CrCoNi Medium-entropy Alloy Research Articles

Strengthening mechanism of CrCoNi medium-entropy alloy from the partially recrystallized structure to the fully recrystallized heterogeneous structure

Vast amounts of strengthening methods towards the single phase face-centered cubic CrCoNi medium-entropy alloy have been carried out in recent years. In this study, a partially recrystallized structure, as well as the fully recrystallized heterogeneous structure, were introduced in CrCoNi medium-entropy alloy through cold-rolling and subsequent annealing that purposely deployed under gradient temperatures. The annealed specimens were subjected to tensile tests, from which detailed parameters of the mechanical properties were derived. Strengthening from grain boundaries, dislocations, solid solutions are quantified, suggesting the existence of other strengthening mechanisms. We attribute the increased strength to the appearance of extensive shear bands, nanograins, nanotwins, HCP lamellas. In addition, the dual-heterogenous structure is believed to contribute to similar ultimate tensile strengths among the annealed specimens. More importantly, dislocations, stacking faults, twins appeared in this alloy as the deformation modes, contribute to the extraordinary fracture elongation.

Ultra-strong and thermally stable nanocrystalline CrCoNi alloy

Grain refinement to the nanocrystalline regime is the most effective way to strengthen materials but this often deteriorates the grain-size thermal stability and plasticity. Here we manufactured a nanocrystalline face centred cubic CrCoNi medium entropy alloy with columnar grains via magnetron sputtering. Compression of CrCoNi pillars with diameters of ∼ 1 µm revealed a record high yield strength of ∼5 GPa for pillars with face centred cubic structures and engineering plastic strain of > 30%. The alloy possessed an outstanding grain-size thermal stability even at 1073 K. Both nanocrystalline grain size and a high density of nanotwins/stacking faults are critical to the exceptional yield strength. Deformation twinning, grains refinement during deformation, grain boundary sliding and random grain orientation all contribute to the large plasticity. The outstanding thermal stability is attributed to the sluggish diffusion effect and the low energy of twin boundaries.

Dynamically compressive behaviors and plastic mechanisms of a CrCoNi medium entropy alloy at various temperatures

Dynamically compressive behaviors and plastic mechanisms of a CrCoNi medium entropy alloy at various temperatures

High strength-ductility synergy in a laser welded dissimilar joint of CrCoNi medium-entropy alloy and stainless steel

CrCoNi medium-entropy alloy (MEA) and its welded joints has great potential applications in engineering structures since they present superior ultimate tensile strength (UTS) and ductility. However, the mechanical performance of the welded joints of CrCoNi and traditional metal alloys remains mysterious. Here, we applied laser welding to join CrCoNi and stainless steel (SS) 301L to achieve a dissimilar MEA-SS301L joint and investigated its microstructure evolution and mechanical properties. The fusion zone becomes a face-centered cubic single-phase non-equiatomic FeCrCoNi high-entropy alloy (HEA). Its UTS and ductility is 482 MPa and 46% at 300K, respectively; and is respectively enhanced to 791 MPa and 59% at 77K. Our dissimilar joint displays a strength-ductility synergy that exceeds that of all other dissimilar HEA joints and most similar HEA joints. Electron backscattered diffraction, transmission electron microscopy and molecular dynamics simulation uncover that extensive dislocation activities, stacking fault networks, deformation twins, and interactivity between dislocations and twin boundaries are the origin of this high strength-ductility synergy in our joint.

Effects of cold-rolling and subsequent annealing on the nano-mechanical and creep behaviors of CrCoNi medium-entropy alloy

The influences of thermomechanical treatment on the microstructural evolution and nano-mechanical behaviors of CrCoNi medium-entropy alloy (MEA) were systematically studied through a series of nanoindentation experiments. Cold-rolling induced substantial grain refinement and during subsequent annealing, the CrCoNi MEA exhibited a highly temperature-dependent recrystallized microstructure characterized by significant variations in grain size, dislocation density, and twin fraction with increasing annealing temperature. The hardness of the cold-rolled sample increased with annealing temperature, reaching its highest value at 600 °C and then decreasing gradually. Annealing was also found to influence the creep behaviors of the CrCoNi MEAs. A cold-rolled sample annealed at 700 °C exhibited the highest resistance to creep. The results of a detailed microstructure examination, calculations, and theoretical analysis revealed that annealing twins play a key role in increasing both the strength and the creep resistance of the CrCoNi MEAs. Short-range order clusters also contributed to the strength and creep resistance.

