AlCoCrCuFeNi High-entropy Alloy Research Articles

Laser shock peening of AlCoCrCuFeNi high-entropy alloy fabricated by laser powder bed fusion: An enhanced oxidation-resistance mechanism at high-temperature

The high-temperature oxidation behaviors of laser powder bed fusion (LPBF)-fabricated AlCoCrCuFeNi high-entropy alloy (HEA) with and without laser shock peening (LSP) were investigated over different exposure durations and temperatures. Results revealed that LSP converted the surface stress from tensile to compressive, while also inducing grain refinement and high-density dislocations. Under high temperatures, the coupled effects of grain refinement and compressive residual stress resulted in the formation of dense protective oxidation scales, which inhibited the inward diffusion of oxygen. LSP improved the high-temperature oxidation resistance of LPBF-fabricated AlCoCrCuFeNi HEA, and the enhanced oxidation-resistance mechanisms were discussed.

Structure and magnetism of AlCoCrCuFeNi high-entropy alloy

We report on the investigation of magnetic and structural properties of AlCoCrCuFeNi, which is known to crystallize in a dual phase solid solution: the face-centred cubic (FCC) or the body-centred cubic (BCC). The results of neutron (NPD) and synchrotron powder diffraction (SXRD) allow to partially resolve magnetic information coming from BCC and FCC phases, which is impossible in the bulk magnetic measurements. Electron diffraction (PED) revealed that AlCoCrCuFeNi forms dendritic microstructure with the Cu-rich FCC phase and the Ni-rich BCC phase. Lattice parameters obtained from PED method are in good agreement with parameters obtained after refinement on the basis of powder X-ray diffraction measurements. The local crystal and electronic structure around Co was studied using Co K X-ray Absorption Spectroscopy (XAS). The magnetic measurements show that AlCoCrCuFeNi reveal a ferromagnetic transition at about 330 K and displays magnetic hysteresis loop at the room temperature. Results from NPD suggest that the magnetic moment is mostly located in the BCC subsystem. The alloy shows soft magnetic properties. Saturated magnetizations (Ms), remanence ratio (Mr/Ms) and coercivity (Hc) of the cast are estimated to be 45.10 emu/g, 5.1 % and 56 Oe at 300 K, respectively. Finally, the BCC-FCC phase transformation up to 673 K (400 °C) was investigated using temperature dependent NPD, where a possible second BCC phase was identified.

Wear characteristics of laser-deposited AlCoCrCuFeNi high entropy alloy with finite element analysis

BackgroundWear is a destructive phenomenon and one of the principal causes of material failure in moving components during surface interaction while in service. AlCoCrCuFeNi high-entropy alloy with its many properties is a potential material for aero-engine applications attributed to its outstanding relatively lightweight, high strength, good thermal, oxidation, and corrosion resistance properties. Hence, the investigation into the tribological behaviour of AlCoCrCuFeNi high-entropy alloys is essential to reduce maintenance costs and prolong the service life of this advanced material for aerospace applications. Most AlCoCrCuFeNi high-entropy alloy compositions were fabricated via arc melting, which has been reported to have defects attributed to slow solidification, consequently reducing the mechanical properties of the alloy with limited reports on other fabrication methods. Therefore, there is a need for the use of advanced manufacturing techniques for fabricating these alloys to improve the tribological properties. In this study, AlCoCrCuFeNi high-entropy alloy was fabricated via laser metal deposition. The influence of the laser processing parameters, rapid solidification, and the applied load on the tribological properties of the as-built alloys under dry conditions has been studied for aerospace applications. The counter ball rolling friction analysis was also investigated using COMSOL Multiphysics.ResultsThe results showed that at a high laser power of 1600 W and a scan speed of 12 mm/s, the lowest wear rates and highest hardness values were observed. The average coefficient of friction at room temperature was 0.1 and 0.3 at a speed of 21 m/s. The dominant wear mechanism at room temperature was abrasive wear as the wear rate increased linearly with an increase in load from 10 to 20 N. The scan speed had the most significant influence on the wear behaviour of the as-built high-entropy alloy attributed to the rapid rate of solidification which occurs at higher scan speeds.ConclusionsThe study examines the wear characteristics of high-entropy alloys fabricated via laser deposition technique in comparison with those fabricated via conventional routes. Although there were similarities in the phase structures of both techniques, the results showed that the wear resistance of the laser-deposited high-entropy alloy was comparatively higher than the same alloy prepared via conventional methods. Laser additive manufacturing was concluded to be a more successful method in fabricating high-entropy alloys.

