CoCrFeMnNi High-entropy Alloy Research Articles

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.

Atomistic insights on the deformation mechanisms of amorphous/crystalline dual-phase high entropy alloys under nanoindentation

The amorphous/crystalline (A/C) dual-phase nanostructured model is an effective method to improve the mechanical properties of high entropy alloys (HEAs). However, the deformation mechanism of the dual-phase HEAs during nanoindentation is still unclear. Here, the effect of the amorphous layer position and thickness on the deformation behavior and mechanical properties of the A/C dual-phase CoCrFeMnNi HEAs under nanoindentation is investigated by molecular dynamics simulation. The results show that the amorphous phase has a significant impact on the mechanical behavior of the HEAs, and the amorphous layer plays a role in absorbing dislocations and hindering their continued movement. The embedding position of the amorphous layer has almost no effect on the elastic deformation of the HEAs. However, during the plastic deformation stage, as the insertion position of the amorphous layer in the HEAs moves downwards, the nanoindentation force of the HEAs shows an obvious increasing trend, which promotes deformation strengthening. The results also indicate that the amorphous layer thickness affects its ability to absorb dislocations, and the dislocation lengths in the HEA with smaller amorphous phase thickness are significantly higher than those in the models with larger amorphous phase thickness.

Corrosion Resistance Enhancement of CoCrFeMnNi High-Entropy Alloy with WC Particle Reinforcements via Laser Melting Deposition.

In the present work, a WC particle-reinforced CoCrFeMnNi high-entropy alloy (HEA) was fabricated by laser melting deposition (LMDed). The LMDed CoCrFeMnNi high-entropy alloy (CoCrFeMnNi) composite is primarily comprised of a face-centered cubic (FCC) crystal structure. However, in the case of CoCrFeMnNi with 2.5 wt.% WC, it exhibits a combination of an FCC matrix and a ceramic phase known as M23C6. The corrosion behavior of CoCrFeMnNi and CoCrFeMnNi with 2.5 wt.% WC particle in 0.5 M H2SO4 was comparatively investigated. Compared with CoCrFeMnNi, the passive film formed on the CoCrFeMnNi with 2.5 wt.% WC had a more stable and stronger protective property. The corrosion current density of the CoCrFeMnNi with 2.5 wt.% WC dropped by 149.1% compared to that of the CoCrFeMnNi, indicating that the CoCrFeMnNi with 2.5 wt.% WC had better corrosion resistance than that of the CoCrFeMnNi.

Interfacial microstructure evolution and bonding mechanism transformation of CoCrFeMnNi high-entropy alloy joints fabricated by vacuum hot-compression bonding

Vacuum hot-compression bonding (VHCB) is a promising solid-phase bonding technology, but its feasibility and applicability in high entropy alloy (HEA) are still unclear. Herein, the VHCB process of CoCrFeMnNi HEA was investigated, and the interfacial bonding quality was comprehensively evaluated via microstructure characterization and tensile test. Results exhibit that the VHCB enables to obtain the high-quality CoCrFeMnNi HEA joints, and the excellent bonding performance is attributed to the dissolution of interfacial oxide particles (IOPs) (MnCr2O4) and the migration of interfacial grain boundaries (IGBs). The bonding quality results suggest that the sluggish diffusion of the dissolved elements resulted in partial nanoscale IOPs remaining at the original bonding interface, and the mechanical properties of the joints were not compromised when the IOPs were smaller than 35 nm in size and less than 15/100 μm in amount. Crystallographic analysis indicates that the migration behavior of IGBs is closely associated with the evolution of recrystallized grains and twin boundaries in the interfacial region, and the migration mechanism transformed with the rising bonding temperature. Furthermore, a new interfacial bonding mechanism, i.e., twin-induced interfacial boundary migration, was revealed and its beneficial effect on interfacial bonding quality was demonstrated.

