High-entropy Alloy Powders Research Articles

Synthesis of single-phase high-entropy carbides from a mixture of pre-mechanically alloyed CrNbMoWV HEA powders and carbon

In this paper, single-phase chemically homogeneous high-entropy ceramics (HEC) were synthesized from a mixture of pre-mechanically alloyed CrNbMoWV high-entropy alloy (HEA) powders and carbon. High-entropy CrNbMoWV carbides were obtained for the first time. Mechanical alloying (MA) of a CrNbMoWV alloy with different process inputs times made it possible to obtain a single-phase powder with a body-centered cubic lattice and a uniform distribution of chemical elements. During the sintering of the mixture of pre-mechanically alloyed HEA powders with carbon by SPS single-phase HEC with the MeC chemical formula was formed. The XRD and SEM/EDX results are consistent between themselves. The obtained HEC are characterized by high wear resistance and hardness, with an average value of hardness for carbide sintered at 1400 °C of 1260 HV.

High entropy nanoparticles of CoCrXFeNi (X=Al, Cu, Mn) loaded on activated carbon for efficient degradation of methylene blue

Different from the reported catalysts such as metallic glasses ribbons and ball-milled high entropy alloy powder, high entropy alloy nanoparticles (HEA NPs) have attracted wide attention for compelling and functional catalytic properties to degrade methylene blue (MB) dye. CoCrXFeNi (X = Al, Cu, Mn) HEA NPs with different loadings were synthesized by impregnation-adsorption method on activated carbon (AC). Nanoscale HEA NPs with maximized configuration mixing entropy are composed of a single solid-solution face-centered cubic (FCC) phase and the nanocomposites show distinctive porous architectures and large specific surface area (average S BET = 632.7 m 2 /g) for 10 wt.% loadings. The reaction rates ( k o b s ) of 10 wt.% CoCrAlFeNi and CoCrMnFeNi HEA NPs-AC for MB degradation are 0.1998 and 0.1830 min −1 and show comparable intrinsic catalytic performance to amorphous ribbons and HEA powders. The HEA NPs can effectively decompose MB molecule via nanoscale galvanic cell effect which provides a promising material in dye decomposition and mineralization.

Investigation of shape memory characteristics and production of HfZrTiFeMnSi high entropy alloy by mechanical alloying method

High entropy alloy (HEA) with shape memory effect (SME) has been the subject of great interest for the past few decades. However, with the increased demands for new materials for high thermal applications, the research activities on the multi elemental high entropy shape memory alloys (HESMA) have been increased by many folds recently. The nano crystalline HEA powder with shape memory effect developed in this study, HfZrTiFeMnSi, was produced by mechanical alloying (MA) for the first time. In this method equiatomic ratio of Hf, Zr, Ti, Fe, Mn, and Si were mixed together and milled by MA process for 100 h. The powder formed was of amorphous in nature. Elemental mapping of the powder from SEM-EDS revealed homogeneity of the alloying elements confirming successful fabrication of HfZrTiFeMnSi HEA powder. The DSC studies from ambient to 500 °C of the annealed alloy powders showed reversible austenitic to martensitic (A↔M) transformations. The A↔M transformation hysteresis seemed to vary with the milling time and annealing temperature. The enthalpy values, ΔH, for the transformation were calculated from the DSC plots using tangent method for peak area calculation. Regardless of the annealing temperature, the thermal analysis revealed that the ΔH, equilibrium temperature (T0), and crystallization temperature values decreased with the increasing milling time.

In-situ reactive synthesis and characterization of a high entropy alloy coating by laser metal deposition

