NiCoCr Alloy Research Articles

Mechanical characteristics and electrochemical corrosion of NiCoCr alloys

Previous studies have shown that NiCoCr medium entropy alloys (MEAs) have perfect strength-ductility balance and significant escape fracture toughness. This work aimed to determine the effects of annealing temperature and alloy composition on the mechanical characteristics and corrosion behaviors of the NiCoCr MEAs. The results showed that the grains generated in the NiCoCr MEAs became more petite with the cooling rate increasing. The Vickers hardness (HV) of annealed NiCoCr MEAs increased with increasing annealing temperature. For NiCoCr alloys with different alloy compositions, the potentiodynamic polarization results showed that the NiCoCrFeMn alloy has the highest polarisation resistance, lowest current density, and lowest corrosion rate. However, the composition of Fe2O3-based passivation film makes it highly reactive and easily cleaved by Cl−. The NiCoCr and NiCoCrFe MEAs with high Ni content had a denser and slowly dissolved NiO passivation layer and better corrosion resistance. Moreover, the hardness of corroded NiCoCr MEAs decreased by 7.7%.

Tuning chemical short-range order for simultaneous strength and toughness enhancement in NiCoCr medium-entropy alloys

The pursuit of enhancing strength and toughness remains a critical endeavor in the field of structural materials. This study explores two distinct strategies to overcome the traditional strength-toughness trade-off. Specifically, we manipulate the chemical composition and short-range order (SRO) of the NiCoCr medium-entropy alloy, which has shown remarkable fracture toughness in recent experiments. Utilizing molecular dynamics simulations, we uncover nano-scale deformation mechanisms during crack propagation. Our findings highlight that optimizing the SRO degree leads to improvements in both atomic scale strength and toughness defined as the area underneath stress-strain curves from MD simulations. In contrast, a trade-off between strength and toughness persists when only manipulating the Ni content in the NiCoCr alloy. Based on the simulation results, we establish a strong correlation between toughness, strength, surface energies, and unstable stacking fault energies. These factors are influenced by the chemical composition and SROs in NiCoCr, with SROs acting as strong obstacles to dislocations, thereby contributing to additional strength. The exceptional toughness of NiCoCr with SRO arises from a synergy of intrinsic and extrinsic mechanisms, including dislocation glide, nanobridging during nanovoid coalescence and zigzag crack path. It is found that, in the presence of SRO, intrinsic toughening mechanisms usually associated with crack tip blunting and dissipation can also facilitate the onset of extrinsic toughening mechanisms of nanobridging and zig-zag crack path associated with nanovoid formation and coalescence. This study emphasizes the importance of tailoring SRO in designing materials with enhanced strength and toughness.

Creation of Multi-Principal Element Alloy NiCoCr Nanostructures via Nanosecond Laser-Induced Dewetting.

The multi-principal element alloynanoparticles (MPEA NPs), a new class of nanomaterials, present a highly rewarding opportunity to explore new or vastly different functional properties than the traditional mono/bi/multimetallic nanostructures due to their unique characteristics of atomic-level homogeneous mixing of constituent elements in the nanoconfinements. Here, the successful creation of NiCoCr nanoparticles, a well-known MPEA system is reported, using ultrafast nanosecond laser-induced dewetting of alloy thin films. Nanoparticle formation occurs by spontaneously breaking the energetically unstable thin films in a melt state under laser-induced hydrodynamic instability and subsequently accumulating in a droplet shape via surface energy minimization. While NiCoCr alloy shows a stark contrast in physical properties compared to individual metallic constituents, i.e., Ni, Co, and Cr, yet the transient nature of the laser-driven process facilitates a homogeneous distribution of the constituents (Ni, Co, and Cr) in the nanoparticles. Using high-resolution chemical analysis and scanning nanodiffraction, the environmental stability and grain arrangement in the nanoparticles are further investigated. Thermal transport simulations reveal that the ultrashort (≈100ns) melt-state lifetime of NiCoCr during the dewetting event helps retain the constituent elements in a single-phase solid solution with homogenous distribution and opens the pathway to create the unique MPEA nanoparticles with laser-induced dewetting process.

