CoCrNi Alloy Research Articles

Temperature dependence of the deformation behavior and mechanical response of CoCrNi medium-entropy alloy: Experiment and simulation

Tailoring the microstructure of metallic materials by controlling the fabrication process is crucial to achieving superior mechanical properties. Herein, the mechanism of modifying the microstructure and mechanical properties of medium-entropy CoCrNi alloys was investigated by equal-channel angular pressing (ECAP) at room temperature (RT) and cryogenic temperature (CT) and post-deformation annealing (PDA) and was analyzed by molecular dynamics simulations. It was found that temperature influences the deformation mechanism of the alloy, dislocations, stacking faults, deformation twins, and FCC-HCP phase transitions, which assume different roles at different temperatures. The dislocation density of the ECAP-CT sample is significantly higher than that of the ECAP-RT sample; however, the ECAP-RT sample exhibits unique deformation features of kink bands and hierarchical twin structure, resulting in dynamic grain refinement effects. In addition, the HCP phase strength increment caused by annealing-induced HCP phase transformation is higher in the ECAP-RT sample than in the ECAP-CT sample, thus leading to a substantial increase in strength after PDA. The present study exhibits the strengthening mechanisms and provides a generic approach to obtaining desirable mechanical properties of alloys.

Origins of strengthening and toughening effects in twinned nanocrystalline alloys of low stacking fault energy with heterogeneous grain structure

Low stacking-fault energy (SFE) nanocrystalline alloys with pre-existing nanotwins (PNTs) and heterogeneous grain structure (HGS) exhibit exceptional strength-ductility combinations. This has been mainly ascribed to the multiple concurrent deformation mechanisms promoted by the low SFE, and back stress hardening from the strain gradient imposed by PNTs and HGS via activities of geometric necessary dislocations (GNDs). However, the influence of PNTs on dislocation configurations, and the role of intragranular statistically stored dislocations (SSDs) in the mechanical response are still unclear. Here, we uncover the origins of strengthening and toughening effects in a prototype low SFE multicomponent CoCrNi alloy with PNTs and HGS via atomistic simulations. The results show that HGS leads to significant strain partitioning between the smaller- and larger-grain domains and promotes the strain localization, while PNTs alleviate the strain localization and facilitate homogeneous deformation for better toughness. Moreover, PNTs efficiently block intragranular mobile Shockley partials and enhance the dislocation reaction probability, increasing the densities of sessile SSDs. The strengthening effects from both, dynamically increased sessile structures and plastic resistance induced by localized dislocations, are revealed by quantitative analysis on dislocation distributions. This indicates that the dislocation configurations correlated with strain localization are fortified owing to the presence of PNTs and HGS, which further enhance the flow stress on deformation. The present study demonstrates that, PNTs can enable efficient controlling of dislocation configurations upon deformation, while the strengthening and toughening effects of alloys with heterostructures are achieved by both GNDs correlated with localized strains and SSDs generated via dislocation reactions.

Ultrastrong and ductile (CoCrNi)94Ti3Al3 medium-entropy alloys via introducing multi-scale heterogeneous structures

• A three-level heterogeneous structure was introduced in CoCrNi alloy. • (CoCrNi) 94 Ti 3 Al 3 delivers ultimate tensile strength of 1.6 GPa and strain of 13.1%. • Heterogeneous matrix, γ’ precipitate and defects led to high strength and ductility. The coarsening-grained single-phase face-centered cubic (fcc) medium-entropy alloys (MEAs) normally exhibit insufficient strength for some engineering applications. Here, superior mechanical properties with ultimate tensile strength of 1.6 GPa and fracture strain of 13.1% at ambient temperature have been achieved in a (CoCrNi) 94 Ti 3 Al 3 MEA by carefully architecting the multi-scale heterogeneous structures. Electron microscopy characterization indicates that the superior mechanical properties mainly originated from the favorable heterogeneous fcc matrix (1–40 µm) and coherent spherical γ' precipitate (10–100 nm), together with a high number density of crystalline defects (2–10 nm), including dislocations, small stacking faults, Lomer–Cottrell locks, and ultrafine deformation twins.

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.