In this study, we investigate the influence of in situ reactive synthesis of Ti6Al4V– AlCoCrFeNiCu high entropy alloys by laser metal deposition on the microstructural and mechanical properties of the as-built alloy as opposed to the traditional method of mixing powders via a ball mill prone to contamination and segregation. We explore the capability of a new alloy design by combining two base alloys via in situ reactive alloying, delivering the Ti6Al4V and AlCoCrFeNiCu high entropy alloy powders from multiple powder feeders and regulating their feed rate ratios. The nano mechanical, tribological and microstructural morphologies of the alloys were characterized using a nanoindentation tester, a tribometer, XRD and SEM, respectively. The results showed that satelliting the high entropy alloys powder and the Ti6Al4V powder fraction using double powder feedstock had a homogeneous distribution with dendritic structures. Optimization was achieved at a laser power of 1600 W, a scan speed of 12 mm/s and a powder flow rate of 2 g/min. The surface roughness (Ra) for Ti–6Al–4V, AlCoCrCuFeNi and (Ti–6Al–4V)-(AlCoCrCuFeNi) alloy was 0.5 μm, 0.63 μm and 0.80 μm, respectively. The high wear resistance of the novel Ti6Al4V– AlCoCrFeNiCu alloy was influenced by the hardness of the alloy which was higher than the Ti6Al4V alloy and the AlCoCrFeNiCu alloy. This study successfully defines the capabilities of in situ fabrication of high entropy alloys and presents novel techniques for multiple powder preparation of high entropy alloys using laser additive manufacturing, to permit the next generation of compositionally graded materials for aerospace components.

Crystal structure, Curie temperature, and electromagnetic absorption properties in FexCo30Ni60−xSi5Al5 (x = 30, 35, 40, 45) high entropy alloys

In order to meet the requirements of future national defense for high temperature electromagnetic (EM) absorbing performance, a series of FexCo30Ni60−xSi5Al5 (x = 30, 35, 40, 45) high-entropy alloys (HEAs) powders was prepared and their Curie temperatures (TC) were measured by a self-made Wheatstone bridge. According to the results, varying the Fe/Ni ratio affected the crystal structure, Curie temperature, oxidation resistance, and electromagnetic absorbing properties of the above compounds. Since Fe has a BCC structure and is thus easier to form the solid solutions with Si and Al, the crystal structure of the alloy has changed from FCC toward BCC with increasing Fe dopant content. In turn, the Curie temperature (TC) decreased from 473.68 °C to 358.07 °C, being lower than their initial oxidation temperature (>800 °C). The reflection losses (RL) of powders at room temperature and high temperatures (≤500 °C) were calculated as well. It was found that the flake powders after ball milling gained a larger aspect ratio, resulting in the better absorption effect, which was due to high toughness and low strength characteristics of the initial FCC structure. Furthermore, the permittivity and permeability of alloys upon heating reached impedance matching at a certain temperature, thus achieving the greater RLmax value. Finally, the high-temperature EM absorption characteristics of HEAs were shown to merit a thorough study.

Microstructures and properties of CoCrFeNiX (X=Mn, Cu) high entropy alloys

CoCrFeNiX (X=Mn, Cu) high entropy alloys (HEAs) were prepared by powder metallurgy. The effects of milling time and milling speed on the microstructures and properties of CoCrFeNiX (X=Mn, Cu) HEAs were studied. The results show that at high rotational speed, the particle size of the HEA powders is refined and is tended to be uniform, and obvious solid solution phenomenon occurs; the microstructure of CoCrFeNiMn HEA is mainly composed of BCC solid solution, FCC solid solution and CoNiCr intermetallic compound; with the increase of ball milling time, Cr-rich phase and CoNiCr compound decrease. The microstructure of CoCrFeNiCu HEA is mainly composed of BCC solid solution and FCC solid solution; with the increase of ball milling time, Ni-rich FCC solid solution is gradually replaced by Co-rich and Ni-rich FCC solid solution. CoCrFeNiMn and CoCrFeNiCu HEAs have higher compressive strength, compression ratio and hardness, and they are 1300 Mpa and 1100 Mpa, 34% and 33%, 400 HV and 350 HV, respectively; The friction coefficient curve of CoCrFeNiMn HEA is more stable than that of CoCrFeNiCu HEA.

