Microstructure and mechanical properties of FexCoCrNiMn high-entropy alloys

The microstructure and tensile properties of Fe_xCoCrNiMn high-entropy alloys (HEAs) were investigated. It was found that the Fe_xCoCrNiMn HEA has a single face-centered cubic (fcc) structure in a wide range of Fe content. Further increasing the Fe content endowed the Fe_xCoCrNiMn alloys with an fcc/body-centered cubic (bcc) dual-phase structure. The yield strength of the Fe_xCoCrNiMn HEAs slightly decreased with the increase of Fe content. An excellent combination of strength and ductility was achieved in the Fe_xCoCrNiMn HEA with higher Fe content, which can be attributed to the outstanding deformation coordination capability of the fcc/bcc dual phase structure.     

Tunability of the mechanical properties of (Fe50Mn27Ni10Cr13)100-xMox high-entropy alloys via secondary phase control

The single-phase face-centered cubic (fcc)-structured Fes0Mn27Ni10Cr1 3 high entropy alloy (HEA) exhibits good ductility but low strength,which presents a challenge.By Mo-alloying and thermomechanical treatments,we have designed the (Fes0Mn27Ni10Cr13)100-xMOx (x=0-6 at.％) alloy series with a wide range of mechanical properties.The careful control of secondary phases introduced in the cold-rolled and annealed (Fe50Mn27Ni10Cr13)Mo2 sample resulted in an enhanced tensile strength from 250MPa to 665 MPa,still having ～25 ％ ductility.TEM investigations of this alloy revealed the presence of deformation twins,dislocation cells,and ordered bcc nano-particles embedded in the ductile fcc matrix post-deformation.The observed deformation structures are an indication of successful cooperation between deformation twinning and precipitation strengthening in enhancing the tensile strength at maintained ductility compared to its cast counterpart.This work provides insight into the tunability of the mechanical properties of non-equiatomic HEAs via alloying and thermomechanical processing.     

Irradiation behaviors of two novel single-phase bcc-structure high-entropy alloys for accident-tolerant fuel cladding

High-entropy alloys(HEAs)are potential alternative materials for accident-tolerant fuel cladding due to their excellent irradiation resistance and high-temperature corrosion resistance.In this work,two novel body-centered cubic(bcc)structured Mo0.5NbTiVCr0.25 and Mo0.5NbTiV0.5Zr0.25 HEAs were fab-ricated.Helium-ion irradiation was performed on the two HEAs to simulate neutron irradiation,and the crystal structure,hardness,and microstructure evolution were investigated.The crystal structure of the Mo0.5NbTiVCr0.25 HEA remained stable at low fluences,while amorphization may occur at high fluences in the two HEAs.The irradiation hardening value of the Mo0.5NbTiVCr0.25 was 0.77 GPa at flu-ences of 1×1017 ions/cm2 and 1.49 GPa at fluences of 5×1017 ions/cm2,while the hardening value of the Mo0.5NbTiV0.5Zr0.25 was 1.36 GPa at ion fluences of 5×1017 ions/cm2.In comparison with most of the conventional alloys,the two HEAs showed slight irradiation hardening.The helium bubbles and dislocation loops with small size were observed in the two HEAs after irradiation.This is the first time to report the formation of a dislocation loop in bcc-structure HEAs after irradiation.However,voids and precipitates were not observed in the two HEAs which could be ascribed to the high lattice distortion and compositional complexity of HEAs.This research revealed that the two HEAs show outstanding irradiation resistance,which may be promising accident-tolerant fuel cladding materials.     

V-Nb-Mo-Al-Ga高熵合金的多晶型性与超导性

高熵合金具有结构多晶型性和超导性,是当前研究的重点.然而,多晶型转变仅在非超导的高熵合金中被观察到,且大多是在高压条件下.本文报道了(V0.5Nb0.5)3-xMoxAl0.5Ga0.5(0.2≤x≤1.4)高熵合金中的超导和温度驱动多晶型性.实验结果表明当x=0.2时铸态高熵合金具有单一的体心立方(bcc)结构,而当x值更高时则为bcc和A15的混合结构.经高温退火后,bcc结构向A15结构进行多晶型转变,且所有高熵合金均表现出块体超导电性.对于x=0.2的组分,其bcc晶型直到1.8 K仍不具备超导性,但其A15晶型却在10.2 K表现出超导性,估算零温上临界磁场Bc2(0)为20.1 T,该超导温度(Tc)和磁场强度在已知的高熵合金超导体中均为最高.随着Mo含量x的增加,A15型高熵合金的Tc和Bc2(0)均降低,但Bc2(0)/Tc比值表明在宽的x范围内存在无序诱导的上临界磁场增加.Tc的降低归因于电子比热系数和电声子耦合强度的减小.此外,该高熵合金的Tc对价电子数依赖关系与二元A15超导体和其他结构高熵合金超导体均不同,且表明可以通过降低价电子数目来进一步提高Tc.本文不仅揭示了一类新结构类型的高熵合金超导体,而且提供了高熵合金中依赖于多晶型性超导的首个示例.     

