Non-equiatomic High Entropy Alloy Research Articles

Prediction on Mechanical Properties of Non-Equiatomic High-Entropy Alloy by Atomistic Simulation and Machine Learning

High-entropy alloys (HEAs) with multiple constituent elements have been extensively studied in the past 20 years, due to their promising engineering application. Previous experimental and computational studies of HEAs focused mainly on equiatomic or near equiatomic HEAs. However, there is probably far more treasure in those non-equiatomic HEAs with carefully designed composition. In this study, the molecular dynamics (MD) simulation combined with machine learning (ML) methods was used to predict the mechanical properties of non-equiatomic CuFeNiCrCo HEAs. A database was established based on a tensile test of 900 HEA single-crystal samples by MD simulation. Eight ML models were investigated and compared for the binary classification learning tasks, ranging from shallow models to deep models. It was found that the kernel-based extreme learning machine (KELM) model outperformed others for the prediction of yield stress and Young’s modulus. The accuracy of the KELM model was further verified by the large-sized polycrystal HEA samples. The results show that computational simulation combined with ML methods is an efficient way to predict the mechanical performance of HEAs, which provides new ideas for accelerating the development of novel alloy materials for engineering applications.

Influence of Milling Conditions on the Microstructural Characteristics and Mechanical Properties of Non-equiatomic High Entropy Alloy

Influence of Milling Conditions on the Microstructural Characteristics and Mechanical Properties of Non-equiatomic High Entropy Alloy

Magnetic, electrical and mechanical properties of Fe40Mn40Co10Cr10 high entropy alloy

A prototypical, single-phase, and non-equiatomic high entropy alloy Fe40Mn40Co10Cr10 has been mechanically deformed at room and cryogenic temperatures. Plastic deformation was accommodated via crystallographic slip at room temperature while transformation induced plasticity (TRIP) has been observed in samples deformed at 77 K. The stress-induced martensitic transformation occurred from face-centered cubic (FCC) to hexagonal close-packed (HCP) structures. A detailed electron backscatter diffraction analysis was utilized to detect phase change and evaluate the evolution of the HCP phase volume fraction as a function of plastic strain. Physical properties of undeformed and deformed samples were measured to elucidate the effect of deformation-induced phase transitions on the magnetic and electrical properties of Fe40Mn40Co10Cr10 alloy. Relatively small magnetic moments along with non-saturating magnetic field dependencies suggest that the ground state in the considered material is ferrimagnetic ordering with coexisting antiferromagnetic phase. The temperature evolution of the coercive fields has been revealed for all samples. The magnitudes of the coercive fields place the considered system into the semi-hard magnetic alloys category. The temperature dependence of the zero-field cooled (ZFC) and field cooled (FC) magnetization was measured for all samples in the low field regime and the origin of irreversibility in ZFC/FC curves was discussed. Besides, the temperature dependence of the resistivity in all samples was measured and the possible conduction mechanisms were discussed.

Decelerated grain growth kinetic and effectiveness of Hall-Petch relationship in a cold-rolled non-equiatomic high entropy alloy

The present work deals with the grain growth behavior and mechanical properties of a cold-rolled non-equiatomic metastable Fe50Mn30Co10Cr10 high entropy alloy. The bimodal grain size distribution which was developed during isothermal annealing at the temperature range of 750–1050 °C, was gradually annihilated through increasing the annealing time. This was due to the higher capability of fine grains to grow up compared with coarse grains. The obtained grain growth exponents and grain growth activation energies were well higher than those reported for solid solution alloys and also reported for five component CoCrFeMnNi high entropy alloy. The extremely low grain growth kinetics of the experimented alloy even at high homologous temperature was discussed regarding the solute drag like effect of whole-solute matrix in addition to the sluggish diffusion as an intrinsic properties of high entropy alloys. The variation in mechanical properties of the experimented alloy was mainly discussed relying on the effectiveness of Hall-Petch relationship and the stability of the matrix. It was found that the grain boundaries acted as an effective barrier against dislocation motion, and caused postponement of the strain induced transformation leading to an acceptable strength/ductility balance.

Mechanical properties and microstructural characteristics of non-equiatomic high entropy alloy FeMnCoCrC prepared by powder metallurgy

ABSTRACT In this work, we prepared non-equiatomic high-entropy alloys using mechanical alloying and spark plasma sintering and investigated the microstructural features and mechanical properties. We designed a heat treatment condition to identify the microstructure effect on the mechanical properties and observed the microstructural features by X-ray diffraction. A quantitative phase analysis was carried out by electron backscatter diffraction, and the mechanical properties were measured by a tensile test. It was found that the microstructural change due to plastic deformation depended on the grain size. In addition, we discuss the phase stability of the FeMnCoCrC alloy depending on the microstructural characteristics.

