Microstructure and Mechanical Properties of Co32Cr28Ni32.94Al4.06Ti3 High-Entropy Alloy

High-entropy alloys have good application prospects in nuclear power plants due to their excellent mechanical properties and radiation resistance. In this paper, the microstructure of the Co₃₂Cr₂₈Ni₃₂.₉₄Al₄.₀₆Ti₃ high-entropy alloy was researched using metallurgical microscopy, X-ray diffraction, and scanning electron microscopy. The mechanical properties were tested using a Vickers microhardness tester and a tensile testing machine, respectively. The results showed that Co₃₂Cr₂₈Ni₃₂.₉₄Al₄.₀₆Ti₃ had a single-phase, disordered, face-centered, cubic solid-solution structure and was strengthened by solid solution. The alloy lattice parameter and density were estimated as 0.304 nm and 7.89 g/cm³, respectively. The test results indicated that the alloy had satisfactory mechanical properties with yield stress and tensile strength of about 530 MPa and 985 MPa, respectively.     

Microstructure Refinement of a Transformation-Induced Plasticity High-Entropy Alloy

High-entropy alloys (HEAs) have attracted extensive interest due to their unprecedented structure and mechanical performance. We recently proposed a series of novel corich twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) HEAs with superior tensile properties at room temperature; however, the hot deformation behavior has not been reported. Here, we investigated the dynamic recrystallization behavior and grain refinement of a representative TRIP-HEA, compressed at temperatures of 1123–1273 K with strain rates of 0.1–0.001 s⁻¹. We characterized the impact of the temperature and strain rate on the grain structure evolution. A constitutive equation was constructed to reveal the correlations between the flow stress, strain rate, temperature, and strain. The apparent activation energy was estimated to be ~385.7 kJ/mol. The discontinuous dynamic recrystallization played an important role in the grain refinement, particularly at a relatively higher temperature and a lower strain rate, and the volume fraction and morphology of the recrystallized grains exhibited a strong dependency on the Zener–Hollomon parameter. The study provides guidelines for the grain refinement of HEAs through thermomechanical processing.     

Microstructure and Friction Properties of CoCrFeMnNiTix High-Entropy Alloy Coating by Laser Cladding

To enhance the friction and wear properties of 40Cr steel’s surface, CoCrFeMnNi high-entropy alloy (HEA) coatings with various Ti contents were prepared using laser cladding. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to characterize the phase composition, microstructure, and chemical composition of the samples. The findings demonstrated that the CoCrFeMnNiTi_x HEA coatings formed a single FCC phase. Fe₂Ti, Ni₃Ti, and Co₂Ti intermetallic compounds were discovered in the coatings when the molar ratio of Ti content was greater than 0.5. The EDS findings indicated that Cr and Co/Ni/Ti were primarily enriched in the dendrite and interdendrite, respectively. Ti addition can effectively enhance the coating’s mechanical properties. The hardness test findings showed that when the molar ratio of Ti was 0.75, the coating’s microhardness was 511 HV0.5, which was 1.9 times the hardness of the 40Cr (256 HV0.5) substrate and 1.46 times the hardness of the CrCrFeMnNi HEA coating (348 HV0.5). The friction and wear findings demonstrated that the addition of Ti can substantially reduce the coating’s friction coefficient and wear rate. The coating’s wear resistance was the best when the molar ratio of Ti was 0.75, the friction coefficient was 0.296, and the wear amount was 0.001 g. SEM and 3D morphology test results demonstrated that the coating’s wear mechanism changed from adhesive wear and abrasive wear to fatigue wear and abrasive wear with the increase in Ti content.     