Structure and phase transformations in gas atomized AlCoCrFeNi high entropy alloy powders

In this study, the crystal structure and phase stability of gas atomized equiatomic AlCoCrFeNi powder was investigated. This alloy is usually described as a high entropy alloy forming a solid solution phase stabilized by a high mixing entropy. However, thermodynamic calculations show that the high entropy phase is stable only at very high temperatures close to the melting point and that a mixture of several phases are the most stable state at lower temperatures. This suggest that kinetic effects may influence the phase composition of atomized powder. The unique features of X-ray diffraction, neutron diffraction as well as transmission electron microscopy were used to study the atomic structure of the atomized powder in detail. The results show that the powder crystallises in an ordered B2 (CsCl-type) structure with a preferred site occupation of Al and Fe on the (½ ½ ½) position and Co and Ni on the (0 0 0) position. During heat-treatment of the powder, the B2 phase decomposes into fcc and σ phases and the final phase composition is highly dependent on the heating rate. The effect of heat-treatment on the atomized powder was also investigated and revealed a significant phase transformation with e.g. the formation of σ phase preferably at the surface of the powder particles. The phase content was also dependent on the size fraction of the powder particles. Sintering of green bodies made with different heat cycles showed that the phase composition of the starting material had a significant impact on the final phase composition and microstructure of the sintered components. The results illustrate the importance of well-defined powder materials for powder consolidation, especially additive manufacturing (binder jetting) of high entropy alloys.

Fast mechanical synthesis, structure evolution, and thermal stability of nanostructured CoCrFeNiCu high entropy alloy

A powder of equiatomic CoCrFeNiCu high entropy alloy (HEA) was prepared by short-term (120 min) high energy ball milling (HEBM). Our structural and chemical analysis showed that microsized particles of fcc CoCrFeNiCu with a grain size of 8 nm were obtained after 120 min of HEBM at 694/1388 rpm. The structural/phase evolution of CoCrFeNiCu HEA powder and its thermal stability were explored by high-temperature XRD at 600 °C, 800 °C and 1000 °C, by DSC up to 1500 °C, through the consolidation by SPS at 800 °C and 1000 °C, and characterized using XRD, SEM and EDX analyses. In-situ HT XRD analysis during 5.5 h of annealing showed the involvement of transient phases: the bcc phase that appeared in 1 h of annealing at 600 °C and disappeared at higher temperatures; and the fcc1 phase (Cu-rich) arising in 2 h of annealing at 800 °C and disappearing at 1000 °C in 3 h of annealing. SPS consolidation at 1000 °C and annealing at 1000 °C for 5.5 h were found to result in the formation of single-phase fcc2 CoCrFeNiCu alloy with a lean amount of Cu. The melting points for Cu-rich and Cu-depleted HEAs were found as 1118 °C and 1288 °C (Calphad calculations) and 1115 °C and 1365 °C (DSC measurements), respectively. SPS consolidation at 1000 °C under a pressure of 50 MPa yielded the single-phase fcc CoCrFeNiCu0.5 alloy that turned thermodynamically more favorable than the equiatomic one. Thus, we can suppose thereupon, that the equiatomic fcc phase that appears after 120 min of HEBM is metastable because of the excess of Cu atoms. During annealing in the temperature range 800–1000 °C, the Cu-rich fcc1 phase precipitates from the initial single-phase alloy, while the “mother phase” transforms into the more stable Cu-depleted fcc2 phase. The chemical compositions of Cu-depleted and Cu-rich phases for the SPS-consolidated HEA CoCrFeNiCu alloy (at 800 °C) were determined from TEM–EDX analyses. Optimal combination of short-term HEBM and SPS consolidation can be recommended as a facile route to fabrication of single-phase fcc equiatomic CoCrFeNiCu HEA powders and bulk materials with good structural/elemental homogeneity.

Promoting the sintering densification and mechanical properties of gas-atomized high-entropy alloy powder by adding boron