Deformation behavior of high-entropy alloy system Al – Co – Cr – Fe – Ni achieved by wire-arc additive manufacturing

A non-equiatomic high-entropy alloy (HEA) of the Al – Co – Cr – Fe – Ni system was obtained using wire-arc additive manufacturing technology in the atmosphere of pure argon. The initial wire had 3 conductors with different chemical composition: pure aluminum wire (Al ≈ 99.95 %), chromium-nickel wire (Cr ≈ 20 %, Ni ≈ 80 %), and cobalt alloy wire (Co ≈ 17 %, Fe ≈ 54 %, Ni ≈ 29 %). The resulting sample of high-entropy alloy was a parallelepiped consisting of 20 deposited layers in height and 4 layers in thickness. The alloy had the following elemental composition, detected by energy-dispersive X-ray spectroscopy: aluminum (35.67 ± 1.34 at. %), nickel (33.79 ± 0.46 at. %), iron (17.28 ± 1.83 at. %), chromium (8.28 ± 0.15 at. %) and cobalt (4.99 ± 0.09 at. %). Scanning electron microscopy revealed that the source material has a dendritic structure and contains particles of the second phase at grain boundaries. Element distribution maps obtained by mapping methods have shown that grain volumes are enriched in aluminum and nickel, while grain boundaries contain chromium and iron. Cobalt is distributed in the crystal lattice of the resulting HEA quasi-uniformly. It is shown that during tensile tests, the material was destroyed by the mechanism of intra-grain cleavage. The formation of brittle cracks along the boundaries and at the junctions of grain boundaries, i.e., in places containing inclusions of the second phases, is revealed. It was suggested that one of the reasons for the increased fragility of HEA, produced by wire-arc additive manufacturing, is revealed uneven distribution of elements in microstructure of the alloy and also the presence in material volume of discontinuities of various shapes and sizes.

Isothermal and non-isothermal sintering characteristics of mechanically alloyed nonequiatomic Fe2CoCrMnNi high-entropy alloy powder

ABSTRACT Mechanically alloyed high-entropy alloy (HEA) has taken considerable attention due to its ease of fabrication. Recent trend progresses towards the development of non-equiatomic HEAs to enhance the material properties compared to equiatomic alloys. Sintering behaviour of widely investigated equiatomic FeCoCrMnNi HEA has been reported. However, no report exists on the sintering kinetics of non-equiatomic HEA, when the concentration of a particular element is increased in the alloy powder. The present work attempts to study the isothermal and non-isothermal sintering behaviour of mechanically alloyed non-equiatomic Fe2CoCrMnNi HEA over a temperature range of 950–1100°C in an argon atmosphere. Activation energies for sintering and diffusivity parameters of Fe2CoCrMnNi HEA were calculated from dilatometric measurements through the sintering models. The result indicates the grain boundary diffusion as a dominating sintering mechanism for this alloy powder.

High-temperature age-hardening of a novel cost-effective Fe45Ni25Cr25Mo5 high entropy alloy

Considering the high potential of age-hardening in metastable non-equiatomic high-entropy alloys (HEAs), the high-temperature aging behaviour of a newly developed cost-effective Fe45Ni25Cr25Mo5 HEA with a face-cantered cubic (FCC) matrix was investigated. The alloy showed high age-hardening response at temperatures up to 900 °C and the peak ageing was obtained at 900 °C for 48 h due to the precipitation of more monoclinic Cr5.5Mo1.5Fe6.5Ni1.5-type σ phase. Although the as-cast Fe-enriched HEA composed of FCC and σ phases possessed balanced high tensile strength and ductility compared with previously reported as-cast FCC-dominated HEAs, the peak ageing only led to substantial improvement in compression properties with compression yield strength of 543.7 MPa, ultimate strength of 1098.1 MPa and strain to fracture of 40%. The peak aged alloy exhibited tensile brittleness at room temperature, which was attributed to the restriction of dislocation movement by the precipitation of large needle-shaped intermetallics. Furthermore, the newly developed alloy showed high oxidation and corrosion resistance, which was superior to the 316L stainless steel and was comparable to the Inconel 625 super alloy. Thus, this HEA can be potentially used as a superalloy.