Enhanced strength-ductility synergy in a Ta-doped CoCrNi medium-entropy alloy with a dual heterogeneous structure

The CoCrNi medium-entropy alloy (MEA) has attracted much attention owing to its promising comprehensive properties. However, the low yield strength of the CoCrNi alloy at room temperature is a major hurdle in its engineering application. In this pursuit, the present study proposed a strategy for strengthening the CoCrNi MEA by tailoring the heterogeneous grains resulting from heterogeneous precipitates regulated by thermo-mechanical processing. Specifically, a Ta-doped CoCrNi MEA ((CoCrNi)98Ta2) with a dual heterogeneous structure was designed and developed. The resulting (CoCrNi)98Ta2 alloy exhibited a high yield stress of ∼1200 MPa, an ultimate tensile stress of ∼1500 MPa, and possessed a high fracture elongation of ∼33%. The excellent mechanical properties of the (CoCrNi)98Ta2 alloy were attributed to the hetero-deformation induced (HDI) hardening, which played an important role during the deformation. The microstructural origin of the enhanced HDI stress was a result of the dynamically reinforced heterogeneous grain structure through numerous deformation twins.

Exceptional strength-ductility synergy of additively manufactured CoCrNi medium-entropy alloy achieved by lattice defects in heterogeneous microstructures

• Giant strengthening in SLMed CoCrNi alloys by subsequent deformation and annealing. • CoCrNi alloy delivers yield strength of 1161.6 MPa and strain of 31.5%. • The refined hierarchical microstructure and lattice defects result in high strength. The selective laser melting (SLM) with subsequent cold rolling and annealing is used to produce high-density lattice defects and grain refinement in the CoCrNi medium-entropy alloys (MEAs). The superior comprehensive mechanical properties have been achieved in the as-SLMed CoCrNi alloy after rolling and annealing. The as-SLMed alloys delivered the yield strength of 693.4 MPa, the ultimate tensile strength of 912.7 MPa and the fracture strain of 54.4%. After rolling with 70% reduction in thickness and annealing at 700 °C for 2 h. the yield strength, ultimate tensile strength and fracture strain reached 1161.6 MPa, 1390.8 MPa and 31.5%, respectively. The exceptional strength-ductility synergy is mainly attributed to the refined hierarchical microstructures with coarsening grains at a level of 30 µm and ultrafine grains at a level of 1 µm, and the heritage of dislocation-formed sub-grains and other lattice defects. This investigation demonstrates that the SLM with subsequent rolling and annealing is beneficial to fabricate high strength and ductile MEAs with single face-centered cubic (fcc) structure.

Ductile fracture of high entropy alloys: From the design of an experimental campaign to the development of a micromechanics-based modeling framework

Cantor-type high entropy alloys form a new family of metallic alloys characterized by a combination of high strength and high fracture toughness. An experimental study on the CoCrNi alloy is first performed to determine the damage and fracture mechanisms under various stress states. A micromechanics-based ductile fracture model is identified and validated using these experimental data. The model corresponds to a hyperelastic finite strain multi-yield surface constitutive description coupled with multiple nonlocal variables. The yield surfaces consist of three distinct nonlocal solutions corresponding to three different modes of void expansion within an elastoplastic matrix: a void growth mode governed by a Gurson-based yield surface corrected for shear effects, an internal necking-driven coalescence mode governed by an extension of the Thomason yield surface based on the maximum principal stress, and a shear-driven coalescence mode governed by the maximum shear stress. This advanced formulation embedded in a large strain finite element setup captures the effects not only of the stress triaxiality but also of the Lode variable. In particular, the analysis shows that a failure model accounting for these two invariants of the stress tensor captures the fracture in high-entropy alloys over a wide range of conditions.

Strength-ductility synergy of additively manufactured (CoCrNi)87Al13 medium entropy alloy with heterogeneous multiphase microstructure

In this study, a (CoCrNi)87Al13 medium-entropy alloy was fabricated by using laser-based directed energy deposition (l-DED). The yield strength and ultimate tensile strength of the (CoCrNi)87Al13 alloy were improved by 67% and 59%, respectively, compared with those of a l-DEDed CoCrNi alloy, and the ultimate elongation remained at 24%. A heterogeneous multiphase microstructure with multiscale ordered/disordered body-centered cubic phases was obtained by adding Al during the rapid solidification and thermal cycle history of the l-DED. The heterogeneous multiphase microstructure was closely correlated with the mechanical performance improvement, deformation, and fracture mechanisms of the alloys.