Effect of Al Addition on the Microstructure and Mechanical Properties of AlxCrCoNi Medium Entropy Alloys Prepared via the Magnetron Co‐Sputtering

The effect of Al addition on the microstructural evolution, hardness, and deformation behavior of a nanostructured CrCoNi medium entropy alloy prepared by co‐sputtering is studied in detail. With an increase in the Al content, the hardness of the alloy first increased (reaching 13.8 GPa) and then decreased (10.7 GPa), while the microstructure changed from nanocrystalline to a nanocrystalline/amorphous dual‐phase structure. During plastic deformation, the appropriate amounts of the amorphous region and nanocrystalline mixed structure prevent shearing and grain rotation. Moreover, the amorphous shell also acts as a dislocation source that emitted and absorbed dislocations constantly and prevented grain boundary sliding to some extent. This work provides a novel approach to improve the mechanical properties of nanostructured medium entropy alloy films by controlling their composition and microstructure.

Deformation characteristics of nanolayered dual-phase CrCoNi medium-entropy alloy nanowires

Due to the potential excellent mechanical properties of multicomponent alloys, the design of their microstructures and deformation mechanisms have attracted considerable attention. In this work, using molecular dynamics simulations, the mechanical behaviors of nanolayered face-centered cube/hexagonal close-packed (FCC/HCP) dual-phase CrCoNi medium-entropy alloys (MEAs) were investigated. The mechanical behaviors of the nanolayered dual-phase nanowires with various radii were studied and the effects of nanolayered dual-phase structure were evaluated. It was found that there are two different deformation modes mainly related to the dislocation nucleation position in the FCC phase or the HCP phase, of which in Model I dislocations nucleate from the intersection of phase boundary and surface, while in Model II dislocations nucleate on the surface of HCP phase. The deformation mode and strength change periodically with the variation of the surface disorder degree against radius. Annealing in the temperature range of 600–800 K can optimize the dual-phase structure, change the deformation mode and improve the mechanical performance.

Effect of low temperature rolling on mechanical properties and corrosion resistance of CrCoNi medium entropy alloy

For the casted CrCoNi medium entropy alloy melted by magnetic levitation, the deformation was carried out in 4 passes under the condition of liquid nitrogen temperature (−196 °C), the total reduction was 50%, The microstructure and properties were analyzed after liquid nitrogen low temperature rolled. The experimental data showed that the phase structure of the alloy was not changed under the low-temperature rolling. The grains, crushed and refined, were elongated along the rolling direction. With the increasing of passes and reduction, the tensile strength increased from 585 MPa to 1359 MPa, while the elongation decreased from 37.8% to 5.9%. work With the increasing of work hardening, the tensile strength of the CrCoNi medium entropy alloy gradually increased, while the plasticity dramatic decreased. At the same time, the corrosion resistance of the CrCoNi medium entropy alloy was improved by low temperature cold rolled. The corrosion resistance of as-cast and rolled CrCoNi was much better than 304 stainless steel.

Chemical Domain Structure and its Formation Kinetics in CrCoNi Medium-Entropy Alloy

Chemical Domain Structure and its Formation Kinetics in CrCoNi Medium-Entropy Alloy

Study on Dynamic Tensile Mechanical Behavior and Deformation Mechanisms of CrCoNi Medium Entropy Alloy at Room and Cryogenic Temperature

Study on Dynamic Tensile Mechanical Behavior and Deformation Mechanisms of CrCoNi Medium Entropy Alloy at Room and Cryogenic Temperature

High Ductility Crconi Medium Entropy Alloy Prepared by Liquid Nitrogen Temperature Rolling and Short Time Annealing at Moderate Temperature

High Ductility Crconi Medium Entropy Alloy Prepared by Liquid Nitrogen Temperature Rolling and Short Time Annealing at Moderate Temperature

An Atomic Insight into the Stoichiometry Effect on the Tribological Behaviors of Crconi Medium-Entropy Alloy

An Atomic Insight into the Stoichiometry Effect on the Tribological Behaviors of Crconi Medium-Entropy Alloy

High Strength and Ductility of an Additively Manufactured Crconi Medium-Entropy Alloy Achieved by Minor Mo Doping

High Strength and Ductility of an Additively Manufactured Crconi Medium-Entropy Alloy Achieved by Minor Mo Doping

Strengthening and strain hardening mechanisms in precipitation-hardened CrCoNi medium entropy alloys

The CrCoNiSixAlx (x = 0.1, 0.2) medium entropy alloys (MEAs) were designed to achieve significant improvement in strength and strain hardening via the minor addition of Si and Al elements to the CrCoNi MEA. The B2 + σ precipitation structure was formed on the CrCoNiSi0.2Al0.2 MEA. Compared to CrCoNi MEA, the yield and ultimate tensile strengths of the CrCoNiSi0.2Al0.2 MEA enhanced from 380 MPa to 850 MPa and 790–1287 MPa, respectively, while the uniform elongation was 23±5% that outperformed various advanced steels and Al-added MEAs/HEAs. The Orowan+shearing precipitation hardening mechanisms together with the deformation twins in a face-centered cubic (FCC) matrix were identified to improve the strength and strain hardening capacity. These findings provide guidance for the development of precipitation-hardened MEAs/HEAs with strength-ductility synergy.

