CoCrFeMnNi High-entropy Alloy Research Articles

Study on Mechanical Properties of Nanopores in CoCrFeMnNi High-Entropy Alloy Used as Drug-Eluting Stent.

The CoCrFeMnNi high-entropy alloy is commonly used for vascular stents due to its excellent mechanical support and ductility. However, as high-entropy alloy stents can cause inflammation in the blood vessels, leading to their re-narrowing, drug-eluting stents have been developed. These stents have nanopores on their surfaces that can carry drug particles to inhibit inflammation and effectively prevent re-narrowing of the blood vessels. To optimize the mechanical properties and drug-carrying capacity of high-entropy alloy stents, a high-entropy alloy system with different wide and deep square-shaped nanopore distributions is created using molecular dynamics. The mechanical characteristics and dislocation evolution mechanism of different nanopore high-entropy alloy systems under tensile stress were studied. The results showed that the CoCrFeMnNi high-entropy alloy with a rational nanopore distribution can effectively maintain the mechanical support required for a vascular stent. This research provides a new direction for the manufacturing process of nanopores on the surfaces of high-entropy alloy stents.

Effect of sputtering power and substrate bias on microstructure, mechanical properties and corrosion behavior of CoCrFeMnNi high entropy alloy thin films deposited by magnetron sputtering method

The CoCrFeMnNi high entropy alloy thin films with crystalline (solid solution) structure or amorphous state were fabricated by magnetron sputtering process at different target powers and different substrate bias. Effects of the target power and substrate bias on the characteristics of the CoCrFeMnNi thin films such as microstructures, morphology and surface topography, as well as mechanical properties and corrosion behavior were studied. Phase prediction was calculated by using thermodynamic and kinetic criteria under various deposition conditions. The enthalpy, entropy, δ and Ω parameters, and electronegativity differences influenced the solid solution stability of thin films. These parameters led to the formation of solid solutions with crystalline or amorphous structures under specific limits, which corresponded to the practical XRD and TEM analysis results. Using a substrate bias in the deposition of CoCrFeMnNi thin film changed the amorphous structure to a crystalline (solid solution (BCC + FCC)) at higher target powers (200 and 300 W). On the other hand, the film structure was entirely amorphous at all sputtering target powers without the substrate bias. The mechanical properties, such as hardness, Young's modulus, film strength, and fracture toughness of CoCrFeMnNi thin film, can be enhanced by using the substrate bias and increasing the sputtering power. The potentiodynamic polarization test in 3.5 wt% NaCl aqueous solution indicated that the deposition of CoCrFeMnNi thin films at different sputtering powers and without substrate bias could increase the corrosion resistance of 304 stainless steel substrate. Using the substrate bias of −100 V in the fabrication of CoCrFeMnNi thin film decreased the corrosion current density and increased its polarization resistance.

In-situ microstructural evolution during tensile loading of CoCrFeMnNi high entropy alloy welded joint probed by high energy synchrotron X-ray diffraction

The research regarding high entropy alloys (HEAs) has proved them to be suitable for engineering applications. Nevertheless, assessing their processability is key for industrial deployment. Of special relevance in terms of processing techniques arised welding. In this work, Gas Tungsten Arc Welding (GTAW) was chosen as a processing method to attest for the suitability of the equiatomic CoCrFeMnNi HEA for one of the most common real-world mechanical solicitations, tensile loading. We delve into an in-situ synchrotron X-ray diffraction analysis of the mechanical behavior of a high-performing GTAW CoCrFeMnNi HEA joint. Local analysis of the microstructure evolution, considering the base material, heat affected zone and fusion zone, was performed by tracking changes in the diffracted intensity and lattice strain. Orientation-dependent evolution is highlighted by considering partial azimuthal integration detailing texture impact across the joint. Evidence of strain concentration at specific locations is correlated with the microstructure and overall macroscopic mechanical behavior.

Spatially varied stacking fault energy induced low twinning ability in high entropy alloys

Nanostructured high-entropy alloys (HEAs) are promising candidates for extreme load-bearing applications due to their superior performance. In this work, we investigate the deformation behaviors of CoCrFeMnNi HEA under high-speed impact by molecular dynamics simulations. Compared with Al, Ni, and Cu representing pure metals with low to high stacking fault energies, it is found that the CoCrFeMnNi HEA exhibits remarkably low twinning density under shock, despite its extremely low stacking fault energy. Shear loading is then applied to stacking-faulted HEAs and these pure metals to study the evolution of stacking faults under shear stress. The results further show a low tendency for stacking faults to transform into deformation twinning in HEAs, regardless of the initial density of stacking faults. The energy path for deformation twins and stacking faults was calculated, and a direct comparison of fault energies could not explain the deformation mechanism of HEA. We reveal that the inhomogeneous energy profile of dislocation slip caused by the inherent heterogeneity of HEA leads to dispersed stacking fault propagation, which suppresses twinning formation. These results address the spatially tunable defects and further urgent need for the synergistic design of components and microstructures in HEAs.

