CoCrNi Medium Entropy Alloy Research Articles

The effect of spin-polarization, atomic ordering and charge transfer on the stability of CoCrNi medium entropy alloy

The aim of this paper is to determine the effects of spin-polarization and atomic ordering on the phase stability of the medium entropy alloy CoCrNi by employing density functional theory calculations. We have found that high Cr ordering causes stabilisation of both the fcc and hcp structures compared to fully disordered alloy, while high Ni ordering leads to destabilisation of the hcp structure but has no effect on the fcc structure. The differences in the mutual stability of fcc and hcp structures for alloys with different levels of ordering are accompanied by differences in the distribution of local magnetic moments. However, their analysis revealed a generally increasing trend in the magnitude of the magnetic moment with the increasing number of Cr atoms in the 1st nearest neighbours (NN) shell for all Co, Cr, and Ni elements. Moreover, the charge analysis revealed a charge transfer from Cr to the other two elements in the alloy and also its dependence on the number of Cr atoms in the 1st NN shell. Thus, we have described the clear indirect dependence of the magnetic moment of all elements in this alloy on the ongoing charge transfer. This also points to the possibility that the charge transfer has the more important effect on stabilisation of particular atomic ordering rather than the spin-polarization as the same behaviour of charge transfer was observed also for results obtained without spin-polarization.

Effect of Temperature on the Corrosion Behavior of CoCrNi Medium-Entropy Alloy in NH4Cl Solution

Effect of Temperature on the Corrosion Behavior of CoCrNi Medium-Entropy Alloy in NH4Cl Solution

The effect of annealing on mechanical properties of selective laser melted CoCrNi medium-entropy alloy

The mechanical properties were evaluated in the selective laser melted CoCrNi medium-entropy alloy after the annealing at 800 °C. The as-selective laser melted sample delivered the yield strength of 820 MPa, the ultimate tensile strength of 1090 MPa and the fracture strain of 34%, While corresponding value for annealed sample at 800 °C for 4 h was 630 MPa, 870 MPa and 42%, respectively. High density dislocation induced high strength of as-selective laser melted sample, and higher ratio of the low-angle grain boundaries in sample annealed at 800 °C for 1 h impeded the dislocation movement and improved the strength. Moreover, high density dislocation and phase transformation from a face-centered cubic phase to a hexagonal close-packed phase in better recrystallization sample annealed at 800 °C for 4 h synergistically induced strength-ductility trade-off.

Enhancement on mechanical properties of CoCrNi medium entropy alloy via cold spray additive manufacturing associated with sintering

Equimolar CoCrNi medium entropy alloy (MEA) has a broad potential for industrial applications due to its excellent strength-ductility synergy. In this study, the emerging solid-state cold spray additive manufacturing (CSAM) approach was applied to fabricate bulk CoCrNi MEA deposits for the first time. However, due to its nature, CSAM normally results in limited metallurgical bonding and weak interfacial bonding between deformed particles, particularly for high-strength alloys such as the CoCrNi MEA used in this work. Therefore, a post-sintering treatment with temperatures ranging from 1200 °C to 1350 °C was applied after CSAM to improve the interparticle bonding and strengthen the as-fabricated MEA deposits. During the sintering process, recovery, recrystallization, sintering necks and grain migration for polycrystalline growth were observed at different stages. The sintering treatment significantly enhanced the tensile strength and ductility of the as-fabricated sample from ∼240 MPa and ∼2 % to ∼660 MPa and 43 %. This study validates the potential of the united CSAM-sintering strategy in additive manufacturing of CoCrNi MEA.

Microstructure and texture characterisation of friction stir welded CoCrNi and CoCrFeMnNi multi-principle element alloys

This work investigates the microstructure formed in friction stir welds of FCC alloys, focused on two multi-principal alloys: a CoCrFeMnNi high-entropy alloy (HEA) and a CoCrNi medium-entropy alloy (MEA). A commercial stainless steel AISI 304 is used for comparison. The largest nugget was formed in the MEA, while the smallest was formed in the HEA. Grain refinement occurs in the stirred zone in all welds. Discontinuous dynamic recrystallisation is the predominant restoration mechanism during friction stir welding of the three investigated alloys. A sharp decrement in the Σ3 boundary fraction occurs in the stirred zone of the AISI 304 and HEA welds, while comparable values with the base metal are found for the MEA weld. The peak in the maximum index of crystallographic texture is observed on the advancing side of the stirred zone of the AISI 304 weld. A strong < 001 > θ-fibre texture is formed in the advancing side of the nugget in the AISI 304 from a well-established {123}< 634 > S-type texture in the base metal. Multiple crystallographic texture components without specific fibres are identified in most regions of the welds, indicating the complex shear path history during friction stir welding.

