Nb-rich Phase Research Articles

High-temperature oxidation behavior of multi-phase ceramic particles-reinforced TiAl composites with different microstructures

The high-temperature oxidation behaviors of multiphase ceramic particle-reinforced TiAl composites with different microstructures were investigated. The TiAl composites were prepared via spark plasma sintering by introducing 0.5 wt % graphene oxide and 0.3 wt% SiC into Ti-48Al-2Nb-2Cr. The near-γ equiaxed microstructure, dual-phase microstructure, and nearly fully lamellar microstructure were obtained by sintering at 1200, 1250, and 1300 °C, respectively. Moreover, multiphase ceramic particles were uniformly distributed in the TiAl matrix. Cyclic oxidation was conducted at 950 °C in air for 100 h. The TiAl composite with the nearly fully lamellar microstructure exhibited the best oxidation resistance (mass gain of 1.83 mg/cm2) and that with the near-γ microstructure exhibited the worst oxidation resistance (2.47 mg/cm2). The superior oxidation resistance of the TiAl composite with the nearly fully lamellar microstructure is attributed to the uniform distributions of Ti2AlC particles at the α2/γ lamellae interfaces and Ti5Si3 particles at the lamellae colony boundaries, which hinder atomic diffusion. Moreover, the α2/γ lamellae colonies facilitate the formation of Nb-rich and Cr-rich phases at the interface of the oxide layer and substrate, which act as a protective barrier against atomic diffusion.

Low-loss and low-temperature sintering of SrNb2V2O11 with a large positive τf value as an effective temperature compensator

Novel SrNb2V2O11 microwave dielectric was synthesized and proposed as a temperature compensator for Low-temperature cofired ceramics (LTCC)/Ultra-low-temperature cofired ceramics (ULTCC) applications. The X-ray diffractometer (XRD) patterns matched well with the SrNb2V2O11 phase, exhibiting a monoclinic structure and Cc space group over the entire experiment. The presence of the Nb-rich second phase at 850 °C was attributed to the melting of the samples. The microwave dielectric properties were highly correlated with the chemical bond parameters, which were intensively investigated using the P-V-L theory. The specimen sintered at 820 °C reveals dielectric properties, combining an εr of 74.5, Q × f of 13,000 GHz, and τf of 517 ppm/°C. The large positive τf can make the ceramic an effective temperature compensator for LTCC applications. Moreover, the medium-high εr value can also act as a practical material for dielectric resonators used in the base stations, where a stable environment and limited temperature changes are crucial.

Enhancement of mechanical properties of NbCN-Ni cermets via WC addition

In this study, the microstructural and mechanical properties of sintered NbCN-Ni cermets with various WC contents were investigated. The density and grain size of the NbCN-Ni cermets increased with an increase in the sintering temperature from 1420 to 1510 °C, whereas after WC addition, the average grain size of NbCN decreased significantly. The added WC was initially dissolved in the binder phase and then dissolved in NbCN crystals and formed a white W-rich (Nb,W)CN shell phase and gray Nb-rich (Nb,W)CN core phase structure for WC contents up to 10 wt%. Meanwhile, the formation of the shell-core structure enabled the morphology of NbCN grains to gradually change from polygonal to spherical, which can be attributed to the high solubility of WC in the Ni binder. The hardness and transverse rupture strength (TRS) of the cermets first increased and then decreased with increasing sintering temperature and WC content. At a WC content of 10 wt% and sintering temperatures of 1480 and 1450 °C, the HV10 and TRS of the cermets reached maximum values of 1405 kg/mm2 and 1280 MPa, respectively. The friction coefficient of the NbCN-Ni cermets decreased from 0.7 to 0.55–0.6 after WC addition. Hence, the cermets exhibited excellent wear resistance because the introduction of WC decreased their friction coefficient and increased their hardness.