Dynamic Nanoindentation and Short-Range Order in Equiatomic NiCoCr Medium-Entropy Alloy Lead to Novel Density Wave Ordering.

Chemical short-range order is believed to be a key contributor to the exceptional properties of multicomponent alloys. However, its direct validation and confirmation has been highly elusive in most compounds. Recent studies for equiatomic NiCoCr alloys have shown that thermal treatments (i.e., annealing and aging) may facilitate and manipulate such ordering. In this work, by using molecular simulations, we show that nanomechanical probes, such as nanoindentation, may be utilized toward further manipulation of chemical short-range order, providing explicit validation pathways. By using well established interatomic potentials, we perform hybrid molecular-dynamics-MonteCarlo at room temperature to demonstrate that particular dwell nanoindentation protocols can lead, through thermal MonteCarlo equilibration, to local reorganization under the indenter tip, toward a density-wave stripe pattern. We characterize the novel density-wave structures, which are highly anisotropic and dependent on local, nanoindentation-induced stress concentrations, and we show how they deeply originate from intrinsic features of interelemental interactions. Furthermore, we show that these novel patterns consistently scale with the incipient plastic zone, under the indenter tip, justifying their observation at experimentally feasible nanoindentation depths.

A comparative study on the oxidation behavior and failure mechanisms of conventional NiCoCrAl alloy and in-situ composite AlCoCrFeNi2.1 eutectic high-entropy alloy at 1300 °C

We present a comparative study on oxidation behavior and failure mechanisms of conventional NiCoCrAl alloys doped with Y and Hf (CNA) and in-situ composite AlCoCrFeNi2.1 eutectic high-entropy alloy doped with Y and Hf (ISC-EHEA) at 1300 °C. We demonstrate that the ISC-EHEA has much stronger resistance to surface rumpling and oxide spallation than CNA. Hybrid molecular dynamics (MD) and Monte Carlo (MC) simulations show that the diffusion coefficients of the metal elements in the Al-depletion layer of the ISC-EHEA are 50 % lower than those in the CNA. The low diffusion coefficients lead to low growth stress in the thermally grown Al2O3 scale on the ISC-EHEA and improve the creep resistance of the metal in contact with the scale, thus preventing the occurrence of rumpling. The oxidation rate constant of the ISC-EHEA is ∼32 % lower in comparison to those of the CNA, which is attributed to the coarser columnar Al2O3 grains that effectively mitigate grain boundary diffusion. The rumpling-free metal/oxide interface, the low residual stress and the low oxide growth rate for the ISC-EHEA result in strong resistance to scale spallation. Owing to the strong scale/alloy bonding at the interface and the build-up of strain energy during prolonged oxidation, damage in the Al2O3 scale on the ISC-EHEA is initiated as surface cracks rather than interface decohesion. When re-oxidized at 1300 °C, the ingress of oxygen along the surface cracks results in fast growth of new oxides at the metal/oxide interface, which causes local stress concentration, interface crack propagation and scale spallation.

Preparation and effect of vanadium addition on the mechanical properties of CoCrFeNiVx high-entropy alloy

High-entropy alloys (HEAs) have attracted considerable attention due to their unique phase structures and excellent properties. However, the existing preparation processes for HEAs involve high temperatures, lengthy procedures, and high costs, which limit their practical applications and confine them to the realm of fundamental research. Here, a novel method combining low-temperature solid-phase molten salt electrolysis and hot-press sintering was proposed to prepare CoCrFeNi HEA. Moreover, the study explored the addition of metal V, to modulate the composition of the HEA and improve its mechanical properties. The results showed that the molten salt electrolysis process could achieve the direct deoxidation of metal oxide mixtures, enabling one-step preparation of HEAs. During the early stages of deoxidation, the metals Ni and Co were obtained first and, under the influence of the mixed entropy, formed a non-stoichiometric NiCoCr alloy. As deoxidation progressed, Cr and Fe were successively reduced and further alloyed with the NiCoCr to form the non-stoichiometric CoCrFeNi HEA. Eventually, the aggregated dendritic metal Fe diffused and solid-solved into the non-stoichiometric CoCrFeNi HEA, resulting in the formation of a stoichiometric CoCrFeNi HEA. The CoCrFeNiVx (x = 0, 0.5, 1, 1.5) alloy maintained a single-phase FCC structure, where V effectively improved the hardness of the CoCrFeNi HEA, with minimal impact on its phase structure. V played a role in solid-solution strengthening, grain refinement, and reducing Cr segregation in the alloy. Additionally, the appropriate addition of V simultaneously increased the tensile strength, hardness, and ductility of the CoCrFeNi HEA.

