CoCrNi Alloy Research Articles

Microstructure and mechanical properties of 9 % nickel steel welded joints using a CoCrNi filler wire for cryogenic storage tanks

A medium-entropy alloy of CoCrNi exhibits excellent cryogenic mechanical properties. It is possible to enhance the ductility of weld metal for cryogenic storage tank materials at cryogenic temperatures by employing CoCrNi alloys as filler metal. In this study, a CoCrNi multi-principal filler wire and a 308L stainless steel wire were used for welding 9 % Ni steel. The effects of the CoCrNi filler wire on the microstructure, hardness, mechanical properties, and deformation mechanisms of the weld metals were evaluated and discussed. A remarkable finding was that as the tensile temperature decreased from 298 K to 77 K, the fracture elongation of the CoCrNi joint increased by approximately 34.7 %, rather than decreasing. In contrast, a significant reduction in fracture elongation was observed in the 308L joint. The CoCrNi filler wire could promote the activation of twinning deformation in the weld metal at 77 K, thereby enhancing the ductility of the welded joints at cryogenic temperatures.

Effect of grain size on the deformation mechanism and fracture behavior of a non-equiatomic CoCrNi alloy with low stacking fault energy

Manipulation of stacking fault energy (SFE) plays a significant role in microstructure control and in turn mechanical properties of advanced alloys. In this work, we present the influence of grain size on the mechanical properties and fracture behavior of a non-equiatomic CoCrNi alloy with low SFE. Specimens with controlled grain sizes ranging from 0.61 to 6.4 µm were fabricated through rolling and annealing. A novel SFs-dominated plastic deformation mechanism was discovered. Tensile strength decreases monotonically with increasing grain size, while ductility achieves a peak value at the medium grain size, contradicting with the typical behavior observed in most single-phase face-centered cubic (FCC) metallic materials deformed primarily by dislocation slips and/or twinning. The fracture behavior changes from void coalescence to quasi cleavage with grain coarsening, and the fracture mechanisms were analyzed. Additionally, the evolution of SFs and phase transformation is explored at various deformation strains.

Comprehensive analysis of ordering in CoCrNi and CrNi2 alloys

Chemical Short-Range Order (CSRO) has attracted recent attention from many researchers, creating intense debates about its impact on material properties. The challenges lie in confirming and quantifying CSRO, as its detection proves exceptionally demanding, contributing to conflicting data in the literature regarding its true effects on mechanical properties. Our work uses high-precision calorimetric data to unambiguously prove the existence and, coupled with atomistic simulations, quantify the type of CSRO. This methodology allows us to propose a mechanism for its formation and destruction based on the heat evolution during thermal analysis and facilitates a precise identification of local ordering in CoCrNi alloys. Samples of CoCrNi (Co33Cr33Ni33) and CrNi2 (Cr33Ni66) alloys are fabricated in varying ordered states, extensively characterized via synchrotron X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. Samples with considerably different ordered states are submitted to tensile tests with in-situ synchrotron X-ray diffraction. We demonstrate, despite inducing varied CSRO levels in CoCrNi, no significant alterations in overall mechanical behavior emerge. However, the CrNi2 alloy, which undergoes long-range ordering, experiences significant shifts in yield strength, ultimate tensile stress and ductility.

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.

Optimized control of laser processing parameter on microstructural evolution and mechanical behavior in CoCrNi medium entropy alloy

In this study, the processing parameters of a laser powder bed fusion (LPBF)-fabricated CoCrNi alloy were systematically investigated for their impact on mechanical properties, microstructure, and fracture characteristics when the relative density exceeds 99%. The maximum relative density of 99.89% was attained at a laser power of 225 W, and scan speed of 1000 mm/s, accompanied by a hardness value of 276.83 HV. However, it is important to note that the highest relative density did not correspond to the maximum σUTS. Tensile testing revealed that increasing the laser power at the same scan speeds led to a decrease in elongation. The LPBF-fabricated CoCrNi alloy exhibited enhanced defect tolerance due to its cellular microstructure, which confined ductile dimple sizes to the nanoscale and impeded crack propagation through pinning effects. At scan speeds of 1000 mm/s and energy densities below 83 J/mm³, an increased presence of unmelted powder and pores was observed. Besides, it was confirmed that the <110> crystallographic texture displayed the strongest orientation within the sample. Theoretical calculations indicated that over 50% of the yield strength is attributed to the dislocation structure. These findings contribute to the process optimization of LPBF, providing a valuable reference for adjusting process parameters to enhance material mechanical properties, particularly when targeting relative densities above 99%.