Effects of Ti addition and annealing on microstructure and mechanical properties of CoCrFeMnNi high-entropy alloy

Effects of Ti addition and annealing on microstructure and mechanical properties of CoCrFeMnNi high-entropy alloy

Strengthening mechanisms analysis and tailoring of bimodal grain structures for enhanced strength in CoCrFeMnNi high-entropy alloys

Strengthening mechanisms analysis and tailoring of bimodal grain structures for enhanced strength in CoCrFeMnNi high-entropy alloys

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.

Multi-scale analysis of microstructural evolution and atomic bonding mechanisms in CoCrFeMnNi high-entropy alloys upon cold spray impact

Large interfacial strains in particles are crucial for promoting bonding in cold spraying (CS), initiated either by adiabatic shear instability (ASI) due to softening prevailing over strain hardening or by hydrostatic plasticity, which is claimed to promote bonding even without ASI. A thorough microstructural analysis is vital to fully understand the bonding mechanisms at play during microparticle impacts and throughout the CS process. In this study, the HEA CoCrFeMnNi, known for its relatively high strain hardening and resistance to softening, was selected to investigate the microstructure characteristics and bonding mechanisms in CS. This study used characterization techniques covering a range of length scales, including electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and high-resolution transmission microscopy (HR-TEM), to explore the microstructure characteristics of bonding and overall structure development of CoCrFeMnNi microparticles after impact in CS. HR-TEM lamellae were prepared using focused ion beam milling. Additionally, the effects of deformation field variables on microstructure development were determined through finite element modeling (FEM) of microparticle impacts. The ECCI, EBSD, and HR-TEM analyses revealed an interplay between dislocation-driven processes and twinning, leading to the development of four distinct deformation microstructures. Significant grain refinement occurs at the interface through continuous dynamic recrystallization (CDRX) due to high strain and temperature rise from adiabatic deformation, signs of softening, and ASI. Near the interface, a necklace-like structure of refined grains forms around grain boundaries, along with elongated grains, resulting from the coexistence of dynamic recovery and discontinuous dynamic recrystallization (DDRX) due to lower temperature rise and strain. Towards the particle or substrate interior, concurrent twinning and dislocation-mediated mechanisms refine the structure, forming straight, curved, and intersected twins. At the top of the particles, only deformed grains with a low dislocation density are observed. Our results showed that DRX induces microstructure softening in highly strained interface areas, facilitating atomic bonding in CoCrFeMnNi. HR-TEM investigation confirms the formation of atomic bonds between particles and substrate, with a gradual change in crystal lattice orientation from the particle to the substrate and the occurrence of some misfit dislocations and vacancies at the interface. Finally, the findings of this research suggest that softening and ASI, even in materials resistant to softening, are required to establish bonding in CS.

Process Optimization of CoCrFeMnNi High Entropy Alloy Fabricated by Selective Laser Melting Based on Thermal Mechanism and Molten Pool Behavior Analysis

Process Optimization of CoCrFeMnNi High Entropy Alloy Fabricated by Selective Laser Melting Based on Thermal Mechanism and Molten Pool Behavior Analysis

A novel strategy to enhance the interfacial bonding quality of CoCrFeMnNi high-entropy alloy joints produced by vacuum hot-compression bonding

The strain history of strain rate transient changes was introduced into the vacuum hot-compression bonding (VHCB) process of CoCrFeMnNi high-entropy alloy, which rapidly improved the interfacial bonding quality of the joint. The results of interfacial microstructure characterization indicated that the high-density dislocations generated in the interfacial region during the initial high strain rate stage not only promoted the nucleation of the interfacial recrystallized grains but also induced the transformation of the sub-boundaries and the interfacial grain boundaries (IGBs) to the high mobility twin boundaries (TBs). The accelerated discontinuous dynamic recrystallization process, the growth of TB-type continuous dynamic recrystallization grains, and the interfacial TB migration during the transient VHCB dramatically enhance the IGB migration level of the joints. This study provides an effective strategy for the interfacial regulation of high-quality VHCB joints.