Relation of phase fraction to superplastic behavior of multi-principal element alloy with a multi-phase structure

An Al0.3CoCrNi medium-entropy alloy was annealed in two conditions to differentiate the phase fraction of nano-scaled B2 and sigma precipitates in FCC microstructure, processed by high-pressure torsion, and subjected to a superplasticity test. The Al0.3CoCrNi with different microstructure exhibited distinct superplastic performance, with maximum elongation of 1900% and 1735% from tensile deformation at 1073 K at the strain rate at 5 × 10−3 s−1 and 10−2 s−1, respectively. The microstructural analysis revealed that the superplasticity is dominantly governed by the grain boundary sliding mechanism with the accommodation of intragranular dislocations. The Al0.3CoCrNi alloy, after superplastic deformation, consists of the mixture of FCC, B2, and sigma phases with different phase fractions. The comparison of this multi-phase structured alloy at two conditions and the sister alloy, Al0.5CoCrFeMnNi high-entropy alloy, suggests that increment of B2 and sigma phase fraction is beneficial to promote high-strain rate superplasticity by limiting the grain growth.

Superb strength-ductility synergy in a medium-entropy CoCrNi alloy via reinforced TRIP effect

In this study, a thermodynamically metastable medium-entropy Co37Cr34Ni29 (at.%) alloy exhibiting superior strength-elongation synergy at ambient temperature was fabricated successfully via tailoring the processing route. It was discovered that after homogenizing and hot-rolling processes, the microstructure of the alloy was composed of two-type face-centered-cubic (FCC)-structure phases: Co-type FCC structure (metastable) phase and Ni-type FCC structure (stable) phase. During the tension process, the metastable Co-type FCC-structure phase has transformed gradually into Co-type hexagonal close-packed (HCP)-structure phase in a restrained environment characterized by the formation of networks of nano-spacing HCP phase bundle. This kind of transformation could provide intensified transformation-induced plasticity (TRIP) effect during the plastic straining process, thus, the alloy presented superior strength-elongation synergy (UTS × TE > 88 GPa%) compared with other ductile FCC-structure alloys with TRIP effect.

TiC strengthened CoCrNi medium entropy alloy: Dissolution and precipitation of TiC and its effect on microstructure and performance

In order to improve the strength and corrosion resistance of CoCrNi medium entropy alloy, TiC strengthened CoCrNi medium entropy alloy (CoCrNi/(TiC)x (x=0.1, 0.2, 0.4)) was designed by addition of different amounts of TiC. The effects of TiC content on the microstructure, mechanical properties, and corrosion resistance of the alloy were investigated. It was found that the precipitation morphologies of TiC changed from lamellar eutectic to needle structure with the increase of TiC content, and finally formed mixed needled and bulk TiC particles. TiC appears as a dissolution−precipitation phenomenon in the CoCrNi alloy, which is important for the mechanical properties and corrosion resistance of the CoCrNi/(TiC)x alloy. The strength of alloy was enhanced obviously after the addition of TiC. The compressive yield strength of CoCrNi/(TiC)0.4 alloy reached 746 MPa, much larger than that of the CoCrNi medium entropy alloy, 108 MPa. Additionally, the addition of TiC was found to improve the corrosion resistance of CoCrNi medium entropy alloy in the salt solution.

Recent progress in the CoCrNi alloy system

The exceptional mechanical properties, particularly at cryogenic temperatures, of the equiatomic CoCrNi alloy are documented in numerous published studies. Similar to the equiatomic CoCrFeMnNi (so called Cantor alloy), from which the ternary alloy was derived, the CoCrNi ternary possesses low stacking fault energy that promotes complex deformation modes, as well as the activation of deformation twinning at ambient temperatures and increased strain. In addition to outstanding deformation mechanisms, chemical short-range order and face-centered cubic (FCC)-hexagonal close packed (HCP) transitions have been verified in this alloy and prove to be key factors contributing to the alloy's notable properties. The relationship between stacking fault energy and FCC→HCP phase transitions has been developed over the years through other low stacking fault materials, but the question that arises is: do well established physical metallurgical mechanisms require modification when applied to systems such as CoCrNi given their compositional complexity? Local chemical order plays an important role in that it brings the deviation from the random solid solution behavior generally expected from complex concentrated alloys. In this review, the fundamental atomistic deformation mechanisms of the CoCrNi alloy will be reviewed with a focus on deformation substructures and chemical short-range ordering. Recent studies on microstructural engineering through thermo-mechanical processing and efforts to enhance the tensile properties of the CoCrNi derived systems with minor alloying additions are discussed. Finally, future directions of research, which involve applying current understanding of the underlying mechanisms towards alloy design strategies, are discussed.

