CoCrNi Medium Entropy Alloy Research Articles

Grain size dependent deformation microstructure evolution and work-hardening in CoCrNi medium entropy alloy

This study clarified the grain size dependence of the deformation microstructure evolution and work-hardening behavior in CoCrNi medium entropy alloy. We fabricated fully recrystallized specimens with coarse-grained (CG) and ultrafine-grained (UFG) specimens by severe plastic deformation and subsequent annealing processes. Tensile deformation was applied to the specimens at room temperature. The UFG specimen exhibited both high strength and high ductility compared to conventional UFG metals due to the high work-hardening ability. In the CG specimen, three distinct types of deformation microstructures consisting of dislocations and deformation twins developed depending on grain orientations, similar to the single-crystalline specimens. In the UFG specimen, widely extended stacking faults and randomly-tangled dislocations were found to coexist in most grains. Deformation twins were found to nucleate without evidence of dislocation reactions regardless of grain orientations, implying abnormal nucleation mechanisms of deformation twins in the UFG specimen. Dislocation densities quantified by in-situ synchrotron XRD measurements during tensile deformation were higher in the UFG specimen than those in the CG specimen and conventional UFG metals. Our analysis showed that the work-hardening behavior of the specimens was primarily controlled by increases in dislocation density as well as the introduction of planar defects during deformation. Through comparisons with the CG specimen and conventional UFG metals, we concluded that the excellent work-hardening ability of the UFG specimen was mainly due to the evolution of unique deformation microstructures and rapid increase in dislocation density, which could be due to inhibited dynamic recovery in the MEA.

Microstructure and properties of the welding heat-affected zone in CoCrNi medium-entropy alloy

Microstructure and properties of the welding heat-affected zone in CoCrNi medium-entropy alloy

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.

Effect of ZrC particle on microstructure and corrosion behavior of CoCrNi medium-entropy alloy fabricated by powder plasma arc cladding

Medium-entropy alloys (MEAs) show significant potential as corrosion-resistant coatings. However, their industrial applications are limited by fabrication challenges and low efficiency. This study synthesized CoCrNi(ZrC)x MEA coatings using powder plasma arc cladding (PPAC) technology. The CoCrNi(ZrC)x MEAs consist of columnar and equiaxed grains. Incorporating ZrC refines the grain structure, achieving maximum refinement at x = 3. This refinement enhances the coating hardness by approximately 40 % (from 165 ± 3.36 to 229 ± 4.83 HV0.1), mainly due to Hall-Petch strengthening, solid solution effects, and grain boundary and dispersion strengthening. In 3.5 wt% NaCl solution, corrosion resistance was notably improved, as evidenced by a substantial decline in corrosion pits and an 81 % (from 4.52 to 0.86 μA/cm²) reduction in icorr. This enhancement is primarily due to ZrC reducing the exposed surface area of the coating and providing protection through localized Cr segregation at anodic sites. This study is the first to introduce ZrC ceramic particles into CoCrNi MEAs, advancing the development of corrosion-resistant coatings.

Microstructures and wear properties of CoCrFexNi (x = 0, 1) medium-entropy alloy coatings prepared by laser cladding

PurposeHigh-nitrogen steel is a common material for the manufacture of drilling equipment such as drill collars. However, its poor surface properties often limit its applications. The purpose of this paper is to find a way to enhance the surface performance of high-nitrogen steel, which is expected to improve the wear resistance of high-nitrogen steel.Design/methodology/approachThe CoCrNi and CoCrFeNi medium-entropy alloy (MEA) coatings were prepared on high-nitrogen steel substrate by laser cladding technology. The microstructure, phase composition and element distribution of the fabricated coatings were investigated using scanning electronic microscopy, electron backscatter diffraction and X-ray diffraction. The phase structure, phase stability and structural stability of the CoCrNi and CoCrFeNi MEAs were investigated in combination with phase diagram calculation and molecular dynamics. Then the wear tests were carried out for coatings.FindingsThe results show that both prepared MEA coatings have good quality and contain a single face-centered cubic phase. The wear performance of MEA coatings is improved by the refinement of grain size and the increase of dislocation density. Due to the addition of Fe atoms, the lattice distortion of the CoCrFeNi system increased, resulting in a higher dislocation density of the coating. Cr atoms in the CoCrFeNi system are the largest, and the local lattice distortions induced by them are greater. Through this study, MEA coatings with high hardness can be expanded to drilling field applications.Originality/valueCoCrFeNi and CoCrNi MEA coatings were successfully prepared on the surface of high-nitrogen steel for the first time without obvious defects. The micromorphology and grain orientation of the different kinds of coatings were discussed in detail. The hardness-strengthening mechanism and structure stability of the coatings were illustrated by experiments and molecular dynamics simulations.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-04-2024-0116/

