Characterization of local chemical ordering and deformation behavior in high entropy alloys by transmission electron microscopy

Short-range ordering(SRO)is one of the most important structural features of high entropy alloys(HEAs).However,the chemical and structural analyses of SROs are very difficult due to their small size,complexed compositions,and varied locations.Transmission electron microscopy(TEM)as well as its aberration correction techniques are powerful for characterizing SROs in these compositionally complex alloys.In this short communication,we summarized recent progresses regarding characterization of SROs using TEM in the field of HEAs.By using advanced TEM techniques,not only the existence of SROs was confirmed,but also the effect of SROs on the deformation mechanism was clarified.Moreover,the perspective related to application of TEM techniques in HEAs are also discussed.

A metastable Fe_(48)Co_(10)Cr_(10)Mn_(32)high-entropy alloy with good damping capacity within an ultra-large temperature regime

1.Introduction Damping alloys are considered to be the most direct and effective approach to reducing unwanted vibration and noise in industrial applications.In particular,it is urgent to develop damping alloys that have highdamping capacity(tanδ)as well as high strength,and a large operating temperature range to meet the demands of extreme working conditions,such as outer space.However,the application temperature of existing damping alloys is limited,which is one of the biggest challenges for developing highperformance damping alloys.For Fe-Mn binary alloys.

Revealing the role of local shear strain partition of transformable particles in a TRIP-reinforced bulk metallic glass composite via digital image correlation

The coupling effects of the metastable austenitic phase and the amorphous matrix in a transformation-induced plasticity(TRIP)-reinforced bulk metallic glass(BMG)composite under compressive loading were investigated by employing the digital image correlation(DIC)technique.The evolution of local strain field in the crystalline phase and the amorphous matrix was directly monitored,and the contribution from the phase transformation of the metastable austenitic phase was revealed.Local shear strain was found to be effectively consumed by the displacive phase transformation of the metastable austenitic phase,which relaxed the local strain/stress concentration at the interface and thus greatly enhanced the plasticity of the TRIP-reinforced BMG composites.Our current study sheds light on in-depth understanding of the underlying deformation mechanism and the interplay between the amorphous matrix and the metastable crystalline phase during deformation,which is helpful for design of advanced BMG composites with further improved properties.

罗霄山脉地区鱼类物种多样性

罗霄山脉是赣江和修水流域与湘江流域的分水岭,是中国生物多样性保护的关键地区之一。然而,罗霄山脉地区的鱼类缺乏系统性的研究,其鱼类物种组成、分布以及受威胁因素尚不清楚。为此,我们于2014–2018年对罗霄山脉地区11条河流的鱼类进行了系统的调查。结果表明,该地区共有鱼类5目17科64属113种,山脉东坡鱼类108种,高于西坡的72种。从生态类型看,罗霄山脉鱼类以肉食性、底栖性、定居性类群为主。区系组成上以东亚江河平原类群为主。从物种多样性看,遂川江、袁水、蜀水和修河的鱼类物种多样性较高,锦江和富水的鱼类物种多样性较低;β多样性指数揭示遂川江与锦江、禾水、富水间鱼类物种出现一定的分化现象。

Improving plasticity of the Zr46Cu46Al8 bulk metallic glass via thermal rejuvenation

In this paper, effects of cryogenic thermal cycling on deformation behavior and thermal stability of the Zr46Cu46AI8 bulk metallic glass （BMG） were studied. The results show that with the increase of the number of cryogenic thermal cycles （CTC）, thermal stability remains almost unchanged, while the plasticity is increased, indicating that the cryogenic thermal cyclic treatment is an effective way to improve plasticity of metallic glasses without distinctly deteriorating thermal stability. Our analysis suggests that the increase in the defect density resulted from the cryogenic thermal treatments are responsible for the plasticity increment. Variation of yield strength can be well interpreted from microstructural percolation which affected by both density and characteristic volume of the defect sites.

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.

High-entropy carbide-nitrides with enhanced toughness and sinterability

High-entropy ceramics (HECs) have attracted much attention due to their huge composition space,unique microstructure,and desirable properties.In contrast to previous studies,which have primarily focused on HECs with one anion,herein,we report a new family of ceramics with both multi-cationic and-anionic structures,i.e.,high-entropy carbide-nitrides (Ti0.33Zr0.33Hf0.33)(C0.5N0.5),(Ti0.25Zr0.25Hf0.25-Nb0.25)(C0.5N0.5) and (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)(C0.5N0.5).These as-synthesized HECs are mainly comprised of a face-centered cubic solid-solution phase accompanied by minor inevitable oxide phases.The formation mechanism of the solid-solution phase is discussed in terms of the lattice size difference and thermodynamic competition between configurational entropy and mixing enthalpy.It is found that the increment in the configurational entropy can effectively lower the sintering temperature and increase the fracture toughness.Particularly,the newly developed (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)(C0.5N0.5) exhibits a decent fracture toughness of 8.4 MPa m1/2and a low sintering temperature of 1750°C,making it promising for ultra-high temperature applications.Our work not only enriches knowledge regarding the HECs categories,but also opens a new pathway for developing HECs with multi-cationic and-anionic structures.