Powder metallurgy (PM) with pressure-assisted sintering is a versatile process for producing high-entropy alloys (HEAs) with a fine grain size and high strength. Unfortunately, HEA powders are extremely difficult to sinter with a general press-and-sinter PM process due to the sluggish diffusion effect in HEAs. One economical means to effectively densify the PM materials is liquid phase sintering (LPS). According to the phase diagrams, boron (B) is a feasible element for facilitating LPS of an AlCoCrFeMoNi powder because B can react with Co, Cr, Fe, Mo, and Ni to generate a low-temperature liquid phase. The objective of this study was thus to conduct the first investigation of the influences of 0.8 wt% B on the sinterability, microstructure, and mechanical properties of the AlCoCrFeMoNi HEA powder. The results showed that after 1200 °C sintering, the densification behaviors of both the AlCoCrFeMoNi and AlCoCrFeMoNi+0.8B alloys were undesirable. However, adding 0.8 wt% B to AlCoCrFeMoNi and sintering at 1220 °C significantly increased the densification and reduced the porosity from 11 vol% to 1 vol%. The onset temperature for liquid generation was identified as 1205 °C by differential scanning calorimetry curve. The microstructure of the AlCoCrFeMoNi alloy sintered at 1220 °C was face-centered cubic and sigma phases. The 0.8 wt% B additive led to the generation of Mo2FeB2 boride at the expense of sigma phase. Furthermore, adding 0.8 wt% B to the AlCoCrFeMoNi alloy sufficiently improved the yield strength by 42%, ultimate compressive strength by 66%, and fracture strain by 92%, due to the LPS effect. The AlCoCrFeMoNi powder was successfully sintered by adding B to facilitate LPS. In the future, this alloy design can be also applied to the other HEA powders that contain principal elements that react with B to form a low-temperature liquid.

Investigation of phase constitution and stability of gas-atomized Al0.5CoCrFeNi2 high-entropy alloy powders

In this study, Al0.5CoCrFeNi2 high-entropy alloy powders with uniform elements distribution were prepared by gas atomization process. The effects of heat treatment on the microstructure, phase transformation and mechanical properties were discussed. The FCC phase was observed in the as-atomized powder. The In-situ XRD results indicated that phase transform occurred at around 900 °C. Therefore, the annealing treatment was conducted at 1000 °C to observe the phase transformation. From the XRD of the different annealing times, additional diffraction peaks began to generate after 24 h. With SEM and EDS analyses, the results indicated the Cr element precipitated along the grain boundaries. The longer the annealing time, the more the precipitates. Afterward, EBSD was carried out to identify the crystal orientation of the phase. It was found that the FCC phase was aligned with the Ni3Al crystal structure. On the other hand, the Cr-rich phase formed after the annealing process tended to form the BCC crystal structure of NiAl and Fe, observed from the results of EDS and EBSD. Furthermore, the average hardness of as-atomized powder was 4.46 GPa. After the annealing process, the lattice distortion was eliminated. As a result, the hardness of FCC matrix was significantly decreased to 2.6 GPa. Whereas the hardness of segregated Cr-rich phase was 12.74 GPa. Thus, the average hardness of annealed powder was increased to 6.93 GPa due to the precipitation strengthening resulting from Cr-rich region.

Microstructure and Mechanical Behavior of AlCoCrFeNi HEA Particles Reinforced 6063Al Alloy Matrix Composites

Lightweight composites consisting of 6063 aluminum alloy matrix reinforced with AlCoCrFeNi high-entropy alloy (HEA) particles were fabricated by the powder metallurgy through hot pressing and hot extrusion under vacuum. A detailed microstructural characterization was carried out along with the analysis of mechanical properties. The results revealed that the observed uniform distribution of HEA powders and a good bonding strength resulted in an effective strengthening and increased plastic strain of the reinforced composite. The tensile properties of composites were effectively improved by the addition of 10 and 30 vol % of reinforcing HEA particles.

Bimodal Microstructure Design of CrMnFeCoNi High-Entropy Alloy Using Powder Metallurgy

Single-phase equiatomic high-entropy alloy (HEA); CrMnFeCoNi, exhibits high strength and ductility at room temperature and low temperature due to the effect of twinning. In this study, a concept of a bimodal microstructure design for HEA using powder metallurgy was proposed. Microstructures of the sintered compact fabricated from HEA powder mechanically-milled using SUJ2 balls were analyzed by EBSD. A network structure of fine grains (Shell), which surrounded the coarse-grained structure (Core), was formed in the HEA compact. The hardness of the HEA with bimodal microstructure were higher than the compact fabricated from as-received HEA powder, and the network structure showed high hardness than Core phase. Furthermore, W-rich surface layer was formed on the HEA powder mechanically-milled using WC-Co balls owing to the transfer of WC-Co balls to the surface of HEA powder during mechanical milling. A network structure of the W-rich phase, which surrounded the HEA phase, was formed in the compact fabricated from the mechanically-milled HEA powder. In particular, the hardness of compact fabricated from HEA powder mechanically-milled using WC-Co balls was high due to the high concentration of tungsten and formation of Shell phase. These results indicate that developed method can control the microstructure of HEA; grain size and elementary diffusion distributions.

