Medium-entropy Alloy Research Articles

CNTs Agglomeration Effect on Wear and Mechanical Behaviors of FeCoNiMn Medium Entropy Alloys

CNTs Agglomeration Effect on Wear and Mechanical Behaviors of FeCoNiMn Medium Entropy Alloys

A cost-effective iron-rich medium entropy alloys with the balance of strength and ductility

Iron-rich medium-entropy alloys (MEAs) have attracted attention due to the balance for cost and properties of alloy. In this study, a combined metastability engineering and eutectic high-entropy alloys (EHEAs) design strategy were employed to develop a cost-effective, iron-rich MEAs for the Fe–Cr–Ni–Al MEAs system using CALPHAD simulations. This approach enabled the tailored control of soft and hard phases within the non-isoatomic MEAs system by manipulating the elemental ratios. Specifically, increasing the aluminum (Al) content promoted the stability of the BCC/B2 phase, and the microstructure of the MEA transitioned from a single-phase FCC structure to a mixed FCC + BCC/B2 structure, culminating in a final single-phase BCC/B2 structure. This BCC/B2 phase precipitation strengthening mechanism led to enhanced microhardness and yield strength in MEAs with high Al content. Notably, the Al11-as alloy demonstrated a yield strength of 692.7 ± 30.4 MPa (precipitation strengthening is ∼428.98 MPa, accounting for the true yield strength of the alloy ∼61.9 %), with an ultimate tensile strength reaching a maximum value of 992.3 ± 32.77 MPa. Moreover, the alloy retained a satisfactory elongation of 7.1 ± 2.9 %. These efforts hold promise for the development and application of cost-effective, iron-rich MEAs for the Fe–Cr–Ni–Al MEAs system.

Temperature effect on the microstructural refinement of equiatomic CoCrNi medium entropy alloy via cyclically multi-directional impact

Severe plastic deformation can effectively refine alloy microstructure and improve the mechanical properties of alloys surprisingly. In this work, a new approach of severe plastic deformation was proposed based on split Hopkinson pressure bar, i.e., cyclically multi-directional impact. Using this newly developed technique for microstructural refinement at different temperatures (77 K, 293 K, and 1073 K), bulk equiatomic CoCrNi medium entropy alloys with refined microstructures were obtained, and the stress vs. strain relation of the equiatomic CoCrNi medium entropy alloy during the cyclically multi-directional impact can be in situ measured. The strain rate of the cyclically multi-directional impact was controlled at about 1500 /s. The hardnesses of the CoCrNi medium entropy alloy increase dramatically from 216 Hv to 460 Hv after the cyclically multi-directional impact at 77 K, 484 Hv after the cyclically multi-directional impact at 293 K, and 434 Hv after the cyclically multi-directional impact at 1073 K. The microstructural evolutions of the CoCrNi MEA after cyclically multi-directional impact at three temperatures were characterized and analyzed. This study aims to provide a new approach to microstructural refinement for bulk alloys.

Sunflower-like eutectic solidification in gas atomized Al0.3CrFeNiTi0.3 medium-entropy alloy powders: A comprehensive microstructural study

The addition of larger atomic size elements, such as Al and Ti, to CrFeNi medium-entropy alloys (MEAs) offers a strategic approach to overcome the strength-ductility trade-off. This study focuses on examining the morphological, solidification, phase, and elemental characteristics of Al0.3CrFeNiTi0.3 feedstock powders produced through gas atomization (GA). The rapid cooling rate inherent in GA results in sunflower-like eutectic solidification. This structure exhibits firstly solidified Cr-rich BCC phases filled with cuboidal-shaped L21-type precipitates, each measuring up to ∼20 nm. Their peripheral regions display outward and radial growth of L21 and Fe-rich BCC eutectic phases, while FCC phase develops at the remaining sites. Intermediate B2 layers between BCC and L21 phases are considered as kinetic buffers, which enhance the interfacial stability. The size and shapes of powders determine their cooling rates, leading to a transition from comparably large sunflower-like eutectic structures to finer equiaxed grains alongside reduced elemental segregation with decreasing powder size. Nano-hardness measurements demonstrate superior mechanical strength in the peripheral regions due to the higher volume fraction of the L21 phase and its optimal interfacial coherency with the Fe-rich BCC phase. Furthermore, the slightly higher concentrations of Al and Ti in Fe-rich BCC phase (compared to the Cr-rich BCC phase) induce a better lattice distortion effect.