Mechanical and thermal expansion behaviour of TiC-reinforced CoCrFeMnNi high entropy alloy prepared by mechanical alloying and spark plasma sintering

ABSTRACT In this study, TiC-reinforced CoCrFeMnNi high-entropy alloy (HEA) composites were prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The phase composition, microstructure, mechanical and thermal expansion behaviour of composite HEAs were investigated. The results reveal that the addition of TiC has no effect on the crystal structure, however, the microstructure and mechanical properties show a strong dependency on the TiC content. Compared to the original HEA, the composite HEA shows decreased grain size, resulting in TiC nanoparticles (NPs) retarding grain growth by pinning the grain boundaries. With increasing TiC content from 0 to 4 wt-%, significant increases in the hardness from 410 to 480 HV and compressive yield strength from 680 to 1100 MPa, which is mainly due to the grain boundary and dispersion strengthening effects. Moreover, the thermal expansion curves show linear increments up to 800°C and decrease with increasing TiC content.

Influence of Heat-Treatment on Microstructures and Mechanical Properties of CuCrFeMnNi High-Entropy Alloy

High entropy alloys (HEAs) are defined as a multi-element alloy including more than 4 elements with near equi-atomic percentage. In general, the configurational entropy of the HEAs is known to be sufficient to stabilize a single solid solution, such as body-centered cubic (BCC), face-centered cubic (FCC) and hexagonal-closed pack (HCP). Compared to BCC single-phase alloys, FCC single-phase alloys draw extensive attention because they are advantageous in manufacturing and processing. FCC-based HEAs show excellent ductility but limited strength, so many research on improving strength has been conducted. Outstanding mechanical properties with a balance of strength and ductility are rarely achieved in single-phase FCC-based HEAs. This is why most alloys for structural applications exhibit a multi-phase microstructure. In this study, we aimed to develop multi-phase FCC-based HEA with superior mechanical properties than single-phase CoCrFeMnNi HEA, via Co substitution in CoCrFeMnNi HEA by Cu, which has a high mixing enthalpy. It was found that the CuCrFeMnNi HEA is composed of two FCC phases and one BCC phase. The CuCrFeMnNi HEA was cold-rolled, and subsequently aged at 500, 700, 900<sup>o</sup>C for 1 hour. As the annealing temperature increased, the volume fraction of the FCC phase (FCC1 + FCC2) increased and the residual stress was gradually relieved by recrystallization. Furthermore, small amount of sigma phase was formed at 900<sup>o</sup>C. The effect of the microstructural evolution on the mechanical properties, such as hardness and tensile properties at room temperature, will be discussed.

Mechanical properties and deformation behavior of equiatomic CoCrFeMnNi high-entropy alloy foam: A molecular dynamics study

In recent years, the aerospace and mechanical industries have employed metal foams for High Energy Capacity applications. However, the porous nature of such metallic foams results in a drop in tensile properties. High Entropy Alloys (HEAs) have been reported to overcome the strength-ductility trade-off as opposed to conventional metals and their alloys. Based on this, an equiatomic nano porous CoCrFeMnNi HEA is investigated using molecular dynamics simulation to elucidate the deformation behavior and mechanical performance at different atom distributions, percent porosity, and pore locations. The findings indicated that the mechanical properties are independent of atom distributions. However, with an increase in percent porosity, a decline in the ultimate tensile strength was observed. Up to 66% reduction in tensile strength was recorded. A similar trend was observed under temperature variations. By varying pore location, either within or on the structure, the pores within the foam structure led to a significant reduction in properties. The deformation mechanism of the HEA foam was established via analyses of the dislocation distributions, stacking faults, crack nucleation, and propagation. The findings offer new insights for material design to produce high-performance HEA foams and a comprehension of its failure mechanism under loading.

Early-stage surface oxidation of the equiatomic CoCrFeMnNi high entropy alloy studied in situ by XPS

The mechanisms of initial oxide growth on the equiatomic CoCrFeMnNi high entropy alloy were studied in situ by X-ray photoelectron spectroscopy on clean, oxide-free surfaces exposed stepwise to low-pressure gaseous oxygen. Cr(III), Mn(III), Fe(III), Co(II) but no Ni(II) oxides were formed at 100 °C, and Mn(II) at 200 °C. Data quantification revealed a transition mechanism from selective initial oxidation to 3D kinetically-controlled growth, leading to stratification of the oxide film. Formation of the Co-rich outer layer above the Cr-rich inner layer first formed implies predominant cationic outward oxide growth supported by the overall stoichiometry of the film.