Plasma spheroidized MoNbTaTiZr high entropy alloy showing improved plasticity

Refinement of the harder phase in two-phase MoNbTaTiZr high-entropy alloys (HEA) prepared by a powder metallurgy route has enhanced strength and plasticity of the alloy. The MoNbTaTiZr HEA powder used in this study was prepared by hydrogenation–dehydrogenation followed by plasma spheroidization, having an inherent dual-phase structure (bcc1 and bcc2) similar to the arc melted alloy of the same composition but with much finer morphology of the bcc2 phase. The homogenous and fine distribution of the harder Zr-rich phase (bcc2) with ~10% volume fraction embedded inside the Ta-rich phase (bcc1) evaded the strength–ductility trade-off. The shear-band delocalization in the bcc1 matrix phase improved the strength as well as the plasticity of the alloy. The alloy has potential in biomedical implants because it shows excellent corrosion resistance and lower magnetic susceptibility, making it an MRI (magnetic resonance imaging)-friendly alloy.

Microstructural Characterization of TiC-Reinforced Metal Matrix Composites Fabricated by Laser Cladding Using FeCrCoNiAlTiC High Entropy Alloy Powder

TiC-reinforced metal matrix composites were fabricated by laser cladding and FeCrCoNiAlTiC high entropy alloy powder. The heat of the laser formed a TiC phase, which was consistent with the thermodynamic calculation, and produced a coating layer without interfacial defects. TiC reinforcing particles exhibited various morphologies, such as spherical, blocky, and dendritic particles, depending on the heat input and coating depth. A dendritic morphology is observed in the lower part of the coating layer near the AISI 304 substrate, where heat is rapidly transferred. Low heat input leads to an inhomogeneous microstructure and coating depth due to the poor fluidity of molten pool. On the other hand, high heat input dissolved reinforcing particles by dilution with the substrate. The coating layer under the effective heat input of 50 J/mm2 had relatively homogeneous blocky particles of several micrometers in size. The micro-hardness value of the coating layer is over 900 HV, and the nano-hardness of the reinforcing particles and the matrix were 17 GPa and 10 GPa, respectively.

Particle size effects on dislocation density, microstructure, and phase transformation for high-entropy alloy powders

Recrystallization and phase transformation reactions are driven by energy stored in the parent phase due to previous processing history. Grain boundaries, lattice strain, and dislocation networks may affect subsequent microstructural evolution, especially during metal powder consolidation processes. In this work, AlCrFe2Ni2 eutectic high-entropy alloy powders over a wide range of size distribution were used to investigate the correlation between dislocation density and microstructure as a function of particle size. Line profile analysis of the X-ray diffraction patterns of the as-received (quenched) samples shows that the dislocation density increases linearly with decreasing particle size. Based on microstructure analysis of the as-quenched and the annealed samples, it is found that the correlation is associated with the grain boundary length which increases with decreasing particle size, revealing that the key source of dislocation density is dislocations within the grain boundaries. The grain boundary energy acts as a driving force for the metastable to stable phase transformation of AlCrFe2Ni2, showing that the phase transformation kinetics is a function of particle size. This work shows a direct experimental observation and quantitative analysis that in metallic powder systems particle size is one of the key parameters which affects the dislocation density and the phase transformation kinetics most probably due to the different cooling rates achieved during powder production.

High-temperature mechanical and thermochemical properties of NbMoTaW refractory high-entropy alloy coatings produced via Nano Particle Deposition System (NPDS)

In this study, we investigated the oxidation resistivity and mechanical properties of NbMoTaW refractory high-entropy alloy (HEA)-based coatings at high temperatures. The coating layers were produced using mechanically alloyed HEA powders via Nano Particle Deposition System (NPDS). The as-milled powder is comprised of W-rich BCC1 and NbMoTa-rich BCC2 phases. After the deposition process, the W-rich BCC1 phase remained, while NbMoTa-rich BCC2 phases were transformed into complex NbMo-rich and Ta-rich oxides. Consequently, the NbMoTaW RHEA composite coating exhibited outstanding oxidation resistivity and high nanoindentation hardness at high temperatures compared to that of commercial coating materials, possibly owing to the combination of the W-rich solid solution phase and internal oxides.