A promising new class of irradiation tolerant materials: Ti2ZrHfV0.5Mo0.2 high-entropy alloy

Recently, high-entropy alloys (HEAs) or multi-principal-element alloys with unprecedented physical, chemical, and mechanical properties, have been considered as candidate materials used in advanced reactors due to their promising irradiation resistant behavior. Here, we report a new single-phase body-centered cubic (BCC) structured Ti₂ZrHfV_0.5Mo_0.2 HEA possessing excellent irradiation resistance, i.e., scarcely irradiation hardening and abnormal lattice constant reduction after helium-ion irradiation, which is completely different from conventional alloys. This is the first time to report the abnormal XRD phenomenon of metallic alloys and almost no hardening after irradiation. These excellent properties make it to be a potential candidate material used as core components in next-generation nuclear reactors. The particular irradiation tolerance derives from high density lattice vacancies/defects.     

Simultaneous enhancement in strength and ductility of Fe_(50)Mn_(30)Co_(10)Cr_(10) high-entropy alloy via nitrogen alloying

The effect of nitrogen on microstructural evolution and tensile properties of transformation-induced plasticity(TRIP)Fe₅₀Mn₃₀Co₁₀Cr₁₀HEAs was investigated.Nitrogen was fully introduced in solid solution by pressure-induced melting technique.Nitrogen addition turned the TRIP alloy to a twinning-induced plasticity(TWIP)alloy,and simultaneously improved the strength and elongation.For the nitrogen-doped HEA,the high yield strength is mainly resulted from the friction stress via interstitial strengthening effect,and the high ductility is originated from retained high strain-hardening capability via the successive onset of dislocation accumulation and deformation twinning.The strain-hardening behavior and microstructural evolution at specified strains were revealed.     

Nanophase precipitation and strengthening in a dual-phase Al0.5CoCrFeNi high-entropy alloy

Precipitation-hardening in fcc-based high-entropy alloys (HEAs) have usually been realized by introducing complex intermetallic compounds.In this study,enhanced strength is ascribed to the existence of L12 precipitates and B2/bcc conjoint phases in the fcc matrix.The nano-size particles in the Al0.5 CoCrFeNi HEA are produced by cold-rolling,followed by intermediate-temperature-annealing at 650 ℃.For L12 ordering,the initial granular structure has transformed into lamella structure and then kept stable when the holding time prolonged to 200 h.The formation of this conjoint B2/bcc driven by the concentration profiles takes place when the diffusion process of elements is sufficient after long-time aging.Based on the microstructure analysis,changes in mechanical properties are associated with the shape,size scale and volume fraction of the precipitates.The peak ultimate tensile stress reaches 1221.5 MPa,1.97 times compared with the as-cast alloy,remaining plasticity of 21.3％.     

Corrosion behavior of Al0.4CoCu0.6NiSi0.2Ti0.25 high-entropy alloy coating via 3D printing laser cladding in a sulphur environment

High-entropy alloys(HEAs) are of great interest in materials science and engineering communities owing to their unique phase structure.HEAs are constructed with five or more principal alloying elements in equimolar or near-equimolar ratios.Therefore,they can derive their performance from multiple principal elements ratherthan a single element.In this work,three-dimensional printing laser cladding was applied to produce an Al_0.4CoCu_0.6NiSi_0.2Ti_0.25 HEA coating.The experimental results confirmed that the laser cladding could be used to produce a thin coating of 120 μm in thickness.In the high-temperature laser cladding process,some Fe elements diffused from the substrate to the coating,forming a combination of face-centred cubic and body-centred cubic phase structures.The HEA coating metallurgically bonded well with the substrate.Owing to the increased dislocation density and number of grain boundaries,the HEA coating was harder and had a stronger hydrophobicity than X70 steel.The electrochemistry results showed that the HEA coating had better corrosion resistance than X70 steel.Aluminium oxides formed on the surface of the HEA coating had a certain protective effect.However,because of the laser cladding,the HEA coating generated cracks.In future work,the laser cladding technology will be improved and heat treatment will be implemented to prevent formation of cracks.     