Low cycle fatigue properties of CoCrFeNiMn high-entropy alloy with heterogeneous microstructure

Heterogeneous microstructure is a hot topic in materials science which facilitates tackling the dilemma of strength-ductility trade-off in engineering materials including high-entropy alloys. The present investigation was conducted on the impact of different microstructures including heterogeneous microstructure, constructed by thermomechanical treatment including cold rolling followed by annealing, on the monotonic and cyclic behavior of a CoCrFeNiMn high-entropy alloy. The tailored microstructure contained three features: non-recrystallized as a hard domain, ultrafine-recrystallized, and fine-recrystallized regions. A desirable combination of strength and ductility including UTS of ∼1.1 GPa together with an elongation of ∼15 % was achieved due to benefiting from hetero-deformation-induced hardening and activation of deformation twins in the fine recrystallized grains. Regarding low-cycle fatigue behavior, at the strain amplitude of 0.4 %, heterogeneous microstructure exhibits remarkable fatigue properties, including high maximum stress and extraordinary lifetime (more than 2 × 105). The compelling reasons for these desirable properties are triggering deformation twins in fine recrystallized grain, the absence of brittle sigma phase as a good location of crack initiation, and the non-recrystallized area as a hard domain for suppressing crack propagation. The results suggested that the heterogeneous microstructure has significant benefits during applying low strain amplitude, however, at high strain amplitude, grain refinement is not recommended, and coarse-grained microstructure provides better properties. The present investigation is a step forward to understand the significance of heterogeneous microstructures to improve fatigue properties of high- or medium-entropy alloys.

Effects of cutting tool geometry on material removal of a gradient nanograined CoCrNi medium entropy alloy.

CoCrNi medium-entropy alloys (MEAs) have attracted extensive attention and research because of their superior mechanical properties, such as higher ductility, strength, and toughness. This study uses molecular dynamics (MD) simulations to investigate the cutting behavior of a gradient nanograined (GNG) CoCrNi MEA. Moreover, it explores the influence of relative tool sharpness and rake angle on the cutting process. The results show that an increase in the average grain size of the GNG samples leads to a decrease in the average resultant cutting force, as predicted by the Hall-Petch relationship. The deformation behavior shows that grain boundaries are crucial in inhibiting the propagation of strain and stress. As the average grain size of the GNG sample increases, the range of shear strain distribution and average von Mises stress decreases. Moreover, the cutting chips become thinner and longer. The subsurface damage is limited to a shallow layer at the surface. Since thermal energy is generated in the high grain boundary density, the temperature of the contact zone between the substrate and the cutting tool increases as the GNG size decreases. The cutting chips removed from the GNG CoCrNi MEA substrates will transform into a mixed structure of face-centered cubic and hexagonally close-packed phases. The sliding and twisting of grain boundaries and the merging of grains are essential mechanisms for polycrystalline deformation. Regarding the cutting parameters, the average resultant force, the material accumulation, and the chip volume increase significantly with the increase in cutting depth. In contrast to sharp tools, which mainly use shear deformation, blunt tools remove material by plowing, and the cutting force increases with the increase in cutting-edge radius and negative rake angle.