CoCrFeMnNi High-Entropy Alloy Thin Films Electrodeposited on Aluminum Support

We investigate the corrosion behavior on the microstructure of CoCrFeMnNi high-entropy thin film potentiodynamic alloys electrodeposited on aluminum support. Using electrolytes based on dimethyformamide (DMF), (HCON(CH₃)₂), dimethylsulfoxide (DMSO, (CH3)2SO), an acetonitrile (AN, CH3CN) organic system (4:1 vol. ratio), LiClO4 in different concentrations and CoCl2, CrCl3 × 6H2O, FeCl2 × 4H2O, MnCl2 × 4H2O and NiCl2 × 6H2O in different conditions, the HEA components were co-deposited on an aluminum foil substrate. Using the CALPHAD method and the MatCalc PC software, the structure of the HEA system under study was investigated by calculating the characteristic kinetic and thermodynamic criteria. The influence of each metal from HEA inside the solid solution zone and over the formation of the solid solution phase were obtained from the ratio between the derived parameters. The electrochemical measurements, which demonstrate that on an aluminum support the HEA thin films have lower corrosion resistance, were performed at ambient temperature in an aerated artificial solution. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to investigate the microstructure of HEA thin films before and after corrosion in artificial seawater. This complex study demonstrates that the electrodeposition of HEA thin films on an aluminum support is more difficult and that the resistance to corrosion is much lower compared to that in the previous work on HEA deposits on a copper support.

Enhanced forming accuracy and mechanical property of CoCrFeMnNi high entropy alloy by ultrasonic and cryogenic fields-assisted deformation

A novel method for enhancing the forming accuracy and mechanical property of high-entropy alloy micro-parts was proposed using the combination of ultrasonic and cryogenic fields. The influence on the shape factor, microstructure, and hardness of compressed specimens was discussed, respectively. The results show that with the addition of cryogenic fields, the increase in forming accuracy is more significant during ultrasonic vibration-assisted forming due to the improved lubrication condition, and the dominant mechanism changes from dislocation slip to twinning deformation, which can effectively weaken the softening effect and enhance the material hardness.

Boost in mechanical strength of additive manufactured CoCrFeMnNi HEA by reinforcement inclusion of B4C nano-particles

As a way to improve the strength of high-entropy alloys (HEAs), reinforcement by composite fabrication of HEAs has recently been investigated in various ways. In this study, CoCrFeMnNi+B4C (2 wt%) high-entropy composite (HEC) parts were fabricated using the direct energy deposition (DED) process of pre-ball milled powders. The grain growth is severely hindered by B4C nano-particles during the layer-by-layer printing of CoCrFeMnNi+B4C HEC compared to CoCrFeMnNi HEA. In addition, as a collateral effect, the sequence of elemental segregation in the dislocation cellular boundary differs in CoCrFeMnNi HEA and CoCrFeMnNi+B4C HEC. Additional segregation of Cr, Mn, and C elements into the cell boundary has been observed. In fact, unlike the CoCrFeMnNi HEA, the segregation of Cr, Mn, and C provides a presence of carbide-rich regions because of the partial decomposition of B4C nano-particles. Furthermore, the B4C nano-particle promotes the pinning effect of moving dislocation leading to the high dislocation density in the matrix, which contributes the biggest portion of strength reinforcing of CoCrFeMnNi+B4C HEC. Besides, the well-distributed B4C nano-particles boost the final strength of the HEC component by providing an additional dispersion hardening effect. Therefore, by co-activation of various strengthening mechanisms, the CoCrFeMnNi+B4C HEC showed magnificent yield and ultimate tensile strengths of 1024 ± 20 MPa and 1264 ± 16 MPa, respectively, with more than 7% of total elongation, which is much higher mechanical strength than the ones reported in the literature.