Microstructure and Magnetic Field-Induced Strain of a Ni-Mn-Ga-Co-Gd High-Entropy Alloy

The effect of a high-entropy design on martensitic transformation and magnetic field-induced strain has been investigated in the present study for Ni-Mn-Ga-Co-Gd ferromagnetic shape-memory alloys. The purpose was to increase the martensitic transition temperature, as well as the magnetic field-induced strain, of these materials. The results show that there is a co-existence of β, γ, and martensite phases in the microstructure of the alloy samples. Additionally, the martensitic transformation temperature shows a markedly increasing trend for these high-entropy samples, with the largest value being approximately 500 °C. The morphology of the martensite exhibits typical twin characteristics of type L1₀. Moreover, the magnetic field-induced strain shows an increasing trend, which is caused by the driving force of the twin martensite re-arrangement strengthening.     

Phase Transformation Induced by High Pressure Torsion in the High-Entropy Alloy CrMnFeCoNi

The forward and reverse phase transformation from face-centered cubic (fcc) to hexagonal close-packed (hcp) in the equiatomic high-entropy alloy (HEA) CrMnFeCoNi has been investigated with diffraction of high-energy synchrotron radiation. The forward transformation has been induced by high pressure torsion at room and liquid nitrogen temperature by applying different hydrostatic pressures and large shear strains. The volume fraction of hcp phase has been determined by Rietveld analysis after pressure release and heating-up to room temperature as a function of hydrostatic pressure. It increases with pressure and decreasing temperature. Depending on temperature, a certain pressure is necessary to induce the phase transformation. In addition, the onset pressure depends on hydrostaticity; it is lowered by shear stresses. The reverse transformation evolves over a long period of time at ambient conditions due to the destabilization of the hcp phase. The effect of the phase transformation on the microstructure and texture development and corresponding microhardness of the HEA at room temperature is demonstrated. The phase transformation leads to an inhomogeneous microstructure, weakening of the shear texture, and a surprising hardness anomaly. Reasons for the hardness anomaly are discussed in detail.     

Wear-Resistant TiC Strengthening CoCrNi-Based High-Entropy Alloy Composite

In order to improve the wear resistance of CoCrNi alloy, TiC was introduced into the alloy and wear-resistant CoCrNi/(TiC)_x composites were designed. The effects of TiC contents on the microstructure, mechanical properties, and wear resistance of CoCrNi matrix were investigated, respectively. It was found that the TiC produced dissolution and precipitation process in CoCrNi alloy, and a large number of needled and blocky TiC particles were precipitated in the composites. The compressive yield strength of CoCrNi/(TiC)_x composites increased with the increasing TiC content. Compared with the CoCrNi alloy, the yield strength of CoCrNi/(TiC)_x composites increased from 108 to 1371 MPa, and the corresponding strengthening mechanism contributed to the second phase strengthening. The wear resistance of CoCrNi/(TiC)_x composites was also greatly improved due to the strengthening of TiC. Compared with the CoCrNi alloy, the specific wear rate of CoCrNi/(TiC)_1.0 alloy was reduced by about 77%. The wear resistance of CoCrNi/(TiC)_x composites was enhanced with the increasing content of TiC addition.     

Microstructure and Corrosive Wear Properties of CoCrFeNiMn High-Entropy Alloy Coatings

In order to improve the wear resistance of offshore drilling equipment, CoCrFeNiMn high-entropy alloy coatings were prepared by cold spraying (CS) and high-speed oxygen fuel spraying (HVOF), and the coatings were subjected to vacuum heat treatment at different temperatures (500 °C, 700 °C and 900 °C). The friction and wear experiments of the coatings before and after vacuum heat treatment were carried out in simulated seawater drilling fluid. The results show that CoCrFeNiMn high-entropy alloy coatings prepared by CS and HVOF have dense structure and bond well with the substrate. After vacuum heat treatment, the main peaks of all oriented FCC phases are broadened and the peak strength is obviously enhanced. The two types of coatings achieve maximum hardness after vacuum heat treatment at 500 °C; the Vickers microhardness of CS-500 °C and HVOF-500 °C are 487.6 and 352.4 HV_0.1, respectively. The wear rates of the two coatings at room temperature are very close. CS and HVOF coatings both have the lowest wear rate after vacuum heat treatment at 500 °C. The CS-500 °C coating has the lowest wear rate of 0.2152 mm³ m⁻¹ N⁻¹, about 4/5 (0.2651 mm³ m⁻¹ N⁻¹) of the HVOF-500 °C coating. The wear rates and wear amounts of the two coatings heat-treated at 700 °C and 900 °C decrease due to the decrease in microhardness. The wear mechanisms of the coatings before and after vacuum heat treatment are adhesive wear, abrasive wear, fatigue wear and oxidation wear.     