Oxidation behavior and corrosion resistance of CoCrFeNi high entropy alloy compared with FeCoNi medium entropy alloy

The entropy-based alloy with excellent oxidation and corrosion resistance is of great significance for practical applications in high-temperature environments. Thus, the difference in anti-oxidation property and corrosion behavior of FeCoNi MEA and CoCrFeNi HEA in a simulated marine environment was studied via a series of measurements. The results reveal that the HEA possesses superior oxidation and corrosion resistance in comparison with the MEA, which can be demonstrated by comparing the oxidation rate and thickness of oxide layer, corrosion current density (icorr) as well as corrosion morphology after polarization. The microstructure and composition of the oxide layer are the primary reasons resulting in the difference of oxidation and corrosion behaviors of the two alloys. The oxide layer with a compact, even and intact structure is produced on the HEA surface, which hinders the diffusion of detrimental ions and oxygen, whereas the loose, porous and cracked oxide layer consisting of inner Fe3O4 and outer CoO exhibits poor protection against corrosion and oxidation of the MEA. For the HEA, the preferential combination of Cr and O elements forms the protective oxide of Cr2O3, effectively enhancing corrosion resistance and inhibiting oxidation. However, the low protective CoO and relatively active Fe3O4 grow on the MEA surface, which cannot provide a good barrier to restrain oxidation and corrosion.

Heterogeneous structure and dual precipitates induced excellent strength-ductility combination in CoCrNiTi0.1 medium entropy alloy

Developing advanced structural materials with high strength and excellent ductility is crucial for meeting aerospace and military requirements. High/medium entropy alloys (H/MEAs) have garnered significant attention due to their superior mechanical properties. In this study, through severe deformation followed by heat treatment at temperatures in the range of 700–900 °C, we successfully obtained a fully recrystallized structure with double precipitates in non-equiatomic CoCrNiTi0.1. Tensile tests at room temperature revealed that the alloy annealed at 700 ° C for 1 h exhibited exceptional mechanical properties, with a yield strength of 1575 MPa, tensile strength of 1735 MPa, and ductility of 15 %. The ultrahigh yield strength is primarily attributed to the synergistic strengthening mechanisms including precipitation strengthening, grain-boundary strengthening, and HDI strengthening. The FCC/L12 interface facilitated dislocation transfer during deformation, reducing stress concentration and promoting uniform deformation. Additionally, deformation mechanisms such as stacking faults and trace twinning contributed to the alloy's excellent ductility, ensuring substantial work-hardening capability and achieving a remarkable strength-ductility combination. These findings not only offer an effective strategy for enhancing the mechanical performance of non-equiatomic MEAs but also deepen our understanding of synergistic strengthening and deformation mechanisms in alloys.

Achieving superior strength-ductility synergy and enhanced corrosion resistance in a novel medium-entropy alloy via grain refinement strategy

The effect of grain size on mechanical properties and corrosion resistance of Ni62Cr10V28 MEA was investigated. The findings indicated that grain refinement significantly enhanced the corrosion resistance of the investigated alloys, attributed to the distinctive characteristics of grain boundaries and the fast formation rate of passive film. Additionally, the investigated alloys demonstrated a yield strength ranging from 518.7 MPa to 714.2 MPa, accompanied by an elongation ranging from 46.29 % to 50.48 %, thereby attaining a superior strength-ductility balance. Overall, grain refinement demonstrates a favorable synergistic effect on both mechanical properties and corrosion resistance.

Mechanical characteristics and electrochemical corrosion of NiCoCr alloys

Previous studies have shown that NiCoCr medium entropy alloys (MEAs) have perfect strength-ductility balance and significant escape fracture toughness. This work aimed to determine the effects of annealing temperature and alloy composition on the mechanical characteristics and corrosion behaviors of the NiCoCr MEAs. The results showed that the grains generated in the NiCoCr MEAs became more petite with the cooling rate increasing. The Vickers hardness (HV) of annealed NiCoCr MEAs increased with increasing annealing temperature. For NiCoCr alloys with different alloy compositions, the potentiodynamic polarization results showed that the NiCoCrFeMn alloy has the highest polarisation resistance, lowest current density, and lowest corrosion rate. However, the composition of Fe2O3-based passivation film makes it highly reactive and easily cleaved by Cl−. The NiCoCr and NiCoCrFe MEAs with high Ni content had a denser and slowly dissolved NiO passivation layer and better corrosion resistance. Moreover, the hardness of corroded NiCoCr MEAs decreased by 7.7%.

