AlxCoCrFeNi High-entropy Alloys Research Articles

The sliding wear behaviour of CoCrFeMnNi and AlxCoCrFeNi high entropy alloys at elevated temperatures

High entropy alloys (HEAs) show promise as materials for structural applications, even at elevated temperatures. However, their wear behaviour over a wide range of temperatures has not been extensively studied. CoCrFeMnNi and AlxCoCrFeNi HEAs were subjected to pin-on-disc dry sliding wear at temperatures between 25 °C and 900 °C against an alumina ball, and the tribological performance benchmarked against AISI 304 and Inconel 718. A detailed characterisation of the wear tracks using electron microscopy and surface profilometry revealed a transition in wear mechanism from abrasive wear at room-temperature to oxidative and delamination wear above 600 °C. The wear performance of the HEAs, AlCoCrFeNi in particular, is substantially enhanced with increasing temperature, surpassing that of Inconel 718 at 900 °C. The enhanced wear performance of the HEAs above 600 °C is attributed to the formation of a compact oxide scale in the contact region, and relative subsurface strengthening in the form of a fine-grained recrystallised structure containing precipitation hardening phases.

Balancing Scattering Channels: A Panoscopic Approach toward Zero Temperature Coefficient of Resistance Using High-Entropy Alloys.

Designing alloys with an accurate temperature-independent electrical response over a wide temperature range, specifically a low temperature coefficient of resistance (TCR), remains a big challenge from a material design point of view. More than a century after their discovery, Constantan (Cu-Ni) and Manganin (Cu-Mn-Ni) alloys remain the top choice for strain gauge applications and high-quality resistors up to 473-573 K. Here, an average TCR is demonstrated that is up to ≈800 times smaller in the temperature range 5-300 K and >800 times smaller than for any of these standard materials over a wide temperature range (5 K < T < 1200 K). This is achieved for selected compositions of Alx CoCrFeNi high-entropy alloys (HEAs), for which a strong correlation of the ultralow TCR is established with the underlying microstructure and its local composition. The exceptionally low electron-phonon coupling expected in these HEAs is crucial for developing novel devices, e.g., hot-electron detectors, high-Q resonant antennas, and materials in gravitational wave detectors.

Investigation of Clusters and Their Effect on Grain Growth in Single Phase AlxCoCrFeNi High Entropy Alloys

Investigation of Clusters and Their Effect on Grain Growth in Single Phase AlxCoCrFeNi High Entropy Alloys

Effect of hot isostatic pressing on the microstructure and mechanical properties of additive manufactured AlxCoCrFeNi high entropy alloys

Three high entropy alloys (HEAs), based on the AlxCoCrFeNi alloy system have been prepared by direct laser fabrication (DLF) with aluminium molar fractions (x) of 0.3, 0.6 and 0.85. These three alloys had FCC, duplex FCC + BCC, and BCC crystal structures, respectively. The effect of hot isostatic pressing (HIP) on alloy density, microstructure and mechanical properties of these DLF bulk high entropy alloys was studied for the first time. HIP was found to decrease the number of large pores (> 5 µm) in the as-deposited alloys, which equated to a marginal increase in density. HIP also induced microstructural coarsening, chemical homogenisation and resulted in a general improvement in the mechanical properties of FCC HEA (x = 0.3). HIP improved the compressive properties of the dual phase HEA (x = 0.6), however, degraded the tensile properties as a result of the coarsening of hard BCC grain boundary precipitates. The mechanical properties were compromised in the high aluminium (x = 0.85) HEA due to the formation of σ-phase at the phase and grain boundaries, which induced a brittle fracture in tension and compression. A continuous cooling transformation (CCT) diagram for the σ-phase was determined by a dilatometric method and the critical cooling rate to inhibit σ-phase formation was found to be 1 K/s.