Effects of the Replacement of Co with Ni on the Microstructure,Mechanical Properties,and Age Hardening of AlCo1−xCrFeNi1+x High-Entropy Alloys

In this study, effects of the replacement of Co with Ni on the microstructure, mechanical properties, and age hardening of high-entropy alloys of AlCo_1−xCrFeNi_1+x (x = molar ratio; x = 0, 0.5, 1, denoted as X₀, X_0.5, and X₁, respectively) were investigated. These three alloys exhibited a dendritic structure comprising an ordered BCC matrix, a BCC phase, and an FCC or an ordered FCC phase. From X₀ to X₁ alloys, the yield stress and compressive stress decreased from 1202 and 1790 MPa to 693 and 1537 MPa, respectively. However, fracture strain increased from 0.15 to 0.42. Peak age hardening at 600 °C for the X₀ alloy was due to the precipitation of the (Cr,Fe)-rich σ phase. Peak age hardening for the X_0.5 and X₁ alloys was observed at 500 °C because of the precipitation of the σ phase and BCC phase, respectively.     

Anisotropic Response of CoCrFeMnNi High-Entropy Alloy Fabricated by Selective Laser Melting

This study investigated the anisotropic characteristics of the microstructural, mechanical and corrosion properties of CoCrFeMnNi high-entropy alloy produced by selective laser melting (SLM) additive manufacturing (AM). Under the extremely high thermal gradient during the SLM process, a columnar solidification structure with a single face-centered cubic (FCC) phase structure was formed. The crystal structure exhibited a regular checkerboard structure in the XOY plane (perpendicular to the building direction), which was composed of {110} direction and a small amount of {100} fiber texture. The cellular-dendritic sub-structures formed in the columnar crystal structure with sizes of about 500 nm in diameter. As for the mechanical properties, the XOY plane exhibited higher ultimate tensile strength and yield strength (σ_0.2) but lower elongation to failure compared to the XOZ plane (parallel to building direction), which reflected the anisotropy of the microstructure. The electrochemical test results of the different planes showed that the XOZ plane exhibited better corrosion resistance in comparison with the XOY plane in the 3.5 wt % NaCl solution, which was on account of the selective attack at the Mn-rich inter-cellular regions and the different structures of the cellular-dendritic sub-structures on different planes.     

Tuning Microstructure and Mechanical Performance of a Co-Rich Transformation-Induced Plasticity High Entropy Alloy

Multi-principal element alloys and high-entropy alloys (HEAs) are emerging metallic materials with unprecedented structures and properties for various applications. In this study, we tuned the microstructure and mechanical performance of a recently designed high-performance Co-rich TRIP-HEA via thermomechanical processing (TMP). The microstructures of the HEA after various TMP routines were characterized, and their correlation with room-temperature tensile performance was clarified. The results showed that grain refinement is an effective strategy for enhancing strength while retaining satisfactory ductility. The formation of incoherent precipitates slightly improves the strength but inevitably sacrifices the ductility, which needs to be considered for optimizing the TMPs. The room temperature tensile yield strength and ultimate tensile strength were increased from 254.6 to 641.3 MPa and from 702.5 to 968.4 MPa, respectively, but the tensile elongation retains a satisfactory value of 68.8%. We herein provide important insights into the regulation of the microstructure and mechanical properties of TRIP-HEAs.     

