Medium-entropy Alloy Research Articles

Simple Scaling as a Tool to Help Assess the Closure-Free da/dN Versus ΔKeff Curve in a Range of Materials

Recent studies have proposed a simple formula, which is based on Elber’s original approach to account for R-ratio effects, for determining the crack closure-free ΔKeff versus da/dN curve from the measured R-ratio-dependent ΔK versus da/dN curves. This approach, which is termed “Simple Scaling,” has been shown to collapse the various R-ratio-dependent curves onto a single curve. Indeed, this approach has been verified for a number of tests on metals, polymers, and a medium-entropy alloy. However, it has not yet been used to help assess/determine the closure-free ΔKeff versus da/dN curve. The current paper addresses this shortcoming and illustrates how to use this methodology to assess the ΔKeff versus da/dN curves given in the open literature for tests on a number of steels, aluminum alloys, STOA Ti-6Al-4V, a magnesium alloy, and Rene 95. As such, it would appear to be a useful tool for assessing fatigue crack growth.

In-situ EBSD study of the active slip systems and substructure evolution in a medium-entropy alloy during tensile deformation

Electron backscatter diffraction (EBSD) coupled with in-situ tensile loading is powerful for investigating the microstructural evolution of alloys. Thermo-mechanically treated f.c.c. medium-entropy alloys (MEAs) typically have high densities of annealing twin boundaries (ATBs), which can not only strengthen but also toughen the MEA via interacting with dislocations. However, the evolution of ATBs and other substructures in plastically-deformed MEA has not yet been revealed. Plastic deformation involving dislocation evolution, active slip systems, lattice rotation, boundary transformation, and grain subdivision in a polycrystalline MEA Ni41.4Co23.3Cr23.3Al3Ti3V6 was studied using in-situ EBSD. The slip was accompanied by heterogeneous lattice rotation among grains and within grains, where inhomogeneous plasticity was accommodated by geometrically-necessary dislocations (GNDs). Both GND and low-angled boundaries (LABs) densities substantially increased with progressive strain, which was mainly concentrated in sites approaching ATBs or grain boundaries (GBs). Located stress, lattice rotation, or curvature caused a loss in the coherence of ATBs, which resulted in integrity loss with increasing strain and promoted a decrease in density by 60 %. Further, lattice rotation incompatibility due to constraints from neighboring grains leads to grain fragmentation into various misorientated volumes, which were separated by LABs or high-angle boundaries (HABs). The grain orientation angle increased with progressive strain and crystallographic 〈111〉 orientation gradually spread toward a tensile direction. Slip systems with maximum Schmid factor were activated first at ε ≥ 3.9 %, which is almost the same with experimental slip traces. Both single slip and double slip occurred during plasticity, where straight slip traces tend to curve due to lattice curvature. Slip transfers are not only controlled by geometric compatibility factor, which can occur between some neighboring grains with low geometric compatibility factor but high Schmid factor.

Phase reversion mediated the dual heterogeneity of grain size and dislocation density in an equiatomic CrCoNi medium-entropy alloy

An ultra-high strain rate (104 s-1) dynamic plastic deformation treatment at liquid nitrogen temperature (LNT-DPD) followed by annealing is carried out to obtain dual heterogeneity of grain size and dislocation density in an equiatomic CrCoNi medium entropy alloy (MEA). Such extreme loading conditions resulted in extensive phase transformation in this MEA. Subsequent annealing at 650°C for 1 h further induced reverse phase transformation and partial recrystallization, forming a complex heterogeneous microstructure characterized by nested trimodal grain sizes and partitioned dislocation density. A superior yield strength of ∼800MPa and a good ductility of ∼40% were simultaneously achieved in this heterogeneous alloy. In order to reveal the effects of grain size and dislocation density distributions on the mechanical property improvements, the underlying deformation mechanisms were systematically discussed. High density of geometrically necessary dislocations (GNDs) would be induced in complex heterogeneous structures during tensile deformation due to strain gradients or partitioning between different regions, which would lead to additional strengthening and work hardening. These results provide a novel approach to overcome the strength-ductility trade-off dilemma for FCC-structured MEAs.