Mechanical Characteristics and Corrosion Behaviour of NiCoCr Medium-Entropy Alloys

In this study, equiatomic NiCoCr medium-entropy alloys (MEAs) were prepared for the experiments using the arc melting method. The cooling rate and annealing effect were analyzed on the microstructure and mechanical properties of the NiCoCr MEAs. The crystal structure, composition changes, surface morphology, and internal defects were explored by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The mechanical characteristics like elasticity modulus, hardness, and cyclic normal compressive loading were determined by a universal testing machine, Vickers hardness tester, and nanoindenter. The results show that the faster the cooling rate, the smaller the grains produced in the sample; otherwise, the larger the grains produced. The AFM results showed that the average surface roughness of the flat area was about 7.23 and 3.73 nm for the low and high cooling rates, respectively. The XRD results showed that the grain sizes determined for the NiCoCr MEAs at annealing temperatures of 300, 450, 600, and 750°C corresponded to 33.68, 18.10, 16.23, and 21.98 nm, respectively. The compression test results showed that the NiCoCr MEAs could withstand an average maximum stress of 45.01 MPa. The average elasticity modulus of the four compression tests was determined to be 2692.33 MPa.The corrosion behavior of the NiCoCr alloy was investigated using electrochemical tests in a NaCl solution.

Separation of static and dynamic displacements in the CrMnFeCoNi high entropy alloy

Assessing the local lattice strains in high-entropy alloys (HEAs) is essential if their mechanical properties are to be rationalised and the validity of the highly distorted lattice hypothesis is to be determined. To accomplish this, direct measurements of local distortions need to be made, and the thermal component separated. In this study, variable temperature neutron total scattering measurements were made on the exemplar HEA CrMnFeCoNi, along with pure nickel and an alloy of Ni-37.5Co-25Cr(at.%). A number of methods of determining local atomic displacements from such measurements were used and their efficacy for determining local lattice strains discussed. Using these methods, the local lattice strains have been effectively quantified. The data suggest the HEA does have a comparatively higher strain than the NiCoCr alloy, however, the alloying strains are small in comparison to the thermal component in all cases. As such, whether the local lattice strains are significant and can class the alloy as highly strained remains inconclusive.

Defect-induced inhomogeneous atomic environments in complex concentrated alloys

The multiple elements in equal ratios in complex concentrated alloys (CCAs) favor the formation of local atomic environments like segregation or chemical ordering observed in recent experiments. Such special local atomic environments may pin dislocations on the slip plane, resulting in a higher flow stress for the escape of dislocations and hence a dynamic strain aging (DSA) effect in the form of stress drop. Here, Monte Carlo molecular dynamics (MC/MD) simulations are used to elucidate the atomic atmospheres around dislocations and stacking faults in the NiCoCr alloy system and quantitatively determine the stress drop effect. Increased chemical ordering and segregation around stacking faults and dislocation cores are observed, but detailed analysis shows that atomic segregation around stacking faults (Suzuki atmospheres) contributes the most to stress drop, compared with segregation around dislocation cores (Cottrell atmospheres) and enhanced ordering (Fisher) effects. The MD simulated stress drops are in good agreement with theoretically predicted and experimental values. This work fills an important gap in the understanding of the DSA effect in H/MEAs.

Equilibrium versus non-equilibrium stacking fault widths in NiCoCr

First principles calculations in the NiCoCr medium-entropy alloy predict a negative stacking fault energy (SFE) at T=0 K, implying an infinite Shockley partial dissociation distance. Many experiments at room temperature (RT) show however a finite dissociation. This discrepancy has been suggested due to solute strengthening that prevents the partial separation. Here, atomistic simulations in a model NiCoCr alloy having a negative SFE show that solute strengthening can limit partial separation at T=0 K but, the solute-induced barriers are easily overcome at RT and time scales of only 1 ns. Under experimental conditions (time scales of hundreds of seconds and longer), solute pinning is therefore insufficient to limit dissociation. Finite partial separations are thus presumably due to a positive stacking fault free energy at RT or short-range-order effects. It is argued here that the former is more likely than the latter.