The effects of grain size and temperature on mechanical properties of CoCrNi medium-entropy alloy

The mechanical properties and deformation mechanisms of CoCrNi medium-entropy alloy are studied through molecular dynamics simulations. The effects of temperature and average grain size on the elastic modulus, Poisson's ratio, yield stress, and maximum flow stress are investigated. The constant pressure molecular dynamics method is used to calculate the elastic modulus and Poisson's ratio of the alloy at different temperatures and average grain sizes. Simple tension simulations are conducted to determine the yield stress and maximum flow stress as a function of average grain size. The study also analyzes the dislocation behavior near grain boundaries at different temperatures using molecular dynamics simulations. The Hall-Petch and inverse Hall-Petch relationships are employed to describe the grain size-dependent deformation behavior of the alloy.

Study on Corrosion Behavior of CoCrNi Medium-Entropy Alloy in Acidified Coastal Seawater Environment

Study on Corrosion Behavior of CoCrNi Medium-Entropy Alloy in Acidified Coastal Seawater Environment

Chemical inhomogeneity inhibits grain boundary fracture: A comparative study in CrCoNi medium entropy alloy

Grain boundary (GB) fracture is arguably one of the most important reasons for the catastrophic failure of ductile polycrystalline materials. It is of interest to explore the role of chemical distribution on GB deformation and fracture, as GB segregation becomes a key strategy for tailoring GB properties. Here we report that the inhomogeneous chemical distribution effectively inhibits GB fracture in a model CoCrNi medium entropy alloy compared to a so-called ‘average-atom’ sample. Atomic deformation kinematics combined with electronic behavior analysis reveals that the strong charge redistribution ability in chemical disordered CrCoNi GBs enhances shear deformation and thus prevents GB crack formation and propagation. Inspects on the GBs with different chemical components and chemical distributions suggest that not only disordered chemical distribution but also sufficient “harmonic elements” with large electronic flexibility contribute to improving the GB fracture resistance. This study provides new insight into the influence mechanism of GB chemistry on fracture behavior, and yields a systematic strategy and criterion, from the atoms and electrons level, forward in the design of high-performance materials with enhanced GB fracture resistance.

Untra-fine-grained equiatomic CoCrNi medium entropy alloys with high density stacking faults and strengthening mechanisms

This study introduces a powder metallurgy route (a combination of mechanical alloying (MA) and spark plasma sintering) to enhance the mechanical properties of equiatomic CoCrNi medium-entropy alloys (MEAs) using multiple strengthening mechanisms. The yield strength of MEA with an average grain size of 0.41 μm produced by MA is 777 ± 21 MPa, which is 121% higher than that of MEA with grains of 4.55 μm prepared from commercial powders. Additionally, the yield strength of the untra-fine-grained alloy is 231% higher than that of the one produced by casting. Such an enhancement of the strength is attributed to the coordination of the ultra-fine grains, twins, and high-density stacking faults. Moreover, the influence of twins and stacking faults was investigated using molecular dynamic simulations. The intersection of ultra-high density stacking faults generates many immobile dislocation locks and separates the grains into several small parts, leading to a dynamic grain-refinement effect. These findings indicate that MA is an effective way to produce MEAs with superior strength.

Strengthening of CoCrNi medium entropy alloy with gadolinium additions

A series of (CoCrNi)100–xGdx (x = 0/0.1/0.3/0.5/1.0) medium entropy alloys (MEAs) was fabricated by high-vacuum arc melting. The effects of rare-earth element, Gd, additions on the evolution of microstructures and mechanical properties of CoCrNi MEAs were studied. The phase and microstructure were examined using X-ray diffraction, scanning electron microscope and transmission electron microscope. The tensile properties were measured by material testing system (MTS) and the hardness was measured by both Vickers indenter and nanoindentation. As the Gd concentration increased, the phase transformed from face-centered cubic (FCC) to hexagonal close-packed (HCP) precipitates in FCC matrix. Compared to the single phase CoCrNi MEA with the yield strength of 400 MPa and fracture strength of 880 MPa, CoCrNi99.5Gd0.5 MEA exhibited a much higher yield strength of 751 MPa and ultimate tensile strength of 1216 MPa, while the ductility still maintained a high value of ∼55%. The enhanced mechanical properties were ascribed to solid solution, precipitation and grain refinement strengthening mechanisms. The relations among compositions, microstructures and mechanical properties of (CoCrNi)100–xGdx MEAs were discussed.

