Non-equiatomic High Entropy Alloy Research Articles

Mechanical properties of a two-phase high-entropy Fe50Mn30Co10Cr10 alloy down to ultralow temperatures

The mechanical properties comprising the stress-strain characteristics under uniaxial tensile deformation, the acoustic properties from mechanical resonance spectroscopy and—in parallel—the microstructural evolution during deformation of a nonequiatomic high-entropy alloy (HEA) Fe50Mn30Co10Cr10 have been studied in a wide temperature range, including ultralow temperatures down to 0.5 K. In the temperature range 300 to 4.2 K, a strong temperature dependence of the tensile strength occurs, hinting at the thermally activated nature of plastic deformation. Within the range of extremely low temperatures (4.2–0.5 K), however, the alloy exhibits anomalies of the yield strength, as well as discontinuous plasticity. Over the whole temperature range, the dynamic Young’s modulus of tensile deformed samples shows a reduction of absolute values compared to those of the undeformed ones, and at temperatures < 30 K a change of the temperature dependence from almost linear to power-law type. At all temperatures down to 0.5 K, the alloy’s plasticity stays as high as 50% as a consequence of a deformation driven martensitic phase transformation from fcc to hcp lattice (TRIP effect). Considering the ultralow deformation temperatures, the tensile strength reaches record values of 1513 MPa at 4.2 K, and still of 1274 MPa at 0.5 K, each being paired with significant strain hardening. These results suggest the HEA Fe50Mn30Co10Cr10 as a promising structural material for use in cryogenic environments down to extremely low temperatures.

Mechanical behaviors of equiatomic and near-equiatomic face-centered-cubic phase high-entropy alloys probed using in situ neutron diffraction

The equiatomic CoCrFeMnNi Cantor alloy, a face-centered-cubic (FCC) single-phase high-entropy alloy (HEA), has attracted considerable attention owing to its high strength and good ductility over a wide temperature range. The mechanical performance of this alloy was improved by reducing the stacking fault energy (SFE) through composition modification, and thus, a series of near- or non-equiatomic HEAs that are stronger and more ductile than their predecessor have been developed. However, the plastic-deformation behavior and strengthening mechanisms have not yet been fully discovered. In this study, we investigated the yielding and hardening behaviors of the Cantor alloy and FCC-phase Co-rich HEAs with different SFEs by in situ neutron diffraction combined with the first-principles method and electron-microscopy characterizations. The Co-rich HEAs exhibited a higher intrinsic yield strength than the Cantor alloy, mainly because of the larger shear modulus or modulus misfit, and grain refinement being more effective in improving the yield strength of low-SFE HEAs. Furthermore, higher flow stresses and better ductility of the Co-rich HEAs are attributed to the greater dislocation density and a larger number of stacking faults, which enhanced the strain-hardening rate during tensile deformation. The low SFE promoted mechanical twinning, and martensitic transformation contributed to higher strain-hardening rates. The present study provides deep insight into the yielding and hardening of FCC-phase HEAs, the understanding of which is a prerequisite for developing high-performance materials.

Formation of Cu-Ni enriched phases during laser processing of non-equiatomic AlSiCrMnFeNiCu high entropy alloy nanoparticles

Al-rich non-equiatomic high entropy alloy nanoparticles (HEA NPs) were synthesized by ablating Al40(SiCrMnFeNiCu)60 (at%) target in deionized water using nanosecond Nd:YAG pulsed laser operating at a wavelength of 1064 nm and having 8 ns pulse duration. The synthesized NPs have retained the composition and structural phases (B2 type AlFe and Cr5Si3) of the target. The colloidal solution of these NPs was further processed with a 532 nm laser. Upon laser processing, compositional and structural changes in HEA NPs are systematically investigated by using electron microscopy and spectroscopic techniques. The cyclic reheating of NPs during laser processing triggers the formation of Cu-Ni nanoprecipitate over the processed NPs. Owing to their similar ionic sizes and having FCC structural phase, Cu-Ni segregated together during the molten stage and evolve as the precipitate upon solidification. Amongst all the binary combinations of elements present in the HEA NPs, Cu-Ni phases are thermodynamically favored. Based on our experimental results, a possible growth mechanism of Cu-Ni enriched nanoprecipitate is discussed.