Redistribution of Nb and other alloying elements in Nb-doped Zr alloy under high dose ion irradiation

ABSTRACT To understand the microstructural changes in Nb-doped zirconium alloy cladding at high irradiation doses, the Mitsubishi Developed Alloy (MDA) cladding was ion- irradiated up to 40 dpa at 400 ◦ C and the distribution of alloying elements was in- vestigated using atom probe tomography (APT) and scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (STEM-EDS). APT analysis revealed two types of Nb nanoclusters with different Nb contents after the ion irradia- tion. The Nb concentration in the matrix decreased significantly with ion irradiation up to 10 dpa and then decreased slowly to 40 dpa. STEM-EDS analysis estimated that the secondary phase precipitates (SPPs) of Zr(Fe,Cr,Nb)2 and Zr(Fe,Cr)2 were formed before the ion irradiation. Although Fe, Cr, and Nb were dissolved from the SPPs during the ion irradiation, these alloying elements still remained in the SPPs after 40 dpa irradiation. Other Fe-Cr nanoclusters, Nb-rich phase, and Fe-rich phase were also formed during the ion irradiation. These radiation-induced precip- itates were observed until after 40 dpa irradiation and were expected to be stable under irradiation conditions.

Effect of lattice defects on crystal structure and microwave properties of Ba(Ga1/2Ta1/2)O3 doped Ba(Zn1/3Nb2/3)O3 ceramics

Traditionally, B-site cation ordering structure in Ba(B′1/3B″2/3)O3 based ceramics with complex perovskite structure is considered to be the dominant factor in determining the dielectric loss. The crystal structure of BZN-BGT ceramics changes to the B-site disordering from B-site 1:2 ordering accompanying with remarkably reduced dielectric loss. Ba(Ga1/2Ta1/2)O3 doping also inhibits the formation of Nb-rich secondary phases induced by Zn volatilization on the surface of BZN-BGT ceramics as well. The conductive activation energy Ea, an energy required for electrons to escape from the traps, increases to 0.55 eV from 0.40 eV, indicating the decrease of lattice defects concentration. As Ba(Ga1/2Ta1/2)O3 doping concentration increases, the dielectric thermal relaxation process of BZN-BGT ceramics weakens and the electrical conductivity decreases. These results indicate that the dielectric loss of BZN-BGT ceramics containing volatile Zn could be controlled by the lattice defects and secondary phases rather than B-site 1:2 ordering structure.

Spinodal Zr–Nb alloys with ultrahigh elastic admissible strain and low magnetic susceptibility for orthopedic applications

Metallic biomaterials, such as stainless steels, cobalt–chromium–molybdenum (Co–Cr–Mo) alloys, and titanium (Ti) alloys, have long been used as load-bearing implant materials due to their metallic mechanical strength, corrosion resistance, and biocompatibility. However, their magnetic susceptibility and elastic modulus of more than 100 GPa significantly restrict their therapeutic applicability. In this study, spinodal Zr60Nb40, Zr50Nb50, and Zr40Nb60 (at.%) alloys were selected from the miscibility gap based on the Zr–Nb binary phase diagram and prepared by casting, cold rolling, and aging. Their microstructure, mechanical properties, corrosion resistance, magnetic susceptibility, and biocompatibility were systematically evaluated. Spinodal decomposition to alternating nanoscale Zr-rich β1 and Nb-rich β2 phases occurred in the cold-rolled Zr–Nb alloys during aging treatment at 650 °C. In addition, a minor amount of α phase was precipitated in Zr60Nb40 due to the thermodynamic instability of the Zr-rich β1 phase. Spinodal decomposition significantly improved the mechanical strength of the alloys due to nanosized dual-cubic reinforcement. The Zr–Nb alloys showed an electrochemical corrosion rate of 94–262 nm per year in Hanks’ solution because of formation of dense passive films composed of ZrO2 and Nb2O5 during the polarization process. The magnetic susceptibilities of the Zr–Nb alloys were significantly lower than those of commercial Co–Cr–Mo and Ti alloys. The cell viability of the Zr–Nb alloys was more than 98 % toward MC3T3-E1 cells. Overall, the spinodal Zr–Nb alloys have enormous potential as bone-implant materials due to their outstanding overall mechanical properties, extraordinary corrosion resistance, low magnetic susceptibility, and sufficient bicompatibility. Statement of significanceThis work reports on spinodal Zr–Nb alloys with heterostructure. Spinodal decomposition significantly improved their mechanical strength due to the nanosized dual-cubic reinforcement. The Zr–Nb alloys showed large corrosion resistance in Hanks’ solution because of formation of dense passivation films composed of ZrO2 and Nb2O5 during the polarization process. The magnetic susceptibilities of the Zr–Nb alloys were significantly lower than those of commercial Co–Cr–Mo and Ti alloys. The cell viability of the Zr–Nb alloys was more than 98 % toward MC3T3-E1 cells. The results demonstrate that spinodal Zr–Nb alloys have enormous potential as bone-implant materials due to their outstanding overall mechanical properties, high corrosion resistance, low magnetic susceptibility, and sufficient biocompatibility.