Effect of Co-content on microstructure, phases, and mechanical properties of laser additive manufactured Cox(CrNi)100-x alloy

This study describes the combination of mechanical properties of the CoCrNi medium entropy alloy system by tailoring its non-equiatomic compositions. The Laser Directed Energy Deposition (LDED) based Additive Manufacturing (AM) technique is deployed to fabricate the Cox(CrNi)100-x (x=40, 50, 60) alloy system. All compositions exhibit the formation of solidification substructures, with the refinement of subgrain size correlated with an increase in Co-content. The Co40 alloy exhibited a single-phase FCC structure, while Co50 (3.1 % ε-martensite) and Co60 (53.6 % ε-martensite) exhibited dual FCC+HCP structure during deposition. The combined effect of dislocation slip, deformation twinning, and deformation-induced HCP ε-martensite transformation strengthened the Co40 alloy during deformation. However, the Transformation-Induced Plasticity effect (TRIP) resulted in greater HCP ε-martensite transformation during deformation. Due to limited slip systems in the HCP structure, the Co60 alloy showed lower strength. The TRIP effect is highly dominant in Co60 alloy due to the prevalent HCP phase fraction in the initial samples. This led to stress concentration at FCC grain boundaries and ε-martensite, resulting in early failure of the alloy. With the increase in Co-content, the deformation mechanism changed from dislocation slip to deformation twinning to HCP ε-martensitic transformation. The LDED-built Cox(CrNi)100-x alloy system showed a good combination of strength and ductility. The study suggests that additive manufacturing is an easier and more suitable technique for obtaining compositions with the desired combination of mechanical properties.

Nano-dispersion strengthened and twinning-mediated CoCrNi medium entropy alloy with excellent strength and ductility prepared by laser powder bed fusion

Laser powder bed fusion (LPBF) was explored to produce CoCrNi medium entropy alloys (MEAs) from pre-alloyed powder with a relatively high oxygen concentration (870 ppm). Microstructure, strength, and toughness were investigated, revealing the effect of oxide dispersion strengthening (ODS). LPBF CoCrNi MEAs had a porosity of <0.10 % and superior mechanical properties compared to CoCrNi-based MEAs reported in the literature. Notably, the CoCrNi MEAs denoted as S1 and S2, exhibited an excellent combination of yield strength (>735 MPa), a UTS > 1 GPa, and a total elongation of ∼ 50 %. High strength results from the high number density of nano chromium oxides (average particle size ∼20 nm), which were in situ formed during LPBF, contributing to a calculated dispersion strengthening of ∼220 MPa. The deformation mechanism was investigated by TEM characterization of the specimen after tensile testing, revealing a large number of nano-twins for S1, cellular structure and nano-twins formed for S3. Extremely low porosity and deformation twinning ensure steady plastic deformation in LPBF CoCrNi, enabling high ductility. The prospect of ODS via LPBF provides a pathway for the strengthening and toughening of medium to high entropy alloys (HEAs).

