Single-phase Fcc Research Articles

Exceptional strength-ductility synergy in additively manufactured (CoCrNi)90Al5Ti5 medium-entropy alloy by heat treatment

Powder Plasma Arc Additive Manufacturing (PPA-AM) technique has tremendous potential for the practical application in medium-entropy alloys (MEAs). In this study, we investigated the effect of heat treatment on microstructural evolution, mechanical properties, and the work hardening behavior in the PPA-AM processed (CoCrNi)90Al5Ti5 MEA. The results show that the As-built specimen is a single-phase FCC structure, displaying <001> strong FCC texture in the deposition direction. The stacking faults (SFs) and dislocations were observed in the alloy, which formed the stacking fault networks and the Lomer-Cottrell locks structure. The plasticity of the alloy increased significantly after the high-temperature heat treatment, which can be attributed to the modifications of the microstructure, such as the weakening of the texture strength and the reduction of the stresses. Low-temperature heat treatment decreases the density of dislocations in the alloy, but promotes the generation of the co-lattice phase. The evolution of the dislocation density, texture strength, and precipitates significantly influenced the strain hardening behavior and mechanical properties. The tensile results showed that the strength and plasticity of the samples were increased after two-step heat treatment. The yield strength, ultimate tensile strength, and elongation to failure were 733 MPa, 1080 MPa, and 22.3%, respectively, which were 16.2%, 20.3%, and 15.5% higher than the As-built samples. This work elucidates the underlying mechanisms of heat treatment on the microstructure and mechanical properties of the alloy.

An investigation to microstructural and texture evolution during the cold rolling of Al0.3CoCrFeNi high entropy alloy

In the present study, Al0.3CoCrFeNi high entropy alloy (HEA) was subjected to thickness reductions through cold rolling process in order to thoroughly investigate the evolution of microstructure and deformation texture. Important aspects of deformed microstructures such as the formation of twinning bundles and shear bands in the specimens were monitored. The evolution of texture during the cold rolling process was measured by X-Ray Diffraction (XRD) and Electron Back Scatter Diffraction (EBSD) technique which revealed the formation of Copper type texture at the initial stages of rolling due to slip deformation mechanism. Originated from twinning and followed by shear banding, the texture was subsequently developed toward predominantly Brass type. Microstructural features and texture evolution behavior of the investigated single phase FCC alloy was attributed to its low SFE level. Visco Plastic Self Consistent (VPSC) technique was coupled in the research to simulate the material behavior and to examine the contribution of slip and twinning systems. Benefiting VPSC justifications, the texture transition from Copper type to Brass type was related to lower CRSS value of slip in comparison to twinning at the small strains which was then grows faster at the later stages of rolling facilitating the onset of twinning and providing the texture transition basement.

Crevice Corrosion Simulation of Single-Phase FCC Co–Cr–Fe–Ni–V High Entropy Alloy

Crevice Corrosion Simulation of Single-Phase FCC Co–Cr–Fe–Ni–V High Entropy Alloy

Composition design of gold-platinum alloy with ultra-low magnetic susceptibility: First-principles and experimental validation

Gold-platinum (Au-Pt) alloy has aroused considerable attention due to its ultra-low magnetic susceptibility (MS) in testing mass (TM) on spacecraft. However, the effect of Au content on the properties of the alloy has not yet been understood. In this study, the composition design of Au-Pt alloy with ultra-low MS was achieved through density functional theory (DFT) and experimental methods. The elastic, thermal properties and electronic structure were systematically investigated, the composition range was further optimized and Au75Pt25 was determined to be the most suitable alloy for TM material. The phase composition of this alloy after cold rolling and solid solution was characterized, indicating a single-phase FCC structure. In addition, there is a good validation between the experimental Vickers hardness and the DFT results. This work provides new insights into the compositional optimization of Au-Pt alloys and lays the foundation for alloy development.