Effect of Ce on the localized corrosion behavior of non-equiatomic high-entropy alloy Fe40Mn20Cr20Ni20 in 0.5 M H2SO4 solution

The effect of rare earth cerium (Ce) on the localized corrosion behavior of the non-equiatomic Fe40Mn20Cr20Ni20 high-entropy alloys were investigated in 0.5 M H2SO4 solution. The corrosion resistance of the alloys increased with addition of Ce, which owing to the protectiveness and compactness of passive film was enhanced. The composition and shape of inclusions were modified by Ce, which resulted in the alteration of the localized corrosion behavior. A linear relationship between the doping density and thickness of the passive film was identified. The discussion based on the point defect model (PDM) was carried out and give a qualitative analysis. Data and code availabilityThe raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Effect of Ce on the pitting corrosion resistance of non-equiatomic high-entropy alloy Fe40Mn20Cr20Ni20 in 3.5wt% NaCl solution

This study focuses on the corrosion resistance of non-equiatomic Fe 40 Mn 20 Cr 20 Ni 20 high entropy alloys (HEAs) with addition of various cerium (Ce) concentrations in 3.5 wt% NaCl solution. The results of potentiodynamic polarization tests demonstrated that cerium can improve the pitting resistance of the HEAs in a considerable manner. Notably, the critical pitting potential ( E pit ) of samples increased dramatically with the addition of cerium, from − 58 mV SCE without cerium to + 265 mV SCE with 0.15 at% cerium in 3.5 wt% NaCl solution. Further the potentiodynamic polarization measures suggested that the impact of cerium addition on the pitting resistance of Fe 40 Mn 20 Cr 20 Ni 20 HEAs is not sensible to the salt concentration. In addition, electrochemical impedance spectroscopy (EIS) and Mott-Schottky tests showed that the addition of rare earth cerium led to the increase of the passive film thickness and the reduction of the donor density in the passive film, which partly accounts for the observed enhanced pitting resistance of the HEAs. • Adding cerium can effectively improve the pitting resistance of Fe 40 Mn 20 Cr 20 Ni 20 HEAs. • The thickness of passive film formed on the HEAs increase with addition of cerium. • The passive film formed on the HEAs with cerium is more protective than Ce-free HEAs.

Ultra-High Mixing Entropy Alloys with Single bcc, hcp, or fcc Structure in Co–Cr–V–Fe–X (X = Al, Ru, or Ni) Systems Designed with Structure-Dependent Mixing Entropy and Mixing Enthalpy of Constituent Binary Equiatomic Alloys

Non-equiatomic high-entropy alloys (HEAs) for which the mixing (Smix), configuration (Sconfig), and equivalent ideal (Sideal) entropies satisfy Smix > Sconfig = Sideal were reported for Co–Cr–V–Fe–(Al, Ru, or Ni) systems. Three Co20Cr20Fe20V10X30 (X = Al, Ru, or Ni) alloys (referred to as Al30, Ru30, and Ni30 alloys) were studied here using conventional arc melting and subsequent annealing. The X-ray diffraction profiles revealed that the Al30, Ru30, and Ni30 alloys annealed at 1600 K for 1 h exhibited B2 ordered, hcp, and fcc structures, respectively. A single structure was verified by scanning electron microscopy observations combined with elemental mapping via energy-dispersive X-ray spectroscopy. Thermodynamic calculations of Smix normalized by the gas constant (Smix/R) revealed that Al30, Ru30, and Ni30 alloys at 1600 K had Smix/R = 0.833, 1.640, and 1.618, respectively, where the latter two alloys exceeded Sconfig/R = 1.557. A compositionally optimized Al-containing HEA for Smix with a single bcc structure was computationally predicted and verified experimentally for the Al6Co27Cr34Fe19V14 alloy (Al6 alloy). The non-equiatomic Al6 alloy with Sconfig/R = 1.480 exhibited Smix/R of 1.703 at 1600 K, surpassing Sconfig/R = ln 5 = 1.609 for the exact equiatomic (EE) quinary alloy. The bcc Al6, hcp Ru30, and fcc Ni30 alloys were regarded as ultra-high mixing entropy alloys (UMHEAs) according to Smix > Sconfig. Structure-dependent Smix and the mixing enthalpy of constituent binary EE alloys are useful for future UHMEAs as a subset of HEAs.