Effect of Nb content variations on the microstructure and mechanical properties of NiCrMo welded joints

The effects of varying the Nb content (1.04 wt%-1.55 wt%) in NiCrMo weld metals on the microstructure, tensile strength and cryogenic impact toughness were investigated. The microstructure of the three different weld metals was observed to be composed of austenite matrix and precipitates by characterization techniques such as SEM, EDS, EBSD, and TEM. The precipitates within the grains were NbC and Ni3Nb phases, and the precipitates on the grain boundaries were NbC phases. With the increase of Nb content, the average grain size of austenite decreased from 205.3 μm to 161.4 μm, and the area fraction of precipitates increased from 1.41% to 3.97%. The cryogenic impact value of the welded joints decreased from 97 J to 76.3 J, while the tensile strength increased from 682.5 MPa to 710 MPa, indicating that the increase in the area fraction of the Nb-rich phase had a deleterious effect on the cryogenic impact toughness and was conducive to the elevation of the tensile strength. When the Nb content in the weld metal was 1.25 wt%, the comprehensive mechanical properties of NiCrMo welded joints were the best, with a tensile strength of 695.5 MPa and a cryogenic impact value of 86.7 J.

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.

Dual heterogeneous structure enabled ultrahigh strength and ductility across a broad temperature range in CrCoNi-based medium-entropy alloy

Developing alloys with exceptional strength-ductility combinations across a broad temperature range is crucial for advanced structural applications. The emerging face-centered cubic medium-entropy alloys (MEAs) demonstrate outstanding mechanical properties at both ambient and cryogenic temperatures. They are anticipated to extend their applicability to elevated temperatures, owing to their inherent advantages in leveraging multiple strengthening and deformation mechanisms. Here, a dual heterostructure, comprising of heterogeneous grain structure with heterogeneous distribution of the micro-scale Nb-rich Laves phases, is introduced in a CrCoNi-based MEA through thermo-mechanical processing. Additionally, a high-density nano-coherent γ' phase is introduced within the grains through isothermal aging treatments. The superior thermal stability of the heterogeneously distributed precipitates enables the dual heterostructure to persist at temperatures up to 1073 K, allowing the MEA to maintain excellent mechanical properties across a wide temperature range. The yield strength of the dual-heterogeneous-structured MEA reaches up to 1.2 GPa, 1.1 GPa, 0.8 GPa, and 0.6 GPa, coupled with total elongation values of 28.6 %, 28.4 %, 12.6 %, and 6.1 % at 93 K, 298 K, 873 K, and 1073 K, respectively. The high yield strength primarily stems from precipitation strengthening and hetero-deformation-induced strengthening. The high flow stress and low stacking fault energy of the dual-heterogeneous-structured MEA promote the formation of high-density stacking faults and nanotwins during deformation from 93 K to 1073 K, and their density increase with decreasing deformation temperature. This greatly contributes to the enhanced strain-hardening capability and ductility across a wide temperature range. This study offers a practical solution for designing dual-heterogeneous-structured MEAs with both high yield strength and large ductility across a wide temperature range.