Phase composition and its effect on the mechanical performance of an AlCoCrFeNiTi high-entropy alloy

The concept of High-Entropy Alloy (HEA) expanded the research field of advanced metallic materials for various applications, like the development of ultra-hardness ballistic protection materials for national security. Although there can be hundreds of compositions, carefully selecting the constituent is mandatory to improve their mechanical behavior, keeping in mind their microstructural array based on chemical composition. In the present study, the AlCoCrCuFeNi HEA was modified, replacing copper with titanium, looking for the formation of a Ti-rich BCC phase for hardening effects. Samples were prepared following the powder metallurgy route, including mechanical alloying favoring the generation of a nanocrystalline microstructure. Studies based on structure, microstructure and nanoindentation testing on each phase of the alloy were performed to determine the correlation between their composition, crystalline structure and mechanical properties. Evidence showed the formation of three main micrometric phases (two BCC and one tetragonal) coexisting with a nanometric dispersion of rounded precipitates. Nanoindentation testing shows that the main hardening effect was related to the tetragonal phase formation by a solid-solution strengthening mechanism. This phase reached the highest hardness (14.9 GPa); meanwhile, the richest Ti phase showed the lowest elastic modulus, titanium favors the material ductility.

Effect of high magnetic field assisted heat treatment on microstructure and properties of AlCoCrCuFeNi high-entropy alloy

High magnetic field assisted heat treatment process was used to regulate the microstructure and properties of AlCoCrCuFeNi high-entropy alloy. Results show that applying magnetic field can refine the FeCoCr-rich phase and suppress the large regional segregation of Cu-rich phase. The strength of the alloy increases greatly from 1482 MPa to 1795 MPa and the elongation also increases from 23% to 27%. The saturation magnetization is depressed after the magnetic field is applied. These variations are attributed to the decrease of Cu-rich phase segregation, grain refinement, and also the slightly decreased volume fraction of BCC phase upon applying high magnetic field.

Microstructure and properties of AlCoCrCuFeNi high-entropy alloy solidified under high magnetic field

High static magnetic field was used to regulate the microstructure and properties of AlCoCrCuFeNi high-entropy alloy. Result shows that applying high magnetic field does not change the microstructure obviously but can improve the content of ferromagnetic FeCr-rich BCC phase. The yield stress and plastic strain were improved simultaneously, and saturation magnetization value increased from 27.3 emu/g to 34.5 emu/g. Our research shows that introducing high magnetic field is a good method to tailor the mechanical and magnetic properties of high-entropy alloys.

Microstructural and mechanical properties of AlCoCrCuFeNiSix (x = 0 and 0.9) high entropy alloys

In this work, the effect of addition of Si on the phase evolution and mechanical properties of AlCoCrCuFeNiSix alloy system has been studied. The High Entropy Alloys (x = 0 and 0.9) have been synthesised by powder metallurgy route which includes mechanical alloying (MA) and spark plasma sintering. X-ray diffraction technique was performed to understand the alloying behaviour and to investigate the phase formation of the high entropy alloys. The samples after spark plasma sintering comprised mainly of body centered cubic structured phase with a small extent of face centered cubic structured phase. With the addition of silicon, the XRD peak intensity of body centered cubic appears strong compared to the face centered cubic structured phase. Moreover, no intermetallic is observed in AlCoCrCuFeNi high entropy alloy system. However, further accumulation of Silicon results in the evolution of sigma (σ) phase.

Investigation on microstructure and properties of laser cladded AlCoCrCuFeNi high entropy alloy coating by ultrasonic impact treatment

The application of high entropy alloy (HEA) as coating materials has become a research hotspot. The combination of laser cladding technology and ultrasonic impact treatment (UIT) may provide an opportunity for better performance of HEA and it is possible to enhance the application prospect of HEA. To research the effects of UIT on microstructure and properties of laser cladded HEA coating, AlCoCrCuFeNi HEA coating was prepared by means of laser cladding and UIT was applied to strengthen the surface properties of AlCoCrCuFeNi HEA coating. The phase structures of laser cladded AlCoCrCuFeNi HEA coating were composed of FCC and BCC phases. There were no new phases formed in the AlCoCrCuFeNi HEA coating after UIT, while the BCC phase volume fraction was increased. Additionally, a hardening layer with a depth of ~15 μm was formed at the top surface and the average values of grain sizes were refined from 34.4 μm to 18.6 μm. As expected, the hardness of AlCoCrCuFeNi HEA coating with UIT was significantly improved to 715 HV0.2 benefitted from the nano crystal structure and enhanced plastic deformation and then decreases gradually along the depth to approximately the same hardness value as that of without UIT (530 HV0.2). More impressively, the impact hardening AlCoCrCuFeNi HEA coating exhibited higher corrosion resistance in 3.5% NaCl solution resulting from the lower surface roughness and fewer microstructure defects.