Low-cycle fatigue behavior and deformation mechanisms of a dual-phase Al0.5CoCrFeMnNi high-entropy alloy

We uncover the low-cycle fatigue response and deformation mechanisms of a dual-phase Al0.5CoCrFeMnNi high-entropy alloy (HEA) by comparing to CoCrFeMnNi HEA. Al0.5CoCrFeMnNi demonstrates higher cyclic stress resistance, meanwhile maintaining comparable cyclic strain resistance at low-to-medium strain amplitudes. Microstructural investigations revealed dislocation slip as the primary deformation mechanism, which changes from planar slip to wavy slip with increasing strain amplitude. The enhanced cyclic stress resistance is related to precipitation hardening and improved solid solution strengthening. Meanwhile, the comparable cyclic strain resistance is ascribed to their similar deformation mode. Lastly, comparisons with Al0.5CoCrFeNi and CoCrNi alloys provide strategies for tailoring HEAs with enhanced fatigue resistance.

Optimizing transformation-induced plasticity in CoCrNi alloys by combined grain refinement and chemical tuning

Grain refinement increases yield strength, but usually at the cost of ductility reduction. In the present work, we explored the strategy of grain refinement and composition tuning in CoCrNi alloys to optimize mechanical properties. Grain refinement was found to strongly suppress deformation-induced martensitic transformation in metastable CoCrNi alloys, which was utilized to modulate the transformation-induced plasticity (TRIP) effect in combination with composition tuning guided by theoretical calculations. We demonstrated in a non-equiatomic CoCrNi TRIP medium-entropy alloy (MEA) that our approach resulted in an excellent combination of strength and ductility. The proposed strategy is expected to be useful in exploring superior mechanical properties of MEAs and high-entropy alloys in varying systems.

Long-term tensile creep behavior of a family of FCC-structured multi-component equiatomic solid solution alloys

Long-term tensile creep experiments were performed at 973 K on a family of multicomponent equiatomic solid solution alloys with face-centered cubic crystal structures, including quinary CoCrFeMnNi alloy and quaternary CoCrFeNi alloy, together with our previous report on ternary CoCrNi alloy. Analyzing the steady-state and transient creep properties and characterizing the precipitate evolution, it is suggested that dislocation creep be the dominant deformation mechanism for all these alloys. Although CoCrNi shows the highest room-temperature strength and the lowest creep rate at 973 K, the creep lifetime data for all three alloys are similar and can be described by the Monkman-Grant relationship.

Influence of Surface Preparation on Cracking Phenomena in TIG-Welded High and Medium Entropy Alloys

Multi-element systems with defined entropy (HEA—high entropy alloy or MEA—medium entropy alloy) are rather new material concepts that are becoming increasingly important in materials research and development. Some HEA systems show significantly improved properties or combinations of properties, e.g., the overcoming of the trade-off between high strength and ductility. Thus, the synthesis, the resulting microstructures, and properties of HEA have been primarily investigated so far. In addition, processing is crucial to achieve a transfer of potential HEA/MEA materials to real applications, e.g., highly stressed components. Since fusion welding is the most important joining process for metals, it is of vital importance to investigate the weldability of these materials. However, this has rarely been the subject of research to date. For that reason, in this work, the weldability depending on the surface preparation of a CoCrFeMnNi HEA and a CoCrNi MEA for TIG welding is investigated. The fusion welding of longer plates is described here for the first time for the CoCrNi alloy. The welds of both materials showed distinct formation of cracks in the heat affected zone (HAZ). Optical and scanning electron microscopy analysis clearly confirmed an intergranular fracture topography. However, based on the results, the crack mechanism cannot be conclusively identified as either a liquid metal embrittlement (LME) or hot cracking-like liquid film separation.