Anisotropic microstructure, properties and molecular dynamics simulation of CoCrNi medium entropy alloy fabricated by laser directed energy deposition

CoCrNi medium entropy alloy additive part was fabricated by laser directed energy deposition process. Based on an optimized additive manufacturing process parameters, the microstructure, tensile properties and corrosion performance in the XOY and XOZ planes were compared to learn the anisotropic properties of the additive part. The majority of the additive parts consisted of equiaxed grains, a small proportion consisted of elongated needle-like grains. The equiaxed and elongated needle-like grains in the XOY plane were finer compared to that in the XOZ plane. The corrosion electrochemical activity is uniform in the XOY plane, but varies at different heights in the XOZ plane base on scanning vibrating electrode technique results. Electron backscatter diffraction results near the fracture indicated a larger and relatively more homogeneous residual stress peak in the XOY plane. The tensile and yield strengths in the XOZ plane were lower than those in the XOY plane by approximately 98.5 MPa and 19.1 MPa. While the elongation in the XOZ plane was more than twice as high as that in the XOY plane. The reason for the anisotropic tensile behavior was the strong texture in the XOY plane and the relatively random grain orientation in the XOZ plane. Single crystal and polycrystalline molecular dynamics were used to simulate the anisotropic tensile properties in the XOZ and XOY planes. Single crystal simulation results showed that the maximum stress was 19.0 GPa and elongation was 13.0% when the stretching was along the [110] crystal orientation. The minimum stress was 17.1 GPa and elongation is 11.1% elongation when the stretching was along the [11 1‾] crystal orientation. Anisotropy existed for CrCoNi alloys in terms of tensile properties along different orientations. Polycrystalline simulation in the XOY plane results showed that the main cause of anisotropy between them was the tangling of the 1/6<112> dislocations and the 1/6<521> dislocations, which caused obstacles to dislocation motion and thus improved the strength of CoCrNi additive parts. High tensile strength in the XOY plane, not the XOZ plane, was attributed to 1/6<112> and 1/6<521> dislocation tangles at the (1 1‾ 0)/(115) planes.

Formation mechanism of hierarchical twins in the CoCrNi medium entropy alloy

• The competitive mechanisms between primary/conjugate slips and twins are clarified. • The particularity of mechanism of hierarchical twins in CoCrNi MEA is unraveled. • The hierarchical twinning behavior depending on strain rate and deformation temperature is revealed. • The twinning map help predict the formation of hierarchical twins as a reference for experiments. The three-dimensional hierarchical twin network has been proved to be the source of the excellent strength-ductility combination in the CoCrNi medium entropy alloy. Revealing the formation mechanism of hierarchical twins, however, remains a challenge using either the post-mortem or the in-situ microstructural characterization. In this study, the atomistic formation mechanism of hierarchical twins was investigated using molecular dynamics simulations, with special focus on the effects of strain rate and deformation temperature. Compared to the primary twin boundaries kink-driven hierarchical twinning tendency in pure FCC metals, the chemical inhomogeneity in CoCrNi can reduce the necessary kink height to trigger conjugate twins (CTWs), fascinating the formation of twin networks. At room temperature, the plastic deformation is dominated by primary twins (PTWs) and conjugate slips at a relatively lower strain rate (e.g., 5×10 7 s −1 ). The hierarchical twins can be activated in cases of deforming at a higher strain rate (e.g., 2×10 8 s −1 ). Further increasing the strain rate (e.g., 1×10 10 s −1 ) leads to the phase-transformation induced plasticity. At cryogenic temperatures, the hierarchical twins are promoted within a large range of strain rates (e.g., 5×10 7 -1×10 10 s −1 ). A higher temperature leads to the synergy of CTWs and primary slips at a lower strain rate, but hierarchical twins at a higher strain rate. On this basis, a qualitative comparison and scalable trends between experiments and simulations were revealed. The present study would not only provide the basic understanding for the twinning behavior found experimentally, but also contribute to the design of medium/high entropy alloys with excellent mechanical performances by tuning microstructures.