Effect of high-current pulsed electron beam treatment on defect substructure of the high-entropy alloy of Co – Cr – Fe – Mn – Ni system

High-current pulsed electron beam surface treatment is a method of materials modifying, which improves the mechanical properties of metal materials. Due to high-speed heating, evaporation, recrystallization, as well as plastic deformation, dislocations with high density are formed in the surface and, as a result, an increase in indicators of various physical and mechanical properties, such as hardness, wear resistance, etc., is observed. Since currently high-entropy alloys are a relatively new class of materials, the effect of pulsed electron beam treatment on the dislocation substructure has not yet been established. In this work, a non-equiatomic high–entropy alloy of the Co – Cr – Fe – Mn – Ni system, made using a wire-arc additive manufacturing, was subjected to surface treatment using a high-current pulsed electron beam with an energy density of 30 J/cm2. By the method of studying thin foils using transmission electron microscopy, it was found that the treatment does not affect the chemical composition of the alloy, but leads to serious changes in the dislocation substructure. A nonmonotonic change in the scalar density of dislocations was revealed, reaching a maximum value of 5.5·1010 cm–2 at a distance of 25 µm from the irradiation surface. It is shown that an undirected cellular dislocation substructure with cell sizes from 400 nm to 600 nm is formed at this distance from the surface. With further distance from the surface at a distance of up to 45 µm, the dislocation substructure changes from cellular to cellular-mesh. At a distance of 120 – 130 µm, the effect of a high-current pulsed electron beam is not observed – the substructure corresponds to the substructure of the initial alloy with a chaotic distribution of dislocations.

The determining role of carbon addition on mechanical performance of a non-equiatomic high-entropy alloy

• The role of C on the microstructure and mechanical properties are discussed. • The microstructure of the tensile specimen interrupted at different tensile strains are examined by EBSD and TEM. The deformation mechanism of a C-doped iHEA with excellent mechanical properties is discussed. • The contributions of different strengthening mechanism to yield strength are calculated and the comprehensive strengthening mechanism is discussed. The mechanical properties and deformation mechanism of a C-doped interstitial high-entropy alloy (iHEA) with a nominal composition of Fe 49.5 Mn 29.7 Co 9.9 Cr 9.9 C 1 (at.%) were investigated. An excellent combination of strength and ductility was obtained by cold rolling and annealing. The structure of the alloy is consisted of FCC matrix and randomly distributed Cr 23 C 6 . For gaining a better understanding of deformation mechanism, EBSD and TEM were conducted to characterize the microstructure of tensile specimens interrupted at different strains. At low strain (2%), deformation is dominated by dislocations and their partial slip. With the strain increase to 20%, deformation-driven athermal phase transformation and dislocations slip are the main deformation mechanism. While at high strain of 35% before necking, deformation twins have been observed besides the HCP phase. The simultaneous effect of phase transformation (TRIP effect) and mechanical twins (TWIP effect) delay the shrinkage, and improve the tensile strength and plasticity. What's more, compared with the HEA without C addition, the yield strength of the C-doped iHEA has been improved, which can be attributed to the grain refinement strengthening and precipitation hardening. Together with the lattice friction and solid solution strengthening, the theoretical calculated values of yield strength match well with the experimental results.

Microstructural Evolution and Mechanical Properties of Non-Equiatomic (CoNi)74.66Cr17Fe8C0.34 High-Entropy Alloy.

In this study, we manufactured a non-equiatomic (CoNi)74.66Cr17Fe8C0.34 high-entropy alloy (HEA) consisting of a single-phase face-centered-cubic structure. We applied in situ neutron diffraction coupled with electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) to investigate its tensile properties, microstructural evolution, lattice strains and texture development, and the stacking fault energy. The non-equiatomic (CoNi)74.66Cr17Fe8C0.34 HEA revealed a good combination of strength and ductility in mechanical properties compared to the equiatomic CoNiCrFe HEA, due to both stable solid solution and precipitation-strengthened effects. The non-equiatomic stoichiometry resulted in not only a lower electronegativity mismatch, indicating a more stable state of solid solution, but also a higher stacking fault energy (SFE, ~50 mJ/m2) due to the higher amount of Ni and the lower amount of Cr. This higher SFE led to a more active motion of dislocations relative to mechanical twinning, resulting in severe lattice distortion near the grain boundaries and dislocation entanglement near the twin boundaries. The abrupt increase in the strain hardening rate (SHR) at the 1~3% strain during tensile deformation might be attributed to the unusual stress triaxiality in the {200} grain family. The current findings provide new perspectives for designing non-equiatomic HEAs.