Effect of cold rolling and ageing treatment on the interface morphology and mechanical properties of maraging steel/medium-entropy alloy multilayer composite

The cold rolling and subsequent ageing treatment of hot-rolled 18Ni300/CoCrNi multilayer composites were carried out to analyse the high work hardening ability of medium-entropy alloy (CoCrNi alloy) and the ageing precipitate strengthening effect of maraging steel (18Ni300). The results show that with the rise of cold rolling reduction, the ratio constitute layer and interface transition layer thicknesses are gradually decreased, and the interface shape changes from a flat to a wavy state, which is mainly due to the serious work hardening of the CoCrNi layer. Meanwhile, the tensile strength continuously increased. When the multilayer composite is cold-rolled to 0.5 mm, its tensile strength reaches more than 2 GPa, and the fracture elongation remains at approximately 7%. After ageing, the superior tensile strength is as high as 2825 MPa, which is attributed to the synergistic effect of work hardening, precipitation strengthening and strong interface bonding strengthening.

Alloying effects on phase stability, mechanical properties, and deformation behavior of CoCrNi-based medium-entropy alloys at low temperatures

Alloying plays an important role in determining the phase stability and mechanical behavior of medium/high-entropy alloys (M/HEAs). In this work, the effects of Al, Ti, Mo, and W additions on the phase stability, strengthening behavior, and stacking fault energies of CoCrNi alloys are quantitatively investigated by using first-principles calculations. Our results reveal that the Al, Ti, and W additions enhance the structural stability of metastable face-centered cubic structures, whereas Mo is in favor of the formation of hexagonal close-packed structures at low temperatures. Through analyzing the elastic moduli and lattice mismatch based on a Labusch-type model, we show that the solute strengthening effect decreases in the order W > Mo > Ti > Al. The compositional dependence of intrinsic stacking fault energy (ISFE) and unstable stacking fault energy (USFE) of CoCrNi MEAs was calculated, and the results indicate that the Al, Ti, Mo and W additions significantly reduce the USFE, leading to a reduction in the energy barriers of dislocation slips. The alloying effects on the deformation behaviors of CoCrNi MEAs are discussed in terms of the ratio of ISFE to the energy barrier of dislocation slips. The present study not only sheds light on the fundamental understanding of phase stability and deformation mechanisms of M/HEAs but also provides useful guidelines for the alloy design of advanced M/HEAs with superior mechanical properties.

Passivation behavior of CoCrNiZr medium-entropy alloy in the sulfuric acid solutions

The passivation behavior of a medium-entropy alloy (MEA) CoCrNiZrx (x = 0.1, 0.2 and 0.3) in 0.1 M H2SO4 solution was investigated. The roles of Zr in alteration of microstructure, electrochemical characteristics, and passive film chemistry of CoCrNi alloy were evaluated. With the addition of Zr element, the phase structure changes from a single face-centered cubic (FCC) phase to a two-phase structure, with the appearance of Zr-rich HCP phase. The addition of Zr accelerates the degradation process of MEAs in 0.1 M H2SO4, resulting in the increase of anodic current and carrier density and decrease of the film resistance as determined by the electrochemical impedance spectroscopy and Mott-Schottky results. The preferential dissolution of the Zr-rich phase (high thermodynamic activity), the weak compactness of the Zr oxides, the depletion of the Cr oxides, and the increase in carrier density are responsible for the degradation of the Zr-alloying CoCrNi alloy in 0.1 M H2SO4.

Influence of machining on the surface integrity of high- and medium-entropy alloys

High- and medium-entropy alloys (HEAs) are a quite new class of materials. They have a high potential for applications from low to high temperatures due to the excellent combination of their structural properties. Concerning their application as components; processing properties, such as machinability, have hardly been investigated so far. Hence, machinability analyses with a focus on the influence of the milling process and its basic parameters (cutting speed, feed per cutting edge) on the resulting surface integrity of specimens from an equiatomic high- (CoCrFeMnNi) and a medium- (CoCrNi) entropy alloy have been carried out. A highly innovative milling process with ultrasonic assistance (USAM) was compared to conventional milling processes. Recent studies have shown that USAM has a high potential to significantly reduce the mechanical load on the tool and workpiece surface during milling. In this study, the basic machining and ultrasonic parameters were systematically varied. After machining, the surface integrity of the alloys was analyzed in terms of topography, defects, subsurface damage, and residual stresses. It was observed that USAM reduces the cutting forces and increases the surface integrity in terms of lower tensile residual stresses and defect density near the surfaces for the CoCrFeMnNi alloy. It was shown that the cutting forces and the metallurgical influence in the sub surface region are reduced by increasing the cutting speed and reducing the feed rate per cutting edge. With the CoCrNi alloy, the tool revealed severe wear. As a result, for this alloy no influence of the parameters on the machinability could be determined.