations in the shear band boundary region played a key role in allowing this as-cast new HEA to achieve the excellent combination of tensile strength and ductility. Furthermore, it exhibited a lower wear rate, lower coefficient of friction and much higher corrosion resistance in Hank’s solution (electrochemical assessments) than Ti-6Al-4V, making it a promising alternative biomedical alloy.

Optimized deep learning regression assisted wear rate analysis of Cu–AlCoCrCuFe HEA composite

At present, composites have brought materials to a new era with superior characteristics. The proper selection of materials with optimal processing conditions is one of the prime factors in determining the efficiency of composites. This article introduces a new composite by fabricating a copper matrix with reinforced AlCoCrCuFe High Entropy Alloy (HEA). The ultimate theme of this research is to forecast the wear rate of the Cu–AlCoCrCuFe HEA composite. Using the pin on the disk, the wear test is carried out on varying combinations of parameters. Then Taguchi analysis is carried out to get the precise fit parameters. The wear rate mainly depends on the percentage of HEA, sliding distance, sliding velocity, and the load applied. Correlation analysis is performed to determine the exact parameter determining the optimal wear rate and the coefficient of friction. After feature extraction, the parameters are optimized, and the neural network regression is given the optimized parameters. The network has been trained, and predictions are made using it. The model successfully predicts the wear rate, as evidenced by the declining RMSE of 0.28 and rising R2 values up to 92%. The analysis shows a significant decline in the wear rate with HEA additions. In addition, the wear rate increases with a rise in load, sliding velocity, and sliding distance.

An Innovative High-Entropy Alloy Solder for High-reliability Low-Temperature Bonding in 3D Electronic Packaging——Based on nano InSnBiZnAg particles

To address the demand for multilevel package, a low-melting-point high entropy alloy (HEA) solder was developed. It consists of 43In28Sn14Bi9Zn6Ag micro- and nanoparticles with a melting point of about 62.80 °C. We obtained 43In28Sn14Bi9Zn6Ag nanoparticles by combining ultrasonic heating interaction with a "hot shock" process. Upon reflowing at 115 °C for 5 min, a Cu/Zn/Ag ternary intermetallic compound (IMC) formed at the interface between the solder and the copper substrate, leading to a shear strength of 40 MPa. The microstructure of this joint exhibits a complex composition with at least three phases, including Bi3In5 phase, Ag5Zn8 phase, and In0.2Sn0.8 solid solution. The stable Ag5Zn8 phase not only enhances the overall strength of the alloy matrix but also impedes the diffusion of In and Bi atoms, thereby preventing the segregation of the Bi3In5 phase. The high mixing entropy and pronounced lattice distortion in this high entropy alloy hinder the diffusion of elements within the solder matrix, mitigating the issue of excessive interfacial IMC growth. Consequently, the solder joints exhibit little degradation even after a high-temperature aging test. This high-entropy alloy solder has promising potential for multi-level chip stacking in 3D packaging.

Grain growth kinetics, Phase evolution during annealing and strain rate sensitivity of TRIP-assisted, dual-phase, non-equiatomic high entropy alloy

Transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) belong to a recently developed class of HEAs. TRIP-DP-HEAs have been reported to exhibit superior strength-ductility combination at room temperature as well as at high temperatures. Small amounts of carbon additions to TRIP-DP-HEAs are known to further improve the strength as a result of interstitial solid solution and nanoprecipitate strengthening effects. Current investigation compares the grain growth kinetics in a metastable, dual-phase (FCC and HCP), Fe49.5Mn30Co10Cr10C0.5, carbon-containing high entropy alloy (C-DP-HEA) with that in the corresponding carbon-free variant, Fe50Mn30Co10Cr10 HEA (DP-HEA). The alloys after being subjected to cold rolling, were annealed at 900 °C, 1000 °C and 1100 °C for 3, 30, 60 and 120 minutes, followed by water quenching. Slower grain growth kinetics was observed in C-DP-HEA due to Zener pinning by carbide particles. The activation energy for grain growth in C-DP-HEA was determined to be 339.71 kJ/mole, which is larger than that of DP-HEA (282.68 kJ/mole). The microstructures of the C-DP-HEA samples, after annealing at 900 °C for different durations of times and quenching, were investigated. The HCP phase fraction in the microstructure initially decreased with increase in annealing time/grain size and later increased with further increase in annealing time/grain size. This is explained based on the counteracting effect of grain boundary density and back stresses acting in samples having increased grain size. Strain rate sensitivity (SRS) parameter, ‘m’, was evaluated for both C-DP-HEA and DP-HEA. The strain rate jump tests were conducted at two quasi-static strain rates, 1x10-3 s-1 and 5x10-3 s-1, and at room temperature as well as at higher temperatures, 900 °C, 1000 °C and 1100 °C. The ‘m’ value was found to depend on the grain size, test temperature, and the presence of carbon in the alloy.