Microstructural and Mechanical Characteristics of Non-Equiatomic High Entropy Alloy FeMnCoCr Prepared by Spark Plasma Sintering

Microstructural and Mechanical Characteristics of Non-Equiatomic High Entropy Alloy FeMnCoCr Prepared by Spark Plasma Sintering

Dislocation-induced breakthrough of strength and ductility trade-off in a non-equiatomic high-entropy alloy

In conventional metallic materials, strength and ductility are mutually exclusive, referred to as strength-ductility trade-off. Here, we demonstrate an approach to improve the strength and ductility simultaneously by introducing micro-banding and the accumulation of a high density of dislocations in single-phase high-entropy alloys (HEAs). We prepare two compositions (Cr10Mn50Fe20Co10Ni10 and Cr10Mn10Fe60Co10Ni10) with distinctive different stacking fault energies (SFEs) as experimental materials. The strength and ductility of the Cr10Mn50Fe20Co10Ni10 HEA are improved concurrently by grain refinement from 347.5 ± 216.1 µm to 18.3 ± 9.3 µm. The ultimate tensile strength increases from 543 ± 4 MPa to 621 ± 8 MPa and the elongation to failure enhances from 43±2% to 55±1%. To reveal the underlying deformation mechanisms responsible for such a strength-ductility synergy, the microstructural evolution upon loading is investigated by electron microscopy techniques. The dominant deformation mechanism observed for the Cr10Mn50Fe20Co10Ni10 HEA is the activation of micro-bands, which act both as dislocation sources and dislocation barriers, eventually, leading to the formation of dislocation cell structures. By decreasing grain size, much finer dislocation cell structures develop, which are responsible for the improvement in work hardening rate at higher strains (>7%) and thus for the increase in both strength and ductility. In order to drive guidelines for designing advanced HEAs by tailoring their SFE and grain size, we compute the SFEs of Cr10MnxFe70–xCo10Ni10 (10 ≤ x ≤ 60) based on first principles calculations. Based on these results the overall changes on deformation mechanism can be explained by the influence of Mn on the SFE.

An investigation into the dynamic recrystallization behavior of a non-equiatomic high entropy alloy

The recrystallization behavior of a well-known non-equiatomic high entropy alloy by the nominal chemical composition of Fe–27Mn–10Cr–10Co, at.% was investigated. The compression tests were performed in the temperature range of 400–1000 °C by the interval of 200 °C under the strain rate of 0.001 s−1. Initial microstructure consist of face centred cubic and hexagonal phases by the average grain size of 90, and 11 μm, respectively. Interestingly, the occurrence of dynamic recrystallization was observed during compression testing at warm temperature regime (400 and 600 °C). The appreciable grain refinement, the corresponding flow curve softening, and the detailed analysis of the hardening curves according to the Poliak and Jonas method, verified the occurrence of dynamic recrystallization at such warm temperature regime. The required stored energy was provided through the shear reversion of thermally induced martensite before any straining. In contrast, the softening mechanisms was not strong enough to compensate the strain hardening of the single phase microstructure. This further approved the significant role of the martensite phase to trigger the occurrence of dynamic recrystallization. The intensified substructure development and the random microtexture of the recrystallized grains well indicate the contribution of continuous recrystallization mechanism. The microstructure was also considerably refined (by the mean grain size of ~1.7 μm) through compressive deformation at 1000 °C. In fact, the deformation temperature was high enough to compensate the inhibitory effects of sluggish diffusion and severe lattice distortion.

Formation of fully equiaxed grain microstructure in additively manufactured AlCoCrFeNiTi0.5 high entropy alloy

In this work, the non-equiatomic high entropy alloy AlCoCrFeNiTi0.5 was additively manufactured via the laser engineered net shaping (LENS™) process. Contrary to the columnar grain microstructure commonly observed in previously reported alloys, the as-deposited AlCoCrFeNiTi0.5 specimens exhibit a fully equiaxed grain microstructure in a wide range of temperature gradients G (85 to 1005 K/mm) and solidification velocities V (5 to 20 mm/s). The main microstructural characteristics were found to be B2-structured proeutectic dendrites delineated by lamellar or rod-like B2/A2 eutectic structures. The formation of this microstructural feature can be discussed with the aid of Scheil’s solidification model. The proeutectic B2-structured dendrites were frequently found to be fragmented, which may provide profuse effective nucleation sites, and hence promote equiaxed grain formation. Furthermore, we estimated the volume fraction ϕ values of equiaxed crystals at solidification front for various G - V combinations established in this paper, which can provide a theoretical basis for our experimental findings. The current work provides guidelines for producing fully equiaxed alloys by the additive manufacturing (AM) process.