Microstructural comparison of hot isostatically pressed Mo[sbnd]10Nb alloys before and after hot rolling

In this work, hot isostatic pressing (HIP) and HIP followed by hot rolling (HIP+HR) were employed to process high-performance Mo10Nb alloys, respectively. The relative density, O content, oxide types, grain size and shape, and texture type of HIP-processed Mo10Nb alloys before and after HR were systematically investigated. The results indicate that the relative densities of Mo10Nb alloys prepared by the two methods are both over 98%. The oxygen content of HIP-processed and (HIP+HR)-processed Mo10Nb alloys is 638 ± 38 ppm and 2297 ± 152 ppm, respectively. X-ray photoelectron spectrometry (XPS) confirms that the main oxides in HIP-processed Mo10Nb alloys are MoO2, MoO3, NbO and Nb2O5 while those in (HIP+HR)-processed Mo10Nb alloys are stable MoO3 and Nb2O5 resulting from the oxygen contamination during HR. More importantly, HR crushes large Nb-rich phase particles and elongates equiaxed grain in HIP-processed Mo10Nb alloys. The high temperature during HR leads to the moderate growth of grains. Noteworthy, two weak textures along {001}〈110〉 and {111}<110 > orientations were found in HIP-processed Mo10Nb alloys after HR, which could be caused by the incomplete development of deformation textures and recrystallization textures. This work helps us to clarify the microstructure difference of HIP-processed Mo10Nb alloys before and after HR, which can provide some valuable science and technology guidance for fabrication of refractory alloys by powder metallurgy and the following plastic deformation.

Effect of Mo/Nb ratio variations on the microstructure and cryogenic impact toughness of NiCrMo deposited metals

Effects of varying the Mo/Nb (in weight percent) ratio in ENiCrMo-6-deposited metals on the microstructure and cryogenic impact toughness of the metals were investigated. When the Mo/Nb ratio decreased from 6.09 to 3.28, the Nb-rich phase on the grain boundaries was transformed from un-continuous precipitation to continuous precipitation, the area fraction and average size of the precipitates within the grains were significantly increased, and a eutectic-type Laves phase appeared. Although the austenitic matrix of the deposited metals indicated a ductile fracture mode, the precipitates significantly deteriorated the cryogenic impact toughness. As the average size and area fraction of the precipitates increased, the cryogenic impact value decreased significantly.

Impact of incorporating FeNbC into weld flux on the abrasive wear resistance of coatings produced by SAW in a microalloyed steel

Owing to the global shortage of raw materials and an increase in their prices, there is a growing demand for engineering solutions to increase the lifespan and durability of equipment and components. Therefore, this study aims to combine surface engineering and welding engineering to produce a niobium-rich coating using submerged arc welding (SAW) deposition. SAW is a cost-effective technique that allows high deposition rates and technical simplicity, which can enhance mechanical properties and resistance to abrasive wear of components. This research involves the addition of a FeNbC powder alloy in percentages of 5, 10, and 15 wt% to a neutral commercial SAW flux, as an alternative to adding Nb to the microstructure of the deposited coating. The coating was characterized by optical microscopy to analyze the microstructure, such as the presence of phases; microhardness through a Vickers micro-durometer, and resistance to abrasive wear through the loss of mass using a rubber wheel-type abrasometer. The wear mechanisms were evaluated using scanning electron microscopy. The results showed that a Nb-rich coating can be deposited via SAW, and the coatings successfully increased microhardness by up to 110% and resistance to abrasive wear to values higher than the base metal used (microalloyed steel). The microstructure formed was rich in Fe2Nb and NbC, proving the formation of Nb-rich phases. Additionally, the mechanism of abrasive wear was predominantly plastic for the base metal and changed to micro-cutting and micro-plowing after the addition of up to 15% of FeNbC.