Research on the influence of ultra-low temperature extreme environments on the surface quality and fatigue life of FeCoCrNiAl0.6 high entropy alloy

This study examines the impact of low-temperature cooling on the surface integrity and fatigue properties of the FeCoCrNiAl0.6 high entropy alloy during machining. Utilizing Abaqus2023/Fe-safe simulations, liquid nitrogen cooling, and fatigue tensile tests, it was observed that low-temperature machining improves surface quality and increases tensile strength and fatigue life compared to room temperature machining. At a cutting speed of 2800 mm/min while the temperature drops to −120 °C, the surface roughness can be minimized to the maximum extent possible. Yield strength and tensile strength increased by 7.6 % and 12.5 %, respectively, at this condition, compared to room temperature. Moreover, material strength improves significantly under low-temperature conditions, with yield and tensile strengths at −120℃ being 1.16 and 1.4 times higher than at room temperature. The fatigue life of specimens also improved under low-temperature conditions, with lives reaching 2.82 × 106 cycles at −120℃, an 86.9 % increase over room temperature. The study concludes that optimal machining performance is achieved at higher cutting speeds and shallower cutting depths under low-temperature conditions.

High entropy alloys amenable for laser powder bed fusion: a thermodynamics guided machine learning search

Although there is considerable research interest in high-entropy alloys (HEAs), only a small fraction of the potential compositional space was explored hitherto. While additive manufacturing techniques such as the laser powder bed fusion (LPBF) can be gainfully employed to accelerate discovery of new HEAs with promising properties, the printability of such alloys could restrict such an exploration. Keeping this in view, a machine learning (ML) based alloy design strategy, which takes thermodynamic parameters such as the liquid temperature range (LR), liquid viscosity (LV), average expansion (AE) during the solidification of the alloy, and shear/bulk moduli ratio (G/B) of solidified alloy into account, is developed for assessing the printability of HEAs using LPBF. First, a printability evaluation chart that has six different printing capacity regions is obtained by considering LR, LV, AE, G, and B. Then, ML is utilized to predict the candidate chemical compositions within them that are amenable to LPBF. On this basis, an alloy with the composition Co7.5Cr7.5Nb7.5Ni10.0Ti67.5 (at.%) was identified, and its process window range and printability were validated through experiments. The range of thermodynamic parameters suitable for the manufacturability was determined as LV >5.5 mPa•s, LR >1600 ℃, AE <25%, and G/B <0.5. The effects of thermodynamic parameters on the printing process window was systematically investigated. Furthermore, Shapley Additive exPlanations (SHAP) algorithm was utilized to evaluate the influence of various elements on the thermodynamic parameters, which could, in turn, guide the chemical composition design so as to achieve HEAs with good printability. This work provides the thermodynamic parameter index for LPBF and offers opportunities to use ML for accelerating the design of HEAs that are amenable to LPBF.

Tensile and impact behavior of 3D-printed (FeCoNi)86Al7Ti7 high entropy alloy at ambient and cryogenic temperatures

High-entropy alloys (HEAs) demonstrate superior mechanical properties at room temperature, highlighting potential for engineering applications. With the increasing demand for advanced materials, especially in modern applications facing rigorous conditions, it becomes imperative to gain profound insights into the dynamic behavior of HEAs. In this study, the (FeCoNi)86Al7Ti7 HEA, renowned for exceptional printability and room-temperature mechanical properties, has been fabricated by selective laser melting (SLM), and its tensile and impact behaviors at ambient and cryogenic temperatures are systematically investigated. Experimental results reveal enhanced tensile properties as temperature decreases, achieving a high ultimate strength of 1313.6 MPa and impressive ductility of 24.8 % at 77 K. This improvement is attributed to the synergistic deformation mechanisms, including dislocation slip, stacking faults, Lomer and Lomer-Cottrell locks, which maintain a high strain-hardening rate and thus suppress strain localization. In contrast, impact properties deteriorate due to constrained dislocation mobility at high strain rates and low temperatures, with Charpy impact energy decreasing from 8.1 J at 293 K to 5.1 J at 77 K. Consequently, cracks readily initiate at cellular boundaries or defects, propagating in an intergranular manner and inducing localized deformation. This study provides valuable insights for the future applications of 3D-printed HEAs in extreme operational environments.