Design and optimization of the composition and mechanical properties for non-equiatomic CoCrNi medium-entropy alloys

• The essential criterion was provided for the composition design of CoCrNi medium-entropy alloys (MEAs) with exceptional strength-ductility combinations. • The analysis on phase stability, deformation mechanisms and mechanical properties is combined in the essential criterion. • The optimal composition space of CoCrNi MEAs was predicted with the adoption of the essential criterion and first principles calculations. • The validity of essential criterion was proved by systematic comparison with numerous experimental data. The development of multi-principal element alloys (MPEAs, also called as high- or medium-entropy alloys, HEAs/MEAs) provides tremendous possibilities for materials innovation. However, designing MPEAs with desirable mechanical properties confronts great challenges due to their vast composition space. In this work, we provide an essential criterion to efficiently screen the CoCrNi MEAs with outstanding strength-ductility combinations. The negative Gibbs free energy difference ∆ E FCC-BCC between the face-centered cubic (FCC) and body-centered cubic (BCC) phases, the enhancement of shear modulus G and the decline of stacking fault energy (SFE) γ isf are combined as three requisites to improve the FCC phase stability, yield strength, deformation mechanisms, work-hardening ability and ductility in the criterion. The effects of chemical composition on ∆ E FCC-BCC , G and γ isf were investigated with the first principles calculations for Co x Cr 33 Ni 67- x , Co 33 Cr y Ni 67- y and Co z Cr 66- z Ni 34 (0 ≤ x, y ≤ 67 and 0 ≤ z ≤ 66) alloys. Based on the essential criterion and the calculation results, the composition space that displays the negative Gibbs free energy difference ∆ E FCC-BCC , highest shear modulus G and lowest SFE γ isf was screened with the target on the combination of high strength and excellent ductility. In this context, the optimal composition space of Co-Cr-Ni alloys was predicted as 60 at.%–67 at.% Co, 30 at.%–35 at.% Cr and 0 at.%–6 at.% Ni, which coincides well with the previous experimental evidence for Co 55 Cr 40 Ni 5 alloys. The validity of essential criterion is further proved after systematic comparison with numerous experimental data, which demonstrates that the essential criterion can provide significant guidance for the quick exploitation of strong and ductile MEAs and promote the development and application of MPEAs.

A novel CoCrNi medium entropy alloy enhanced AgCuTi composite filler to braze Kovar alloy and sapphire

A novel CoCrNi medium entropy alloy enhanced AgCuTi composite filler to braze Kovar alloy and sapphire

Comparison of Corrosion Behavior of CoCrNi Medium-Entropy Alloy with CoCrFeMnNi High-Entropy Alloy in SO2-Polluted Marine Environment

Comparison of Corrosion Behavior of CoCrNi Medium-Entropy Alloy with CoCrFeMnNi High-Entropy Alloy in SO2-Polluted Marine Environment

Microstructure, mechanical properties, and strain rate sensitivity of vacuum arc melted CoCrNi medium entropy alloy

Vacuum arc melting is a prevalent and low-cost technique to synthesize several metals and alloys. This study reports the microstructure, mechanical properties, and high strain rate testing of vacuum arc melted CoCrNi Medium Entropy Alloy (MEA). CoCrNi is a popular MEA owing to its better combination of strength and ductility, and the alloy also exhibits superior fatigue properties to the well-known first-generation CoCrFeMnNi high entropy or Cantor alloy. The CoCrNi MEA is found to be a single-phase face-centered cubic (fcc) solid solution and microstructural investigation revealed near-equiaxed coarse grains. Since strain hardening is an important mechanism for strengthening of CoCrNi at higher strains, the high strain rate tests are performed at strain rates of 103, 203, and 303 S-1. The strain hardening effect is dominant at higher strain rates, and a positive strain rate sensitivity of 0.256 ± 0.03 is observed.

Dislocation behavior in initial stage of plastic deformation for CoCrNi medium entropy alloy

The effects of crystal orientation on the plastic deformation behavior for typical CoCrNi medium entropy alloy under tensile and compressive loading were investigated by molecular dynamics simulations. The Lomer-Cottrell (L-C) structure and parallel hexagonal close packed (HCP) bands are produced in the [111] and [11̅0] orientations at the initial stage of tensile deformation, respectively. However, perfect dislocations and two types of non-slip dislocations (Hirth and Stair-rod) are generated in the [111] and [11̅0] orientations during compressive deformation, respectively. The L-C structure produced by tensile loading in the [111] orientation and Hirth/Stair-rod dislocations produced in the [11̅0] orientation under compression act as a dislocation pinning and limit the continued glide of the dislocations. The dislocation reactions under various conditions were analyzed, which is helpful to understand the plastic deformation mechanism of CoCrNi alloy.