High corrosion resistance of nanograined and nanotwinned Al0.1CoCrFeNi high-entropy alloy processed by high-pressure torsion

This study examines the corrosion resistance of ultrafine-grained Al0.1CoCrFeNi high-entropy alloy processed by high-pressure torsion (HPT), focusing on stored energy, diffusion, and the high-entropy effect. Microstructural analysis revealed a single-phase FCC structure with numerous defects and reduced crystallite size following the HPT process. Samples with higher HPT turns formed high-angle grain boundaries, equiaxed nanograins, and numerous nanotwins, contributing to a notable increase in Vickers microhardness from 159 Hv for the initial sample to 550 Hv after 5 turns of HPT. Corrosion behavior is found to be influenced by stored energy and chromium diffusion. Low HPT turns increased the corrosion rate due to sluggish diffusion of chromium and high stored energy in the form of dislocations. Conversely, corrosion resistance improved with increased HPT turns due to fast diffusion through non-equilibrium grain boundaries and the formation of nanotwins with lower stored energy. These findings suggest that a relatively low entropy plays a critical role in the chromium segregation from the solid solution for forming a passive film and enhancing corrosion resistance.

High-temperature tribological properties of (TiZrNbMoTa)N and (TiZrNbMoTa)CN ceramic coatings

As dry-cutting tool coatings, high-entropy ceramics (HECs) exhibit excellent hardness, toughness, and wear resistance properties. However, the hardening mechanism of HEC coatings and their mechanical and frictional properties under high-temperature conditions have not been studied. In this study, we successfully prepared (TiZrNbMoTa)N high-entropy nitride (HEN) and (TiZrNbMoTa)CN high-entropy carbon nitride (HECN) coatings using reactive magnetron sputtering technology. The strengthening mechanism and thermodynamic properties of HEN and HECN coatings were systematically studied using the Debye-Grüneisen quasi-harmonic approximation model combined with density functional theory (DFT) calculations. At room temperature (RT, 25 °C) and high temperature (HT, 500 °C), the microhardness of HECN is 62 % and 56 % higher than that of HEN, respectively. Especially at RT, the microhardness of HECN reaches 46.37 GPa. Through in-situ XRD, in-situ nanoindentation, and high-temperature friction and wear analysis, we conducted in-depth research on the phase stability, high-temperature mechanical properties, and high-temperature tribological properties of HECs coatings in high-temperature. XRD test results show that HEN and HECN coatings exhibit a single-phase FCC structure and maintain good stability under HT. DFT calculation results show that the M − C bond in HECN is stronger than the M − N bond in HEN, which is why the HECN coating is harder. At HT, the wear rate of HECN coating is only reduced by 10 % compared with HEN coating. Different from RT, under high-temperature conditions, the primary wear mechanism of the coating is oxidative wear. The coating's red hardness and friction coefficient (COF) have a limited impact on the coating's high-temperature anti-wear performance. This research provides a theory and technical basis for designing and preparing protective coatings with high hardness and low friction coefficient.

Annealing-induced multi-heterogeneous microstructure strengthening in FeCoCrNiMo0.2 high-entropy alloys

In this study, by conducting cryogenic rolling (CR) followed by short-term annealing (CRA), we achieved the annealing-induced hardening in a single-phase FCC FeCoCrNiMo0.2 high-entropy alloy by introducing multiple heterostructures structures. Through this approach, the yield strength, ultimate tensile strength and tensile fracture elongation of CRA alloy were increased from 1116.8 MPa to 1286.1 MPa, 1363.2 MPa to 1576.3 MPa and 5.4–6.9 %, respectively. Furthermore, a broader steady state of strain softening behavior was found in the CRA alloy, which is attributed to the additional strengthening and hardening effects resulting from high density of deformation nanotwins and phase transformations. Additionally, the research results show that the ultra-high strength and sufficient ductility of CRA alloy are attributed to the synergistic effect of multiple mechanisms, such as strengthening induced by heterogeneous microstructure of σ precipitate/HCP phase/FCC matrix phase, and high work hardening ability caused by the heterogeneity of high density deformation nanotwins, geometrically necessary dislocations (GND), stacking faults (SFs) and Lomer-Cottrell locks (L-C locks) nano-scale defects. This work provides useful insights for alloys that involves multiple heterogeneous microstructures strengthening and provides a promising approach for the design and manufacture of high-strength structural materials.