Structural-regulation of Laves phase high-entropy alloys to catalytically enhance hydrogen desorption from MgH2

Magnesium hydride (MgH2) is a promising hydrogen storage material, but slow hydrogen desorption kinetics and poor cycling stability still limit its development. Catalytic doping was demonstrated to provide effective solution for it. In this work, we develop novel Zr-based high-entropy alloys (HEAs) catalysts with different Laves phase structures to improve the hydrogen storage in MgH2. It was demonstrated that the catalyst grain size, defect density and lattice distortion have a great influence on the performance of HEAs catalysts. Specially, the C15 type Laves phase HEA exhibits a better catalytic effect than that of C14 type. The introduction of HEAs with "hydrogen pump" effect would shift the rate-limiting step of MgH2 hydrogen desorption from two-dimensional nucleation growth to two-dimension phase boundary migration, which makes it exhibit both great better hydrogen desorption kinetics enhanced by nine times and cycling stability with 92% capacity retention after 30 cycles. This work not only highlights the impact of Laves phase crystal structure regulation on HEAs catalysts, but also provides insights into the design and development of HEAs catalysts to improve the hydrogen storage performance of MgH2.

Effect of H+ pre-irradiation on irradiation hardening and microstructure evolution of FeCoCrNiAl0.3 alloy with He2+ irradiation

FeCoCrNiAlx-based high entropy alloys (HEAs) exhibit excellent high-temperature strength, good radiation tolerance and corrosion resistance. However, studies on light-ion irradiation damage are relatively lacking. The influence of hydrogen and helium effect on HEAs deserves further study, given their complex interaction during different sequence irradiation at different irradiation conditions. In the present study, FeCoCrNiAl0.3 HEA was irradiated under two different modes at 723 K, one is single He2+ and the other is pre-iradiation of H+ followed by He2+. Microstructure evolution and irradiation hardening were studied by transmission electron microscopy and nanoindentation technique. There is no distinct difference of the irradiation hardening rate between sequential H+-He2+ and single He2+ irradiated samples. A large number of irradiation-induced nano-defects were observed after pre-irradiation of H+, which act as sinks to trap the subsequent helium atoms in the H+-He2+ irradiated sample. Fine helium bubbles and dislocation loops with high density dispersed in both irradiated samples. Compared with single He2+ irradiation, pre-irradiation of H+ resulted in a decrease in bubble density and an increase in loop density in H+-He2+ irradiated sample, but had little effect on their size. It is speculated that the critical concentration of bubble formation originating from H-He-V clusters is supposed to be higher than that of HeV clusters at elevated temperatures in FeCoCrNiAl0.3 alloy, leading to the aggregation and redistribution of He atoms at the H-He-V clusters rather than the formation of stable bubble nucleus after succeeding He2+ irradiation. The influence of pre-irradiation of H+ on FeCoCrNiAl0.3 alloy on the formation of helium bubbles and the relationship between bubble, loop and irradiation hardening are discussed.

Chemical dealloying derived nanoporous FeCoNiCuTi high-entropy bifunctional electrocatalysts for highly efficient overall water splitting under alkaline conditions

In response to society’s urgent drive to significantly decrease its reliance on fossil fuels in the future, there has been continuing efforts to utilize hydrogen (H2) generated by water electrolysis as clean and sustainable substitute for the traditional fossil fuels. A significant challenge in water electrolysis revolves around creating affordable and efficient electrocatalysts to speed up the chemical reactions involved. Our new study introduces a bifunctional catalyst made from a high-entropy alloy (HEA), capable of efficiently catalyzing both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The findings reveal that a complex chemical composition of FeCoNiCuTi HEA can significantly modify its electronic structure, resulting in an impressive electrocatalytic performance. Specifically, FeCoNiCuTi HEA catalyst exhibits an overpotential of 64.9 mV at a current density of 10 mA cm−2 for hydrogen evolution with a Tafel slope of 36.81 mV dec−1 and double-layer capacitance (Cdl) of 34.1 mF cm−2. For oxygen evolution, HEA achieves an overpotential of 175 mV at 10 mA cm−2, a Tafel slope of 44.31 mV dec−1, exhibiting the performance superior to noble catalysts. Theoretical calculations support that the intricate chemical composition of high-entropy alloy plays vital role to enhance its electrocatalytic capabilities. This research represents a straightforward efficient method to create electrocatalysts for water electrolysis using non-noble metals by leveraging high-entropy effect.

