Microstructures and properties of high-entropy alloys

This paper reviews the recent research and development of high-entropy alloys (HEAs). HEAs are loosely defined as solid solution alloys that contain more than five principal elements in equal or near equal atomic percent (at.%). The concept of high entropy introduces a new path of developing advanced materials with unique properties, which cannot be achieved by the conventional micro-alloying approach based on only one dominant element. Up to date, many HEAs with promising properties have been reported, e.g., high wear-resistant HEAs, Co_1.5CrFeNi_1.5Ti and Al_0.2Co_1.5CrFeNi_1.5Ti alloys; high-strength body-centered-cubic (BCC) AlCoCrFeNi HEAs at room temperature, and NbMoTaV HEA at elevated temperatures. Furthermore, the general corrosion resistance of the Cu_0.5NiAlCoCrFeSi HEA is much better than that of the conventional 304-stainless steel. This paper first reviews HEA formation in relation to thermodynamics, kinetics, and processing. Physical, magnetic, chemical, and mechanical properties are then discussed. Great details are provided on the plastic deformation, fracture, and magnetization from the perspectives of crackling noise and Barkhausen noise measurements, and the analysis of serrations on stress–strain curves at specific strain rates or testing temperatures, as well as the serrations of the magnetization hysteresis loops. The comparison between conventional and high-entropy bulk metallic glasses is analyzed from the viewpoints of eutectic composition, dense atomic packing, and entropy of mixing. Glass forming ability and plastic properties of high-entropy bulk metallic glasses are also discussed. Modeling techniques applicable to HEAs are introduced and discussed, such as ab initio molecular dynamics simulations and CALPHAD modeling. Finally, future developments and potential new research directions for HEAs are proposed.     

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.     

Microstructures and mechanical properties of Nb-alloyed CoCrCuFeNi high-entropy alloys

Nb has a positive effect on improving the mechanical properties of metal materials, and it is expected to strengthen CoCrCuFeNi high-entropy alloys (HEAs) with outstanding ductility and relatively weak strength. In this paper, the alloying effects of Nb on the microstructural evolution and the mechanical properties of the (CoCrCuFeNi)_100-xNb_x HEA were investigated systematically. The result shows that Nb promotes the phase transition from FCC (face-centered cubic) to Laves phase, and the volume fractions of Laves phase increase from 0% to 58.2% as the Nb content increases. Compressive testing shows that the addition of Nb has a positive effect on improving the strength of CoCrCuFeNi HEA. The compressive yield strength of (CoCrCuFeNi)_100-xNb_x HEAs increases from 338 MPa to 1322 MPa and the fracture strain gradually reduces from 60.0% (no fracture) to 8.1% as the Nb content increases from 0 to 16 at.%. The volume fraction increase of hard Laves phase is the key factor for the strength increase, and the reduction of the VEC (valence electron concentration) value induced by the addition of Nb is beneficial for the increase of the Laves phase content in these alloys.     

Microstructural evolution and mechanical properties of CoCrFeNiMnTix high-entropy alloys

This study was conducted to investigate the effect of titanium addition on the microstructure and properties of an equitaomic CoCrFeNiMn high-entropy alloy. Homogenized microstructures of CoCrFeNiMnTi_x (x = 0.1 and 0.3) alloys consist of face-centered cubic phase; however, addition of more titanium led to formation of a (chromium, titanium)-rich σ phase in CoCrFeNiMnTi_0.4 alloy. The average electron hole number calculations indicate the higher possibility of σ phase formation by adding more titanium. Furthermore, addition of an atom like titanium with a larger atomic radius in comparison with other elements can affect stability of face-centered cubic structure. Chromium and manganese has a destabilizing influence on the single face-centered cubic phase and manganese may reject chromium to facilitate the formation of a (chromium, titanium)-rich phase in alloys containing more than 5.5 at.% titanium (x>0.3). The mechanical properties revealed an improvement in strength without losing the ductility drastically by adding titanium up to 5.5 at.% (x = 0.3). Nevertheless, the strength remarkably increased and ductility significantly decreased in CoCrFeNiMnTi_0.4 alloy due to formation of brittle σ phase in the microstructure.     