Process Map Definition for Laser Metal Deposition of VDM Alloy 780 on the 316L Substrate

VDM Alloy 780 is a novel Ni-based superalloy that allows for approximately 50 °C higher operating temperatures, compared to Inconel 718, without a significant decrease in mechanical properties. The age hardenable NiCoCr Alloy combines increased temperature strength with oxidation resistance, as well as improved microstructural stability due to γ′-precipitation. These advantages make it suitable for wear- and corrosion-resistant coatings that can be used in high temperature applications. However, VDM Alloy 780 has not yet been sufficiently investigated for laser metal deposition applications. A design of experiments with single tracks on 316L specimens was carried out to evaluate the influence of the process parameters on clad quality. Subsequently, the quality of the clads was evaluated by means of destructive and non-destructive testing methods, in order to verify the suitability of VDM Alloy 780 for laser metal deposition applications. The single-track experiments provide a basis for coating or additive manufacturing applications. For conveying the results, scatter plots with regression lines are presented, which illustrate the influence of specific energy density on the resulting porosity, dilution, powder efficiency, aspect ratio, width and height. Finally, the clad quality, in terms of porosity, is visualized by two process maps with different mass per unit lengths.

Dislocation plasticity in equiatomic NiCoCr alloys: Effect of short-range order

Equiatomic NiCoCr solid solutions have been shown by recent experiments and atomistic simulations to display exceptional mechanical properties that have been suggested to be linked to nanostructural short-range order (SRO) features that may arise from thermal treatments, such as annealing or/and aging. Here we use hybrid Monte Carlo--molecular dynamics simulations to gain further insights of thermal effects on the SRO formation as well as the edge dislocation plasticity mechanisms of equiatomic NiCoCr face-centered cubic solid solution. For that purpose, we utilize two well-known NiCoCr interatomic potentials, one of which displays well-documented SRO, believed to be linked to experimental evidence and labeled as the Li-Sheng-Ma potential, while the other (Farkas-Caro) does not. We use these two potentials to discern short-range ordering (from inherent randomness in random solid solutions) and understand how SROs influence dislocation depinning dynamics in various thermal annealing scenarios. In this context, we used robust, scale-dependent metrics to infer a characteristic SRO size in the Li-Sheng-Ma case by probing local concentration fluctuations which otherwise indicate uncorrelated patterns in the Farkas-Caro case in a close agreement with random alloys. Our Voronoi-based analysis shows meaningful variations of local misfit properties owing to the presence of SROs. Using relevant order parameters, we also report on the drastic increase of chemical ordering within the stacking fault region. More importantly, we find that the Li-Sheng-Ma potential leads to excellent edge dislocation depinning strength with low stacking fault width. Our findings indicate an enhanced roughening mechanism due to the SROs-misfit synergy that leads to significant improvements in dislocation glide resistance. We argue that the improvements in alloy strength have their atomistic origins in the interplay between nanoscopic SROs and atomic-level misfit properties.

Recent progress on understanding the temperature-dependent irradiation resistance ranking among NiFe, NiCoCr, and NiCoFeCr alloys: A review

Recent progress on understanding the temperature-dependent irradiation resistance ranking among NiFe, NiCoCr, and NiCoFeCr alloys: A review

(Digital Presentation) Industrial Transition and Methods to Apply Metallic Alloys and Composites By Electrodeposition

Faraday Technology Inc. is a research, development and engineering firm developing electrochemical innovations based has pulse/pulse reverse electrolytic principles [1]. With that founding basis Faraday has been able to bridge the gap between academic understanding and small scale demonstration to commercialization of practical components over 30 years of operation.This presentation will discuss a broad range of alloy and composite deposition programs that have been carried out at Faraday. These include functionally graded NiMo, CeO2 inclusion in NiMo, CrAlY inclusions in NiCo, NiCoCr alloy deposition, and so on. We intend the presentation to give a perspective on the challenges associated with transitioning electrodeposition technologies from coupons to components. We also want to demonstrate the potential to achieve success during these challenging transitions by tuning the electrochemical device and method. Finally, we will try to identify similarities and differences in the observed challenges and ways that many could be overcome either by electrochemical engineering, process developments, or program planning.