Comparison of Micro-nano FeCoNiCrAl and FeCoNiCrMn Coatings Prepared from Mechanical Alloyed High-entropy Alloy Powders

Comparison of Micro-nano FeCoNiCrAl and FeCoNiCrMn Coatings Prepared from Mechanical Alloyed High-entropy Alloy Powders

Preparation of nanostructured CoCrFeMnNi high entropy alloy by hot pressing sintering gas atomized powders

Pre-alloyed CoCrFeMnNi high-entropy alloy (HEA) powders were prepared by gas atomization and consequently hot pressing sintered (HPS) at 1100 °C for 2 h for fabrication bulk materials. Sintered equiatomic CoCrFeMnNi HEA exhibited a homogeneous FCC structured single-phase solid solution and equiaxed grains with an average size of ∼16 μm. Analyses using TEM showed that sheet-like metastable structures with a size of ∼55 to 160 nm were formed in the sintered bulks. The nano-sized metastable structures were inherited from the gas atomized CoCrFeMnNi powders with nano-sized crystallites formed during the rapid solidification process. The room temperature yield and ultimate tensile strength of sintered HEA reached 358 and 778 MPa, respectively, and the alloy kept excellent plasticity of ∼28 %. Investigations of the deformation substructures at specific strain levels with EBSD revealed the sintered CoCrFeMnNi HEA kept FCC structured single phase and the main deformation mechanism was dislocation slip. The strengthening mechanism can be attributed to the combination effects of grain refinement and the presence of nano-sized metastable structures.

CoCrFeNi High-Entropy Alloy Thin Films Synthesised by Magnetron Sputter Deposition from Spark Plasma Sintered Targets

Two magnetron sputter targets of CoCrFeNi High-Entropy Alloy (HEA), both in equal atomic ratio, were prepared by spark plasma sintering. One of the targets was fabricated from a homogeneous HEA powder produced via gas atomisation; for the second target, a mixture of pure element powders was used. Economic benefits can be achieved by mixing pure powders in the intended ratio in comparison to the gas atomisation of the specific alloy composition. In this work, thin films deposited via magnetron sputtering from both targets are analysed. The surface elemental composition is investigated by X-ray photoelectron spectroscopy, whereas the bulk stoichiometry is measured by X-ray fluorescence spectroscopy. Phase information and surface microstructure are investigated using X-ray diffraction and scanning electron microscopy, respectively. It is demonstrated that the stoichiometry, phase composition and microscopic structure of the as-deposited HEA thin films are almost identical if the same deposition parameters are used.

Effect of light (X = Mg, Si) and heavy (X = Zn) metals on the microstructural evolution and densification of AlCuFeMnTi-X high-entropy alloy processed by advanced powder metallurgy

ABSTRACT Novel AlCuFeMnTi–X high-entropy alloy (HEA) powder mixture with individual X = Mg, Si, Zn was mechanically alloyed for 45 h and consolidated at 700°C by spark plasma sintering (SPS). The effect of the addition of Mg, Si and Zn was studied on the microstructural evolution, density and microhardness of the developed HEAs. The results demonstrated that milled HEAs were body-centered cubic (BCC) structured (BCC1 and BCC2). The BCC2 fraction was highest in AlCuFeMnTi–Zn followed by AlCuFeMnTi–Si. However, minor Ti, Si and intermetallic compounds (IMC) were observed in AlCuFeMnTi–Mg with BCC1 major phase. After SPS, the alloying improved, and BCC2-rich HEAs were observed for all compositions. An attractive hardness value was obtained in AlCuFeMnTi–Mg due to the dispersed Cu2Mg precipitates in the BCC2/BCC1 matrix followed by AlCuFeMnTi–Si and AlCuFeMnTi–Zn HEA, respectively.