Microstructural evolution in doped high entropy alloys NiCoFeCr-3X (X=Pd/Al/Cu) under irradiation

Commonly studied equatomic single-phase FCC high entropy alloys based on 3d transition metals like NiCoFeCr do not provide adequate strength and radiation resistance at high doses for nuclear structural applications. In the current study, the major alloying effects like lattice distortion, ordering and clustering tendencies were investigated by adding low concentration of Pd, Al, or Cu respectively to study the doping effects on the ion irradiation response of NiCoFeCr alloy. The alloys were irradiated with 3 MeV Ni2+ ions at 500 °C to a fluence of 1 × 1017/cm2 at a beam flux of approximately 2.8 × 1012 ions/cm2/s. The microstructural evolution upon irradiation i.e., formation of dislocation networks, radiation induced segregation and precipitation, and void formation were studied in detail. Post-irradiation characterization results showed that a Pd addition leads to a high void nucleation rate but controlled void growth, which may be attributed to increased lattice distortion. In Al added HEA, our microstructural analysis indicates that radiation induced ordered L12 precipitates do not affect void swelling significantly. Cu addition led to Cu precipitation that drastically suppressed dislocation density and void swelling of the alloy. Additionally, a model was developed to qualitatively describe the trend in void swelling of typical FCC alloys under ion irradiation. This model was able to qualitatively explain the suppression and reappearance of void swelling in ion irradiated alloys that generally occurs near the region with peak implanted ion concentration.

AlxCoCrCuFeNi (X = 0, 0.8) high entropy alloys fabricated by laser powder bed fusion

ABSTRACT The AlxCoCrCuFeNi (x = 0, 0.8) high entropy alloys (HEAs) were prepared by laser powder bed fusion (LPBF), and the effects of volumetric energy density (VED) and heat treatment on microstructures and mechanical properties of HEAs were investigated. The results show that the CoCrCuFeNi HEA has a single FCC phase structure and exhibits good ductility. Its yield strength is relatively low, being 520 MPa. After the alloy composition was adjusted from CoCrCuFeNi to Al0.8CoCrCuFeNi, the HEA changed from FCC single-phase to FCC + BCC dual-phase structure and the hardness and compressive properties were significantly improved. Among them, the Al0.8CoCrCuFeNi HEA prepared with the VED of 167 J/mm3 has the best mechanical properties, with a compressive yield strength of 652 MPa and good plasticity. After annealing at 600°C and 900°C for 1 h, the mechanical properties of HEAs are significantly improved. In particular, the microhardness and compressive yield strength of the Al0.8CoCrCuFeNi HEA annealed at 600°C for 1 h are 378 HV and 810 MPa, respectively. The coordinated deformation between FCC and BCC domains, back stress strengthening, solid solution strengthening and precipitation strengthening after heat treatment make the Al0.8CoCrCuFeNi HEA achieve a balance of high strength and good ductility.

Enhancing the high-temperature oxidation performance of AlCrTiSiN coating by in-situ generation of multi-layer antioxidant composite structures

A novel AlCrTiSiN high-entropy coating with superior thermal stability and high-temperature oxidation resistance, has been produced. The coating is characterized by unique fcc single-phase solid solution structure with uniform composition, severe lattice distortion and low fcc-AlN solubility, which results in the superior thermal stability. The excellent high-temperature oxidation resistance of coating is attributed to the multi-layer composite structures generated by in-situ reaction of element selective diffusion during the oxidation process. The inner dense SiO2-typed amorphous sub-layer and the intermediate (Al,Cr)2O3-typed sub-layer effectively block the O inward diffusion, and thus enhance the high-temperature oxidation resistance of the coating.