Dynamic compression behavior of CoCrFeMnNi high-entropy alloy fabricated by direct energy deposition additive manufacturing

High-entropy alloys (HEAs) have attracted significant interest in recent years because of their unique physical properties, which make them suitable for usage in extreme environments. The fabrication of HEAs using metal additive manufacturing (MAM) has shown potential for improving their performance. This study investigates the mechanical properties of CoCrFeMnNi HEA manufactured through direct energy deposition using compression tests conducted at various strain rates ranging from quasi-static to dynamic (10−4 - 4200 s−1). The effect of the strain rate on the mechanical properties was analyzed, and a Johnson-Cook constitutive model was employed to simulate the dynamic compression test using the finite element method. The temperature and strain distributions during high-strain-rate compression were analyzed, reflecting the local temperature rise in the finite element analysis. The comparison of the strain distribution results of the simulation and microstructure after deformation showed that the microstructural behavior was influenced by strain localization, which occurred more prominently at the edge of the material. Because the CoCrFeMnNi HEA had a low stacking fault energy, twin formation was accelerated at high strain rate deformation by easily overcoming the critical twinning stress. The edge region, where strain localization was activated, exhibited a higher twin fraction than the central region (uniform deformation). The partitioned strain in the edge region was absorbed by twin formation, effectively suppressing the formation of the adiabatic shear band. These findings present significant implications for the design and development of MAM-processed HEAs with improved mechanical properties under extreme environments.

A high-entropy alloy syntactic foam with exceptional cryogenic and dynamic properties

Metal foams are in great demand in extreme service environments with cryogenic temperatures and high strain rates due to their unique properties. However, the reduced ductility and impact toughness under such conditions limit their applications. In this work, we tested the quasi-static and dynamic compression properties of CoCrFeMnNi high entropy alloy syntactic foam at liquid nitrogen temperature (−196 °C). We found that it exhibits ultra-high strengths and energy absorption capacity, especially a superior resistance to embrittlement at cryogenic temperature. Microstructural characterizations reveal that the foam matrix has a high twinning activity and shear localization propensity in cryogenic environments. Dense deformation twins and multiple shear bands intersected, forming a weave-like microstructure that can disperse the deformation and benefits energy absorption. Deformation twins can also strengthen the matrix and delay the crack nucleation and growth. While under dynamic loading, an FCC to HCP phase transformation was activated, forming a nano-laminated dual-phase (NLDP) FCC/HCP structure in the matrix, leading to further strengthening and toughening. Deformation twinning and HCP phase transformation act as additional plastic deformation mechanisms along with stacking faults and shear bands to offset the shortcomings caused by limited dislocation movements at low temperatures and high strain rate dynamic loading, enabling the high strength and energy absorption of the syntactic foam.

Hierarchical toughening mechanisms in face centered cubic CoCrFeMnNi high-entropy alloy at room temperature and cryogenic temperatures

As a class of promising structural material, recently synthesized face centered cubic phased high-entropy alloys (HEAs) exhibit excellent room-temperature (RT) fracture toughness as well as an abnormally increasing one at cryogenic temperatures (CTs). The intrinsic toughening mechanisms are not yet well understood. Attention here is focused on the atomistic crack initiation and propagation in a model HEA CoCrFeMnNi under RT and CTs, by means of atomistic simulations integrated with theoretical analysis. Hierarchical deformation mechanisms of the incipient plasticity; the local amorphization; and the formation, growth, and coalescence of voids are found, based on which the strain energy stored in the material is continually dissipated, resulting in the outstanding fracture toughness of the HEA at both RT and CTs. The origin for low-temperature toughening may be attributed to fewer immobile dislocations at CTs, delaying the occurrence of amorphization and microvoids. The differences in mechanisms for low-temperature toughening of the HEA and low-temperature embrittlement of traditional metals are further comparatively revealed. This study provides mechanistic insights into the fundamental understanding of intrinsic toughening mechanisms in the HEA.

Elemental segregation in CoCrFeMnNi high entropy alloy after intermediate-temperature low cycle fatigue loading

We report on the elemental segregation in CoCrFeMnNi high entropy alloy after low cycle fatigue loading at 550 °C via transmission electron microscopy. The segregation of Fe, Co and Ni elements at the grain boundaries was identified, which is in addition to previously reported precipitation of MnNi-rich and Cr-rich phases. Besides, cyclic straining is suggested to play an assisting role in the precipitation of these phases and the elemental segregation upon cycling CoCrFeMnNi at intermediate temperatures. In addition, various dislocation patterns in the post-fatigued CoCrFeMnNi are compared to those in monotonic loading.