Effect of Ti content on microstructure evolution and mechanical properties of FeCoNi-based medium-entropy alloys

Effect of Ti content on microstructure evolution and mechanical properties of FeCoNi-based medium-entropy alloys

Critical impacts of thermodynamic instability and short-range order on deformation mechanisms of VCoNi medium-entropy alloy

Short-range order (SRO) has made a splash in the recent research on multi-principal element alloys (MPEAs), among which the equiatomic VCoNi alloy was regarded as the most potential prototype. However, in addition to disputes over SRO, there is still a lack of agreements on the phase stability and deformation mechanism of the VCoNi alloy, which should be addressed before the SRO can be independently analyzed. To this end, microstructural evolutions with increasing long-term annealing temperatures (from 700 to 1,200 °C) were first inspected in detail to determine critical temperatures for phase transitions unambiguously. Our results revealed that the VCoNi alloy was still dominated by the face-centered-cubic (FCC) structure with a minor κ phase at 900 °C and became a single FCC solid solution at above 910 °C. Subsequently, by carefully examining dislocation configurations and stacking fault (SF) widths in the FCC phases annealed at 900 °C and 1,200 °C, significant variations in local stacking fault energy (SFE) were unveiled, and the overall SFE decreased with increasing annealing temperature. Combined with the state-of-the-art density function theory (DFT)-based lattice Monte Carlo (MC) simulation, we demonstrated how the rise/decline in SFE can be explained by the greater/lower degree of SRO. This research would not only offer new perspectives for recent controversies over phase stability, deformation mechanism, and SRO characterization technique using electron diffraction but also shed light on the intrinsic relationship between SRO and local stacking fault energy of MPEAs.

Corrosion Behavior of Al-Containing CoNiV Medium Entropy Alloy Coating Fabricated by Laser Cladding.

CoNiV medium entropy alloys (MEAs) have been widely recognized for their superior corrosion resistance. Nonetheless, their extensive application has been hindered by high production costs and complex fabrication processes. In this study, CoNiVAlx MEA coatings were synthesized on AISI 304 stainless steel substrates via laser cladding technology. The microstructure, phase composition, corrosion resistance, and corrosion mechanisms of the coatings were systematically investigated by using advanced characterization techniques, including optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and electrochemical workstation analysis. The ratio of O2-/OH- in the passivation film of the coated surface exhibited a gradual increase with the addition of Al. The formation of the Al-containing precipitated phase L21 was observed at x = 0.3 and 0.4. The results demonstrated that moderate Al doping (x ≤ 0.2) enhanced corrosion resistance by improving the stability of the passivation film and reducing the thermodynamic tendency toward corrosion. In contrast, excessive Al doping (x > 0.2) led to the formation of the L21 phase, which increased the susceptibility to localized corrosion, thus compromising the overall corrosion resistance.

Achieving strength–ductility synergy in novel paramagnetic Fe-based medium-entropy alloys through deep cryogenic deformation

Achieving strength–ductility synergy in novel paramagnetic Fe-based medium-entropy alloys through deep cryogenic deformation

Thermodynamic and elastic properties of laves phase AB2-based alloys and their hydrides: A density functional theory (DFT) study