Influence of Solution Alkalinity and Alternating Current Density on Anti-corrosion Property of CoCrFeMnNi High-Entropy Alloy in Simulated Concrete Pore Solution

Influence of Solution Alkalinity and Alternating Current Density on Anti-corrosion Property of CoCrFeMnNi High-Entropy Alloy in Simulated Concrete Pore Solution

Effects of Selective Laser Melting Process Parameters on Structural, Mechanical, Tribological and Corrosion Properties of CoCrFeMnNi High Entropy Alloy

AbstractThe structural, tribological, mechanical, corrosion, and other properties of materials produced by laser-based powder bed fusion additive manufacturing methods are significantly affected by production parameters and strategies. Therefore, understanding and controlling the effects of the parameters used in the manufacturing process on the material properties is extremely important for determining optimum production conditions and for saving time and materials. This study aimed to determine the optimal laser parameter values for CoCrFeMnNi high-entropy alloy powders using the selective laser melting (SLM) method. The layer thickness was kept constant during experimentation. 5 different laser powers and 10 varying laser scanning speeds were tested, with hatch spacing from 30 to 90%. After determining the optimal laser parameters for SLM, prismatic samples were fabricated in different build orientations (0°, 45°, and 90°), and subsequently, their structural, mechanical, tribological, and corrosion properties were compared. Melt pool morphology could not be obtained at 20—40 and 60W laser powers and at all laser scanning speeds used at these laser powers. At 100 W laser power, 600 mm/s laser scanning speed, and 70% hatch spacing parameters, an ultimate tensile stress of 550 MPa and elongation of 48% were obtained. Among the samples produced in different build orientations, the sample produced with a 0° build orientation exhibited the highest relative density (99.94%), the highest microhardness (201.2 HV0.1), the lowest friction coefficient (0.7025), and the lowest wear and corrosion rates (0.7875 mpy). Additionally, SLM parameters were evaluated to have a significant impact on the performance of all properties of the samples. Graphical Abstract

An integrated computation framework for predicting mechanical performance of single-phase alloys manufactured using laser powder bed fusion: A case study of CoCrFeMnNi high-entropy alloy

Establishing a process-structure-property (PSP) relationship is crucial for understanding and optimizing the mechanical properties of components via laser powder bed fusion (L-PBF) additive manufacturing (AM). However, the parameters pertinent to the PSP relationship are encapsulated in a “black box” with a high dimension, resulting in relying on trial-and-error approaches to elucidate hidden associations becoming impracticable, while the acquisition of microscopic data proves financially burdensome. Thus this paper presents an integrated computation framework based on machine learning (ML) for predicting the tensile yield strengths and post-yield hardening rates of single-phase alloys fabricated through L-PBF, for efficient process optimization purposes. Research herein focuses on single-phase alloys to be built via L-PBF, taking CoCrFeMnNi high-entropy alloy as a study case, and provide an efficient integrated computation solution. The methodology proposed in this paper generates as-built microstructures associated with the varying L-PBF process parameters via cellular automata (CA). Subsequently, the mechanical responses of these microstructures are estimated by employing crystal plasticity finite element (CPFE) simulations. In addition, the approach raised in the paper leverages particle-level microstructure descriptors instead of average feature descriptors for the entire representative volume element (RVE) structure in an ML framework to predict the characteristics of the newly randomly growing grains. This research demonstrate the practicality and instructiveness of the methodology for modeling realistic grains. Then, trained ML model exhibits promising accuracy for the prediction of mechanical properties. This work showcases an integrated computation framework combining advanced materials science and ML techniques for enhancing understanding of the L-PBF process and offering valuable insights into process optimization for L-PBF-built diverse materials.

Plane-Stress Deformation Behavior of CoCrFeMnNi High-Entropy Alloy Sheet under Low Temperatures.

High-entropy alloys are promising candidates expected to be applied in transportation equipment serving in extreme environments due to their excellent properties. CoCrFeMnNi high-entropy alloy is a typical representative of them, and its low temperature performance is excellent. In this study, to evaluate the feasibility of forming HEA shells, the deformation behavior of CoCrFeMnNi under a plane-stress state at lower temperatures was thoroughly studied. Firstly, a thin-walled HEA tube was fabricated using hot extrusion and further formed into a thin shell for uniaxial tensile and biaxial bulging tests. Subsequently, uniaxial tensile tests at cryogenic temperatures were conducted. Both the strength and the ductility improves as the temperature decreases from -160 °C to -196 °C. Then, a systematic low-temperature bulging test was performed using isothermal dome tests and the thickness uniformity analysis of the bulged specimens was carried out. In addition, grain microstructural observation using EBSD was characterized analyze the possible deformation mechanism at the cryogenic temperature under the biaxial stress state. This study, for the first time, investigated the biaxial deformation behavior of HEA. Considering the plane-stress state deformation is the dominant type in the thin-walled shell deformation, this study enables us to provide direct guidance for various sheet-forming processes of HEAs.