Vacancy formation enthalpies of high-entropy FeCoCrNi alloy via first-principles calculations and possible implications to its superior radiation tolerance

Because atoms in high-entropy alloys(HEAs) coordinate in very different and distorted local environments in the lattice sites, even for the same type of constituent, their point defects could highly vary.Therefore, theoretical determination of the thermodynamic quantities(i.e., defect formation enthalpies)of various point defects is rather challenging because each corresponding thermodynamic quantity of all involve constituents is not unique. The knowledge of these thermodynamic quantities is prerequisite for designing novel HEAs and understanding the mechanical and physical behaviors of HEAs. However,to date there has not been a good method to theoretically derive the defect formation enthalpies of HEAs. Here, using first-principles calculations within the density functional theory(DFT) in combination of special quasi-random structure models(SQSs), we have developed a general method to derive corresponding formation enthalpies of point defects in HEAs, using vacancy formation enthalpies of a four-component equiatomic fcc-type FeCoCrNi HEA as prototypical and benchmark examples. In difference from traditional ordered alloys, the vacancy formation enthalpies of FeCoCrNi HEA vary in a highly wide range from 0.72 to 2.89 eV for Fe, 0.88–2.90 eV for Co, 0.78–3.09 eV for Cr, and 0.91–2.95 eV for Ni due to high-level site-to-site lattice distortions and compositional complexities. On average, the vacancy formation enthalpies of 1.58 eV for Fe, 1.61 eV for Cr, 1.70 eV for Co and 1.89 eV for Ni are all larger than that(1.41 eV) of pure fcc nickel. This fact implies that the vacancies are much more difficult to be created than in nickel, indicating a reasonable agreement with the recent experimental observation that FeCoCrNi exhibits two orders of amplitudes enhancement of radiation tolerance with the suppression of void formation at elevated temperatures than in pure nickel.     

Strengthening Mechanisms in Bulk FeCoCrNiAl x (x=0, 0.5, 1.0) High-Entropy Alloys Fabricated by Laser Melting Deposition

Herein, FeCoCrNiAl _x (x = 0, 0.5, 1.0) high-entropy alloys (HEAs) are fabricated by the laser melting deposition (LMD) technique. With the increase of Al content, the LMD-ed microstructure transitions from a single face-centered cubic (FCC) phase to a dual-phase structure containing a small amount of body-centered cubic (BCC) phase (5.3%), and the proportion of the final BCC phase increases significantly to 98.2%. In addition to the compression tests, four strengthening models are used to evaluate the theoretical strength of the three alloys. The addition of Al element as grain refiner can improve the ultimate compressive strength of HEAs; however, the yield strength and plasticity do not improve, as theoretically expected. The FCC phase with more slip systems leads to higher plasticity in the LMD-ed FeCoCrNi HEA but results in lower yield strength. The LMD-ed FeCoCrNiAl_0.5 HEA exhibits the best combination of strength and plasticity. Therefore, to meet the required service requirements, the content of Al in the FeCoCrNiAl _x HEA should be carefully controlled under the premise of considering the actual working conditions.     

Irradiation-Induced Extremes Create Hierarchical Face-/Body-Centered-Cubic Phases in Nanostructured High Entropy Alloys

A nanoscale hierarchical dual-phase structure is reported to form in a nanocrystalline NiFeCoCrCu high-entropy-alloy (HEA) film via ion irradiation. Under the extreme energy deposition and consequent thermal energy dissipation induced by energetic particles, a fundamentally new phenomenon is revealed, in which the original single-phase face-centered-cubic (FCC) structure partially transforms into alternating nanometer layers of a body-centered-cubic (BCC) structure. The orientation relationship follows the Nishiyama–Wasser-man relationship, that is, (011)_BCC || ( 1¯1¯1)_FCC and [100]_BCC || [ 11¯0]_FCC. Simulation results indicate that Cr, as a BCC stabilizing element, exhibits a tendency to segregate to the stacking faults (SFs). Furthermore, the high densities of SFs and twin boundaries in each nanocrystalline grain serve to accelerate the nucleation and growth of the BCC phase during irradiation. By adjusting the irradiation parameters, desired thicknesses of the FCC and BCC phases in the laminates can be achieved. This work demonstrates the controlled formation of an attractive dual-phase nanolaminate structure under ion irradiation and provides a strategy for designing new derivate structures of HEAs.     