Fabrication of high-performance CoCrNi medium entropy alloy by laser powder bed fusion: The effect of grain boundary segregation

A CoCrNi composite material with 3 wt% TiC was successfully fabricated by laser powder bed fusion (LPBF). With increasing content of TiC particles, C gradually segregated along the grain boundaries (GBs). The segregation of C improved the grain coalescence, intensified the attraction of the columnar grains, and increased the consumption of the liquid film. Furthermore, the segregation of C led to the formation of Cr23C6 and core–shell structure (TiO2/TiC) nanoprecipitates along the GBs. These precipitates reduced the mobility of the boundaries and restricted the grain growth along the direction of the maximum thermal gradient. Reinforcing the CoCrNi microstructure with 3 wt% TiC yielded optimal results, as the resultant composite microstructure was free of hot cracks, consisted of small grains, and exhibited a more random texture. The high density of wide stacking faults and Lomer–Cottrell locks hindered cross-slip during further deformation at ambient temperature, and twins dominated at elevated temperatures. These nanoprecipitates hindered dislocation propagation at all temperatures. Therefore, the synergistic strengthening effects from the different types of dislocations and nanoprecipitates endowed the CCN-3M alloy with excellent mechanical properties. Ultimate tensile strengths of approximately 1.34 and 0.88 GPa, yield strengths of approximately 1.02 and 0.75 GPa, and elongations of approximately 15.6% and 25.3% were observed at ambient and elevated temperatures (700 °C), respectively. These results indicate that GB segregation is an effective method for suppressing the evolution of hot cracks, thus expanding the scope of LPBF-fabricated components for industrial applications.

Competitive strengthening between dislocation slip and twinning in cast-wrought and additively manufactured CrCoNi medium entropy alloys

In situ neutron diffraction experiments have been performed under loading in cast-wrought (CW) and additively manufactured (AM) equiatomic CoCrNi medium-entropy alloys. The diffraction line profile analysis correlated the faulting-embedded crystal structure to the dislocation density, stacking/twin fault probability, and stacking fault energy as a function of strain. The results showed the initial dislocation density of 1.8 × 1013m−2 in CW and 1.3 × 1014m−2 in AM. It significantly increased up to 1.3 × 1015m−2 in CW and 1.7 × 1015m−2 in AM near fracture. The dislocation density contributed to the flow stress of 470 MPa in CW and 600 MPa in AM, respectively. Meanwhile, the twin fault probability of CW (2.7%) was about two times higher than AM (1.3%) and the stacking fault probability showed the similar tendency. The twinning provided strengthening of 360 MPa in CW and 180 MPa in AM. Such a favorable strengthening via deformation twinning in CW and dislocation slip in AM was attributed to the stacking fault energy. It was estimated as 18.6 mJ/m2 in CW and 37.5 mJ/m2 in AM by the strain field of dislocations incorporated model. Dense dislocations, deformation twinning, and atomic-scale stacking structure were examined by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM).

Cracks suppression strategies for CoCrNi medium entropy alloy fabricated by laser directed energy deposition

Laser-directed energy deposition (l-DED) additive manufacturing of the CoCrNi medium entropy alloy is prone to cracking, especially with high laser heat input. Electron backscatter diffraction analysis showed crack formation and stress concentration at the high-angle grain boundaries. In addition, element content test results exhibited a higher oxygen content in the additive parts with high laser heat input. Electron probe X-ray microanalysis indicated that the oxygen was distributed uniformly in the additive parts. Corresponding molecular dynamics simulation results revealed that an increase in oxygen content led to a decrease in the tensile strength of the additive parts. Two methods of argon-filled environment l-DED experiments and low-oxygen CoCrNi feeding power (argon gas atomization) were used. Results showed that the number and size of cracks were significantly reduced in the additive parts obtained by a high heat input process when the l-DED was conducted in an argon-filled atmosphere and when powders with lower oxygen content were used. For the crack-free additive parts, the ultimate tensile strengths reached 625 MPa with high laser heat input. Three crack suppression strategies could be proposed from this study, namely, reducing the heat input, performing additive experiments under argon-filled environment, and decreasing the oxygen content of the powder.