Effects of V addition on the mechanical properties at elevated temperatures in a γ''-strengthened NiCoCr-based multi-component alloy

NiCoCr-based multi-component alloys have drawn much attention due to their exceptional ductility and strain hardening capacity. However, insufficient strength-ductility synergy of NiCoCr alloy has always been an issue that prevents it from extensive applications. According to our previous research, the precipitation of γ'' phase can significantly improve the strength-ductility synergy of this alloy system at room temperature. In this study, the effects of V addition on γ'' phase stability and high-temperature mechanical properties have been explicitly investigated. The results indicate that V addition can stabilize the metastable γ'' phase in this alloy system and prevent it from transforming into a stable δ phase at grain boundaries upon 650 °C aging, resulting in improved mechanical properties at elevated temperatures. The specific strength of γ''-strengthened multi-component NiCoCr-based alloy can reach up to 86.2 MPa g −1 cm −3 at 650 °C, which is higher than those of Ni-based superalloys, IN 939 and Waspaloy. This work provides theoretical guidance for the novel design of γ''-strengthened alloy for high-temperature applications.

Surface energies and relaxation of NiCoCr and NiFeX (X = Cu, Co or Cr) equiatomic multiprincipal element alloys from first principles calculations

First principles calculations of the energies and relaxation of unreconstructed low-index surfaces, i.e. (001), (011) and (111) surfaces, in NiCoCr and NiFeX (X = Cu, Co or Cr) equiatomic multi-principal element alloys (MPEAs) are presented. The calculations were conducted for 12-layer slabs represented by special quasi-random supercells using the projector augmented wave method within the generalized gradient approximation. While experimental predictions are unavailable for comparison, the calculated surface energies agree fairly well with those from thermodynamic modeling and a bond-cutting model. In addition, the calculations unveil an important surface structure, namely, that the topmost surface layer is in contraction except for the (001) surface of NiFeCr alloy, the next layer below is in extension, and the bulk spacing is gradually recovered from the subsequent layers down. Additionally, the surface contraction is the most pronounced on the (011) plane, being about 4%–10% relative to the bulk spacings. The results presented here can provide an understanding of surface-controlled phenomena such as corrosion, catalytic activities and fracture properties in these equiatomic MPEAs.

High radiation tolerance of an ultrastrong nanostructured NiCoCr alloy with stable dispersed nanooxides and fine grain structure

The present paper reports the irradiation behavior of nanostructured NiCoCr medium entropy alloy enhanced by Y-Hf-O nanooxides and fine grains produced by powder metallurgy . Ion beam irradiation to a peak dose of 130 dpa at 580°C produced neither detectable void swelling nor irradiation-induced hardening, in contrast to significant void swelling and property degradation of single phase NiCoCr concentrated solid-solution alloy under the similar irradiation condition. The dispersed nanooxides and high density of grain boundaries act as defect sinks that effectively annihilate irradiation-induced interstitials and vacancies. Furthermore, high-density nanooxides maintain their crystalline structures and stabilize the grain boundaries during irradiation.

Interdiffusion behaviors between NiCrFe alloy and low-/medium-/high-entropy alloys

Six groups of fcc solid/solid diffusion couples, including Ni-Cr-Fe/ low-entropy (pure Ni, and NiCo, NiCr, and NiCoCr) alloys, Ni-Cr-Fe/ medium-entropy (CoCrNi) alloys, and Ni-Cr-Fe/ high-entropy (CoCrFeNi) alloys, were prepared, and their composition profiles were experimentally measured. Concentration-dependent interdiffusion and tracer diffusion coefficients in fcc Ni-Co-Cr-Fe system at 1473 K were retrieved with the aid of High-throughput Determination of Interdiffusion Coefficients (HitDIC) software and their reliability was validated. In most cases, D˜CrCrNi>D˜FeFeNi>D˜CoCoNi. The comprehensive comparison between the interdiffusivities/tracer diffusivities in the NiCo, NiCr, NiCoCr low-entropy, CoCrNi medium-entropy, and CoCrFeNi high-entropy alloys and those reported data available in the literatures indicates that the interdiffusivities Co, Cr, and Fe in low-entropy alloys are the largest than those in the other two types of alloys. While the tracer diffusivity in Co-Cr-Fe-Ni HEAs may have no sluggish phenomenon when the absolute temperature is used.