Effect of graphene oxide on CoCrFeNi high-entropy alloy coatings prepared by the electrodeposition method

The performance advantages of high-entropy alloy (HEA) have become a consensus. However, the current preparation methods are mostly focused on thermal processing methods, and the research on the preparation of HEA coatings using electrochemical technology is generally rare. Therefore, in this study, the CoCrFeNi HEA coating was synthesized by electrodeposition technology and the effect of graphene oxide (GO) concentration on the microstructure and tribological property of the HEA coating was further studied. The results showed that during the electrodeposition process, the four atoms of Co, Cr, Fe and Ni were co-deposited to form the HEA coating with a face-centered cubic crystal structure. The introduction of GO did not change the crystal structure of HEA coatings, and GO was also co-deposited with metal atoms to form the HEA-GO composite coating. Meanwhile, GO was distributed in composite coatings in the form of being encapsulated by the metal layer. The addition of GO improved the compactness of HEA coating and reduced its surface roughness. The increase in GO concentration helped to inhibit the formation of stress-induced cracks on the surface of the composite coating, and significantly enhanced the wear resistance of HEA coating. When the GO concentration was 0.2 g/L, the wear resistance was the best. The wear form of HEA composite coatings was independent of GO and its concentration, and was dominated by abrasive wear.

The comparative research on tribological behavior of carbon-doped CoCrFeNiV high entropy alloys in air and vacuum environments

In the field of aviation and aerospace engineering, the application of high-entropy alloys (HEAs) is inevitably needs to withstand the damage of both wear and adhesion in vacuum. Herein, the vacuum tribological behavior of a high performance Co21Cr21Fe21Ni21V14.5C1.5 HEAs (C12) were investigated. In this study, the hardness and compressive strength, significantly improved for Co21Cr21Fe21Ni21V14.5C1.5 (1200 °C, C12) HEA compared to the CoCrFeNiV (C0) base alloy. The focus was put on the effect of C element and sintering temperature at 1000℃ and1200 ℃ (C10 and C12) on the microstructure and vacuum tribological mechanisms of the alloy. It was found that the friction coefficient of the C12 alloy under air can be minimized to lower than 0.37. The wear rate of C12 alloy ((1.31-2.67)×10−4mm3/Nm) is 2-4 times lower than that of C10 alloy (2.91-8.31)×10−4mm3/Nm). The wear-resistance of Co21Cr21Fe21Ni21V14.5C1.5 alloy at sinter temperature 1200 °C (C12) is significantly higher than that of alloy sinter temperature at 1000 °C (C10). Therefore, C12 alloy exhibited optimal vacuum tribological properties. This work provides helpful guidance for material design and wear resistance in the field of space tribology.

Effect of Cr content on microstructure, mechanical, and corrosion properties of CoCrxFeMnNi high-entropy alloys fabricated by selective laser melting

The method of using mixed powder for SLM in-situ alloying to prepare high entropy alloys has been verified to be feasible. In this investigation, the effect of Cr addition on microstructure uniformity, mechanical properties, and corrosion resistance of CoCrxFeMnNi High-entropy alloy (with x = 0.4, 0.7, 1.0, 1.3 at. %) prepared by the SLM in-situ alloying technique has been thoroughly investigated. X-ray diffraction and scanning electron microscopy suggest that all CoCrxFeMnNi HEAs samples are composed of a FCC matrix and little residual Cr particles, and all elements of the FCC matrix are evenly distributed, accompanied by a very small amount of Mn evaporation. Electrochemical corrosion tests have shown that with an increase in Cr content, the corrosion resistance of the alloy is significantly improved. The CoCr1.3FeMnNi HEAs possess the best corrosion resistance against the attack of the Cl ion, with a corrosion rate 0.671 × 10−3 mm / year. Furthermore, the Cr element can significantly improve the microhardness, tensile strength, and plasticity of CoCrxFeMnNi HEAs. The CoCr1.3FeMnNi HEAs possess the best mechanical properties, with microhardness 217.8 HV0.5, tensile strength 691 MPa and elongation rate 18.9%, respectively.