Effect of grain size, temperature, and tensile strain rate on mechanical response of CoCrNi medium-entropy alloys

Grain boundary (GB) strengthening of metallic materials faces limitations as grain sizes are reduced to the nanoscale, primarily due to the transition from the strengthening to the softening effects. Understanding the intrinsic mechanisms behind such pronounced transitions is essential for optimizing the mechanical properties of nanocrystalline (NC) alloys. In this work, the critical grain size responsible for this transition is determined for NC CoCrNi medium-entropy alloys using molecular dynamics simulation, and the underlying mechanisms concerning varying temperatures and strain rates are systematically investigated for the samples with the critical grain size. The transition from Hall-Petch (HP) strengthening to Inverse Hall-Petch (IHP) softening is established by decreasing the average grain sizes from 21 nm to 3 nm. The critical grain size for the current alloys is estimated to be 12 nm, and the underlying deformation mechanisms are illustrated as follows: In the IHP regime, GB-mediated deformation becomes dominant, leading to softening. Conversely, in the HP regime, dislocation slip dominates the deformation mode, contributing to strengthening. It is found that an increase in temperature from 77 K to 1100 K leads to a decrease in the average flow stress and Young's modulus. Meanwhile, the deformation mechanisms change from dislocation slip (77K–700K) to GB-mediated deformation (700 K–1100 K). Furthermore, the underlying mechanisms correlating to the critical strain rate are uncovered. It can be attributed to the shift in deformation mechanisms from dislocation slip at relatively low strain rate regimes (5 × 107 s−1-2 × 109 s−1) to dislocation drag at relatively high strain rate regimes (2 × 109 s−1-5 × 109 s−1). Our atomistic insights into the tensile response of NC CoCrNi may provide a crucial basis for designing superior properties to meet the growing demands of advanced engineering fields.

Effect of interlayer scan rotation on structure-property relationship of directed energy deposited CoCrNi medium entropy alloy

Laser-based Additive Manufacturing (AM) is sought to be a prevalent technique to synthesize medium to high entropy alloys. However, the anisotropy, temperature distribution, and residual stresses in Laser-based AM are some of the major problems that need to be addressed. The effect of interlayer scan rotation on microstructure, texture evolution, and tensile properties of laser-based directed energy deposited CoCrNi are investigated in this study. The electron backscatter diffraction studies revealed the strong 〈001〉 cubic crystallographic texture along the build direction (YZ-plane) without scan rotation. However, there is no preferred grain orientation observed with interlayer scan rotations. The 〈111〉 fiber texture and 〈111〉〈101〉 fiber texture, respectively, are obtained with 67° and 90° scan rotation strategies. The randomized grain orientation due to fiber texture formation enhanced strength significantly with little improvement in ductility. In addition, the substructure refinement is also observed after incorporating scan rotation. Moreover, the present work suggests that nearly isotropic strength and elongation are obtained with 67° rotations.

Significantly lowering friction and wear of CoCrNi medium-entropy alloy film via massive interstitial carbon-induced amorphization

CoCrNi medium-entropy alloy (MEA) exhibits exceptional damage-tolerance, particularly at cryogenic temperatures, but demonstrates poor wear resistance. In this study, we propose a strategy to significantly reduce friction and wear of CoCrNi MEA film through the introduction of massive interstitial carbon-induced amorphization. The microstructure of as-sputtered CoCrNi film reveals an anisotropic columnar nanograin structure, featuring a primary face-centered-cubic (fcc) phase and a minor hexagonal-close-packed (hcp) phase. Upon the introduction of 16 at.% interstitial carbon (CoCrNi-C), a remarkable phase transformation occurs, resulting in an isotropic amorphous structure. Under low-load and low-sliding-velocity conditions, the CoCrNi-C film exhibits a 21% reduction in coefficient of friction (CoF) and 17% reduction in wear rate. More notably, when subjected to higher loads and sliding velocities, the CoCrNi film experiences rapid failure within the initial 100 cycles, manifesting as microcracks, breakage, and peeling. In contrast, the amorphous CoCrNi-C film demonstrates remarkable durability, sustaining over 5000 cycles without failure. The wear rate exhibits a negative correlation with load but a positive correlation with sliding velocity. This enhanced wear resistance can be attributed to the amorphous nature of the CoCrNi-C film, which inhibits local strain accumulation, preventing strain localization that typically leads to crack initiation and propagation in the CoCrNi film. Additionally, the formation of a protective oxide layer contributes to the improved wear resistance of CoCrNi-C film under higher loads and lower sliding velocities. This study provides valuable insights into the design of wear-resistant CoCrNi-based MEA coatings through massive interstitial carbon-induced amorphization.