Laser powder bed fusion of CoCrNi composite with high fraction of TiC: The microstructure and mechanical properties at different temperatures

At present, high fraction nano-TiC containing CoCrNi-4wt%TiC composite has been obtained through laser powder bed fusion (LPBF) of the blends of elemental powders and nano-TiC particles. Due to the addition of high fraction nano-TiC particles, there is a significant improvement in the strength of the composite to 1886 MPa at 77K, 1456 MPa at 298K, 528 MPa at 1073K, 300 MPa at 1173K, respectively. Characterized by high scanning speed and high laser power, the laser scan strategy combining is effective in preventing the nano-TiC particles from agglomeration, and the nano-carbides show a heterogeneous distribution with a higher number density and a finer particle size at the bottom of melting pool, the mechanism of which is discussed. The second-phase strengthening of the high-density nano-carbides combining dislocation strengthening or grain boundary strengthening raises the yielding strength of the composite compared to CoCrNi alloy at cryogenic or elevating temperature, respectively. Combining with the deformation-induced nano stack faults/twins plasticity and heterogeneous distribution induced (HDI) strengthening, the addition of high-density nano-carbides contributes to the high tensile strength and good plasticity of the composite at the temperature in a wide range.

Quantifying short-range order using atom probe tomography

Medium- and high-entropy alloys are an emerging class of materials that can exhibit outstanding combinations of strength and ductility for engineering applications. Computational simulations have suggested the presence of short-range order (SRO) in these alloys, and recent experimental evidence is also beginning to emerge. Unfortunately, the difficulty in quantifying the SRO under different heat treatment conditions has generated much debate on the atomic preferencing and implications of SRO on mechanical properties. Here we develop an approach to measure SRO using atom probe tomography. This method balances the limitations of atom probe tomography with the threshold values of SRO to map the regimes where the required atomistic neighbourhood information is preserved and where it is not. We demonstrate the method with a case study of the CoCrNi alloy and use this to monitor SRO changes induced by heat treatments. These species-specific SRO measurements enable the generation of computational simulations of atomic neighbourhood models that are equivalent to the experiment and can contribute to the further understanding and design of medium- and high-entropy alloys and other materials systems where SRO may occur.

On the Origin of Recovery‐Induced Strengthening in CoCrNi Alloy

Strengthening during low‐temperature annealing is a common observation in several nanocrystalline and ultrafine‐grained materials. The origin of such strengthening is generally attributed to various recovery processes leading to restricted dislocation motion. Herein, the observations on recovery‐induced strengthening in cold‐rolled (CR) equiatomic CoCrNi alloys are presented. An increase in hardness is observed for samples deformed to thickness reduction of 35–80% after 2 h of annealing at 500 °C. A detailed microstructural investigation of 80% CR and subsequently annealed samples suggests that both dislocation density and local chemical composition play important role in strengthening. Further, uniaxial tensile tests elucidate the importance of mobile dislocations on strength and ductility of annealed samples.

Critical temperature-dependent shear band formation in CoCrNi alloy under high-temperature dynamic compression