A novel high-entropy alloy with multi-scale precipitates and excellent mechanical properties fabricated by spark plasma sintering

Body-centered-cubic (BCC) high entropy alloys (HEAs) usually exhibit high strength but poor ductility. To overcome such strength-ductility trade-off, a novel (FeCr)₄₅(AlNi)₅₀Co₅ HEA was presented in this paper, which was designed and fabricated with mechanical alloying (MA) followed by spark plasma sintering (SPS), and has a heterogeneous microstructure with multi-scale precipitates. Electron microscopy characterization revealed that the sizes of the precipitates range from nano (<300 nm), sub-micron (300～800 nm) to micron (>1 μm). The bulk HEA exhibits excellent mechanical properties, of which the compressive yield strength, fracture strength, and plasticity at room temperature can reach 1508 MPa, 3106 MPa and 30.4 %, respectively, which are much higher than that of most HEAs prepared by Powder Metallurgy reported in the literatures, suggesting that the HEA developed is highly promising for engineering applications. The excellent mechanical properties of the bulk HEA can be attributed to that the multi-scale precipitates are fully coherent with the matrix, which could reduce the misfit strain at the interface, and relieve the stress concentration during deformation.     

Short-range ordering and its effects on mechanical properties of high-entropy alloys

In this letter,we briefly summarize experimental and theoretical findings of fo rmation and characterization of short-range orderings(SROs)as well as their effects on the defo rmation behavior of high-entropy alloys(HEAs).We show that existence of SROs is a common yet key structural feature of HEAs,and tuning the degree of SROs is an effective way for optimizing mechanical properties of HEAs.In additional,the challenges concerning about formation mechanism and characterization of SROs in HEAs are discussed,and future research activities in this regard are also proposed.     

激光直接沉积CoCrFeNiMn高熵合金:气孔-组织结构-拉伸性能之间的关系EI北大核心CSCD

采用激光直接沉积技术成功制备等原子CoCrFeNiMn高熵合金。研究沿试样沉积高度方向上的气孔的大小、数量和组织结构及室温(293 K)和低温(77,200 K)下试样的拉伸性能。结果表明:CoCrFeNiMn合金表现出定向结晶规律,在合金底部区域晶界处形成伴有长形气孔的树枝状柱状晶,随着区域靠近试样顶部,晶粒形态转变为等轴晶粒。而在试样顶部区域,气孔形状呈圆形且数量大大降低。比较在77,200 K和293 K温度下的合金的相应拉伸性能可知:试样顶部区域选取的77 K拉伸试样具有更好的性能,但在中部区域的293 K拉伸试样和在底部区域中的200 K拉伸试样的伸长率相似,这是由于试样不同的气孔率和组织结构的差异所致。     

难熔高熵合金研究进展

高熵合金是近年来的新兴领域,与传统合金不同,其一般是由五种或者五种以上主要元素组成,每种主元的含量在5％～35％(原子分数)之间,多种元素混乱排列却拥有简单的相结构,高熵合金的优点显著,发展空间巨大.以难熔金属元素为基础的难熔高熵合金近年来大受关注,含有3种及以上的难熔金属组成的高熵合金称为难熔高熵合金,由于难熔金属的...     

Laves-phase formation criterion for high-entropy alloys

In this study, we have analysed Laves-phase formation in high-entropy alloys (HEAs). For that purpose, the AlCr_xNbTiV and Al_xCrNbTiVZr (x?=?0, 0.5, 1, 1.5) alloys were produced and examined. It was found that the AlNbTiV and AlCr_0.5NbTiV alloys had single-phase body-centred cubic structure, while the other alloys contained Laves phase. Analysis has demonstrated that Laves-phase formation in the produced and in the other HEAs, which are predominantly composed of Al and the elements of 4–6 groups and tend to form body-centred cubic solid solutions, can be predicted by the atomic size mismatch, δ_r, and the Allen electronegativity difference, Δχ_Allen, parameters. It was shown that Laves-phase formation is observed when δ_r?>?5.0% and Δχ_Allen?>?7.0%.     

Liquid-phase separation of immiscible CrCuxFeMoyNi high-entropy alloys

Taking into consideration the large positive mixing enthalpy of Cu–Mo, in this work, the relative content of Cu and Mo elements was adjusted to investigate the solidification process and microstructure formation of CrCu_xFeMo_yNi high-entropy alloys. For CrCu_0.5FeMo_yNi (y?=?0.5, 1) high-entropy alloys, many Cu-rich spheres were observed in the as-solidified microstructures, while macroscopic phase separation structures were obtained in CrCuFeMo_yNi (y?=?0.5, 1) alloys. Liquid-phase separation occurred in high-entropy alloys when the enthalpy of mixing (ΔH_mix）was positive, the valence electron concentration (VEC) was in a range of 8.09–8.44 and the difference in atomic radii (δ) was between 3.39 and 4.25. For lower positive ΔH_mix, Cu-rich spheres were observed, while for higher positive ΔH_mix, macroscopic phase-separated structures were obtained.     