Deformation mechanism of a metastable medium entropy alloy strengthened by the synergy of heterostructure design and cryo-pre-straining

Deformation mechanism of a metastable medium entropy alloy strengthened by the synergy of heterostructure design and cryo-pre-straining

Oxygen adsorption, absorption and diffusion in FeCrNi medium entropy alloy: an ab initio study.

Despite tremendous efforts to understand interstitial diffusion in bulk alloys, a clear understanding of the principal elemental effect on surface interstitial diffusion is still lacking. In this study, a first-principles approach is employed to study oxygen interstitial diffusion in FeCrNi medium entropy alloy (MEA) based on principal element content at various subsurface sites. Oxygen adsorption energy on surfaces, solution energy at interstitial sites, and activation energy for oxygen permeation are calculated. The adsorption energy for oxygen cohesion to all investigated surfaces was lowest for the sites containing Cr, suggesting a positive effect of Cr in producing a chromium oxide scale. In addition, we have calculated the contribution of the principal element to the stability of the interstitial sites and the activation energy to diffuse between them. This work provides insights into the formation of chromium scaling based on oxygen adsorption and permeation, with potential implications in the design of oxidation-resistant surfaces for high-temperature applications.

Impact of gradient microstructure on strain hardening via activation of multiple deformation mechanisms in CoCrNi medium entropy alloy

Abstract Face centered cubic (FCC) structural medium/high entropy alloys (MEAs), characterized by excellent strength and ductility, have attracted significant attention by the research community. The incorporation of gradient structures (GSs) further can enhance their mechanical properties. In the present research, we employ the rotation acceleration shot peening technique to introduce a GS within the CoCrNi FCC MEA to investigate underlying mechanisms governing the physical deformation processes during the generation of GSs through processing, which the primary goal is mitigating the intrinsic trade-off between strength and ductility. Through the microstructures analysis along the depth direction, both pre and post uniaxial tensile plastic deformation, we unveiled that the low stacking fault (SF) energy characteristic of the CoCrNi MEA triggered the emergence of diverse defects in the core region. The presence of nanoscale deformation twins, SFs, Lomer–Cottrell dislocation locks and phase transformation from FCC to hexagonal close-packed at twin boundaries synergistically enhanced the strain hardening capacity of the material.

Influence of Al and Ti Alloying and Annealing on the Microstructure and Compressive Properties of Cr-Fe-Ni Multi-Principal Element Alloy

This study meticulously examines the influence of aluminum (Al) and titanium (Ti) on the genesis of self-generated ordered phases in high-entropy alloys (HEAs), a class of materials that has garnered considerable attention due to their exceptional multifunctionality and versatile compositional palette. By meticulously tuning the concentrations of Al and Ti, this research delves into the modulation of the in situ self-generated ordered phases’ quantity and distribution within the alloy matrix. The annealing heat treatment outcomes revealed that the strategic incorporation of Al and Ti elements facilitates a phase transformation in the Cr-Fe-Ni medium-entropy alloy, transitioning from a BCC (body-centered cubic) phase to a BCC + FCC (face-centered cubic) phase. Concurrently, this manipulation precipitates the emergence of novel phases, including B2, L21, and σ. This orchestrated phase evolution enacts a synergistic enhancement in mechanical properties through second-phase strengthening and solid solution strengthening, culminating in a marked improvement in the compressive properties of the HEA.