In numerous engineering applications, the occurrence of highly rapid loading conditions, coupled with significant temperature rise, is prevalent. Under these harsh conditions, shear localization is a prominent material failure mechanism, which may result in catastrophic failure. Generally, materials possessing higher strength, limited work hardening rate, and lower thermal conductivity are more prone to mechanical instability. However, in the current study, a cryo-rolled CoCrNi alloy, characterized by much higher strength and limited work hardening, shows remarkable room and high-temperature mechanical performance under impact loading except at one temperature domain∼600 °C where this material displays mechanical instability by exhibiting adiabetic shearband. To shed light and understand this abnormal mechanical response, an in-depth microstructural study revealed that the ASB formation is closely linked to the recrystallization behavior of this alloy that is invariantly related to the stored energy of cold work, test temperature, and strain rate. Uniquely, the amplified nano-twinning activity at high strain rate and critical temperature promoted the fragmentation of grains and facilitated recrystallization, respectively. Once the recrystallization is completed, the alloy regains work hardening ability even at very high test temperatures, reaching 900 °C due to the deformation of the newly recrystallized matrix. The present work is valuable in delineating the alloy's application domain for rigorous impact and crash-worthiness applications, as there exists a dearth of literature on this material for dynamic scenarios.

Compositional regulation in additive manufacturing of precipitation-hardening (CoCrNi)94Ti3Al3 medium-entropy superalloy: Cellular structure stabilization and strength enhancement

High or medium-entropy alloys that feature high thermal stability and excellent oxidation resistance are promising candidates for elevated temperature applications. The rapid softening of monolithic high or medium-entropy alloys with single face-centered cubic structure at elevated temperatures, however, is a main weakness. In this paper, we report new high strength γ′-hardened ((CoCrNi)94Ti3Al3)98Nb2 medium-entropy alloy through laser powder-bed fusion (L-PBF) followed by ageing. In particularly, the tensile strengths of the aged ((CoCrNi)94Ti3Al3)98Nb2 alloy at 20 °C and 700 °C can reach up to 1.93 GPa and 1.11 GPa, respectively, 112 % and 122 % stronger than the as-built CoCrNi alloy tested at the same condition. A new strengthening mechanism, i.e., elemental segregation induced the cellular structure stabilization, in tandem with other hierarchical microstructure features, including ultrafine γ′ precipitates, dense twin boundaries, and other types of crystallized defects, co-contribute to the superb tensile strength at room and elevated temperatures. Such a simple alloy design and processing strategy outlines a guideline for designing novel multicomponent alloys and/or composites with superior microstructural stability and mechanical response at room and elevated temperatures.

Achieving high ductility at medium temperature in a nonequiatomic CoCrNi alloy

The tensile strength and ductility of face-centered cubic (FCC) multi-principal element alloys (MPEAs) typically decline monotonically with increasing temperature. In this work, we conducted tensile tests at different temperatures and characterized deformation microstructures for a nonequiatomic CoCrNi alloy with low stacking fault energy (SFE). Exceptional ductility is obtained at medium temperature, breaking the general sense of monotonic change with temperature. The effect of deformation mechanisms of dislocations, stacking faults and phase transformation on the ductility was discussed. This work reveals the unique temperature effect on the mechanical properties of metals with very low SFE.

In-situ neutron diffraction study of serration-involved ultra-cryogenic deformation behavior at 15 K

This work focused on the mechanical properties and serration-involved deformation behavior of advanced alloys at 15 K. Evolution of stacking faults and ε-martensite improved the mechanical performance of CoCrNi alloys, and significant strain-induced martensite transformation of DED-SS316L led to superior strength and strain hardening. A magnitude in stress drop was governed by dislocation density, phase type, and lattice defects, irrespective of processing method. FCC {200} notably was influenced recovery behavior after stress drop, and the contribution of strain energy density by serration on tensile toughness was the greatest for HR-CoCrNi.

Microstructure and mechanical properties of multi-principal component CoCrNiMo medium entropy alloys

In traditional high entropy alloy (HEA) systems, achieving a synergistic enhancement of alloy strength and ductility remains a challenge. However, dual-phase high entropy alloys, with their simultaneous high strength and high ductility, have shown potential for industrial applications. In this study, three multi-principal element CoCrNiMo alloys were synthesized by adding varying amounts of Mo to the base medium entropy Co45Cr27Ni28 alloy, which possesses a single face-centered cubic (FCC) phase. The impact of variations in Mo content on the alloy's microstructure and mechanical properties was investigated. Experimental data revealed that all three alloys formed dual-phase (FCC + μ) precipitation-strengthened medium entropy alloys (MEAs), where the μ phase is enriched with Mo, and its quantity increases with the increase in Mo content. With the rise in Mo content, the alloy's hardness increased from 240 ± 20 HV to 668 ± 35 HV, and tensile strength also increased, albeit at the cost of reduced plasticity. The single-phase medium entropy Co45Cr27Ni28 alloy exhibited a tensile strength and elongation rate of 591 ± 15 MPa and 44 ± 3%, respectively. The tensile strength of the dual-phase CoCrNiMo HEAs could be elevated to 857 ± 16 MPa and 1045 ± 25 MPa, while maintaining considerable elongation rates of 16 ± 2% and 5 ± 1%, respectively. This work provides a pathway for strengthening CoCrNi alloys and contributes positively to the development of high-performance HEAs.