Body-centered-cubic martensite and the role on room-temperature tensile properties in Si-added SiVCrMnFeCo high-entropy alloys

We present a new class of metastable high-entropy alloys(HEAs),triggering deformation-induced martensitic transformation(DIMT)from face-centered-cubic(FCC)to body-centered-cubic(BCC),i.e.,BCC-DIMT.Through the ab-initio calculation based on 1st order axial interaction model and combined with the Gibbs free energy calculation,the addition of Si is considered as a critical element which enables to reduce the intrinsic stacking fault energy(ISFE)in SixV(9-x)Cr10Mn5Fe46Co30(x=2,4,and 7 at.％)alloy system.The ISFE decreases from-30.4 to-35.5 mJ/m2 as the Si content increases from 2 to 7 at.％,which well corresponds to the reduced phase stability of FCC against HCP.The BCC-DIMT occurs in all the alloys via intermediate HCP martensite,and the HCP martensite provides nucleation sites of BCC martensite.Therefore,the transformation rate enhances as the Si content increases in an earlier deformation range.However,the BCC-DIMT is also affected by the phase stability of FCC against BCC,and the stability is the highest at the Si content of 7 at.％.Thus,the 7Si alloy presents the moderate transformation rate in the later deformation range.Due to the well-controlled transformation rate and consequent strain-hardening rate,the 7Si alloy possesses the superior combination of strength and ductility beyond 1 GPa of tensile strength at room temperature.Our results suggest that the Si addition can be a favorable candidate in various metastable HEAs for the further property improvement.     

The excellent soft magnetic properties and corrosion behaviour of nanocrystalline FePCCu alloys

The effects of the addition of Cu on the crystallisation behaviour, soft magnetic properties, and corrosion behaviour of Fe_84-xP₉C₇Cu_x (x = 0–1.15) alloys were investigated. The experimental results demonstrate that the glass-forming ability of this alloy was improved and the soft magnetic properties of the alloy system were enhanced by proper Cu addition. FePCCu nanocrystalline alloys with a dispersed α-Fe phase were obtained by appropriately annealing the melt-spun ribbons at 693 K for 2 min. The Fe_83.25P₉C₇Cu_0.75 nanocrystalline alloy exhibited a high saturation magnetic flux density, B _s , of 1.64 T; a low coercivity, H _c , of 3.9 A/m; and a high effective permeability, μ _e , of 21,000 at 1 kHz. These characteristics are superior to corresponding properties of FePC alloys. Furthermore, the corrosion resistance of this nanocrystalline alloy increases when elevating the annealing temperature and was confirmed to be improved with respect to the corresponding amorphous alloy. These results indicate that this alloy is a promising soft magnetic material.     

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.     

Endless recrystallization of high-entropy alloys at high temperature

In traditional physical metallurgy, once recrystallization occurs, it will proceed to 100% along with time even at relatively low temperatures, resulting in the limited thermal stability of partially recrystallized alloys. Here, we proposed the strategy of achieving the endless recrystallization state at high temperature(～0.6Tm) in high entropy alloys for the first time. The partially recrystallized microstructures remained stable after annealing at 700 °C for 1440 h toward endless recrystalliza...     

Effect of alloying elements on microstructure,mechanical and damping properties of Cr-Mn-Fe-V-Cu high-entropy alloys

A series of new Cr-Mn-Fe-V-Cu high-entropy alloys were prepared by arc melting and suction casting. It is found that with the addition of Cu, the structure of the alloys evolved from BCC?+?BCC1 phases to BCC?+?FCC phases. With increase of Cu, the volume fraction of the Cu-Mn-rich FCC phase increased, and the morphology of the FCC phase transformed from granular particles to long strips and blocks. Compared with other reported HEAs, the Cr-Mn-Fe-V-Cu HEAs exhibit a good balance between strength and ductility. The CrMn0.3FeVCu0.06 alloy with granular FCC particles exhibits the highest compressive yield strength (1273?MPa) and excellent ductility (ε_f?=?50.7%). Quantitative calculations for different strengthening mechanisms demonstrate that dislocation and precipitate strengthening are responsible for high strength of the CrMn0.3FeVCu0.06 alloy, while the solid solution strengthening effect is very low because of its small atomic-size difference. In addition, the CrMn0.3FeVCu0.06 alloy exhibits good damping capacity due to its high dislocation and interface damping effects. Therefore, the dislocation density and distribution of FCC phase are the crucial factors for improvement of both mechanical properties and damping capacity of the HEAs.     