Deposition of FeCoNiCrMn high entropy alloy (HEA) coating via cold spraying

High entropy alloys (HEAs) are of great interest in the community of materials science and engineering due to their unique phase structure. They are constructed with five or more principal alloying elements in equimolar or near-equimolar ratio. Therefore, HEAs can derive their performance from multiple principal elements rather than a single element. In this work, solid-state cold spraying (CS) was applied for the first time to produce FeCoNiCrMn HEA coating. The experimental results confirm that CS can be used to produce a thick HEA coating with low porosity. As a low-temperature deposition process, CS completely retained the HEA phase structure in the coating without any phase transformation. The characterization also reveals that the grains in the CSed HEA coating had experienced significant refinement as compared to those in the as-received HEA powder due the occurrence of dynamic recrystallization at the highly deformed interparticle region. Due to the increased dislocation density and grain boundaries, CSed HEA coating was much harder than the as-received powder. The tribological study shows that the CSed FeCoNiCrMn HEA coating resulted in lower wear rate than laser cladded HEA coatings.     

Plastic deformation of bcc metal thin foils without dislocation

Heavy plastic deformation of fcc metal thin foils to fracture has been found recently to proceed without involving dislocations, and it results in the formation of high density of vacancy clusters. Thin foil specimens of bcc metals such as V and Mo were plastically deformed to fracture in in situ elongation experiments under an electron microscope. Morphology of thinning and fracture was found to be similar to fcc metals, and no dislocation was observed during heavy deformation. Electron diffraction analysis at the tip of a crack during deformation confirmed a large elastic deformation of up to 5%. Unlike in fcc metal thin foil specimens, point defect clusters were not observed near fractured tips. This difference is attributed to the difference in vacancy reaction, though the deformation in bcc metals without dislocation most likely does produce vacancies.     

Microstructure,thermodynamics and compressive properties of AlCrCuNixTi (x = 0, 1) high entropy alloys

Two equiatomic high entropy alloys, AlCrCuNi_xTi (x?=?0, 1), were prepared by an arc furnace. Their microstructure, thermodynamics and compressive properties were investigated in as-cast state. The AlCrCuTi alloy consists of a face centred cubic (fcc) phase, two body centred cubic (bcc) phases and an Al₄Cu₉-like phase, while the AlCrCuNiTi alloy contains an fcc phase and two bcc phases. Thermodynamic expressions based on mixing enthalpy matrix facilitate the thermodynamic calculation. The element Cr takes severe segregation during solidification and forms Cr rich phases in both alloys. The addition of Ni to the AlCrCuNiTi alloy inhibits the formation of intermetallic compounds and enhances the yield strength, compressive strength and ultimate strain, but degrades Vickers hardness.     

Self-decay-induced damage production and micro-structure evolution in fcc metals: An atomic-scale computer simulation approach

In this paper, we discuss the development and application of a hybrid molecular dynamics/kinetic Monte Carlo simulation to model defect production and microstructure evolution in irradiated fcc metals. The molecular dynamics results show that the primary damage state produced by high-energy 30 keV recoils in low melting point, high-Z fcc metals such as Pb and Au is dominated by populations of large vacancy and interstitial clusters, and only a small percentage of the damage is produced as isolated point defects. Kinetic Monte Carlo simulations were used to calculate the fraction of the defects produced by the 30 keV recoils that are able to avoid recombination within their nascent cascade and freely migrate through the lattice. We show that because of in-cascade clustering this fraction is different for vacancies and interstitials and depends strongly on temperature. The results are used to provide a qualitative explanation for the experimentally observed differences in void swelling between fcc and bcc metals. The kinetic Monte Carlo simulations were also used to model damage accumulation in Pb under 30 keV self-recoil irradiation as a function of dose rate and temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

添加Be对CoCrFeNi高熵合金组织和性能影响的研究

目的 研究CoCrFeNi高熵合金组织和性能在添加Be后的变化,通过高熵合金固溶体相形成规律,设计从面心立方固溶体转变至含体心立方及金属间化合物的(CoCrFeNi)1-xBex系列高熵合金。方法 通过计算验证(CoCrFeNi)1-xBex系列高熵合金的成分是否落入固溶体区域,并对上述成分高熵合金组织和力学性能进行研究。结果 Be元素的原子数分数为4%时,高熵合金仍为单一的FCC相结构,随着Be元素含量的进一步增加,基体中出现BCC相和金属间化合物。Be的添加使得(CoCrFeNi)1-xBex高熵合金的屈服强度及显微硬度均大大提高,同时密度降低。结论 根据相形成规律设计的(CoCrFeNi)1-xBex系列高熵合金表明,适量添加Be元素可以改善CoCrFeNi高熵合金的综合物理力学性能。     