Machine Learning-Enabled Tomographic Imaging of Chemical Short-Range Atomic Ordering.

In solids, chemical short-range order (CSRO) refers to the self-organization of atoms of certain species occupying specific crystal sites. CSRO is increasingly being envisaged as a lever to tailor the mechanical and functional properties of materials. Yet quantitative relationships between properties and the morphology, number density, and atomic configurations of CSRO domains remain elusive. Herein, it is showcased how machine learning-enhanced atom probe tomography (APT) can mine the near-atomically resolved APT data and jointly exploit the technique's high elemental sensitivity to provide a 3D quantitative analysis of CSRO in a CoCrNi medium-entropy alloy. Multiple CSRO configurations are revealed, with their formation supported by state-of-the-art Monte-Carlo simulations. Quantitative analysis of these CSROs allows establishing relationships between processing parameters and physical properties. The unambiguous characterization of CSRO will help refine strategies for designing advanced materials by manipulating atomic-scale architectures.

Partially recrystallized heterostructure via Mo-doping endows CoCrNi medium-entropy alloy with exceptional strength and ductility

CoCrNi medium-entropy alloy (MEA) has excellent ductility, but the low strength at room temperature limits its practical usage. This study proposes an effective strategy to enhance the strength of CoCrNi MEA without apparent loss of ductility via partially recrystallized heterostructures resulted from doping of large atomic radius element, which was substantiated in a Mo-doping (CoCrNi)98Mo2 MEA. Compared with CoCrNi, the strength of (CoCrNi)98Mo2 is remarkably improved, maintaining the high uniform elongation of ∼24%. The yield strength and ultimate tensile strength increased from 950 MPa, 1287 MPa to 1131 MPa, 1564 MPa, respectively. The difference in deformation between the soft recrystallized region (SRR) and the hard non-recrystallized region (HNRR) leads to an excellent hetero-deformation induced (HDI) strengthening. Additionally, it is easier to reach the critical stress of twinning caused by high HDI stress at the interface of SRR/HNRR, which contributes to activation of high-density twins and produce dynamic Hall-Petch strengthening effect.

Additive Manufacturing of Interstitial Nitrogen‐Strengthened CoCrNi Medium Entropy Alloy

Equiatomic CoCrNi medium entropy alloy (MEA) is known for its excellent mechanical properties, oxidation resistance, and hydrogen embrittlement resistance. Interstitial elements like carbon and nitrogen further enhance these properties. However, there is limited research on interstitial solid solution‐strengthened alloys produced via powder bed fusion‐laser beam (PBF‐LB). This study investigates the role of nitrogen as an interstitial alloying element in CoCrNi MEA using PBF‐LB. Two alloy variants, one without nitrogen and one prealloyed with nitrogen, are analyzed through thermodynamic calculations and experimental validations. A minor nitrogen loss occurs in as‐printed samples compared to prealloyed powders, but the remaining nitrogen stays as an interstitial solid solution without forming detrimental secondary phases. This leads to improved mechanical properties, with yield and ultimate tensile strengths increasing from about 690 and 900 MPa in nitrogen‐free samples to over 750 and 1000 MPa in nitrogen‐containing samples while maintaining the high ductility of the nitrogen‐free counterpart in the as‐printed materials. Additionally, the nitrogen‐containing CoCrNi‐N MEA exhibits ≈35HV higher hardness than the nitrogen‐free variant in both as‐printed and heat‐treated states. These findings highlight the benefits of nitrogen addition in enhancing the mechanical properties of CoCrNi MEA produced by PBF‐LB.