The enhanced static recrystallization kinetics of a non-equiatomic high entropy alloy through the reverse transformation of strain induced martensite

The present work deals with the static recrystallization behavior of a cold rolled non-equiatomic Fe50Mn30Co10Cr10 high entropy alloy. Towards this end, the experimented alloy was cold rolled to a total thickness reduction of 60% and was then isothermally annealed in the temperatures range of 750–850 °C for various periods of time followed by air coolling. It was found that the reverse transformation of the strain induced martensite phase during continues heating results in formation of lath-shaped reversed regions holding high density of dislocation tangles and stacking faults. During isothermal annealing, these highly stored energy regions serve as preferred nucleation sites of statically recrystallized grains. The recrystallization kinetics has been analyzed in terms of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. Interestingly, the Avrami exponent ranged from 1.62 to 1.84 and deviates from the ideal values that was discussed considering the various capability of thermal martensite, strain induced martensite and face center cubic phase for static recrystallization. The activation energy of recrystallization is found to be much lower than that of bulk diffusion of each constituent. This is attributed to the progressive substructure development which provides the high track diffusion through the low angle boundaries.

Combinatorial thin film screening to identify single-phase, non-equiatomic high entropy alloys in the MnFeCoNiCu system

Abstract

Experimental investigation and phase diagram of CoCrMnNi–Fe system bridging high-entropy alloys and high-alloyed steels

An investigation of equiatomic and non-equiatomic high-entropy alloys (HEAs) and medium-entropy alloys (MEAs) was carried out by performing both thermodynamic calculations and experiments. The design strategy of the alloys was based on a constant valence electron concentration (VEC) of 8 while varying the content of Fe in a Fex(CoCrMnNi)100−x where x = 00, 20, 40, and 60. The X-ray diffraction results showed that the alloys have a single face-centered cubic structure at 900 °C. The tensile test results revealed that both the yield strength and ultimate tensile strength decrease as the amount of Fe is increased but the uniform elongation increases. Additionally, the strain-hardening rate is significantly enhanced at higher Fe concentrations due to the activation of deformation twinning as an additional deformation mechanism. Interestingly, the critical twinning stress is significantly reduced as the Fe content is increased from Fe00 to Fe60. As a result, the deformation twins were easily activated in a specimen deformed up to a true strain of 20% for Fe60, unlike the other alloys, which exhibited no deformation twinning at the same true strain. Furthermore, it was revealed that the frequency of deformation twinning increases proportionally with increase in the Fe content.

High-Temperature Age-Hardening of a Novel Cost-Effective Fe <sub>45</sub>Ni <sub>25</sub>Cr <sub>25</sub>Mo <sub>5</sub> High Entropy Alloy

Considering the high potential of age-hardening in metastable non-equiatomic high entropy alloys (HEAs), the high-temperature ageing behaviour of a newly developed cost-effective Fe45Ni25Cr25Mo5 HEA with a face-cantered cubic (FCC) matrix was investigated. The alloy demonstrates a significant high-temperature age-hardening response and a good balance of strength and ductility was achieved with compression yield strength of 543.7 MPa, ultimate strength of 1098.1 MPa and elongation of 40% at the peak aged (900°C for 48 hours) condition. Such high-temperature age response of the Fe45Ni25Cr25Mo5 alloy was attributed to the precipitation strengthening by the monoclinic Cr5.5Mo1.5Fe6.5Ni1.5-type precipitates formed during the ageing.

Unexpected strength–ductility response in an annealed, metastable, high-entropy alloy

The design of non-equiatomic high-entropy alloys (HEAs) opens huge compositional space to develop new materials with exceptional properties. Among them, HEAs with flexible microstructures showed an adaptive phase stability that enhanced the work hardening (WH) ability of the material drastically. With the same motive, here we present a new friction stir processed Fe39Mn20Co20Cr15Si5Al1 HEA that demonstrated an unexpected strength–ductility response just upon annealing. The inter-competing precipitation and grain/twin formation events during low-temperature annealing resulted in an unexpected f.c.c. (γ) → h.c.p. (ɛ) transformation. This unusual phase evolution triggered development of refined, ɛ-dominated microstructure coupled with a uniformly dispersed fine γ phase. The controlled 〈c+a〉 slip and twinning in the ɛ phase along with the transformation of a refined γ matrix resulted in higher elongation of 52% with enhanced ultimate tensile strength of 1.12GPa during deformation. Thus, the metastability-assisted design of ɛ-martensite-dominant HEAs by annealing opens a new path to obtain strong yet ductile alloys, which is otherwise not feasible in conventional steel/HEA designs.