N-Type AlCuFeMn Medium-Entropy Alloy with Reduced Thermal Conductivity: A Prospective Thermoelectric Material

n-Type AlCuFeMn Medium-Entropy Alloy with Reduced Thermal Conductivity: A Prospective Thermoelectric Material

New insights into fatigue anisotropy of an additively manufactured medium-entropy alloy: from the perspectives of crack initiation and crack propagation

ABSTRACT The present work investigates anisotropic fatigue properties of an additively manufactured (AM) medium-entropy alloy (MEA). The fatigue properties of the materials in 0°, 45° and 90° orientations with respect to the build layers were measured. In order to discover the dominant factors for stress level-dependent fatigue anisotropy, crystal plasticity finite element simulations and fatigue crack growth (FCG) experiments were used to evaluate crack initiation and crack propagation behaviours, respectively. The main conclusions are summarised as follows: First, in comparison to grain anisotropy, the difference in fatigue initiation stage is controlled by defect anisotropy. Second, the direct cause of the difference in the FCG rate is considered to be the differences in deflection angle, affected by the incompatibility of the slip planes between adjacent grains. Third, the AM-MEA displays a stress level-dependent fatigue anisotropy. At high-stress level, the fatigue life of the MEA-45° specimen is significantly higher than that of MEA-0° and MEA-90°, while at low-stress levels, the fatigue resistance of MEA-0° is similar to that of MEA-45°. Stress level-dependent fatigue anisotropy is controlled by different ratios of crack initiation and crack propagation. Our work proposes a novel research strategy that qualitatively evaluates fatigue anisotropy of AM materials.

Enhanced cryogenic mechanical properties of heterostructured CrCoNi multicomponent alloy: Insights from in-situ neutron diffraction

Heterostructured materials (HSMs) has been shown to improve the strength-ductility trade-off of conventional alloys but their cryogenic performance has not been studied during real-time deformation. We investigated heterostructured CrCoNi medium-entropy alloy by in-situ neutron diffraction at cryogenic (77 K) and room (293 K) temperatures. The significant mechanical mismatch at interfaces between fine and coarse grains, due to pronounced grain size disparity, resulted in exceptional yield strength of 918 MPa at 293 K. The yield strength further increased to 1244 MPa at 77 K with an excellent uniform elongation of 34 %. The exceptional strength–ductility combination at 77 K can be attributed to enhanced geometrically necessary dislocation pile-up density boosted from high-mechanical mismatch interfaces, as well as higher planar faults, and martensitic phase transformation. Comparison with homogenous counterparts demonstrates the potential of HSMs as a new strategy to improve the mechanical performance of different materials, including medium-/high-entropy alloys for cryogenic applications.

HyStor: An experimental database of hydrogen storage properties for various metal alloy classes

In this work, we introduce the HyStor database, consisting of 1282 metal alloys along with their maximum hydrogen storage capacity (H2wt%) at a given absorption temperature. The curated HydPark database consist of 831 entries. We sourced compositions from research articles and various patent documents, resulting in addition of 451 compositions to the HydPark database. The addition is reflected in the data across all existing classes of alloys. Further, low entropy alloys (LEA), medium entropy alloys (MEA) and high entropy alloys (HEA) have been newly included classes. This has broadened the scope of the database to encompass the latest materials of interest for hydrogen storage. HyStor contains representation of 54 elements, with a temperature range of 200–800 K, and H2wt% ranging from 0.1 to 7.19. We conducted thorough checks for duplicate entries, erroneous data, and conflicting compositions within the database to ensure data quality. Furthermore, we conducted multiple tests to identify potential outlier compositions. The data curation and updation reflects into slight improved error metrics of the HYST model, reducing the Mean Absolute Error (MAE) from 0.31 to 0.29 and increasing the R2 score from 0.77 to 0.79.

Enhanced corrosion resistance of NbTaMoW medium entropy alloy coatings in simulated PEMFC environments: Experimental and computational insights

An anti-corrosive and conductive NbTaMoW medium entropy alloy (MEA) coating was deposited on 304 stainless steel using a dual-cathode glow discharge technology. This MEA coating exhibits a single body-centered cubic phase with about 10 nm grains, and forms a passivation film with significantly enhanced corrosion resistance at any given HF concentration. The lower adsorption energy of Nb atoms for oxygen atoms leads to the enrichment of Nb oxides in the passivation film, which is beneficial for hindering the rapid diffusion of F− ions. These findings underscore the potential of NbTaMoW MEA coatings to improve the durability and efficiency of PEMFCs.