Effect of initial twins on mechanical properties of additive manufacturing CoCrNi alloy and molecular dynamic simulation

The influence of twins on laser additive CoCrNi MEA was analyzed by experiments and molecular dynamic simulation. Results showed that the traditional laser additive CoCrNi MEA was prone to slip in the (111) direction. EBSD analysis showed an obvious 60°(111} axial angle pair relationship, which is related to the slower cooling rate. The results of molecular dynamic simulation showed that the yield strength of CoCrNi MEA increases first and then decreases, and the material has a critical twin density. When the twin density is less than the critical twin density, the strength of the material increases due to the thinning effect of the twin boundary on the grain. When the twin density is greater than the boundary twin density, the nucleation and proliferation of the twin boundary and the junction of the two become the dominant factors of the material deformation. When the twin density is far from the critical value, the twin spacing becomes smaller, the dislocation sources increase, the dislocation nucleation and proliferation increase, and the strength of the material decreases.

Wear-resistant CoCrNi multi-principal element alloy at cryogenic temperature

Traditional high strength engineering alloys suffer from serious surface brittleness and inferior wear performance when servicing under sliding contact at cryogenic temperature. Here, we report that the recently emerging CoCrNi multi-principal element alloy defies this trend and presents dramatically enhanced wear resistance when temperature decreases from 273 to 153 K, surpassing those of cryogenic austenitic steels. The temperature-dependent structure characteristics and deformation mechanisms influencing the cryogenic wear resistance of CoCrNi are clarified through microscopic observation and atomistic simulation. It is found that sliding-induced subsurface structures show distinct scenarios at different deformation temperatures. At cryogenic condition, significant grain refinement and a deep plastic zone give rise to an extended microstructural gradient below the surface, which can accommodate massive sliding deformation, in direct contrast to the strain localization and delamination at 273 K. Meanwhile, the temperature-dependent cryogenic deformation mechanisms (stacking fault networks and phase transformation) also provide additional strengthening and toughening of the subsurface material. These features make the CoCrNi alloy particularly wear resistant at cryogenic conditions and an excellent candidate for safety–critical applications.

Nano-precipitation strengthening regulated by nanotwins in CoCrNi alloy with super-high strength

The number and size of precipitates directly influence the strengthening of the high-temperature alloys. As special coherent interfaces, nano-twinned (NT) boundaries are pre-set in CoCrNi-based superalloy to regulate the distribution of nano-precipitates for further strengthening the mechanical property. High density of NTs with twin spacings of 2–200 nm is produced in CoCrNi alloy by surface mechanical rolling treatment (SMRT), and L12 (Ni3Ti)-type precipitates are induced by the low-energy NT boundaries, as well as grain boundaries, after aging at 700 °C for 4 h. The CoCrNi alloy with L12 nano-precipitates exhibits a super-high mechanical property, characterized by 2019 MPa of a yield strength (σy) and 2300 MPa of an ultimate tensile strength (σu), much higher than the CoCrNi alloy counterpart of 840 (σy) and 1100 (σu) MPa, respectively. Meanwhile, the treated CoCrNi alloy also delivers a reasonable elongation of 7.0%. The super-high strength is mainly ascribed to the high density L12 nanoprecipitates grown on the NT boundaries, while the elongation is sustained by the NTs boundaries.