A first-principles method based on the density functional theory (DFT) was employed in conjunction with quasi-harmonic Debye model to investigate the structural, elastic, and thermodynamic properties of a series of AB2-based Laves phases alloys and their hydrides. Simulation results revealed that the studied materials possess the Laves phase structure with lattice parameters comparable to experimental findings. For the first time, to the best of our knowledge, mixing entropy determined using Debye model was used to classify alloys into (Low- Medium-High Entropy Alloys) LEA, MEA and HEA categories rather than the commonly used ideal solution model, which is often inaccurate and ignores the impact of temperature. Lattice analyses of the alloy materials indicated that cell volume increases with the addition of elements, while the enthalpy of hydride formation indicates that hydrogen absorption in these alloys is exothermic and that the 3.00 H/F.U configuration is energetically stable. The alloys and their hydrides are metallic with no band gap at the Fermi level. The thermodynamic properties were studied using the quasi-harmonic Debye model and it was found that Bulk modulus decreases with increasing volume, and the hydrides possess lower bulk modulus compared to their metallic counterparts. Moreover, Debye temperature decreases with the gradual addition of elements, indicating weaker chemical bonds in ternary and other alloys. All hydrides have lower Debye temperature than their parent alloy materials. Finally, The alloys are classified into low-, medium-, and high-entropy alloys based on mixing entropy calculated using the Debye model. TiMn2 is classified as a low-entropy alloy, ZrMn2, Ti0.5Zr0.5Mn2, and Ti0.5Zr0.5MnFe as medium-entropy alloys, and Ti0.5Zr0.5(MnCr)2, Ti0.5Zr0.5Mn2/3Fe2/3Cr2/3, and Ti0.5Zr0.5Mn0.5Fe0.5Cr0.5Ni0.5 as high-entropy alloys.

Edge Dislocation and Grain Boundaries Effects on the Mechanical Properties in NiCoAl$N i C o A l$ Medium Entropy Alloy

Medium‐entropy alloys (MEAs) are becoming increasingly popular owing to their superior mechanical features. For the first time, using molecular dynamics (MD) simulations, investigation is done on how grain boundaries (GBs) and dislocations affect the mechanical properties of face‐centered cubic (FCC) equiatomic MEA. Lattice distortion, elastic constants and moduli, machinability index, Vickers hardness, and Kleinman parameters to explore how grain boundaries affect MEA are analyzed. It tends to become more rigid based on the link between its elastic moduli and GBs. Interestingly, the analyses of Cauchy pressure, Poisson's, and Pugh's ratios all indicate the ductility of the studied MEA. The investigation of anisotropy factors indicates that MEA exhibits anisotropy, which is reduced by the addition of GBs. Mechanical properties of the considered MEA, such as ductility, fracture toughness, and stability, are significantly influenced by the GBs. The obtained results show that studying these phenomena can help design structural nano‐metal materials with superior mechanical properties, allowing them to be used in various applications.

Effect of the deformation temperature and strain on the strain rate sensitivity of fcc medium-entropy alloys

The primary objective of the present investigation is to elucidate the operative micromechanisms influencing the strain rate sensitivity and activation volume in (FeCrNi)99Si1 and FeMnNi medium-entropy alloys. Room-temperature nanoindentation experiments at different loading rates were performed to study the evolution of the strain rate sensitivity and activation volume in (FeCrNi)99Si1 and FeMnNi medium-entropy alloys. The (FeCrNi)99Si1 samples were subjected to plane strain deformation by rolling at 77 and 300 K to study the effect of temperature on the strain rate sensitivity, while the FeMnNi and (FeCrNi)99Si1 samples were subjected to simple shear deformation by high-pressure torsion at 300 K to examine the effect of strains. Contrary to the well-documented trend observed in fcc metals and alloys, where the strain rate sensitivity typically increases with decreasing grain size, the present study reveals a distinct behavior for the current alloys. Similarly, these alloys are characterized by extremely low activation volumes of a few tens of b3 compared to 100–1000 b3 for conventional fcc metals and alloys in the microcrystalline grain size regime. Unlike conventional fcc metals and alloys, there is an insignificant change in the activation volume of the current high-/medium-entropy alloy (H/MEA) with decreasing grain size from the microcrystalline to nanocrystalline regime. The unique evolution of strain rate sensitivity and activation volume in H/MEAs is explained in terms of the evolution of distinct dislocation structures as well as synergistic operation of additional mechanisms such as twinning, phase transformation from fcc to hcp phases, cluster strengthening, and short-range ordering due to the aperiodic energy landscape existing in MEAs.