Superior strain-hardening and strength-ductility synergy by hierarchical L12 phases in a highly (Al, Ti)-added medium-entropy alloy

A novel precipitate-strengthened Co–Cr–Ni medium-entropy alloy with a pure “FCC + L12” dual-phase structure is developed by adding a high concentration (6 at.%) of Al and Ti elements. After aging heat treatment at 700 °C for 4 h, the hierarchical L12 phases, composed of primary large-sized L12 phases (∼70 nm) and secondary L12 particles (∼14 nm), are achieved. The designed alloy featuring hierarchical L12 phases exhibits excellent strength-ductility combinations and strain-hardening ability, properties that are much improved over their counterparts with homogeneous dispersed L12 phases (∼13 nm) and without L12 phases. Especially, the yield strength (1320 MPa), ultimate tensile strength (1886 MPa), and average strain-hardening rate (5.15 GPa) are obtained in the alloy with hierarchical L12 precipitates deformed at cryogenic temperature. The Orowan bypass mechanism of the primary L12 phases, along with the shearing mechanism of the secondary L12 phases, collectively contributes to the exceptional strength of the alloy with hierarchical L12 precipitates. The penetration of dislocations and stacking faults (SFs) in the primary L12 phases, combined with hetero-deformation-induced (HDI) hardening attributed to these hierarchical L12 precipitates, may be a key factor in improving strain-hardening properties of the alloy.

Engineering medium-entropy alloy nanoparticle nanotubes for efficient oxygen reduction

Fuel cells, as promising energy conversion devices, realize the direct conversion of hydrogen fuel chemical energy into electrical energy. The kinetics of cathodic oxygen reduction reaction (ORR) is slow and plays a decisive role in the output efficiency of fuel cells. Platinum catalysts have been commercialized, but their high cost, relatively low catalytic activity and poor cycling durability limit the development of fuel cells. The introduction of non-platinum components is an effective strategy to minimize the cost and maximize the activity. In this study, PtPdCu medium entropy alloy nanoparticle nanotubes (PtPdCu MEANPTs) that demonstrate high catalytic activity and long durability are presented. Combined with density functional theory calculations, the introduction of Pd and Cu modulates surface electronic structure and optimizes the oxygen adsorption energy, thus enhancing the catalytic activity. The 0D/1D hybrid structure exposes more surface sites and inhibits particle maturation. The atomic migration and rearrangement are emerged during the initial stage of potential cycling process, induing a palladium-rich outer layer and preventing the oxidation of platinum atoms. The best-performing Pt24Pd28Cu48 MEANPTs deliver the impressive ORR activity (1.42 A/mgPt at 0.9 V vs. RHE, which is 6.17 times that of commercial Pt/C) and durability (retaining 2.87 times the initial activity of benchmark catalyst after 30,000 potential cycles). Theoretical calculations have confirmed the regulation of alloying on the surface oxygen affinity and revealed the true active sites on the catalyst surface. This work explores the research direction of platinum-based multi-element catalysts with low platinum content and high catalytic activity.

Pursuing ultrahigh strength–ductility CoCrNi-based medium-entropy alloy by low-temperature pre-aging