Microstructural evaluation and corrosion behaviour of Inconel 82 and Inconel 625 overlays on 304L stainless steel weld used in nuclear fuel reprocessing plant

Weld overlays are considered the most cost-effective surface modification technique to mitigate corrosion in the nuclear industry. Considering this, an attempt to overlay the 304L weld metal at some critical weld locations of the nuclear fuel reprocessing plant with Inconel alloy was initiated. The work focuses on the microstructural evaluation and corrosion behaviour of Inconel 82 and Inconel 625 weld overlays on 304L stainless steel (SS) weld metal in a nitric acid and chloride medium. Scanning electron micrographs of the 304L SS weld metal revealed a duplex microstructure containing δ-ferrite as a minor phase in the austenite matrix. The microstructure of the Inconel 82 and Inconel 625 weld overlays was found to be fully austenitic, with Nb-rich laves phases at the inter-dendritic region. The hardness of the Inconel weld overlays was found to be higher than that of the 304L SS weld metal. X-ray diffraction measurements showed austenite and δ-ferrite in the 304L SS weld metal and a single austenite phase in the Inconel weld overlays. Potentiodynamic anodic polarization experiments carried out in 4M and 8M nitric acid indicated superior passive film stability in the Inconel weld overlays when compared to the 304L SS weld metal. Comparatively, Inconel 625 weld overlay was found to be much superior to Inconel 82 weld overlay in nitric acid medium. The 304L SS weld metal exhibited inferior pitting resistance in chloride medium by virtue of the high δ-ferrite present in the weld fusion zone. The pitting resistance of the Inconel 82 weld overlay marginally improved, while a substantial enhancement occurred in the Inconel 625 weld overlay.

Effect of high and low Nb content in multicomponent Nb–Ni–Ti–Zr–Co alloy on its structure, hardness and hydrogen permeability

In the present work, novel multicomponent Nb–Ni–Ti–Zr–Co alloys were synthesized by arc melting. Two alloys with low and high Nb contents and an equimolar high-entropy alloy (HEA) were produced and compared. The influence of the elemental composition on their microstructure, phase composition, lattice constants, hardness and hydrogen permeability was investigated. The synthesized Nb–Ni–Ti–Zr–Co alloys are characterized by near-equimolar BCC-(Nb, Ni, Ti, Zr, Co) and Nb-rich BCC-Nb(Ni, Ti, Zr, Co) phases. The equimolar Nb20Ni20Ti20Zr20Co20 alloy had the highest concentration of BCC-(Nb, Ni, Ti, Zr, Co) phase (84 vol %), while the Nb74Ni6Ti9Zr5Co6 alloy had the highest concentration of Nb-rich BCC phase (95 vol %). First-principles study of the lattice parameters for the Nb–Ni–Ti–Zr–Co alloy revealed that the lattice parameter of the BCC phase decreases with Ni and Co addition and increases with Nb, Zr addition. Titanium does not significantly affect the lattice constant of BCC phase for compositions close to equimolar (10–30 at. %). The microhardness of the alloys decreases with increasing Nb content, which is mainly due to the proportion of softer Nb-rich BCC phase and microstructural changes. The synthesized Nb15Ni20Ti15Zr30Co20 and Nb20Ni20Ti20Zr20Co20 alloys demonstrate high hydrogen permeability at 400 °C that makes them promising for hydrogen purification membranes. It is shown high-entropy Nb20Ni20Ti20Zr20Co20 alloy exhibit highest resistance to hydrogen embrittlement, while high Nb content alloys are strongly susceptible to embrittlement.