Revealing the inhomogeneous nature of microstructure and texture evolution in the cold-rolled CoCrFeMnNi alloy during static recrystallization

In the present investigation, equiatomic CoCrFeMnNi high entropy alloy (HEA) was subjected to 80% cold rolling (as-rolled) followed by isothermal annealing treatment at 700 °C for different time periods. Microstructural characterization was performed using electron back-scattered diffraction (EBSD) and electron channeling contrast imaging (ECCI) techniques on the as-rolled and annealed samples. The as-rolled microstructure consisted of elongated grains mainly composed of strong α-fiber and weak γ-fiber textures. The as-rolled sample showed the formation of ingrain SBs in the γ-fiber grains, which served as preferential sites for grain nucleation. The annealing treatment of as-rolled samples at 700 °C cause static recovery (SRV) before 5 minutes of heat-treatment, whereas static recrystallization (SRX) was observed after 5 minutes of heat-treatment in the CoCrFeMnNi alloy samples. A faster rate of recrystallization kinetics was observed after 15 minutes of heat-treatment. The annealing treatment reduced the intensity of α-fiber and enhanced the Copper, Cube and P ({011}<122>) texture components. Copper components originated from coarse recrystallized grains, which were the results of localized grain growth phenomena. Unusual behavior of banded featured α-fiber grains was observed during the annealing process. These banded grains in the partially annealed samples consisted of subgrain-boundaries and showed a delayed response to recrystallization as compared to grains with other orientations. Stored energy (SE) and Taylor factor (M) calculations were also used to understand the delayed recrystallization response of the banded featured/retained deformed grains. The microhardness value of the as-rolled sample was approximately 450 Hv, which decreased to around 230 Hv for the fully recrystallized grains.

Effects of single and simultaneous Fe, He ions irradiations on CoCrFeMnNi high entropy alloy

Simultaneous irradiation was performed on CoCrFeMnNi high entropy alloy (HEA) at room temperature using Fe ions and He ions, and single Fe ions irradiation and single He ions irradiation were used as comparisons. The synergistic effect of the two ions on irradiation-induced defects such as dislocation loops and He bubbles, as well as the effect of irradiation damage on the nano-hardness were investigated. As the irradiation dose increased, a distribution with small and dense dislocation loops was first formed in CoCrFeMnNi HEA, and then these dislocation loops merged and grew. However, compared to pure metal, CoCrFeMnNi HEA entered the dislocation loop merging stage at a higher threshold dose due to its higher defect recombination capacity and higher critical stress of initial dislocation motion. Compared with the single Fe ions irradiation, the addition of He in the simultaneous irradiation accelerated the merging and growth of dislocation loops and decreased the density of dislocation loops. At the same He dose, simultaneous irradiation produced a higher vacancy concentration than single He ions irradiation, which increased He mobility, leading to wider He bubble layer and larger He bubble size. CoCrFeMnNi HEA hardened after irradiation with all three parameters in the order of single He ions irradiation > single Fe ions irradiation > simultaneous Fe+He ions irradiation. This study provided a deeper understanding of the irradiation damage mechanism of HEA in complex irradiation environment.

Synergistic effects of Monel 400 filler wire in gas metal arc welding of CoCrFeMnNi high entropy alloy

Weldability plays a crucial role in the journey of high entropy alloys towards their engineering applications. In this study, gas metal arc welding was performed to join an as-rolled CoCrFeMnNi high entropy alloy using Monel 400 as the filler wire. The present research findings demonstrate a favorable metallurgical chemical reaction between the Monel 400 filler and the CoCrFeMnNi high entropy alloy, resulting in compositional mixing within the fusion zone that promotes a solid-solution strengthening effect, counteracting the typical low hardness associated to the fusion zone of these alloys. The weld thermal cycle induced multiple microstructure changes across the joint, including variations in the grain size, existing phases and local texture. The grain size was seen to increase from the base material toward the fusion zone. An FCC matrix and finely sparse Cr-Mn-based oxides existed in both base material and heat affected zone, while in the fusion zone new FCC phases and carbides were formed upon the mixing of the Monel 400 filler. The role of the filler material on the fusion zone microstructure evolution was rationalized using thermodynamic calculations. Texture shifted from a γ-fiber (in the base material) to a strong cubic texture in the fusion zone. Digital image correlation during tensile testing to fracture coupled with microhardness mapping revealed that, stemming from the process-induced microstructure changes, the micro and macromechanical response differed significantly from the original base material. This study successfully established a correlation between the impact of the process on the developed microstructural features and the resultant mechanical behavior, effectively assessing the processing-microstructure-properties relationships towards an improved understanding of the physical metallurgy associated to these advanced engineering alloys. In conclusion, this work provides an important theoretical framework and practical guidance for optimizing the engineering applications of high entropy alloys.