Developing high-performance alloys with gigapascal strength and excellent ductility is crucial for modern engineering applications. The concept of multi-component high/medium entropy alloys (H/MEAs) provides an innovative approach to designing such alloys. In this work, we developed the Co1.5CrNi1.5Al0.2Ti0.2 MEA, which exhibits outstanding mechanical properties at room temperature through low-temperature pre-aging followed by annealing treatment. Tensile testing reveals that the MEA possesses an ultrahigh yield strength of 20±0785 MPa, an ultimate tensile strength of 2365 ± 70 MPa, and exceptional ductility of 15.8% ±1.7%. The superior tensile properties are attributed to the formation of fully recrystallized heterogeneous structures (HGS) composed of ultrafine grain (UFG) and fine grain (FG) regions, along with discontinuous precipitation of coherent nano-size lamellar L12 precipitates. The mechanical incompatibility between the UFG region and the FG regions during deformation induces the accumulation of a large number of geometrically necessary dislocations at the interface, resulting in strain distribution and hetero-deformation-induced (HDI) stress accumulation, contributing significantly to HDI strengthening. HDI strengthening, precipitation strengthening, and grain boundary strengthening are the primary mechanisms responsible for the ultra-high yield strength of the MEA. During deformation, the dominant deformation mechanisms include dislocation slip, deformation-induced stacking faults, and Lomer–Cottrell locks, with minor deformation twinning. The synergistic interaction of these multiple deformation modes provides the MEA with excellent work hardening capability, delaying plastic instability and achieving an excellent combination of strength and ductility. This study provides an effective strategy for synergistically strengthening MEAs by combining HDI strengthening with traditional strengthening mechanisms. These findings pave the way for the development of advanced structural materials with high performance tailored for demanding applications in engineering.

He irradiation resistance performance in CrNiCo, CrFeNiCo, and CrFeMnNiCo multi-principal element alloys

In this study, the He irradiation resistance of CrFeMnNiCo high-entropy alloy (HEA) and CrNiCo and CrFeNiCo medium-entropy alloys (MEAs), which have better mechanical properties than HEAs, was investigated. Thin-film samples of CrFeMnNiCo, CrNiCo, and CrFeNiCo were irradiated with up to 2 × 1020 He/m2 of 5 keV ions at 673, 773, and 873 K, respectively. In all samples, He bubble formation was observed when irradiated at 1–3 × 1019 He/m2, which depended on the irradiation temperature. At low temperatures of 673 and 773 K, even when as the irradiation dose increased to 2 × 1020 He/m2, the differences in cavity swelling due to He bubble formation among the three alloys were not large. The relative resistance (degree) to cavity swelling for the three investigated alloys was as follows: CrNiCo MEA, CrFeMnNiCo HEA, and CrFeNiCo MEA. First-principles calculation results revealed that the formation of He-di-vacancy clusters was not possible in the CrFeNiCo MEA. This is believed to be the cause of the cavity swelling decrease in the CrFeNiCo MEA. In contrast, after 2 × 1020 He/m2 at 873 K, the cavity swelling of the CrNiCo and CrFeNiCo MEAs, especially CrNiCo, was approximately four times higher than that of the CrFeMnNiCo HEA. It is believed that the He irradiation resistance of the MEAs deteriorated because of element segregation owing to high–temperature irradiation.

New insight for designing high-strength medium-entropy alloy with resistance to intermediate-temperature embrittlement by nitrogen-induced Ti-rich precipitation mechanism

Intermediate-temperature embrittlement is a common problem faced by many multi-component alloys. In this study, we propose the introduction of nitrogen into a lightweight TiVNb medium-entropy alloy to enhance solution strengthening and induce titanium segregation. Titanium-rich precipitates in turbine blade-like structures near the grain boundaries of a body-centred cubic solution matrix yield alloys that exhibit tensile ductility at intermediate temperatures. The titanium-rich precipitates dominantly have a face-centred cubic structure coupled with a small amount of hexagonal close-packed structure. The size of the titanium-rich phase increases with increasing nitrogen content from 0.5 % to 1.5 %, as does the alloy strength. The tensile yield strength of TiVNb-N1.5 reaches 1247 MPa at room temperature and 623 MPa at 800 °C, with a strain of 10 %. These values indicate that the prepared alloy overcomes the ultra-low strain of the base alloy at intermediate temperatures. By combining the MRS-Swap algorithm with the density functional theory, the formation mechanism of Ti-rich phases is deduced. This study further elucidates the types and pathways of interstitial elements that can induce FCC-Ti formation.