High‐Performance Microwave‐Derived Multi‐Principal Element Alloy Coatings for Tribological Application

The authors report the development of AlxCoCrFeNi (x = 0.1 to 3) high entropy alloy (HEA) coatings using a simple and straightforward microwave technique. The microstructure of the developed coatings is composed of a cellular structure and diffused interface with the substrate. The microstructure of the HEA coatings varies as a direct function of Al content. An increase in Al fraction shows structural transformation from FCC to BCC along with the evolution of σ and B2 as the major secondary phases. The diffusion of Mo from the substrate enhances the mixing entropy and promotes σ‐phase formation. The HEA coatings show significantly high hardness compared to SS316L substrate steel (227 HV) with a maximum value of 726 HV observed for three‐molar composition. The fracture toughness exhibits an inverse correlation with the Al fraction with the highest value of around 49 MPa m1/2 observed for Al0.1CoCrFeNi coating. The equimolar coating composition shows lowest erosion rates among all the tested samples due to optimum combination of the mechanical properties. The erosion resistance of the equimolar coating is 2 to 5 times higher than steel substrate and around 1.5 times higher than the non‐equimolar counterparts depending upon the impingement angles.

Homogenization of AlxCoCrFeNi high-entropy alloys with improved corrosion resistance

The present work investigates the homogenization effect of 1250 °C heat treatment on the AlxCoCrFeNi high-entropy alloys (HEAs). The multi-phase microstructures with chemical segregations are inevitable with the increased Al content in the alloys, which cause work function variations and localized corrosion. After heat treatment, the homogenization effect revealed by the microstructure simplification and chemical-segregation reduction leads to the decreased work function variations and the improved corrosion resistance. Thermodynamic calculations that are reliable to predict the phase transformations of the AlxCoCrFeNi HEAs, indicates a further enhancement in corrosion resistance through annealing could be guided for many other HEAs systems.

In-situ electrochemical-AFM study of localized corrosion of AlxCoCrFeNi high-entropy alloys in chloride solution

In-situ electrochemical (EC)-AFM is employed to investigate the localized corrosion of the AlxCoCrFeNi high-entropy alloys (HEAs). Surface topography changes on the micro/sub-micro scale are monitored at different applied anodizing potentials in a 3.5 wt% NaCl solution. The microstructural evolutions with the increased Al content in the alloys are characterized by SEM, TEM, EDS and EBSD. The results show that by increasing the Al content, the microstructure changes from single solid-solution to multi-phases, leading to the segregations of elements. Due to the microstructural variations in the AlxCoCrFeNi HEAs, localized corrosion processes in different ways after the breakdown of the passive film, which changes from pitting to phase boundary corrosion. The XPS results indicate that an increased Al content in the alloys/phases corresponds to a decreased corrosion resistance of the surface passive film.

Direct laser deposition cladding of AlxCoCrFeNi high entropy alloys on a high-temperature stainless steel

AlxCoCrFeNi (x=0.3, 0.6 and 0.85) high entropy alloy (HEA) claddings were produced by coaxial direct laser deposition (DLD) on a 253MA austenitic steel substrate using a mixture of blended elemental powders. The effect of key processing variables on the formation of HEA claddings and the compositional mixing between the deposited layer and the substrate was investigated through a three-level parametric study on laser power, laser scanning speed, laser beam size, powder feeding rate and hatch distance. Using selected parameters, HEA claddings mostly free of defects were successfully manufactured with very minimal dilution. With an increase in the Al mole fraction from 0.3 to 0.6 and 0.85, the HEA claddings displayed an evolution of crystal structure from FCC, to FCC+BCC and BCC, accompanied by an increase in microhardness. The increased Al content also resulted in reduced microstructural stability of the coatings and hence higher level of thermal softening upon isothermal treatment at 1000°C.