Atomic-scale evidence of chemical short-range order in CrCoNi medium-entropy alloy

High (or medium)-entropy alloys (H/MEAs) are complex concentrated solid solutions that may develop chemical short-range order (CSRO). In this regard, CrCoNi, the prototypical face-centered-cubic MEA, has recently kindled a debate in the H/MEA community, as it is uncertain if CSRO can possibly form in such a multi-principal-element solution, where no equilibrium or metastable intermetallic compounds have ever been seen or predicted. To answer this challenging question, here we present firm experimental evidence for the CSRO from electron diffraction as well as atomic-resolution chemical mapping, under an appropriate zone axis. We also develop a methodology to reliably determine the locations of atomic columns from the line scan profiles in the chemical maps, as well as a quantitative covariance-based correlation analysis of the column chemical compositions to reveal the spatial correlations between various atomic pairs. The detailed chemical information affirms the tendency for like-pair avoidance and unlike-pair preference, specifies the preferred atomic packing and plane stacking by the three constituent species, and suggests a proposed atomic configuration that constitutes the CSRO motif. The fraction of CSRO regions is moderately lowered after either plastic deformation or high-temperature heating. A comparison is also made with previous attempts to identify CSROs in H/MEAs.

Surface hardening of high- and medium-entropy alloys by mechanical attrition at room and cryogenic temperatures

Surface hardening by mechanical attrition at room and cryogenic temperatures has been studied on CrMnFeCoNi high- and CrCoNi medium-entropy alloys. The hardness gradient is produced by severe plastic deformation through dislocation slip and mechanical twinning, finally leading to an ultrafine or even nanocrystalline structure. The hardness of samples after surface attrition at cryogenic temperature is slightly less than that of samples deformed at room temperature. A small amount of deformation-induced martensitic transformation is only observed in a narrow surface layer of the medium-entropy alloy severely deformed under cryogenic conditions. Reasons for these observations are discussed.

Experimental investigation of friction behaviors for CrCoNi medium entropy alloy in reciprocating sliding

Medium/high entropy alloys have some excellent properties, such as high strength, high plasticity, high toughness, high wear resistance, etc., which are better than those of traditional metal alloys. The mechanical properties of medium/high entropy alloys have always been the focus of academic research, but research on their interface mechanical behaviors is still missing. In this paper, we have designed a fretting friction and wear testing system, whose accuracy has been verified. A series of friction experiments have been carried out on the medium entropy alloy–steel friction pair. The friction responses have been investigated, and the factors affecting the friction coefficient have been revealed. Results show that the friction coefficient increases with the increase in the maximum horizontal displacement and decreases with the increase in the normal pressure. The sliding velocity and the normal load together affect the friction coefficient between the friction pairs. Compared with the commonly used titanium alloy and nickel alloy, the friction coefficient between the medium entropy alloy–steel friction pair is smaller during the fretting cycle process. Despite the experimental results, a theoretical model is also proposed to qualitatively explain the corresponding experimental phenomena. Our research sheds light on revealing the interfacial mechanical properties of the medium entropy alloy and provides a basis for tribological design and its internal mechanism.

Exceptional high-strain-rate tensile mechanical properties in a CrCoNi medium-entropy alloy

Alloys with combined outstanding strength and excellent ductility are highly desirable for many structural applications. However, alloys subjected to deformation at very high strain rates and/or cryogenic temperatures usually suffer from very limited ductility. Here, we demonstrate that a bulk CrCoNi medium-entropy alloy presents exceptional combination of high strength and excellent ductility during deformation at high strain rates over a wide temperature range. Full tensile stress-strain curves at a high strain rate of 2000 s−1 and temperatures down to 77 K were successfully obtained using an electromagnetic Hopkinson tension bar system attached with a cooling device, revealing high true ultimate tensile strength (σUTS,T) of 1.8 GPa and true strain of ∼54% at σUTS,T. These outstanding mechanical properties were mainly attributed to profuse deformation twinning. Both high strain rate and cryogenic temperature promoted deformation twinning. Grain refinement caused by deformation twinning, dislocation slip and dynamic recrystallisation added to work hardening and the excellent tensile strain.

Martensitic transformation in CrCoNi medium-entropy alloy at cryogenic temperature

We investigated the in situ deformation behavior of the CrCoNi medium-entropy alloy at a cryogenic temperature of 140 K and compared it with deformation at room temperature. The sample exhibited higher strength and larger ductility at the cryogenic temperature. The CrCoNi alloy remained single-phase face-centered cubic at room temperature, while deformation at 140 K resulted in a martensitic transformation to the hexagonal close-packed structure. The phase transformation, an additional deformation mechanism to stacking faults, twinning, and dislocation slip, resulted in a higher work hardening at cryogenic temperature. The study addresses the structure metastability in the CrCoNi alloy, which led to the formation of epsilon-martensite from the intrinsic stacking faults.

Hot Deformation and Workability of a CrCoNi Medium Entropy Alloy

Hot Deformation and Workability of a CrCoNi Medium Entropy Alloy