Exceptional Strength-Ductility Combinations of a CoCrNi-Based Medium-Entropy Alloy via Short/Medium-Time Annealing after Hot-Rolling.

Strong yet ductile alloys have long been desired for industrial applications to enhance structural reliability. This work produced two (CoCrNi)93.5Al3Ti3C0.5 medium-entropy alloys with exceptional strength-ductility combinations, via short/medium (3 min/30 min) annealing times after hot-rolling. Three types of intergranular precipitates including MC, M23C6 carbides, and L12 phase were detected in both samples. Noticeably, the high-density of intragranular L12 precipitates were only found in the medium-time annealed sample. Upon inspection of the deformed substructure, it was revealed that the plane slip is the dominant deformation mechanism of both alloys. This is related to the lower stacking fault energy, higher lattice friction induced by the C solute, and slip-plane softening caused by intragranular dense L12 precipitates. Additionally, we noted that the stacking fault and twinning act as the mediated mechanisms in deformation of the short-time annealed alloy, while only the former mechanism was apparent in the medium-time annealed alloy. The inhibited twinning tendency can be attributed to the higher energy stacking faults and the increased critical twinning stress caused by intragranular dense L12 precipitates. Our present findings provide not only guidance for optimizing the mechanical properties of high/medium-entropy alloys, but also a fundamental understanding of deformation mechanisms.

Effect of Deposit Scale on Mechanical Properties of In-Situ Alloyed CrCoNi Medium Entropy Alloys Formed by Directed Energy Deposition

Directed energy deposition (DED), as an additive manufacturing technology, has shown unique advantages in multi-material additive manufacturing and remanufacturing. In this study, two types in-situ alloyed CrCoNi medium entropy alloys that have thin-walled structures with different thicknesses (T1 and T2) were manufactured by the DED process, and the mechanisms of differences in relative density, microstructure, and mechanical properties at different heights were systematically analyzed. In terms of microstructure, the T1 and T2 samples along the building direction exhibit significant differences in crystallographic orientation, grain size, and dislocation density, which are related to the local temperature gradient differences caused by the scanning path and heat accumulation. In terms of mechanical properties at different heights of the two types of thin-walled structures, the yield strength is higher but the elongation is lower at the bottom position of sample, while the yield strength is lower but the elongation is higher at the middle and top positions. The differences of mechanical properties at different heights of the T1 and T2 samples are related to the microstructure and relative density. This finding provides new insights for the design and performance analysis of complex thin-walled structures formed by additive manufacturing.

Carbon-microalloying enhances strength-ductility synergy of (FeCoNi)90Al10 medium-entropy alloy via tailoring precipitation

The metastable high/medium-entropy alloys (H/MEAs) composing L12 nano-precipitates exhibit outstanding mechanical properties. In this work, the effect of carbon (C)-microalloying on the precipitation behavior and mechanical properties of (FeCoNi)90Al10 MEA was systematically investigated. With the addition of 0.2 at. % C, the yield strength of the alloy increased from 290.4 ± 1.3 MPa to 343.1 ± 2.8 MPa, the ultimate tensile strength increased from 648.5 ± 3.5 MPa to 686.1 ± 3.2 MPa, and the uniform elongation slightly increased after ageing for 45 min. C-microalloying resulted in a decrease of 0.85 kJ/mol and 0.26 kJ/mol in Gibbs free energy for face-centered cubic matrix to L12 and B2 phase transitions, facilitating the segregation of Al atom. This promoted the precipitation of L12 and B2 phases during ageing treatment, which enhanced ordering strengthening. The discontinuous precipitation was promoted at grain boundary after C-microalloying, providing an impetus for the movement of grain boundary. Meanwhile, the movement of grain boundary and the pinning of grain boundary by body-centered cubic precipitates resulted in the formation of grain boundary serration, which could release intergranular strain localization during deformation. Moreover, C-microalloying promoted the formation of planar slip and Taylor lattice during the deformation process, thus improving the plasticity. This finding provides a significant reference for C-microalloying and precipitate control of metastable MEAs.