Direct alpha Ta formation on TaN by resputtering for low resistive diffusion barriers

The Ta/TaN bilayer exhibits the best performance in the Cu metal multilevel interconnects, because it provides good coherence between Cu and dielectric layer. In the Ta/TaN bilayer, Ta has two phases: alpha-phase of body center cubic is preferred due to its lower resistivity (15-60 microOmega-cm), whereas beta-phase of tetragonal should be avoided due to high resistive (approximately 150-250 microOmega-cm). However, beta Ta most commonly forms on fcc TaN. Here we provide a simple scheme to bypass this high resistive phase by resputtering TaN prior to Ta deposition. We found that, with surface treatment by argon ion bombardment for enough time, alpha Ta phase can be directly formed, which is supported both by X-ray diffraction and resistivity measurement. Depth profiles of all elements from Auger electron spectroscopy reveals that the surface treatment induces a nitrogen deficient surface layer due to different sputtering yield, which causes phase changes from fcc TaN to hcp Ta2N followed by bcc Ta(N) and provide a favorable lattice constant for Ta alpha-phase formation.     

Nanostructured amorphous Fe_(29)Co_(27)Ni_(23)Si_9B_(12) high-entropy-alloy:an efficient electrocatalyst for oxygen evolution reaction

Proximal configu ration of dissimilar metal atoms in amorphous high-entropy-alloys(HEAs) always re sult in interatomic d-band ligand effect,dense defect distribution,coordinatively unsaturated sites,high potential energy,and loose atom bonding.Herein,nanostructured amorphous Fe₂₉Co₂₇Ni₂₃Si₉B₁₂ HEA ribbon is fabricated via a melt spinning method combined with electrochemical corrosion etching process,which is applied as the potential oxygen evolution reaction electrocatalyst.It is found that there are micro/nano pits on the surface of etched amorphous Fe₂₉Co₂₇Ni₂₃Si₉B₁₂ ribbons.Various elements of HEAs bond with each other to form a highly disordered configu ration,which could result in an optimized bonding energy and enhanced intrinsic catalytic activity.The electrocatalysis activity measurements indicate that the amorphous HEA endows a much higher activity than the crystalline one,which is further improved by the electrochemical etching treatment.Especially,the HEA ribbon etched for 3 h requires a low overpotential of 230 mV to afford 10 mA cm^-2 current density.In addition,density functional theory calculations demonstrate that the amorphous structure can weaken the interaction between the surface of Fe₂₉Co₂₇Ni₂₃Si₉B₁₂ alloy and the intermediates,leading to an optimized adsorption Gibbs free energy.     

Significant reduction in friction and wear of a high-entropy alloy via the formation of self-organized nanolayered structure

Sliding wear-induced nanolayering and its positive impact on wear resistance have been observed in conventional binary alloys with a matrix of high stacking fault energy (SFE),but this concept has never been reported in high-entropy alloys (HEAs) with low SFE.Here,we design and fabricate a (CoCrFeNi)90Ag10 HEA,consisting of a face-center-cubic (fcc) CoCrFeNi HEA matrix with low SFE and uniformly dispersed Ag precipitates.In comparison with CoCrFeNi,a significant reduction in friction and wear was found in (CoCrFeNi)90Ag10 HEA through the spontaneous formation of nanolayered subsurface microstructure during wear.The finding suggests a novel approach for designing HEAs that can achieve low friction and wear.     

Phase‐Transformation Ductilization of Brittle High‐Entropy Alloys via Metastability Engineering

High‐entropy alloys (HEAs) in which interesting physical, chemical, and structural properties are being continuously revealed have recently attracted extensive attention. Body‐centered cubic (bcc) HEAs, particularly those based on refractory elements are promising for high‐temperature application but generally fail by early cracking with limited plasticity at room temperature, which limits their malleability and widespread uses. Here, the “metastability‐engineering” strategy is exploited in brittle bcc HEAs via tailoring the stability of the constituent phases, and transformation‐induced ductility and work‐hardening capability are successfully achieved. This not only sheds new insights on the development of HEAs with excellent combination of strength and ductility, but also has great implications on overcoming the long‐standing strength–ductility tradeoff of metallic materials in general.