Precipitation growth of σ-phase along the interface in CoCrFeMnNi high-entropy alloy

The equiatomic CoCrFeMnNi high-entropy alloy (HEA) has a face-centered cubic (FCC) single-phase structure with a good balance of yield strength and elongation due to the twinning-induced plasticity (TWIP). In order to increase the usability of FCC-structured HEA, it is necessary to develop its mechanical properties. The hard precipitation of the σ-phase might be a promising strengthening mechanism. Furthermore, developing an advanced manufacturing process is necessary to apply the σ-phase-containing HEAs. This study focused on σ-phase formation during annealing after hot compression of a CoCrFeMnNi HEA. The σ-phase was primarily observed near the high-angle grain boundaries and acted as the obstacle for the boundary migration, resulting in grain growth during annealing. In addition, the development of the σ-phase according to the interface type was investigated. This microstructure increased the hardness of the alloy and may contribute to the further development of high-strength HEAs.

Corrosion Resistance of CoCrFeMnNi High Entropy Alloy Compared with 2205 Duplex Stainless Steel in a Simulated Concrete Pore Solution

Corrosion Resistance of CoCrFeMnNi High Entropy Alloy Compared with 2205 Duplex Stainless Steel in a Simulated Concrete Pore Solution

Microstructural evolution and tensile property of gradient microstructure CoCrFeMnNi high-entropy alloy induced by pre-torsion

Gradient-structure was obtained in CoCrFeMnNi high-entropy alloys utilizing both the forward pre-torsion and forward-reverse pre-torsion preparation technique. The slope between hardness and r/R is high corresponding to increase in the pre-torsion angles attributable to the gradient structure after pre-torsion processing. Under 2% tensile strain, the change of dislocation and twin morphology takes place as pre-torsion angle increases, i.e., from discrete dislocations, to dislocation tangles and to dislocation cells, and from single twins to secondary twins. According to tensile test results, yield strength of samples increases but fracture strain linearly decreases with increasing pre-torsion angles as: Yield strength=905.47−11.67elongation. Yield strength of FRPTed specimens is slightly higher than that of FPTed specimens under the same pre-torsion angles with the maximum increment of 56 MPa. Strength mechanism indicates that the improvement of yield strength of the HEAs is mainly attributed to gradient dislocation and twin strengthening. In addition, a strength predictive model, which takes gradient dislocation and twin strengthening into consideration, is proposed, and predictive yield strength match well with the experimental data of FPTed- and FRPTed- samples.

Effects of grain size and Al addition on the activation volume and strain-rate sensitivity of CoCrFeMnNi high-entropy alloy

The activation volume (V*) and strain-rate sensitivity exponent (m) of CoCrFeMnNi and Al0.5CoCrFeMnNi high entropy alloys (HEAs) with various grain sizes (ranging between 2.4 and 356 μm) were measured at different strain rates and strain levels at room temperature. As the strain rate decreased, the plastic strain decreased, but the grain size and V* increased. The enhanced solid-solution strengthening by addition of aluminum decreased V*. The Hassen plot was modified to capture the grain-size dependence of V* by considering the grain-size dependence of the dislocation density. As the plastic strain increased, the strain rate, grain size and m decreased. The behavior of m could be quantitatively predicted by using equations derived for the grain-size-dependent V*and flow stress. The difference in the grain-size dependence of m between conventional face centered cubic (FCC) metals and FCC HEAs over the nanograin size range could not be explained in terms of a relatively large Hall-Petch slope in the latter compared with the former but was explainable in terms of a substantially higher probability of activation of grain-boundary diffusion-controlled grain-boundary-sliding mechanism at nanograin sizes in the former than in the latter.

Study on effect of chloride and temperature on corrosion behavior of CoCrFeMnNi high entropy alloy

The effect of chloride ion concentration and solution temperature on the corrosion behavior of CoCrFeMnNi high entropy alloy (HEA) was studied by electrochemical measurements, atomic force microscope (AFM) and immersion test. The result shows that increasing [Cl-]/[OH-] ratio or solution temperature enhances the electrochemical reaction ability, weakens the protection of passive film, and generates more severe pitting corrosion. The negative impact of temperature on the corrosion behavior of the HEA is more obvious than that of chloride concentration. A synergistic effect is produced between solution temperature and chloride ion concentration, which significantly destructs the denseness of passive film, and greatly decreases the anti-corrosion properties of the HEA. The pits occurred on the HEA could be ascribed to the selective dissolution of Fe, Co and Ni elements.