Effect of Al addition on Nb-rich phase precipitation behavior in ferritic stainless steel

The precipitation behavior of Nb-rich phase in Al-free and 1Al Nb-bearing ferritic stainless steels (FSSs) at 700 °C were investigated. The results show that Nb-bearing carbides and Laves phases with mutual transformation relationships are precipitated in both Al-free and 1Al Nb-bearing FSSs at the early stage of aging. At this stage, the coarsening rate of Laves phase is higher than that of Nb2C phase, and the addition of Al can promote the precipitation of Laves phase and weaken the precipitation of (Nb, Ti)2C phase. As the aging time extended to 60 min, the Nb-rich phase in the two experimental steels was mainly Laves phase, and the intragranular Laves phase showed feather like aggregation distribution. On the one hand, the added Al can be enriched in the Laves phase to produce a tailing morphology, on the other hand, it can reduce the diffusion rate of Nb atoms in the matrix, thereby weakening the aggregation state of the intragranular Laves phase. During the coarsening process of Laves phase, the addition of Al can reduce the coarsening rate of Laves phase by 7.45 times.

Liquation cracking in laser powder bed fusion-fabricated Inconel718 of as-built, stress-relieved, and hot isostatic pressed conditions

A critical factor of the laser powder bed fusion (LPBF) additive manufacturing processed Inconel718 is its susceptibility to liquation cracking, primarily caused by the incipient melting of the grain boundary Laves phase or carbide (Nb-rich intermetallic phases). In this study, the liquation cracking behavior of the LPBF-processed Inconel718 was investigated using Gleeble® hot-ductility tests. In particular, the effect of commonly employed post-LPBF heat treatments, such as stress relieving, and hot isostatic pressing (HIP), on the liquation cracking resistance was examined. While the as-built Inconel718 exhibited good liquation cracking resistance, its liquation cracking resistance decreased after the stress relieving and HIP treatment. The variation in liquation cracking susceptibility was attributed to microstructural differences, including grain size, size and volume fraction of the low-melting carbides phase, and the segregation pattern of the solute element Nb. Good liquation cracking resistance of the as-built provides more flexibility in process design involving high temperatures. Meanwhile, HIP treatment can reduce internal defects but presents a significant disadvantage of increased liquation cracking susceptibility. These findings underscore the importance of carefully designing post-processing conditions for additively manufactured parts.

An inspection into the unexpected gamma precipitation in supersaturated U-13 at.% Nb subjected to aging

Phase separation of the U-Nb (13 at.%) alloy at an aging temperature of 723 K was investigated through transmission electron microscopy. Chemical redistribution initiated from a supersaturated state was identified after aging for half an hour before the discontinuous precipitation (DP) process. Such redistribution was accompanied by a fine two-phase structure and non-lamellar morphology. Among the decomposition products at this early stage, a lath-shaped Nb-rich phase was confirmed to be the bcc (γ) phase, with Nb content in the range of 35∼45 at.%. Meanwhile, discrete regions of much smaller size were detected to bare an Nb content approaching the equilibrium of the α phase. It is envisaged that the γ precipitation was a preceding event of the α. Thermodynamic estimation was carried out to reveal the decomposition mechanism responsible for this unexpected behavior. The results were in favor of a nucleation and growth mechanism over the spinodal decomposition. These results provide a deeper understanding and a new sight into the aging phenomenon of U-13Nb alloys, which are also worth consideration in other binary systems slightly outside the coherent spinodal regions such as Fe-20Cr.

Effects of niobium on the microstructure, corrosion, and mechanical behavior of ultra-fine lamellar Al0.3CrFeCoNiNbx eutectic high-entropy alloys