Deformation and annealing behaviors of as-cast non-equiatomic high entropy alloy

The recently developed non-equiatomic high entropy alloy (HEA), Fe80-xMnxCo10Cr10, overcoming the strength-ductility trade-off. Phase stability in Fe80-xMnxCo10Cr10 is reduced via tailoring Mn content, resulting in a single phase or dual-phase microstructure (face-centered cubic (FCC) phase and hexagonal close packed (HCP) phase). In this work, the plastic deformation and annealing behaviors during processing in two as-cast non-equiatomic HEAs with coarse grains, i.e., Fe30Mn50Co10Cr10 with single-phase (FCC) and Fe60Mn20Co10Cr10 with dual-phase (FCC+HCP), are investigated by a combination of electron back-scattered diffraction and transmission electron microscope. Plastic strain of Fe30Mn50Co10Cr10 is mediated by dislocation slip via forming interacting planar arrays of dislocations, {111} deformation twins and micro-shear bands. In Fe60Mn20Co10Cr10, it is revealed a synergy of multiple deformation mechanisms, involved dislocation slip and transformation induced plasticity (TRIP, FCC→HCP). More interestingly, it is revealed {101¯2} deformation twins and ε martensite plates with the special misorientation (70.5°/<112¯0>) in the HCP phase. This work further confirms Mn content can act as a switch for TRIP effect in Fe80-xMnxCo10Cr10 HEA. Upon annealing treatments, profuse annealing twinning boundaries in Fe30Mn50Co10Cr10 and dense boundaries with 70.5°/<112¯0> in Fe60Mn20Co10Cr10 are formed, paving the way for the development of HEA with the superb mechanical performance.

Effect of Al on structure and mechanical properties of Fe-Mn-Cr-Ni-Al non-equiatomic high entropy alloys with high Fe content

Microstructure and mechanical properties of the Fe-Mn-Cr-Ni-Al system non-equiatomic high entropy alloys with a different Al content (x = 0–14 at.%) were studied in the present work. The Fe40Mn25Cr20Ni15 alloy was composed of the face-centered cubic (fcc) matrix phase with a small amount of coarse body-centered cubic (bcc) particles. Addition of a small amount of Al (x = 2–6) resulted in an increase in the fraction of the bcc phase to 26% and the formation of fine B2 precipitates within the bcc phase. At higher amounts of Al (x = 10 and x = 14) the microstructure consisted of coarse bcc matrix grains with the B2 precipitates inside. The alloys tend to become stronger with an increase in the Al content from 0 to 10 at.%; further increase in Al concentration did not influence strength considerably. The alloys exhibited pronounced softening with an increase in testing temperature from 25 to 400 °C–600 °C. Ductility of the alloys was high enough (>50%) at all temperatures. A quasi-binary Fe40Mn25Cr20Ni15-Al phase diagram was constructed using a ThermoCalc software and a TCHEA2 database; reasonable agreement between the experimental and predicted phase compositions of the alloys was obtained. It was suggested that an addition of the strong bcc-stabilizing and compound-forming Al to a bcc-prone Fe40Mn25Cr20Ni15 alloy is beneficial for the development of the alloys with the disordered bcc matrix and the embedded B2 precipitates having attractive mechanical properties.

Enhancement of recrystallization texture in non-equiatomic Fe-Ni-Co-Al-based high entropy alloys by combination of annealing and Cr addition

Formation of strong <001> recrystallization texture in new non-equiatomic Fe-Ni-Co-Al-based high entropy alloys has been investigated. Optimal annealing temperature and time to obtain strong <001> texture is subsequently determined through microstructure and recrystallization texture study of cold-rolled NCACB (34.95Fe-27.5Ni-17.5Co-11.5Al-8.5Cr-0.05B at.%) at 1200 °C and 1300 °C for different annealing times. It has also been found that the recrystallization texture is influenced by grain growth and the relative grain size (d/t, d-grain size, t-sheet thickness). Furthermore, contribution of chemical composition to <001> texture has been examined in two additional cold-rolled non-equiatomic high entropy alloys, NCAB (Fe-27.5Ni-17.5Co-11.5Al-0.05B at.%) and NCATB (Fe-27.5Ni-17.5Co-11.5Al-2.5Ta-0.05B at.%), under the same optimal annealing condition used for NCACB. Comparison of texture intensity of NCAB, NCATB, and NCACB demonstrates that Cr is very effective in the formation of strong <001> recrystallization texture of Fe-Ni-Co-Al-based alloys. Based on low-angle boundary statistics, it is hypothesized that decrease in stacking fault energy is responsible for the enhanced recrystallization texture in NCACB. Additionally, it is demonstrated employing a Zener-type model for the pinning of grain boundaries by second-phase particles, that precipitated NiAl particles facilitate the abnormal grain growth in NCACB. The tensile tests results show that strong recrystallization texture and bamboo-like structure improve the ductility of NCACB.