Mechanical-thermal coupling fatigue failure of CoCrFeMnNi high entropy alloy

High entropy alloys exhibit excellent performance under different-temperature conditions. In order to investigate the effect of temperature on the mechanical-thermal coupling fatigue failure of classical CoCrFeMnNi high entropy alloy to accelerate the application of high entropy alloys, the uniaxial tensile and fatigue tests at temperatures ranging from RT to 500 °C were conducted, and statics simulation analysis of fatigue specimens under fatigue loading conditions was conducted. The results of the uniaxial tensile tests illustrated that the yield stress and tensile strength of the alloy decreased with increasing temperature. In contrast, the elongation after the fracture of the alloy increased with temperature. The fatigue test results revealed that the fatigue life decreased as the temperature increased. The fracture morphology of the fatigue specimens and the flank morphology and dislocation distribution near the crack nucleation zone were collected to support the investigation. Accordingly, the reasons for the high-temperature-induced decrease in fatigue crack nucleation and propagation lives were systematically investigated. The main reasons for the higher-temperature-induced decrease in crack nucleation lives were a decrease in the yield stress of the alloy, more easily triggered dislocation motion and stress homogenization. The reasons for the decrease in fatigue crack propagation lives induced by higher temperatures were the more prone occurrence of dislocation motion, a decrease in yield stress, and a decrease in the area of the crack propagation region.

Enhancement in interfacial bonding quality of CoCrFeMnNi high-entropy alloy vacuum hot-compression bonding via surface shot peening

Vacuum hot-compression bonding (VHCB) is an advanced solid-state bonding technique. To expand its application in high-entropy alloys (HEAs), a new strategy to enhance the interfacial bonding quality of CoCrFeMnNi HEA VHCB joints by surface shot peening (SP) was proposed, and the effects of SP treatment on the interfacial bonding behavior and mechanical properties of VHCB joints were investigated. The results revealed that SP treatment introduced a nanocrystalline layer and plastic deformation layer with non-uniform strain on the specimen bonding surface, and a hierarchical grain size gradient fine microstructure was formed by a static recrystallization process under the thermal activation condition before VHCB. During the bonding process, the fine microstructure of the SP-treated layer was retained and provided a large number of grain boundaries and dislocations, which facilitated atomic diffusion and plastic flow in the interfacial region and promoted the closure of interfacial voids and the dissolution of interfacial oxide particles. Additionally, interfacial crystallographic and tensile test analyses indicated that the SP treatment induced a multiple interfacial grain boundary (IGB) migration mechanism in the VHCB joints, which dramatically improved the IGB migration level and ultimately led to a complete recovery of the mechanical properties of the CoCrFeMnNi HEA VHCB joints.

Strength-Ductility Mechanism of CoCrFeMnNi High-Entropy Alloys with Inverse Gradient-Grained Structures.

The microstructures and mechanical properties of equiatomic CoCrFeMnNi high-entropy alloys (HEAs) treated with various processing parameters of laser surface heat treatment are studied in this paper. The typical inverse gradient-grained structure, which is composed of a hard central layer and a soft surface layer, can be obtained by laser surface heat treatment. A much narrower gradient layer leads to the highest yield strength by sacrificing ductility when the surface temperature of the laser-irradiated region remains at ~850 °C, whereas the fully recrystallized microstructure, which exists from the top surface layer to the ~1.05 mm depth layer, increases the ductility but decreases the yield strength as the maximum heating temperature rises to ~1050 °C. Significantly, the superior strength-ductility combination can be acquired by controlling the surface temperature of a laser-irradiated surface at ~1000 °C with a scanning speed of ~4 mm/s due to the effect of hetero-deformation-induced strengthening and hardening, as well as the enhanced interaction between dislocation and nanotwins by the hierarchical nanotwins. Therefore, retaining the partial recrystallized microstructure with a relatively high microhardness in the central layer, promoting the generation of hierarchical nanotwins, and increasing the volume proportion of gradient layer can effectively facilitate the inverse gradient-grained CoCrFeMnNi HEAs to exhibit a desirable strength-ductility synergy.