Achieving excellent mechanical and robust lubrication behavior in the CoCrNi medium-entropy alloy via in-situ graphite

The unsatisfactory wear resistance and lubrication performance of CoCrNi multi-principal elements alloys (MPEAs) have emerged as critical challenges, impeding their extensive utilization as advanced engineering materials, despite their desirable mechanical and physical properties. Incorporating graphite to obtain MPEAs-based self-lubricating composites is believed to improve the wear resistance. However, it is challenging to prepare composites with high strength, large plasticity, and excellent anti-friction performance simultaneously. The present study reports the fabrication of novel CoCrNi MPEA-based self-lubricating composites, consisting of in-situ graphite and high-hardness silicides/carbides as reinforcement, prepared by powder metallurgy and reactive sintering. The 20.6 vol% Gr@CoCrNi–SiC composite exhibits an optimal trade-off between strength and ductility, along with anti-friction and wear resistance which is superior to the reported composites with different solid lubricants. The wear mechanism is attributed to the coordinated deformation mechanism between the CoCrNi FCC matrix and silicides/carbides, and the stabile lubrication effect supported by the structural transformation from the original polycrystalline lattice structure to core-shell nanocomposite structure (similar to diamond-like carbon) of the in-situ graphite. This simple, economical and practical strategy contributes to the development of MPEA self-lubricating composites with excellent comprehensive properties.

Thermomechanical conversion of CoCrNi medium entropy alloy subjected to transient reverse loading

Heat dissipation of CoCrNi alloy is investigated under fast reverse loading. The Taylor-Quinney coefficient (TQC) decreases during the initial strain hardening, while the subsequent kinematic hardening is associated with gradually constant TQC of about 50 %, indicating the cyclic plastic work equally divided into heat and cold work stored in CoCrNi.

Temperature effect on the microstructural refinement of equiatomic CoCrNi medium entropy alloy via cyclically multi-directional impact

Severe plastic deformation can effectively refine alloy microstructure and improve the mechanical properties of alloys surprisingly. In this work, a new approach of severe plastic deformation was proposed based on split Hopkinson pressure bar, i.e., cyclically multi-directional impact. Using this newly developed technique for microstructural refinement at different temperatures (77 K, 293 K, and 1073 K), bulk equiatomic CoCrNi medium entropy alloys with refined microstructures were obtained, and the stress vs. strain relation of the equiatomic CoCrNi medium entropy alloy during the cyclically multi-directional impact can be in situ measured. The strain rate of the cyclically multi-directional impact was controlled at about 1500 /s. The hardnesses of the CoCrNi medium entropy alloy increase dramatically from 216 Hv to 460 Hv after the cyclically multi-directional impact at 77 K, 484 Hv after the cyclically multi-directional impact at 293 K, and 434 Hv after the cyclically multi-directional impact at 1073 K. The microstructural evolutions of the CoCrNi MEA after cyclically multi-directional impact at three temperatures were characterized and analyzed. This study aims to provide a new approach to microstructural refinement for bulk alloys.

Effects of cutting tool geometry on material removal of a gradient nanograined CoCrNi medium entropy alloy.

CoCrNi medium-entropy alloys (MEAs) have attracted extensive attention and research because of their superior mechanical properties, such as higher ductility, strength, and toughness. This study uses molecular dynamics (MD) simulations to investigate the cutting behavior of a gradient nanograined (GNG) CoCrNi MEA. Moreover, it explores the influence of relative tool sharpness and rake angle on the cutting process. The results show that an increase in the average grain size of the GNG samples leads to a decrease in the average resultant cutting force, as predicted by the Hall-Petch relationship. The deformation behavior shows that grain boundaries are crucial in inhibiting the propagation of strain and stress. As the average grain size of the GNG sample increases, the range of shear strain distribution and average von Mises stress decreases. Moreover, the cutting chips become thinner and longer. The subsurface damage is limited to a shallow layer at the surface. Since thermal energy is generated in the high grain boundary density, the temperature of the contact zone between the substrate and the cutting tool increases as the GNG size decreases. The cutting chips removed from the GNG CoCrNi MEA substrates will transform into a mixed structure of face-centered cubic and hexagonally close-packed phases. The sliding and twisting of grain boundaries and the merging of grains are essential mechanisms for polycrystalline deformation. Regarding the cutting parameters, the average resultant force, the material accumulation, and the chip volume increase significantly with the increase in cutting depth. In contrast to sharp tools, which mainly use shear deformation, blunt tools remove material by plowing, and the cutting force increases with the increase in cutting-edge radius and negative rake angle.