This study investigates the effects of the element, Nb, on the microstructure, corrosion, and mechanical behavior of the Al0.3CrFeCoNiNbx (in molar ratio) high-entropy alloys (HEAs). It is found that the Nb addition facilitates the formation of the Nb-rich Laves phase for the HEAs. The HEAs show a hypoeutectic structure [the primary face-centered-cubic (FCC) phase] at x = 0.25–0.4, eutectic structure at x = 0.45, and hypereutectic structure (the primary Laves phase) at x = 0.5–0.75. In the 3.5 mass% NaCl solution, the Al0.3CrFeCoNiNbx HEAs are spontaneously passivated and exhibit a low corrosion rate of less than 10−3 mm/year as a result of the formation of the protective passive films enriched in the chemically-stable Cr- and Nb-oxides. Compared with the FCC phase, the Laves phase displays a better corrosion resistance because of the higher concentration of Nb-oxides in its passive film. The Al0.3CrFeCoNiNb0.45 eutectic HEA possesses superior corrosion resistance among the HEAs, which is ascribed to a high concentration, and more homogeneous distribution of Nb-oxides in the passive film, as well as higher electrochemical impedance and thicker passive film. Furthermore, the Al0.3CrFeCoNiNbx HEAs with larger fractions of the Laves phase and ultra-fine eutectic structure present higher strength and hardness. Nevertheless, the plastic strain of the HEAs sharply decreases with increasing the amount of the brittle Laves phase. The results propose an effective approach for developing the HEAs with good corrosion resistance, high strength, and hardness.

Thermal stability and deformation mechanisms in Ni-Co-Fe-Cr-Al-Ti-Nb-type nanoparticle-strengthened high-entropy alloys

The precipitate morphologies, coarsening kinetics, elemental partitioning behaviors, grain structures, and tensile properties were explored in detail for L12-strengthened Ni39.9Co20Fe15Cr15Al6Ti4–xNbxB0.1 (x = 0 at.%, 2 at.%, and 4 at.%) high-entropy alloys (HEAs). By substituting Ti with Nb, the spheroidal-to-cuboidal precipitate morphological transition, increase in the coarsening kinetics, and phase decomposition upon aging at 800 °C occurred. The excessive addition of Nb brings about the grain boundary precipitation of an Nb-rich phase along with the phase decomposition from the L12 to lamellar-structured D019 phase upon the long-term aging duration. By partially substituting Ti with Nb, the chemically complex and thermally stable L12 phase with a composition of (Ni58.8Co9.8Fe2.7)(Al12.7Ti5.8Nb7.5Cr2.3) ensures the stable phase structure and clean grain boundaries, which guarantees the superb high-temperature mechanical properties (791 ± 7 MPa for yielding and 1013 ± 11 MPa for failure) at 700 °C. Stacking faults (SFs) were observed to prevail during the plastic deformation, offering a high work-hardening capability at 700 °C. An anomalous rise in the yield strength at 800 °C was found, which could be ascribed to the multi-layered super-partial dislocations with a cross-slip configuration within the L12 particles.

Laves phase assisted the passive behaviors of Co-free non-equiatomic Cr-Fe-Ni-Nb eutectic high-entropy alloys

A novel Co-free non-equiatomic Cr-Fe-Ni-Nb eutectic high entropy alloys (EHEAs) were developed, and the microstructures, corrosion behaviors, and mechanism of as-cast EHEAs were explored in 3.5 wt% NaCl solution. The quaternary EHEAs demonstrated the typical lamellar microstructure, evidencing that a dual-phase microstructure composed of FCC (Nb-lean) and Laves (Nb-rich) phases. With increase of Nb, the volume fraction of the Laves phase increased, while the interlayer distance decreased from 340 nm to 150 nm, leading to significantly improvement of corrosion resistance. The Co-free EHEAs displayed the distinguished combination of a wide passive region (approximately 1300 mV), a low corrosion current density of 6.31 × 10−8 A cm−2, and a superior repassivation ability, with preeminent corrosion resistance compared to the established anticorrosion alloys. Reasonable control of the Nb content could reduce of the point-defect density in the passive films. Especially, for Cr8Fe8Ni13Nb5 EHEAs, the critical pitting potential (1148 mV) was increased by nearly three times higher than that of SS304. The outcome of our study enriched the designing system of the eutectic high entropy alloys with excellent corrosion resistance.