Synthesis of AlxCoCrFeNi high-entropy alloys by high-gravity combustion from oxides

A new method for preparing high-entropy alloys (HEAs), combustion synthesis under high-gravity, is presented in this paper, including several highly-exothermic aluminothermic reactions. The microstructures and mechanical properties of the AlxCoCrFeNi HEAs prepared by the above method were investigated. With the increase of the Al content, the crystal structures evolved from face-centered cubic (FCC) to body-centered cubic (BCC) structures, which was proved to be in line with the previous results. The microstructure is strongly related to the synergistic effect of high gravity and Al contents. Massively-dispersive nano-precipitation had been found in the Al0.75CoCrFeNi alloy. The yield strength increases while the plasticity decreases with the increase of Al in AlxCoCrFeNi alloys. All those results confirmed that the new method was potentially a high-throughput technology, which could greatly shorten the processing period and reduce the energy consumption during the production of high performance materials.

Understanding the mechanical behaviour and the large strength/ductility differences between FCC and BCC AlxCoCrFeNi high entropy alloys

The deformation behaviour of three direct laser fabricated AlxCoCrFeNi high entropy alloys were examined. One alloy had a face-centered cubic (x = 0.3) structure, one had a two-phase structure comprising an ordered (B2) and disordered BCC phase (x = 0.85), and the third alloy contained a mixture of all three phases (x = 0.6). The deformation behaviour of all three alloys was examined by mechanical testing, scanning electron microscopy and transmission electron microscopy. The dislocation density of specimens was measured using x-ray diffraction. For the FCC alloy, it was found that the correlation between dislocation density, applied stress and compressive strain all benchmark closely with the behaviour of austenitic stainless steel. It is therefore found that this alloy does not show any unique deformation behaviours of note, and like austenitic steels, show a low yield point, moderately high work hardening rates and excellent plasticity. For the case of the alloy with a high aluminium concentration, which had a two-phase B2+BCC microstructure, an exceptionally high yield strength of 1400 MPa was observed. Nano-hardness examination of this alloy showed that the B2 phase had an exceptionally high hardness value. Since the alloy contained a large volume fraction of this hard intermetallic B2 phase, is was concluded that the B2 phase is responsible for the high strength of this alloy. It was shown that this alloy behaves like a composite, and its strength can be predicted closely using a simple rule of mixtures approach. The poor tensile ductility of BCC high entropy alloys has been suggested to be the result of the inability of the B2 phase to accommodate shape change, leading to interphase cracking that results in brittle fracture characteristics.

Effects of nanotwins on the mechanical properties of AlxCoCrFeNi high entropy alloy thin films

In this work, we investigate the plastic deformation characteristics (hardness, strain rate sensitivity and activation volume) of nanotwinned AlxCoCrFeNi (x=0, 0.1, 0.3) high entropy alloy thin films (HEAFs). The extremely thin nanotwins with thickness of ~2nm soften these HEAFs, but enhance their strain rate sensitivity. We combine the detwinning softening process with conventional strengthening mechanisms to elucidate the mechanical properties of AlxCoCrFeNi HEAFs. These findings provide deep insights into design strategies to explore the promising high entropy alloys, especially the nanotwinned HEAFs at micro- and nano-scale.

Abnormal temperature dependence of impact toughness in AlxCoCrFeNi system high entropy alloys

In this work, the effect of Al contents and the associated phase transformations on the impact toughness of AlxCoCrFeNi high-entropy alloys (HEAs) (x denotes Al molar ratio; x = 0, 0.1, 0.75, and 1.5) in the as-cast state were investigated at T = 77 K, 200 K, and 298 K. For the alloys with x = 0 and 0.1, the alloys have the FCC structure, and an inverse temperature dependence of impact toughness between 298 K and 77 K was observed. The enhanced impact toughness (397.87 J) at 77 K is attributed to the capability of extensive nano-twinnings as well as ductile dimple fracture. Increasing Al contents to x = 0.75 and 1.5 induces the formation of brittle BCC phase and ordered B2 compound and accordingly embrittlement, leading to tremendously reduced Charpy-impact energies. The present study demonstrates that FCC AlxCoCrFeNi system HEAs with low Al contents (x < 0.5) may be well suitable for cryogenic applications.