Investigation of Co-Cr-Fe-Mn-Ni Non-Equiatomic High-Entropy Alloy Fabricated by Wire Arc Additive Manufacturing

Fabrication of high-entropy alloys (HEAs) is a crucial area of interest for materials scientists since these metallic materials may have many practical uses. Wire arc additive manufacturing (WAAM), unlike other additive technologies, has tangible benefits for making large-sized components, but manufacturing the wire from HEAs is still very limited. Recent studies suggested tackling this problem using a combined cable composed of wires consisting of pure elements as feeding material. However, not all compositions of HEAs can be obtained by the pure elements’ wires because the number of them is limited. This study aims to examine phase composition, chemical elements distribution, microstructure, and mechanical properties of a Co-Cr-Fe-Mn-Ni HEA, which was not previously obtained by the WAAM. The cable-type wire used in this study is composed of two wires which consist of Cr, Fe, Mn, and Ni, and one pure Co wire. The phase composition, chemical elements distribution, microstructure, and mechanical properties were investigated. The prepared high-entropy alloy has non-equiatomic chemical composition with a single-phase FCC crystal structure with homogeneously distributed elements inside the grains. The microstructure examinations showed dendrite structure which is typical for WAAM processes. The compressive yield strength of the alloy is ~279 MPa, the ultimate compressive strength is ~1689 MPa, the elongation is 63%, and the microhardness is ~150 HV, which was found to be similar to the previously fabricated Co-Cr-Fe-Mn-Ni alloys by other methods. Fracture analysis confirmed the ductile behavior of deformation by the presence of dimples.

Effects of Al or Mo Addition on Microstructure and Mechanical Properties of Fe-Rich Nonequiatomic FeCrCoMnNi High-Entropy Alloy

In this work, a Fe-rich nonequiatomic Fe40Cr15Co15Mn10Ni20 high-entropy alloy was successfully prepared based on phase analysis and cost reduction. Fe40Cr15Co15Mn10Ni20 high-entropy alloy with a single-phase face-centered cubic (FCC) structure was strengthened by the addition of 11 at.% Al or 10 at.% Mo, and the variations of phase and mechanical properties of the strengthened alloys were subsequently investigated. It has been found that the addition of 11 at.% Al led to the formation of FCC and body-centered cubic (BCC) dual-phase structure in the Fe40Cr15Co10Mn4Ni20Al11 alloy, while its yield strength (σ0.2) and tensile strength increased from 158 ± 4 MPa and 420 ± 20 MPa to 218 ± 7 MPa and 507 ± 16 MPa, respectively, as compared to the single-phase FCC structure Fe40Cr15Co15Mn10Ni20 alloy. The addition of 10 at.% Mo introduced intermetallic compounds of μ and σ phases, which resulted in improved yield strength of 246 ± 15 MPa for the Fe40Cr15Co10Mn5Ni20Mo10 alloy. However, the alloy exhibited premature brittle fracture due to the existence of a large number of intermetallic compounds, which led to deteriorated tensile strength of 346 ± 15 MPa. The findings of this work suggest that the introduced secondary phases by the addition of Al and Mo can effectively strengthen the high-entropy alloy; however, the number of intermetallic compounds should be controlled to achieve a combination of high strength and good ductility, which provides a reference for the follow-up study of nonequiatomic high-entropy alloys.