Atomistic interpretation of extra temperature and strain-rate sensitivity of heterogeneous dislocation nucleation in a multi-principal-element alloy

Incipient plasticity of crystals is usually initiated from versatile pre-existing crystalline defects of different geometries and length scales that formed during materials fabrication and processing, which is closely associated with the mechanical properties. Here we study the heterogeneous dislocation nucleation behaviors from an existing nanoscale void embedded in a prototypical multi-principal-element (MPE) NiCoCr alloy via atomistical exploration of possible minimum energy pathways and analyzes of the dislocation kinetics by a continuum-level mechanistic model, in comparison with the same plastic mechanism in an elemental face-centered cubic copper. It is found that the rough nucleation pathway of dislocation brings about extra thermal softening and strain-rate sensitivity for the critical nucleation stress of dislocation in the MPE alloy, which reproduces the experimental observations. The extra temperature softening is revealed to be associated with the high total entropy of the MPE alloy, in which the configurational entropy plays a dominating role rather than the vibrational counterpart. The extra strain rate sensitivity is caused by the relatively small activation volume in MPE alloy. Our strategy offers physical understanding of the experimental phenomena with atomistic details, which also emphasizes the critical role of configurational disorder in the dislocation accommodated plasticity in the generic complex concentrated alloys.

Physical properties of face-centered cubic structured high-entropy alloys: Effects of NiCo, NiFe, and NiCoFe alloying with Mn, Cr, and Pd

This paper reports a comprehensive study of electrical and thermal transport properties of a series of face-centered cubic structured high-entropy alloys by alloying Mn, Cr, and Pd elements in NiCo, NiFe, and NiCoFe alloys. X-ray diffraction revealed a single-phase Cu-type cubic structure, and scanning electron microscopy displayed elongated grained microstructures in all alloys. Like NiCo, NiFe, and NiCoFe alloys, the alloys containing Cr/Mn/Pd exhibit metallic behavior; however, their electrical transport properties, such as residual resistivity, residual resistivity ratio, and temperature coefficient of resistivity, vary significantly due to the increase of chemical disorder and defects. The analysis of resistivity of these alloys further showed different scattering mechanisms at low temperatures. Interestingly, the electrical resistivity of NiCoCr, NiCoFeCr, and NiCoFeMn alloys is nearly linear at low temperatures, most likely related to the Mott-Ioffe-Regel limit. Additionally, the NiCoMnCr and NiCoFeMnCr alloys exhibit a minimum in resistivity at low temperatures, which can be explained by the weak localization effect. The Seebeck coefficient measurements reveal that the charge carrier for thermoelectric transport in NiCo, NiFe, and NiCoFe is changed from electrons to holes with Mn alloying. In contrast, a sign reversal of the charge carriers observed in the Cr-containing alloys is connected to the compensation of electron and hole carriers. Furthermore, the NiCoCr, NiCoFeCr, NiCoMnCr, and NiCoFeMnCr alloys show a negative phonon drag effect at low temperatures due to electron-phonon interaction. The measured thermal conductivity behaves similarly in all alloys, except for a considerable reduction in magnitude in Cr/Mn/Pd-containing alloys. This is attributed to a significant decrease of electronic thermal conductivity due to an increased electron scattering by disorders and lattice distortions and a substantial modification of band structure. There is almost an equal contribution of electronic and lattice to the total thermal conductivity in Cr/Mn/Pd-containing alloys, suggesting a semimetallic nature. The temperature dependence of lattice thermal conductivity of these alloys is described by different phonon scattering mechanisms.