Secondary phases in AlxCoCrFeNi high-entropy alloys: An in-situ TEM heating study and thermodynamic appraisal

Secondary phases, either introduced by alloying or heat treatment, are commonly present in most high-entropy alloys (HEAs). Understanding the formation of secondary phases at high temperatures, and their effect on mechanical properties, is a critical issue that is undertaken in the present study, using the AlxCoCrFeNi (x = 0.3, 0.5, and 0.7) as a model alloy. The in-situ transmission-electron-microscopy (TEM) heating observation, an atom-probe-tomography (APT) study for the reference starting materials (Al0.3 and Al0.5 alloys), and thermodynamic calculations for all three alloys, are performed to investigate (1) the aluminum effect on the secondary-phase fractions, (2) the annealing-twinning formation in the face-centered-cubic (FCC) matrix, (3) the strengthening effect of the secondary ordered body-centered-cubic (B2) phase, and (4) the nucleation path of the σ secondary phase thoroughly. The present work will substantially optimize the alloy design of HEAs and facilitate applications of HEAs to a wide temperature range.

Corrosion of Al xCoCrFeNi high-entropy alloys: Al-content and potential scan-rate dependent pitting behavior

The present work investigates the influence of Al-content and potential-scan-rate on stable/metastable pitting of Al xCoCrFeNi high-entropy alloys in a 3.5wt.% NaCl solution. The increased Al content causes the volume fraction of the Cr-depleted phase to increase, resulting in thicker/dispersive passive films that degrade the localized corrosion resistance. The varied stable/metastable pitting behavior under different potential scan rates indicates that pit generation is influenced by the properties of passive films and the interactions between chloride ions and the active sites. Furthermore, a mean-field theory is employed to predict the scan-rate dependent, power-law distribution of current fluctuations.

Phase stability and microstructures of high entropy alloys ion irradiated to high doses

The microstructures of AlxCoCrFeNi (x = 0.1, 0.75 and 1.5 in molar ratio) high entropy alloys (HEAs) irradiated at room temperature with 3 MeV Au ions at the highest fluence of 105, 91, and 81 displacement per atom, respectively, were studied. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses show that the initial microstructures and phase composition of all three alloys are retained after ion irradiation and no phase decomposition is observed. Furthermore, it is demonstrated that the disordered face-centered cubic (FCC) and disordered body-centered cubic (BCC) phases show much less defect cluster formation and structural damage than the NiAl-type ordered B2 phase. This effect is explained by higher entropy of mixing, higher defect formation/migration energies, substantially lower thermal conductivity, and higher atomic level stress in the disordered phases.

The fcc-bcc crystallographic orientation relationship in AlxCoCrFeNi high-entropy alloys

This paper concentrates on the crystallographic-orientation relationship between the various phases in the Al-Co-Cr-Fe-Ni high-entropy alloys. Two types of orientation relationships of bcc phases (some with ordered B2 structures) and fcc matrix were observed in Al0.5CoCrFeNi and Al0.7CoCrFeNi alloys at room temperature: (1−10)bcc//(200)fcc, [001]bcc//[001]fcc, (b) (1−11)B2//(2−2 0)fcc, [011]B2//[112]fcc.

The influence of Al elements on the structure and the creep behavior of AlxCoCrFeNi high entropy alloys