Formation of Cuni Enriched Phases During Laser Processing of Non-Equiatomic Alsicrmnfenicu High Entropy Alloy Nanoparticles

Formation of Cuni Enriched Phases During Laser Processing of Non-Equiatomic Alsicrmnfenicu High Entropy Alloy Nanoparticles

Enhancing the magnetocaloric response of high-entropy metallic-glass by microstructural control

Non-equiatomic high-entropy alloys (HEAs), the second-generation multi-phase HEAs, have been recently reported with outstanding properties that surpass the typical limits of conventional alloys and/or the first-generation equiatomic single-phase HEAs. For magnetocaloric HEAs, non-equiatomic (Gd36Tb20Co20Al24)100−xFex microwires, with Curie temperatures up to 108 K, overcome the typical low temperature limit of rare-earth-containing HEAs (which typically concentrate lower than around 60 K). For alloys with x = 2 and 3, they possess some nanocrystals, though very minor, which offers a widening in the Curie temperature distribution. In this work, we further optimize the magnetocaloric responses of x = 3 microwires by microstructural control using the current annealing technique. With this processing method, the precipitation of nanocrystals within the amorphous matrix leads to a phase compositional difference in the microwires. The multi-phase character leads to challenges in rescaling the magnetocaloric curves, which is overcome by using two reference temperatures during the scaling procedure. The phase composition difference increases with increasing current density, whereby within a certain range, the working temperature span broadens and simultaneously offers relative cooling power values that are at least 2-fold larger than many reported conventional magnetocaloric alloys, both single amorphous phase or multi-phase character (amorphous and nanocrystalline). Among the amorphous rare-earth-containing HEAs, our work increases the working temperature beyond the typical <60 K limit while maintaining a comparable magnetocaloric effect. This demonstrates that microstructural control is a feasible way, in addition to appropriate compositional design selection, to optimize the magnetocaloric effect of HEAs.

Thermodynamic modelling to predict phase stability in BCC + B2 Al–Ti–Co–Ni–Fe–Cr high entropy alloys

This paper examines the potential of thermodynamic modelling as a simple and inexpensive means for assessing phase stability in a series of non-equiatomic high entropy alloys and compares with CALPHAD calculations to demonstrate an appropriate level of simplifying assumptions. The modelling was motivated by alloys from the Al–Ti–Co–Ni–Fe–Cr system, which were produced by iteratively following the natural compositional segregation of the two-phase BCC + B2 microstructure present in a Al2TiCoNiFeCr alloy after casting and heat treatment. This produced a range of multicomponent B2-type alloys with different volume fractions of a BCC secondary phase. The solubility limits and traditional empirical thermodynamic driving forces for phase stability were investigated to explain the formation of the two phases. Limitations of prior semi-empirical models are highlighted, with advancements demonstrated by accounting for contributions from the effect of ordering on configurational entropy, the difference in enthalpy from intermetallic compounds, and thermal influences on both entropy and enthalpy. The new models are compared against the current leading thermodynamic modelling approach, CALPHAD, with excellent correlation. This work outlines a methodology to predict and design phase constitution in future high-performance BCC + B2 alloys and, more generally, it demonstrates the value of models with temperature-dependent thermodynamic quantities for exploring new, complex compositional regions.

A molecular dynamics study of the influence of nucleation conditions on the phase selection in Fe50Mn30Cr10Co10 high entropy alloy

Non-equiatomic high entropy alloys with single or multiphase microstructures offer opportunities to optimize the mechanical properties through compositional variation. The Fe-Mn-Cr-Co alloys have shown the potential to activate nanotwinning at large strains even at room temperatures, thereby enhancing both ductility and strength. This paper presents a molecular dynamics study of phase formation during solidification of Fe50Mn30Cr10Co10 using a modified embedded atom method potential. The evolution of the phases has been investigated using adaptive common neighbour analysis, composition analysis, and radial distribution function. In contrast to the reported microstructures containing FCC and HCP phases, a BCC phase with the average composition of the alloy has resulted under homogeneous conditions. On the other hand, heterogeneous nucleation from an initial FCC seed resulted in a dual-phase microstructure of FCC and HCP at 300 K in agreement with the experiments. Interestingly, annealing at 900 K did not alter the phase fractions in samples from both nucleation conditions. Analysis based on the empirical thermodynamic parameter suggested by Ye et al. [Materials Today (2016), Vol19, p349–362] and valence electron concentration (VEC) supports the formation of both types of microstructures observed here. Hence, biasing the nucleation process is believed to strongly influence the type of phases formed in this alloy.