Periodic spinodal decomposition in double–strengthened medium–entropy alloy

Achieving an optimal balance between strength and ductility in advanced engineering materials has long been a challenge for researchers. In the field of material strengthening, most approaches that prevent or impede the motion of dislocations involve ductility reduction. In the present study, we propose a strengthening approach based on spinodal decomposition in which Cu and Al are introduced into a ferrous medium–entropy alloy. The matrix undergoes nanoscale periodic spinodal decomposition via a simple one-step aging procedure. Chemical fluctuations within periodic spinodal decomposed structures induce spinodal hardening, leading to a doubled strengthening effect that surpasses the conventional precipitation strengthening mechanism. Notably, the periodic spinodal decomposed structures effectively overcome strain localization issues, preserving elongation and doubling their mechanical strength. Spinodal decomposition offers high versatility because it can be implemented with minimal elemental addition, making it a promising candidate for enhancing the mechanical properties of various alloy systems.

Analysis of the temperature-dependent plastic deformation of single crystals of quinary, quaternary and ternary equiatomic high- and medium-entropy alloys of the Cr-Mn-Fe-Co-Ni system

ABSTRACT Temperature-dependent plastic deformation behaviors of single crystals of quaternary and ternary equiatomic medium-entropy alloys (MEAs) belonging to the Cr-Mn-Fe-Co-Ni system were investigated in compression at temperatures in the range 9 K to 1373 K. Their critical resolved shear stresses (CRSSs) increase with decreasing temperature below room temperature. There is also a dulling of the temperature dependence of CRSS below 77 K due to dislocation inertial effects that we attribute to a decrease in the phonon drag coefficient. These behaviors were compared with those of previously investigated single crystals of the equiatomic Cr-Co-Ni and Cr-Fe-Co-Ni MEAs, and the equiatomic Cr-Mn-Fe-Co-Ni high-entropy alloy (HEA). The temperature dependence of CRSS and the apparent activation volumes below room temperature can be well described by conventional thermal activation theories of face-centered cubic (FCC) alloys. Above 673 K, there is a small increase in CRSS, which we believe is due to elastic interactions between solutes and mobile dislocations, the so-called Portevin-Le Chatelier (PL) effect. The CRSS at 0 K was obtained by extrapolation of fitted CRSS vs. temperature curves and compared with predictions from solid solution strengthening models of HEA and MEAs.

Microstructure evolution and twinning-induced plasticity (TWIP) in hcp rare-earth high- and medium-entropy alloys (HEAs and MEAs) due to tensile deformation

The microstructure evolution due to the tensile deformation of the equiatomic quinary high-entropy alloy Ho-Dy-Y-Gd-Tb (HEA-Fb) is assessed. HEA-Fb has extraordinarily similar alloying elements. It is one of the few hexagonal-close-packed single-phase representatives of HEA. HEA-Fb is compared to the equiatomic quaternary medium-entropy alloy (MEA) Ho-Dy-Gd-Tb with no Y (4-Y). For a hexagonal HEA, in contrast to the cubic HEA, little information on plastic deformation and underlying mechanisms is available. A detailed study using electron microscopy-based multi-scale characterization (SEM, S/TEM, and STEM-EDS) explains significant differences between the ductile behavior of the quaternary MEA 4-Y and the brittle behavior of the quinary HEA-Fb at room temperature. Twinning during plastic deformation is decisive for ductility, which challenges the widely discussed high-entropy effect on the mechanical behavior of the HEA. For the quaternary MEA 4-Y, a twinning-induced plasticity effect is found. In the latter, oxidized twins are present in the undeformed state. In both alloys, the twin orientations are indexed as [2¯201], while the matrices have the perpendicular [112¯0] orientation. Additionally, the analysis of twin structures confirms the importance of twin boundaries as obstacles for dislocations and stacking fault mobilities. The results are discussed in the context of the existing knowledge gaps in the field of hexagonal MEAs and HEAs.