Microstructures and properties of equimolar AlCoCrCuFeNi high-entropy alloy additively manufactured by selective laser melting

An equimolar hexahydroxy AlCoCrCuFeNi high-entropy alloy (HEA) was produced by selective laser melting (SLM) gas-atomized powders, with emphasis on investigating its microstructure and properties. SLM printed AlCoCrCuFeNi HEA shows high crack sensitivity with faced centered cubic (FCC) and body-centered cubic (BCC) phases. High-angle grain boundary, segregation of Cu element and the misfit between BCC and FCC phase account for the crack formation during SLM process. The content of FCC phase gradually increases from 30.55% to 40.20% as VED increases from 52.08 J/mm3 to 83.3 J/mm3. The formation of BCC is restrained owing to rapid solidification. Moreover, unique fine columnar grains growing along the building direction are perpendicular to the boundary of molten pools and present (001), (111) and (101) grain orientation. Also, the fine grains and the high-density dislocation network induced by rapid cooling of the SLMed AlCoCrCuFeNi HEA are observed. The SLMed AlCoCrCuFeNi HEA possesses outstanding properties with a Vickers hardness of 710.4 Hv compared to its counterpart prepared by traditional methods of 500 Hv.

Heat Treatment Influence on Tribological Properties of AlCoCrCuFeNi High-Entropy Alloy in Hydrogen Peroxide-Solution

Tribological properties of AlCoCrCuFeNi high-entropy alloy were studied after annealing at various temperatures. X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, microhardness tester and pin-on-disc tribometer analyses were performed to reveal the microstructure, composition, microhardness and tribological behavior variations. With the heat treatment temperature increasing, time taken for friction coefficient going through the rapidly dropping down and then into the stable period increases, the white sheets structures and their size in dendrite area of the AlCoCrCuFeNi alloy becomes bigger, however BCC content declined dramatically. Then, the average friction coefficient of the AlCoCrCuFeNi/Si3N4 sliding friction pair increase from 0.037 to 0.115, and the pin wear loss increase from 3 to 11 µm.

Solidification behavior and morphological evolution in laser surface forming of AlCoCrCuFeNi multi-layer high-entropy alloy coatings on AZ91D

The AlCoCrCuFeNi high-entropy alloy (HEA) coatings after one, two and three layers were laser cladded on the AZ91D substrate. Towards the coating interface, the substrate is composed of Mg dendrites and interdendritic Mg + Q eutectics. The formed coatings can be categorized into a low narrow composite region and a top thick dense HEA region. The dominated microstructures in the dense HEA region are columnar dendrites. Nevertheless, the average primary spacing of columnar dendrites became smaller when the coating grew thicker mainly due to the temperature gradient decreasing slowly. Moreover, the planar morphology at re-melted boundary of each clad layer is absence because of the local conditions being suitable for epitaxial growth. Additionally, it is found that the columnar to equiaxed transition (CET) had occurred in the top area of the three-layer HEA coating. This evolution of the microstructure can be explained by the CET theory combined with the sluggish diffusion kinetics and the abnormal variation of the thermal conductivity with the temperature of the AlCoCrCuFeNi HEA. Additionally, the problem associated with dilution by Mg can be completely suppressed by the HEA coating after three layers.

Concurrence of spinodal decomposition and nano-phase precipitation in a multi-component AlCoCrCuFeNi high-entropy alloy

The concurrence of spinodal decomposition and nano-phase precipitation in an AlCoCrCuFeNi high-entropy alloy produced by arc melting, spray casting and melt spinning was investigated in the present work. It is shown that the present high cooling rate cannot inhibit the concurrence of spinodal decomposition and nano-phase precipitation. In the solidification and annealing process, three typical structures including Cu-rich nano-precipitates of FCC structure, Al-Ni-rich plate of B2 structure, and Fe-Cr-rich plate of A2 structure were observed; meanwhile, the width of modulated plate decreased with the increase of the cooling rate. An interesting phenomenon was observed in the as-annealed microstructure that Al-Ni and Fe-Cr spinodal plates were weakened or vanished rather than strengthened, due to the atom diffusion and composition homogenization. Different from spinodal decomposition plates, the Cu-rich nano-precipitates grew into rod-like phase from point-like or oval phase in the whole process. The intersection angle of newborn rod-like nano-phases tended to 60°, following the K-S crystallographic relationship.