Study on Microstructure of Fiber Laser Welding of CoCrCuFeNi High Entropy Alloy

A CoCrCuFeNi high-entropy alloy was successfully welded in this study using fiber laser welding. The effects of the welding parameters on the microstructure and mechanical properties were studied. Three zones were formed: the fusion zone, partial melting zone, and base metal. The base metal exhibited a typical dendrite structure, and the Cu element segregated in the interdendrite. The fusion zone consisted of fine equiaxed crystals and columnar crystals with the same crystalline structure as the base metal. The fusion zone exhibited minimal compositional microsegregation after laser welding. Electron backscatter diffraction results showed that the low-angle grain boundary fraction in the fusion zone increased. Furthermore, some dislocations and dislocation pile-ups were present in the fusion zone, and the densities of the dislocations and dislocation pile-ups were higher than those of the base metal. The hardness of the fusion zone was considerably higher than that of the base metal, while the ultimate tensile strength and elongation values were lower than those of the base metal for all conditions. The ultimate tensile strength and the elongation increased gradually and then decreased with increasing laser power. The maximum ultimate tensile strength exceeded that of the base metal by 90%.     

High-Temperature Oxidation Behavior of AlTiNiCuCox High-Entropy Alloys

In this study, the high-temperature oxidation behavior of a series of AlTiNiCuCo_x high-entropy alloys (HEAs) was explored. The AlTiNiCuCo_x (x = 0.5, 0.75, 1.0, 1.25, 1.5) series HEAs were prepared using a vacuum induction melting furnace, in which three kinds of AlTiNiCuCo_x (x = 0.5, 1.0, 1.5) alloys with different Co contents were oxidized at 800 °C for 100 h, and their oxidation kinetic curves were determined. The microstructure, morphology, structure, and phase composition of the oxide film surface and cross-sectional layers of AlTiNiCuCo_x series HEAs were analyzed using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and X-ray diffraction (XRD). The influence of Co content on the high-temperature oxidation resistance of the HEAs was discussed, and the oxidation mechanism was summarized. The results indicate that, at 800 °C, the AlTiNiCuCo_x (x = 0.5, 1.0, 1.5) series HEAs had dense oxide films and certain high-temperature oxidation resistance. With increasing Co content, the high-temperature oxidation resistance of the alloys also increased. With increasing time at high temperature, there was a significant increase in the contents of oxide species and Ti on the oxide film surface. In the process of high-temperature oxidation of AlTiNiCuCo_x series HEAs, the interfacial reaction, in which metal elements and oxygen in the alloy form ions through direct contact reaction, initially dominated, then the diffusion process gradually became the dominant oxidation factor as ions diffused and were transported in the oxide film.     

The Effect of Scandium on the Structure,Microstructure and Superconductivity of Equimolar Sc-Hf-Nb-Ta-Ti-Zr Refractory High-Entropy Alloys

In this study, we investigate the scandium-containing Sc-Hf-Nb-Ta-Ti-Zr system of refractory high-entropy alloys (HEAs). Using the arc-melting method, we synthesized nine equimolar alloys (five 4-, three 5- and one 6-component), with all of them containing Sc. The alloys were characterized by XRD, electron microscopy and EDS, while superconductivity was investigated via electrical resistivity, specific heat and the Meissner effect. The results were compared to the parent Hf-Nb-Ta-Ti-Zr refractory HEAs, forming a single-phase body-centered cubic (bcc) structure and quite homogeneous microstructure. The addition of Sc produces a two-phase structure in the Sc-Hf-Nb-Ta-Ti-Zr alloys, with one phase being bcc and the other hexagonal close-packed (hcp). The hcp phase absorbs practically all Sc, whereas the Sc-poor bcc phase is identical to the bcc phase in the Hf-Nb-Ta-Ti-Zr parent system. Upon the Sc addition, the microstructure becomes very inhomogeneous. Large bcc dendrites (10–100 µm) are homogeneous in the central parts, but become a fine dispersion of sub-micron precipitates of the bcc and hcp phases close to the edges. The interdendritic regions are also a fine dispersion of the two phases. Superconductivity of the Sc-Hf-Nb-Ta-Ti-Zr alloys originates from the bcc phase fraction, which demonstrates identical superconducting parameters as the bcc Hf-Nb-Ta-Ti-Zr parent alloys, while the Sc-containing hcp phase fraction is non-superconducting.     