Mechanical properties and energy absorption of medium-entropy alloy triply periodic minimal surface cellular structures fabricated via selective laser melting

Triply periodic minimal surfaces (TPMSs) are extensively used as effective tools for the microarchitectural designing of cellular structures. CoCrNi medium-entropy alloy, known for its high strength and high ductility, is an ideal candidate for fabricating TPMS-based structures. However, few studies have focused on this topic. This study investigated the compressive mechanical and energy absorption properties of CoCrNi D-sheet TPMS cellular structures fabricated via selective laser melting (SLM). The surface morphology of the fabricated D-sheet TPMS structure was observed through scanning electron microscopy, and the deformation and failure mechanisms of the structure were analyzed by the quasi-static compression test. The results indicated that the SLM-ed CoCrNi D-sheet TPMS structure exhibited stable collapse mechanisms compared to other metal-based TPMS structures. Meanwhile, the energy absorption characteristics of the modified finite element (FE) model agreed well with the experimental results. Furthermore, the impact of the level constant on the energy absorption performance of the D-sheet TPMS structure was investigated using the FE model. Thus, an optimal D-sheet (OD-sheet) TPMS structure with lower density and higher energy absorption capacity was obtained. Additionally, the Gibson–Ashby prediction model was established to aid in selecting CoCrNi OD-sheet TPMS structures with controlled relative densities for energy absorption applications.

Ultra-high strength of additively manufactured CoCrNi medium entropy alloy with high-fraction TiC

In this work, the novel CoCrNi composite with as high fraction as 5 wt% TiC reinforcements were additively manufactured (AMed) by laser powder bed fusion (LPBF) of blended powders of nano-C particles, spherical micro-Ti powders and spherical CoCrNi powders, instead of blended powders of nano-TiC and CoCrNi powders. This method resulted in fully melting of C and Ti elements during laser fusion and subsequent precipitation of nano-TiC during solidification. The agglomeration of nano-TiC in the matrix is reduced by this method, resulting in an 1800 MPa ultrahigh strength, simultaneously maintaining a considerable elongation of 12 %. Rise in the fractions of TiC from 0 to 5 wt% reduces intensity of the texture and grain size in the matrix and convert the strong 〈101〉 texture to relative weak 〈100〉 texture along building direction (BD).

Shock response of gradient nanocrystalline CoCrNi medium entropy alloy

A comprehensive investigation of shock-induced deformation and spallation mechanisms in nanocrystalline CoCrNi medium-entropy alloys (MEAs) is conducted by large-scale molecular dynamics simulations, encompassing both gradient and homogeneous nanocrystalline structures. Under the shock velocity ranging from 0.2 km/s to 1 km/s, the effects of grain heterogeneity on shock wave propagation, spallation and defect evolution are examined. An inverse Hall–Petch relationship, where the Hugoniot elastic limit (HEL) increases with larger grain sizes is observed within the grain size range of 3 to 12 nm for homogeneous nanocrystalline samples. Compared to the localized strain distribution either within grain interior (GI) or at grain boundaries (GB) in homogeneous nanocrystalline structures, gradient-grained counterparts exhibit compatible deformation along shock direction, which coordinates the load partitioning between GB and GI. As a result, lower densities of dislocation and stacking fault are triggered within the GI, while shear localization is mitigated at GB, leading to higher spall strength observed in the gradient nanocrystalline MEA samples at the same shock velocity. The results shed light on the insights of deformation and spallation mechanisms in gradient-grained MEAs under dynamic loading, which may open up a new avenue for the design of advanced structural MEAs with high strength and toughness.