Laser Powder Bed Fusion Processing of Low Cost CoCrFeNiMoxNby High Entropy Alloys with Promising High-Temperature Properties via In Situ Alloying Commercial Powders

A blend of only commercial powders, including Ni625, CoCrF75, and 316L, were used as the raw material for fabricating non-equiatomic CoCrFeNiMoxNby high entropy alloys (HEAs) through laser powder bed fusion (PBF-LB/M) via in situ alloying, instead of using pure elemental powders, thus reducing the raw materials cost. The rapid cooling inherent in the PBF-LB/M process facilitated the dissolution of Mo and Nb, resulting in a single FCC phase characterized by high relative densities. High-temperature tensile tests were conducted at room temperature, 700 °C, 800 °C, and 900 °C, revealing mechanical properties that surpassed those reported in existing HEA literature. The remarkable strength of the HEAs developed in this study primarily stemmed from the incorporation of Mo and Nb, leading to the precipitation of Mo and Nb-rich lave phases at elevated temperatures. While constraining elongation when confined to grain boundaries, these precipitates enhanced strength without compromising elongation when distributed throughout the matrix. This work is a feasibility study to explore the usage of commodity compositions from the market to develop HEAs using PBF-LB/M, which opens the possibility of using scraps to further the development of new materials. Consequently, this study presents a rapid and cost-effective approach for HEA development, improving efficiency and sidestepping the direct utilization of critical raw metals for sustainable manufacturing. Moreover, this work also underscores the outstanding mechanical performance of these HEAs at high temperatures, paving the way for the design of innovative alloys for future high-temperature applications.

Recrystallized hard zone and resultant tri-modal microstructure produces superior mechanical properties in a single-phase heterostructured high-entropy alloy

Heterostructured high-entropy alloys (HS-HEAs) integrate two innovative concepts in metallic materials that encompass alloy design and microstructural engineering. HS-HEAs have demonstrated outstanding mechanical properties. However, it remains challenging to explore these materials due to the intricate nature of their composition and microstructure. In this study, we introduced heterostructures into a single-phase FCC HEA through cold rolling and partial recrystallization. This process resulted in an exceptionally high density of nanoscale recrystallization twins, leading to significant hardening and the formation of heterogeneous hard zones in recrystallized regions with small grain sizes. This tri-modal heterostructure results in improved balance between strength and ductility, high zone boundary density and strong hetero-deformation induced (HDI) effects, and a unique two-stage strain partitioning mechanism that postpones necking. As a result, the alloy exhibits enhanced strength and ductility, resulting in superior mechanical properties. The identification of the recrystallized hard zones is crucial for gaining a proper understanding of the hetero-zones in HEAs and provides valuable insights for the future design of HS-HEAs.

Annealing-induced hardening in single-phase FCC, equiatomic CrCoNi and non-equiatomic CrCoNiMo medium-entropy alloys

A two-step processing route including cold working (primary hardening) followed by annealing-induced (secondary) hardening is proposed to strengthen single-phase face-centered cubic CrCoNi-based medium-entropy alloys (MEAs). Here, we investigate two compositions (in at.%): Cr33·3Co33·3Ni33.3 and Cr27·3Co33·3Ni33·3Mo6. The former has received considerable interest and is now regarded as a model MEA. The latter was designed to investigate how replacing 6 at.% Cr with Mo affects mechanical properties and annealing-induced hardening. It is worth mentioning that this latter composition is similar to that of the industrial MP35 N alloy (Cr22·9Co35·4Ni35·5Mo6.2), which is well-known to exhibit a strong annealing-induced hardening, but Cr27·3Co33·3Ni33·3Mo6 is outside its specification. In MP35 N, it has long been thought that Mo segregation at planar defects is responsible for annealing-induced hardening. In the present work, we show that the strength of both Cr33·3Co33·3Ni33.3 and Cr27·3Co33·3Ni33·3Mo6 can be increased by ∼25% after post-deformation annealing, thus demonstrating that Mo is not essential to cause annealing-induced hardening. Either another element, which could be Cr as revealed by atom probe tomography, is able to segregate at planar defects and induce annealing-induced hardening, or different mechanisms produce a similar strengthening effect. Based on these results, we discuss different models that can be used to quantify annealing-induced hardening in CrCoNi-based MEAs.