Corrosion Behavior of the AlCoCrFeNi2.1 Eutectic High-Entropy Alloy in Chloride-Containing Sulfuric Acid Solutions at Different Temperatures

In this work, the influence of temperature on the corrosion behavior of AlCoCrFeN_i2.1 eutectic high-entropy alloy in a chloride-containing sulfuric acid solution was investigated using electrochemical measurement, X-ray photoelectron spectroscopy, and scanning electron microscopy. Results show that the passive film of AlCoCrFeN_i2.1 is stable in chloride-containing sulfuric acid solutions at low temperatures, while an unstable film forms on the alloy at high temperatures. Furthermore, temperature changes the proportion of hydroxide and oxide in Fe and Cr, but it has no noticeable effect on Al and Ni, which is a significant factor on the passive behavior. L12 phase exhibits good corrosion resistance at different temperatures. Pitting occurred on B2 phase in the chloride-containing sulfuric acid solution at a low temperature of 5 °C, while pitting and dissolution take place on AlCoCrFeNi_2.1 in the acid solution at room temperature and above.     

Structure and Superconductivity of Tin-Containing HfTiZrSnM (M = Cu,Fe, Nb,Ni) Medium-Entropy and High-Entropy Alloys

In an attempt to incorporate tin (Sn) into high-entropy alloys composed of refractory metals Hf, Nb, Ti and Zr with the addition of 3d transition metals Cu, Fe, and Ni, we synthesized a series of alloys in the system HfTiZrSnM (M = Cu, Fe, Nb, Ni). The alloys were characterized crystallographically, microstructurally, and compositionally, and their physical properties were determined, with the emphasis on superconductivity. All Sn-containing alloys are multi-phase mixtures of intermetallic compounds (in most cases four). A common feature of the alloys is a microstructure of large crystalline grains of a hexagonal (Hf, Ti, Zr)₅Sn₃ partially ordered phase embedded in a matrix that also contains many small inclusions. In the HfTiZrSnCu alloy, some Cu is also incorporated into the grains. Based on the electrical resistivity, specific heat, and magnetization measurements, a superconducting (SC) state was observed in the HfTiZr, HfTiZrSn, HfTiZrSnNi, and HfTiZrSnNb alloys. The HfTiZrSnFe alloy shows a partial SC transition, whereas the HfTiZrSnCu alloy is non-superconducting. All SC alloys are type II superconductors and belong to the Anderson class of “dirty” superconductors.     

A Feasibility Study of High-Entropy Alloy Coating Deposition by Detonation Spraying Combined with Laser Melting

In this work, a new two-stage approach to the deposition of high-entropy alloy coatings is proposed. At the first stage, a composite precursor coating is formed by detonation spraying of the metal powder mixtures. At the second stage, the precursor coating is re-melted by a laser, and the formation of multi-component solid solution phases can be expected upon solidification. The feasibility of the proposed approach was validated using three different mixtures of Fe, Ni, Cu, Co and Al powders. It was shown that detonation spraying allows forming composite coatings with a uniform distribution of the lamellae of different metals. The results of the structural analysis of the laser-treated coatings suggest that complete alloying occurred in the melt and face-centered cubic solid solutions formed in the coatings upon cooling.     

Effect of Grain Size on Carburization Characteristics of the High-Entropy Equiatomic CoCrFeMnNi Alloy

In this study, the carburization characteristics of cast and cold-rolled CoCrFeMnNi high-entropy alloys (HEAs) with various grain sizes were investigated. All specimens were prepared by vacuum carburization at 940 °C for 8 h. The carburized/diffused layer was mainly composed of face-centered cubic structures and Cr₇C₃ carbide precipitates. The carburized/diffused layer of the cold-rolled specimen with a fine grain size (~1 μm) was thicker (~400 μm) than that of the carburized cast specimen (~200 μm) with a coarse grain size (~1.1 mm). In all specimens, the carbides were formed primarily through grain boundaries, and their distribution varied with the grain sizes of the specimens. However, the carbide precipitates of the cast specimen were formed primarily at the grain boundaries and were unequally distributed in the specific grains. Owing to the non-uniform formation of carbides in the carburized cast specimen, the areas in the diffused layer exhibited various carbide densities and hardness distributions. Therefore, to improve the carburization efficiency of equiatomic CoCrFeMnNi HEAs, it is necessary to refine the grain sizes.     