Fabrication and characterization of WC-AlCoCrCuFeNi high-entropy alloy composites by spark plasma sintering

Mechanically alloyed AlCoCrCuFeNi high-entropy alloy (HEA) powders were used as binder to fabricate WC/HEA composites by spark plasma sintering. The effects of sintering temperature, duration time and binder content on microstructures and mechanical properties were studied. Compared with cobalt binder, the HEA binder has an advantage on the inhibition of WC grain growth due to the sluggish diffusion effect, and the average WC grain size decreases with increasing HEA binder content. Furthermore, the fracture toughness of WC/HEA composites firstly increases with increasing Vickers hardness and then decreases. A good comprehensive mechanical property was obtained after sintering at 1250 °C for 5 min with 10 wt.% HEA binder under a sintering pressure of 30 MPa. The Vickers hardness is 1922 HV30 and the fracture toughness is 10.41 MPa m1/2, which are higher than those of commercial WC/Co composites. Therefore, the AlCoCrCuFeNi high-entropy alloy has a potential to be a binder for WC.

Strengthening mechanisms in equiatomic ultrafine grained AlCoCrCuFeNi high-entropy alloy studied by micro- and nanoindentation methods

A single phase fcc based nanocrystalline solid solution in equiatomic AlCoCrCuFeNi high-entropy alloy (HEA) has been synthesized using ball milling. The milled powders were of “plate-like” morphology and possessed a precise lattice parameter of 3.641 Å. Compaction of ball milled powders into bulk components using spark plasma sintering (SPS) at 1023 K led to the precipitation of ordered bcc (B2). Detailed structural and microstructural investigations on the sintered alloy indicate the presence of bimodal grain size distribution with average grain sizes of 112 ± 46 nm and 1550 ± 500 nm, solid solutions (fcc and B2 phases), dislocations and twin boundaries. A high hardness value of 6.5 ± 0.1 GPa was measured for the sample sintered at 1023 K/15 min using Vickers microindentation. Comprehensive analysis on probable strengthening mechanisms suggests that frictional stress, Taylor hardening, Hall-Petch strengthening, solid solution strengthening and twin boundary strengthening mechanisms are responsible. The Taylor hardening arising from intersection of dislocations and grain boundary (Hall-Petch) strengthening arising from grain boundary-dislocation interactions together account for 85% of the observed flow stress. The Tabor's ratio, (H/σflow) attained a value of 2.7 which is in close agreement with that for conventional polycrystalline materials. Nanoindentation at a peak force of 8000 μN yielded a high hardness value of 8.13 ± 0.15 GPa and an elastic modulus of 172 ± 10 GPa. A low strain rate sensitivity of 0.0084 and an activation volume of 13 b3 (b is 0.23 nm) were measured, suggesting that grain boundaries, twin boundaries and interphase boundaries (fcc/B2) are influential in governing the deformation kinetics.

The effect of Cu rejection in laser forming of AlCoCrCuFeNi/Mg composite coating

AlCoCrCuFeNi/Mg composite coating was formed by means of laser melt injection using a direct blown powder method with the aim of investigating the effect of Cu rejection in laser forming of AlCoCrCuFeNi high-entropy alloy (HEA) particle reinforced composite coating on magnesium substrates. It is found that the distribution of rejected Cu was uneven within the formed coating. As a distance from the substrate increased, a higher Cu content was detected in the area between the reinforcing HEA particles. Accordingly, the dendritic and cellular structures of CuMg2 were obtained in the upper region of the composite coating due to a high Cu content. With the Cu concentration decreasing, the microstructure was gradually dominated by columnar Mg when moving towards the substrate. Although microhardness values increased, room temperature dry sliding wear properties of the formed coating were adversely influenced by the Cu rejection due to the formation of the brittle CuMg2 phase. Additionally, the Cu rejection could promote the BCC-structured formation within the HEA particles, and therefore it may cause a change in deformation behavior of the reinforcement due to the increase of hardness and the decrease of plasticity of the reinforcing HEA particles.