The influence of microstructural evolutions on electrochemical corrosion and passive behavior in precipitation-strengthened high-entropy alloys

Effects of microstructural evolutions on electrochemical corrosion properties and passive film characteristics of precipitation-strengthened (Ni2Co2FeCr)92Al4Nb4 high-entropy alloys (HEAs) were investigated in a saline environment. Compared with single-phase FCC alloys, the appearance of nano-sized L12 particles increases the metastable pitting activities and slightly reduces the pitting potential, while the lamellar D019 phase further deteriorates the localized corrosion resistance. The chemical composition and oxide state of passive films are closely related to the different precipitation scenarios. The corrosion morphologies of the single-phase and L12-strengthened alloys are dominated by pitting, whereas the intergranular corrosion (IGC) and intragranular dealloying-like corrosion occur in the D019-strengthened alloy.

Achieving ultra-strong and ductile CoNi-based FCC multi-principal element alloys via alloying with refractory Mo and W

FCC single-phase multi-principal element alloys (MPEAs) often suffer from insufficient strength, which limits their wide applications. Here a CoNi-based FCC MPEA (Co44Ni44Mo9W3, at. %) was designed by alloying with refractory Mo and W. The Co44Ni44Mo9W3 alloy possesses an ultrahigh yield strength of ∼1050 MPa and good ductility of ∼30 % at ambient temperature. The excellent mechanical properties were correlated to its superior lattice friction stress and grain boundary strengthening compared to most reported FCC MPEAs and conventional alloys. By density-functional theory calculations, such extraordinary mechanical properties were further attributed to its high lattice distortion, low intrinsic stacking fault energy and high unstable stacking fault energy due to the addition of Mo and W. Transmission electron microscopy investigations revealed that the high strength-ductility combination arises from the synergistic effect of planar slip bands, stacking faults and deformation nano-twins.

Grain-selective growth makes Al0.1CoCrFeNi high entropy alloys strong and ultra-ductile

The trade-off between strength and ductility of structural materials has always been a concern, and the pursuit of higher strength often comes with the inevitable sacrifice of ductility, which poses challenges for the design and manufacture of high-performance engineering materials. In this study, we introduce a novel concept called grain-selective growth (GSG) to address the strength-ductility trade-off by effectively triggering the strain hardening capacity of the single-phase FCC HEAs. Specifically, we prepared an Al0.1CoCrFeNi high-entropy alloy with a heterogeneous grain structure using cyclic rolling and annealing. By employing this approach, our GSG HEA achieves an ultra-ductility with ∼53% uniform elongation and an ultimate tensile strength of 1165 MPa. Rapidly accumulating geometrically necessary dislocations in the domain boundary region, as revealed through electron backscatter diffraction analysis, have led to the development of high-level heterogeneous deformation-induced (HDI) stress. In addition, we also observed synergistic strain hardening effects generated by various deformation mechanisms, such as dislocation pile-ups, stacking faults (SFs), deformation twinning, dislocation walls, and SF networks. This study provides valuable insights for the manufacturing of high-performance heterostructure materials, making it possible to develop advanced alloys with higher strength and ductility, suitable for a wide range of engineering applications.

Mechanical properties of CoCrFeNiV high-entropy alloys with different Cu element contents under different strain rates

In this study, the alloys used contained Co, Cr, Fe, Ni, and V as the primary elements, while 6%–10% Cu was used as the added element. The high-entropy alloys (HEAs) were made in an electric-arc vacuum furnace from which specimens in the form of cylinders with a 5-mm diameter and 5-mm length were fabricated. An XRD analysis revealed that alloys with 6% and 8% Cu had a single-phase FCC structure, and a weak BCC peak only appeared in the (110) direction in alloys with 10% Cu. A universal testing machine was used for testing at the strain rates of [Formula: see text][Formula: see text]s[Formula: see text], [Formula: see text][Formula: see text]s[Formula: see text], and [Formula: see text][Formula: see text]s[Formula: see text], and the high strain rate was tested using a split Hopkinson pressure bar (SHPB) under strain at 3000[Formula: see text]s[Formula: see text], 4000[Formula: see text]s[Formula: see text], and 5000[Formula: see text]s[Formula: see text], respectively. The specimens did not fracture during the quasi-static compression and dynamic impact tests, which displayed better ductility. The surface of the three alloys had extensive dendrites and non-equiaxed crystal structures. In an EDS composition analysis, the Cr and V elements were more frequently observed in the intergranular region than in the interdendritic region.