Microstructural Evolution and Tensile Properties of Al0.3CoCrFeNi High-Entropy Alloy Associated with B2 Precipitates

The room-temperature strength of Al_0.3CoCrFeNi high-entropy alloys (HEAs) is relatively low owing to its intrinsic fcc structure. In the present study, the as-cast HEAs were subjected to cold rolling and subsequent annealing treatment (800, 900, and 1000 °C) to adjust the microstructures and tensile properties. This treatment process resulted in the partial recrystallization, full recrystallization, and grain coarsening with increasing the annealing temperature. It was found that the large and spherical B2 precipitates were generated in the recrystallized grain boundaries of three annealing states, while the small and elongated B2 precipitates were aligned along the deformation twins in the non-recrystallized region of the 800 °C-annealing state. The former B2 precipitates assisted in refining the recrystallized grains to quasi ultra-fine grain and fine grain regimes (with the grain sizes of ~0.9, ~2.2, and ~7.2 μm). The tensile results indicated that the decreased annealing temperature induced the gradual strengthening of this alloy but also maintained the ductility at the high levels. The yield strength and ultimate tensile strength in 800 °C-annealed specimen were raised as high as ~870 and ~1060 MPa and the ductility was maintained at ~26%. The strengthening behavior derived from the heterogeneous microstructures consisting of quasi ultra-fine recrystallized grains, non-recrystallized grains, deformation twins, dislocations, and B2 precipitates. Current findings offer the guidance for designing the HEAs with good strength and ductility.     

A Lightweight AlTiVNb High-Entropy Alloy Film with High Strength-Ductility Synergy and Corrosion Resistance

The simultaneous improvement of mechanical and corrosion resistance is of great significance for engineering applications. In this work, a novel lightweight amorphous structure AlTiVNb high-entropy alloy (HEA) film was fabricated by magnetron sputtering. The compression test of the AlTiVNb HEA film nanopillar exhibits a high compressive strength of up to 3.6 GPa and deformability approaching 58%. The high strength is affected by the disordered state, the nanostructure, and the lattice distortion effect, while the high ductility comes from the ductile shear band and the island structure. In addition, the AlTiVNb HEA film shows a current density of 4.90 × 10⁻⁸ A/cm² and a potential of −0.234 V in the 3.5% NaCl solution, comparable to that of the 316L stainless steel. The chemical disorder state, cocktail effect, and homogeneous amorphous structure contribute to excellent corrosion resistance. This finding offers new insights into high-performance HEA films with robust mechanical and anticorrosion performances for microelectronic devices and mechanical metamaterials.     

Refractory Metal Intermetallic Composites,High-Entropy Alloys,and Complex Concentrated Alloys: A Route to Selecting Substrate Alloys and Bond Coat Alloys for Environmental Coatings

This paper considers metallic ultrahigh-temperature materials (UHTMs) and the alloying behaviour and properties of alloys and their phases by using maps of the parameters δ (based on atomic size), Δχ (based on electronegativity), and valence electron concentration (VEC), and discusses what connects and what differentiates material groups in the maps. The formation of high-entropy or complex concentrated intermetallics, namely 5-3 silicides, C14 Laves and A15 compounds, and bcc solid solutions and eutectics in metallic UHTMs and their co-existence with “conventional” phases is discussed. The practicality of maps for the design/selection of substrate alloys is deliberated upon. The need for environmental coatings for metallic UHTMs was considered and the design of bond coat alloys is discussed by using relevant maps.