Influence of microstructure of CoCrNi medium entropy alloy on its corrosion behavior

Purpose The purpose of this paper is to reveal the effect of microstructure on the corrosion behavior of CoCrNi alloy in 3.5 Wt.% NaCl solution. Design/methodology/approach The as-cast CoCrNi alloy was prepared by arc melting, and the cold-rolled and annealed alloys were prepared by processing the as-cast alloy. Findings The experimental results showed that a protective passivation film was formed on the surfaces of these CoCrNi MEA, and the stability and compactness of alloys increased in the sequence of cold-rolled, as-cast and annealed CoCrNi alloys. The annealed CoCrNi alloys had the best pitting resistance. Originality/value This study proposes the effect of the microstructure of CoCrNi alloy on corrosion resistance.

Achieving Excellent Strength-Ductility Balance in Single-Phase CoCrNiV Multi-Principal Element Alloy.

CoCrNi alloys exhibit excellent strength and ductility. In this work, the CoCrNiV multi-principal alloy with single-phase fine grained (FG) structure was prepared by rolling and heat treatment. The characteristics of deformation microstructures and mechanical properties were systematically investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicate that the CoCrNiV alloy successfully attains a yield strength of 1060 MPa while maintaining a uniform elongation of 24.1%. The enhanced strength originates from FG structure and severe lattice distortion induced by V addition. Meanwhile, the exceptional ductility arises from the stable strain-hardening ability facilitated by dislocations and stacking faults. The deformation mechanisms and the optimization strategies for attaining both strength and ductility are thoroughly discussed.

Effects of V Addition on the Deformation Mechanism and Mechanical Properties of Non-Equiatomic CoCrNi Medium-Entropy Alloys.

Equiatomic CoCrNi medium-entropy alloys exhibit superior strength and ductility. In this work, a non-equiatomic CoCrNi alloy with low stacking fault energy was designed, and different fractions of V were added to control the stacking fault energy and lattice distortion. Mechanical properties were evaluated by tensile tests, and deformation microstructures were characterized by transmission electron microscope (TEM). The main deformation mechanisms of CoCrNiV alloy with low V content are dislocation slip, stacking faults, and deformation-induced HCP phase transformation, while the dominant deformation patterns of CoCrNiV alloy with high V contents are dislocation slip and stacking faults. The yield strength increases dramatically when the V content is high, and the strain-hardening behavior changes non-monotonically with increasing the V content. V addition increases the stacking fault energy (SFE) and lattice distortion. The lower strain-hardening rate of 6V alloy than that of 2V alloy is dominated by the SFE. The higher strain-hardening rate of 10V alloy than that of 6V alloy is dominated by the lattice distortion. The effects of V addition on the SFE, lattice distortion, and strain-hardening behavior are discussed.

Achieving ultrahigh strength in oxide-dispersion-strengthened CoCrNi alloy via in situ formation of coherent Y-Ti-O nanoprecipitates

Oxide-dispersion-strengthened CoCrNi alloys were fabricated via in situ oxidation by adding Ti and Y, and non-in-situ oxidation by direct addition of Ti and Y2O3, referring to as Y-ODS and Y2O3-ODS alloys, respectively. Transmission electron microscopy (TEM) and atom probe tomography (APT) characterizations reveal that both alloys consist of an ultrafine-grained face-centered-cubic (fcc) matrix, a high number density of nanoscale Y-Ti-O precipitates and a small number of (Cr0.75Ti0.25)2O3 oxides. However, the nanoscale Y-Ti-O precipitates in the two alloys show distinct phase and microstructure. The Y2O3-ODS alloy contains only incoherent orthorhombic Y2TiO5 nanoprecipitates, but the Y-ODS alloy also contains a high density of fully coherent pyrochlore Y2Ti2O7 nanoprecipitates. The Y-ODS alloy achieves an ultrahigh yield strength of 1660 MPa, which is 320 MPa higher than that of the Y2O3-ODS one, but maintains the same ductility. Quantitative analysis of the strengthening mechanism indicates that such large difference in strength is mainly attributed to the presence of coherent Y2Ti2O7 nanoprecipitates in Y-ODS alloy. This study should provide significant insight into the design of ODS high/medium-entropy alloys via in situ oxidation during mechanical alloying and consolidation.