Reinforcement and substrate interaction in laser surface forming of AlCoCrCuFeNi particle reinforced AZ91D matrix composites

AlCoCrCuFeNi high-entropy alloy (HEA) particles were laser melt injected on AZ91D substrates using a one-step blown powder method to investigate the interaction between the reinforcement and the substrate in laser surface forming of AlCoCrCuFeNi HEA particle reinforced AZ91D metal matrix composites (MMCs). The results show that some Cu was rejected into the substrate melt during the processing. The Cu content in the melt has a great impact on the microstructure evolution of the formed MMC. When the Cu concentration reached a level of about 30 at.%, the CuMg2 phase was formed instead of the predicted λ1 phase. As the Cu content decreases, the λ1 phase as well as different Mg + CuMg2, Mg + λ1 and Mg + Q eutectics will be formed depending on the Cu concentration in the remaining melt. Moreover, decreasing the Cu content in the AlCoCrCuFeNi HEA particles can also affect the wear properties of the formed MMC because it could result in a plastic deformation to the brittle-like transition of the reinforcement.

Mechanical Behavior and Microstructure Evolution during Superplastic Deformation of the Fine-Grained AlCoCrCuFeNi High Entropy Alloy

Characteristics of mechanical behavior during superplastic flow and associated microstructural evolution in the wrought AlCoCrCuFeNi high-entropy alloy were studied. The alloy had complex microstructure with fine grain/particle size of ≈2.1 μm. 4 different phases with volume fractions from 7% to 46% and different deformation characteristics were found in the alloy. Very high tensile elongations of up to 1240% were observed during deformation at temperatures of 800°C–1000°C and at strain rates of 10-4 s-1–10-1 s-1 despite presence of pronounced softening stage followed by steady state flow stage. Microstructure of the alloy after tensile testing was studied in detail. Phase transformations were analyzed employing thermodynamic modeling and their role in strain accommodation is discussed.

Two-step Laser Cladding of AlCoCrCuFeNi High-entropy Alloy on AZ91-Mg

AlCoCrCuFeNi high entropy alloy (HEA) coatings were formed on AZ91-Mg substrates using a two-step laser cladding technique that was adopted to solve the HEA melting problem due to the large difference in their melting temperatures. First, a layer composed of partially melted HEA powders in an AZ91 Mg matrix was produced, i.e. a composite layer, followed by laser cladding on the composite layer surface. The results showed that the top layer of the coating had a bcc solid solution structure and no intermetallic compound was detected. The dilution behaviour and the reactions between the HEA powder and the matrix were studied. In the composite layer, the Cu from the HEA powder was the main element that entered into the substrate melt, however, no serious dilution occurred. The solidification reactions at the HEA powder surface were determined using the corresponding Mg-Al-Cu phase diagrams that were constructed based on the CALPHAD approach.

Equation of State of an AlCoCrCuFeNi High-Entropy Alloy

The pressure–volume (P–V) relationship of the AlCoCrCuFeNi high-entropy alloy (HEA) at room temperature has been studied using in situ high-pressure energy-dispersive x-ray diffraction with synchrotron radiation at high pressures. The equation of state of the AlCoCrCuFeNi HEA is determined by the calculation of the radial distribution function. The experimental results indicate that the HEA keeps a stable face-centered-cubic + body-centered-cubic structure in the experimental pressure range from 0 GPa to 24 GPa.

Laser surface forming of AlCoCrCuFeNi particle reinforced AZ91D matrix composites

Abstract Traditionally, the laser melt injection (LMI) technique can only be used for forming ceramic particles reinforced metal matrix composites (MMCs) for enhancing surface properties of lightweight engineering materials. In this research, the LMI method was employed to form metal particles reinforced MMCs on AZ91D instead. This was viable because of the unique properties of the AlCoCrCuFeNi high-entropy alloy (HEA) metal particles used. The large difference in melting point between the HEA and the substrate material (AZ91D), and the limited reaction and the lack of fusion between the HEA and Mg have made it possible that a metal particles reinforced AZ91D composite material was produced. The reason of limited reaction was considered mainly due to the relatively high mixing enthalpy between the HEA constituent elements and Mg. Although there was some melting occurred at the particles surface with some solute segregation found in the vicinity close to the surface, intermetallic compounds were not observed. With regard to the wear resistance of the MMCs, it was found that when the volume fraction of the reinforcement phase, i.e. the HEA particles, reached about 0.4, the wear volume loss of the coating was only one-seventh of that of the substrate material.