Strength-ductility balance of AlCoCrFeNi2.1 eutectic high-entropy alloy via additive manufacturing

AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA), with its distinctive heterogeneous structure, not only overcomes the drawback of insufficient strength of FCC single-phase high-entropy alloys (HEAs), but also overcomes the drawback of insufficient ductility of BCC single-phase HEAs, making it a promising candidate for engineering application. In addition, EHEAs possessing excellent castability are considered to be suitable for additive manufacturing (AM). The AlCoCrFeNi2.1 EHEA prepared by direct energy deposition (DED) exhibits a dual-phase structure consisting of FCC(L12) and BCC(B2) phases. With the increase of deposition height, its morphology transforms from columnar and dendrite structures to lamellar structure. Furthermore, the tensile strength and yield strength of the DED-ed EHEA samples increase, while the ductility decreases with the increase of deposition height. Especially, the middle region of the DED-ed samples achieves a good strength-ductility balance. The lamellar structure composed of soft FCC(L12) phase and hard BCC(B2) phase produces a hetero deformation induced (HDI) hardening, which improves the strength of the samples and promotes plastic deformation. The FCC(L12)/BCC(B2) phase interfaces not only hinder the movement of a large number of dislocations and cause dislocation pile-up, but also allow some dislocations to slip across, which helps to plastic co-deformation of two phases in the EHEA samples. This work provides useful guidance for the performance improvement of EHEAs using laser additive manufacturing.

(FeCoNi)75Cu25-xSix high entropy alloys prepared by mechanical alloying and spark plasma sintering: Microstructure, deformation behavior and dynamic recrystallization kinetics

The present research aimed to produce (FeCoNi)75Cu25-xSix (x = 0, 5, 10, 15, 20, 25 at.%) high-entropy alloys (HEAs) using mechanical alloying (MA) and spark plasma sintering (SPS). The HEAs containing 0 to 25 at.% of Si are designated as Si0, Si5, Si10, Si15, Si20, and Si25 alloys. The microstructure, phase evolution, and deformation behavior of the HEAs were investigated before and after SPS. Compression tests were conducted on SPSed alloys at room temperature and 800 °C. XRD examination revealed a single-phase FCC structure in Si0 alloy, with Si5 and Si10 alloys consisting of distinct FCC phases. Si15 alloy contained BCC phase and intermetallic compound Ni74Si26. Si20 alloy had BCC and two intermetallics Ni74Si26 and FeSi, while Si25 alloy consisted of intermetallic Ni31Si12 and BCC phases. For Si0, Si5, Si10, and Si15 alloys, cold pressing resulted in ductile failure; for Si20 and Si25 alloys, it caused brittle failure. The Si20 alloy had the highest strength at 1520 MPa, while the Si25 alloy had the lowest strength at 810 MPa. Dynamic recrystallization (DRX) occurs at 800 °C in all alloys with a peak in flow stress followed by a gradual decrease to a steady state stress. Stress-strain curves at 800 °C were analyzed using DRX kinetics, and the Avrami exponent n was between 1 and 2 for all alloys.

The Effect of Heat Treatment on the Microstructure and Mechanical Properties of Plasma-Cladded CoCrFeNiMn Coatings on Compacted Graphite Iron

CoCrFeNiMn high-entropy alloy coatings were deposited on compacted graphite iron (CGI) by plasma transfer arc cladding to strengthen and improve the wear resistance (performance) of the surface. The effects of different heat treatment processes on the microstructure and mechanical properties of the CoCrFeNiMn coatings were investigated. Compared with the deposited coating, the single FCC phase in the heat-treated coatings was retained, the grain size of the columnar dendrites decreased, the spacing between the dendrites increased, and the Cr-rich precipitated phase in the grain boundary increased. The heat treatment process had a positive influence on the microhardness and wear resistance of the coatings. The microhardness of the coatings increased after heat treatment. After heat treatment at 660 °C for 90 min, the coating had the highest microhardness of 563 ± 6.9 HV0.2, and it had the best wear resistance.