Al_xFeCoCrNiCu(x=0.25、0.5、1.0)高熵合金的组织结构和电化学性能研究

研究了不同Al含量AlxFeCoCrNiCu(x=0.25、0.5、1)高熵合金的组织结构,探讨了Al含量对合金电化学性能的影响,并与304不锈钢进行对比。结果表明,制备的高熵合金晶体结构由简单的FCC结构转为FCC和有序BCC结构。与此同时,随着Al含量的增加,合金的硬度越大,从165HV提高到485HV。极化曲线表明,在0.5mol/L H2SO4溶液和1mol/L NaCl溶液中,高熵合金和304不锈钢相比,Al0.5FeCoCrNiCu合金表现出较好耐腐蚀性和抗孔蚀能力。     

Nanocrystalline CoCrFeNiMn high-entropy alloy with tunable ferromagnetic properties

A nanocrystalline CoCrFeNiMn high-entropy alloy(nc-HEA)with nano-multiphase structure was pre-pared by inert gas condensation(IGC)using a laser evaporation source.Encouragingly,the laser-IGC nc-HEA exhibits unexpected ferromagnetic behavior and the Curie temperature(Tc)increased nearly 10 times compared to any CoCrFeNiMn HEAs prepared by various other methods.In addition,the saturation magnetization(Ms)and Tc of the laser-IGC nc-HEA can be controlled via heat treatment,which is result-ing from the formation and structural evolution of magnetic nanophases during annealing.This work widens the design toolbox for high-performance nc-HEAs based upon laser-IGC technique.     

CeO2掺杂对AlCoCrCuFe高熵合金的组织结构与摩擦磨损性能的影响

采用熔铸法制备等摩尔比的AlCoCrCuFe高熵合金。利用X射线衍射仪、扫描电镜、能谱分析仪、显微硬度计和摩擦磨损试验机分别测试CeO2掺杂前后对其物相结构、显微组织和摩擦磨损性能的影响。结果表明:AlCoCrCuFe由BCC和FCC双相组成,合金中掺杂1%(质量分数)CeO2后引起衍射峰强度的显著提高。两种合金显微组织均为典型树枝晶,Cu与Ce元素在晶间富集,枝晶内为调幅分解组织。CeO2的加入使合金显微硬度从441.5HV增加到475.3HV,摩擦因数与质量损失率分别从0.55,1.44%降低到0.4,1.28%。     

Fundamental deformation behavior in high-entropy alloys: An overview

High-entropy alloys (HEAs), as a new class of materials, are nearly equiatomic and multi-element systems, which can crystallize as a single phase or multi-phases. Most of the HEAs described in the literature contain multiple phases (secondary phases, nanoparticles, and so on), rather than a single solid-solution phase. Thus, it is essential to review the typical mechanical properties of both single-phase and multiphase HEAs thoroughly, with emphases on (1) the fundamental physical mechanisms and (2) the difference from conventional alloys. In this paper, mainly based on different mechanical properties, HEAs are classified into four types for the first time, i.e., (a) HEA alloy systems of 3d-transition metals only (Type 1), (b) HEA alloy systems of transition metals with larger atomic-radius elements (Type 2), (c) HEA alloy systems of refractory metals (Type 3), and (4) others (Type 4). Then a number of aspects of mechanical behavior are reviewed and discussed, including the elastic anisotropy, yield strength, high-temperature performance, serration behavior, fracture toughness, and fatigue responses, which may serve as a demonstrative summary for the current progress in the scientific research of HEAs. Several mechanisms that quantitatively explain the mechanical properties of single-phase and multiphase HEAs in terms of basic defects (dislocations, twinning, precipitates, etc.) are discussed. A number of future research activities are suggested, based on the emphasis on developing high-performance structural materials. The review concludes with a brief summary of major mechanical properties and insights into the deformation behavior of single-phase and multiphase HEAs. The comparison and contrast between HEAs and conventional alloys remain the most compelling motivation for future studies. With the integrated experimental and simulation investigations, a clearer picture of the fundamental deformation behavior of single-phase and multiphase HEAs could be explored.     

Local chemical fluctuation mediated ductility in body-centered-cubic high-entropy alloys

High-entropy alloys (HEAs) open up a new horizon for discovering un-explored mechanical properties and deformation mechanisms. Local chemical fluctuations (LCFs) in HEAs were found to have significant influences on their mechanical performance, however, the underlying origins remain unclear. In this work, direct dynamic observation of the interaction between LCFs and dislocations was captured by in situ transmission electron microscopy in a ductile body-centered-cubic (BCC) HfNbTiZr HEA under loading. The observed dislocation pinning induced by LCFs contributes to the increment not only in the strength but also in the ductility due to strongly promoted dislocation interaction. The observed local double cross-slips caused by the LCFs distribute dislocations onto various atomic planes homogenously, which is also beneficial for ductilization in HfNbTiZr. Our findings not only shed light on the understanding of deformation mechanisms of HEAs, but also provide a new perspective to design ductile BCC HEAs.