The microstructure, crystal structure, creep behavior and mechanism of AlxCoCrFeNi (x is in molar ratio) high entropy alloys were examined. The low Al content of Al0.15CoCrFeNi alloy had single FCC structure with columnar cell microstructure, while the high Al content of Al0.60CoCrFeNi alloy contained FCC+BCC duplex crystal structure with columnar dendrite microstructure. The creep property, evaluated by stress relaxation test, showed that the Al0.15CoCrFeNi alloy had a higher creep resistant property than that of the Al0.60CoCrFeNi alloy. The creep constitutive equation for the Al0.15CoCrFeNi alloy was established with an average stress exponent of 5.56 and an average activation energy of 385kJmol−1, while it was with a larger average stress exponent of 8.82 and a smaller average activation energy of 334kJmol−1 for the Al0.60CoCrFeNi alloy. The analysis of apparent activation volume reveals that both alloys were in a rate limiting mechanism of cross-slip. Then, the higher stacking fault energy of the Al0.60CoCrFeNi alloy was considered to result in its higher creep rate. Therefore, it was concluded that the increased Al element tended to reduce the creep resistant property of the AlxCoCrFeNi alloys by increasing the stacking fault energy.

Effects of AL addition on microstructure and mechanical properties of AlxCoCrFeNi High-entropy alloy

The effects of Al on microstructure and mechanical properties of AlxCoCrFeNi (x=0.1, 0.75 and 1.5) high-entropy alloys were systematically studied by using various characterization methods. It was found that the crystalline structure of AlxCoCrFeNi high-entropy alloy varies markedly with Al content, which changes from the initial single face-centered cubic (fcc) to fcc plus ordered body-centered cubic (bcc) structure (B2) and then to a duplex bcc structure (A2+B2) as the Al content is increased. The chemical composition analysis reveals that Al primarily partitions to B2 phase, suggesting Al is a stabilizer of B2 structure. With increasing Al content, more Ni and Al partition to the B2 phase due to the very negative mixing enthalpy of Ni and Al, and another phase enriched in Cr and Fe transforms from fcc to disordered bcc. Nano indentation measurements show that the hardness of AlxCoCrFeNi high-entropy alloy increases with Al content, accompanied by the decrease of ductility. The stability of single-phase solid solution in AlxCoCrFeNi HEAs is deduced from various criteria. Combined with the experiment results of other similar HEA systems, such as AlxCoCrFeNiCu, the effects of Al addition on the microstructure of AlxCoCrFeNi HEAs are discussed based on the Gibbs free energy of all competing phases and the fundamental properties of constituent elements. The aim of current study is to provide experimental evidence to establish a correlation between the microstructure and mechanical properties to search for high-entropy alloys with higher performances.

Irradiation Resistance in Al x CoCrFeNi High Entropy Alloys

The irradiation behavior of AlxCoCrFeNi (x = 0.1, 0.75, and 1.5) high entropy alloys was studied under 3 MeV Au-ions irradiation. The microstructural change and volume swelling due to irradiation were investigated using transmission electron microscopy and atomic force microscopy. The results showed that, with increasing the Al contents, the phase crystal structures of the as-cast samples evolved from face-centered cubic (FCC), to FCC + body-centered cubic (BCC), and BCC and irradiation-induced volume swelling increased. All alloys showed exceptional structural stability when irradiated up to over 50 displacement per atom at 298 K, and the irradiation-induced volume swellings in AlxCoCrFeNi HEAs were significantly lower than conventional nuclear materials under similar irradiation dosages.

Predicting the Crystal Structure and Phase Transitions in High-Entropy Alloys

High-entropy alloys (HEAs) have advantageous properties compared with other systems as a result of their chemistry and crystal structure. The transition between a face-centered cubic (FCC) and body-centered cubic (BCC) structure in the Al x CoCrFeNi high-entropy alloy system has been investigated on the atomic scale in this work. The Al x CoCrFeNi system, as well as being a useful system itself, can also be considered a model HEA material. Ordering in the FCC structure was investigated, and an order–disorder transition was predicted at ~600 K. It was found that, at low temperatures, an ordered lattice is favored over a truly random lattice. The fully disordered BCC structure was found to be unstable. When partial ordering was imposed (lowering the symmetry), with Al and Ni limited specific sites of the BCC system, the BCC packing was stabilized. Decomposition of the ordered BCC single phase into a dual phase (Al-Ni rich and Fe-Cr rich) is also considered.