The Effect of Heat Treatment on the Microstructure of Co50Ni25Al8Ti8Nb3Ta3W3 High Entropy Alloy and Its High- temperature Oxidation Resistance

The design based on high-entropy alloys (HEAs) is undergoing a conceptual expansion from equiatomic alloys to non-equiatomic alloys. To provide the experimental basis for non-equiatomic high-entropy alloys and their oxidation resistance at high temperatures, in this study, a Co50Ni2sAl8Ti8Nb3Ta3W3 high-entropy alloy was prepared by a vacuum arc melting method, and then it was subjected to different aging heat treatments. The phase composition, element composition distribution, and mechanical properties of the alloy were characterized by XRD SEM/EDS. In addition, we have systematically studied the high-temperature oxidation resistance of the alloy and analyzed the high-temperature oxidation mechanism of the alloy. The results showed that the phase composition of the Co50Ni25Al8Ti8Nb3Ta3W3 high-entropy alloy is a single fcc solid solution phase, and the grains of the alloy become coarser after secondary aging. Many oxidation products were produced after high-temperature oxidation of the alloy. The oxidation products of the outermost layer of the alloy are mainly CoNiO2, CoTiO3, and M2O3, and the oxidation products of the intermediate layer are mainly Al2O3, TiO2, and Nb2O3.

Crack-resistant σ/FCC interfaces in the Fe40Mn40Co10Cr10 high entropy alloy with the dispersed σ-phase

We report the role of morphology and size of the σ-phase on mechanical properties of the Fe40Mn40Co10Cr10 non-equiatomic high entropy alloy. The dispersed and fine σ-precipitates formed after cold-rolling and annealing at 700 °C for 1 h lead to an increase in strength without severe ductility loss compared to the material without the σ-phase. However, the coarsened and connected σ-phase formed after prolonged annealing at 700 °C for 100 h results in early failure due to the activation of microcracks along σ/σ interfaces. The σ/FCC interface could act as strong obstacles for dislocation motion and could effectively relieve the stress concentration by activating deformation twins or dislocations. The σ/FCC interfaces are excellent in terms of hardening, accommodation of plastic deformation, and stress relaxation leading to the observed crack resistance. Therefore, we suggest that increasing σ/FCC interfaces by controlling size and dispersion of the σ-phase is essential to develop HEAs with an excellent strength-ductility combination and damage tolerance.

A comparison of the microstructures and hardness values of non-equiatomic (FeNiCo)-(AlCrSiTi) high entropy alloys having thermal histories related to laser direct metal deposition or vacuum remelting

Compared with conventional casting process, laser direct metal deposition (LDMD) can effectively reduce component segregation in non-equiatomic high entropy alloys (HEAs). However, the non-equilibrium thermal history imparted by this method has a significant effect on the microstructure and mechanical properties of the HEA. The present work prepared non-equiatomic (FeNiCo)-(AlCrSiTi) HEAs using the LDMD process, after which some samples were subjected to a subsequent vacuum remelting (VRM) treatment. The variations in phase composition, microstructure and hardness between the HEAs processed using the LDMD or VRM methods were investigated. Differences in phase composition increased with increases in the proportion of the Al, Cr, Si and Ti components. Compared with the HEA samples processed using the VRM method, the LDMD specimens possessed much finer and simpler microstructures mainly comprising solid solution phases. At relatively high concentrations of the Al, Cr, Si and Ti components (32 at% and 40 at% in total), the HEAs processed under LDMD conditions had similar grain morphologies, intermetallic phases and microhardness values, while the VRM specimens presented completely different microstructures and properties.

Effect of Pulsed-Electron-Beam Irradiation on the Surface Structure of a Non-Equiatomic High-Entropy Alloy of the Al–Co–Cr–Fe–Ni System

Effect of Pulsed-Electron-Beam Irradiation on the Surface Structure of a Non-Equiatomic High-Entropy Alloy of the Al–Co–Cr–Fe–Ni System

Effect of Sintering Condition on Tensile Strength of Fe-based Non-equiatomic High Entropy Alloy

Effect of Sintering Condition on Tensile Strength of Fe-based Non-equiatomic High Entropy Alloy