Enhanced plastic deformation capacity in hexagonal-close-packed medium entropy alloys via facilitating cross slip

Alloys with a hexagonal close-packed(HCP)lattice often suffer from intrinsic brittleness due to their in-sufficient number of slip systems,which limits their practical uses.In this paper,nevertheless,we show that remarkably tensile ductility in HCP Hf-Zr-Ti medium entropy alloys(MEAs)was achieved,particu-larly in the MEAs with a higher content of Hf.Both first-principles calculation and experimental analyses reveal that addition of Hf increases basal I2 stacking fault energy and decreases prismatic stacking fault energy in these HCP MEAs,which promotes the source of pyramidal dislocations due to the facilitated cross slips of basal dislocations and eventually give rise to the observed large tensile ductility.Our current findings not only shed new insights into understanding deformation of HCP alloys,but also provide a basis for controlling alloying effects for developing novel HCP complex alloys with optimized properties.

Interpretable machine-learning strategy for soft-magnetic property and thermal stability in Fe-based metallic glasses

Fe-based metallic glasses(MGs)have been extensively investigated due to their unique properties,especially the outstanding soft-magnetic properties.However,conventional design of soft-magnetic Fe-based MGs is heavily relied on“trial and error”experiments,and thus difficult to balance the saturation flux density(Bs)and thermal stability due to the strong interplay between the glass formation and magnetic interaction.Herein,we report an eXtreme Gradient Boosting(XGBoost)machine-learning(ML)model for developing advanced Fe-based MGs with a decent combination of Bs and thermal stability.

Local chemical ordering and its impact on radiation damage behavior of multi-principal element alloys

Multi-principal element alloys(MPEAs)have attracted much attention as future nuclear materials due to their extraordinary radiation resistances.In this work,we have elucidated the development of local chemical orderings(LCOs)and their influences on radiation damage behavior in the typical CrFeNi MPEA by hybrid-molecular dynamics and Monte Carlo simulations.It was found that considerable LCOs consist-ing of the Cr-Cr and Ni-Fe short-range orders existed in the ordered configuration with optimized system energy.Through modeling the accumulation cascades up to 1000 recoils,we revealed that the size of de-fect clusters and dislocation loops is smaller in the ordered configuration than those in the random one,although the former formed more Frenkel pairs(i.e.,self-interstitials and vacancies).In addition,the dis-tribution of dislocation loops is relatively more dispersed in the ordered configuration,and the stair-rod dislocations related to irradiation swelling are also smaller,implying that the existence of LCOs is con-ducive to enhancing radiation damage tolerance.To understand the underlying mechanism,the effects of LCOs on the formation and evolution of defects and radiation resistance were discussed from the aspects of atomic bonding,migration path,and energy of defect diffusion,which provides theoretical guidance for the design of MPEAs with enhanced radiation resistance.

Self-supporting nanoporous Ni/metallic glass composites with hierarchically porous structure for efficient hydrogen evolution reaction

Searching for free-standing and cost-efficient hydrogen evolution reaction(HER)electrocatalysts with high efficiency and excellent durability remains a great challenge for the hydrogen-based energy industry.Here,we report fabrication of a unique hierarchically porous structure,i.e.,nanoporous Ni(NPN)/metallic glass(MG)composite,through surface dealloying of the specially designed Ni_(40)Zr_(40)Ti_(20)MG wire.This porous composite is composed of micrometer slits staggered with nanometer pores,which not only enlarges effective surface areas for the catalytic reaction,but also facilitates the release of H2 gas.As a result,the NPN/MG hybrid electrode exhibited the prominent HER performance with a low overpotential of 78 m V at 10 m A cm^(-2)and Tafel slope of 42.4 m V dec^(-1),along with outstanding stability in alkaline solutions.Outstanding catalytic properties,combining with their free-standing capability and cost efficiency,make the current composite electrode viable for HER applications.

Dynamic deformation behavior of a face-centered cubic FeCoNiCrMn high-entropy alloy

In this study, mechanical tests were conducted oil a face-centered cubic FeCoNiCrMn high-entropy alloy, both in tension and compression, in a wide range of strain rates （10^-4-10^4 s^-1） to systematically investigate its dynamic response and underlying deformation mechanism. Materials with different grain sizes were tested to understand the effect of grain size, thus grain boundary volume, on the mechanical prop-erties. Microstructures of various samples both before and after deformation were examined using elec-tron backscatter diffraction and transmission electron microscopy. The dislocation structure as well as deformation-induced twins were analyzed and correlated with the measured mechanical properties. Plastic stability during tension of the current high-entropy alloy （HEA）, in particular, at dynamic strain rates, was discussed in lights of strain-rate sensitivity and work hardening rate. It was found that, under dynamic conditions, the strength and uniform ductility increased simultaneously as a result of the mas-sive formation of deformation twins. Specifically, an ultimate tensile strength of 734 MPa and uniform elongation of-63% are obtained at 2.3×10^3 s^-1, indicating that the alloy has great potential for energy absorption upon impact loading.

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 environ- ments 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 combina- tion 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.

Eight in one:high-entropy-alloy nanoparticles synthesized by carbothermal shock

Since the Bronze Age to the early 21st century,development of metals and alloys have been limited to one principal element with minor addition of relatively small amounts of other elements,such as steels,titanium alloys,aluminum alloys,and so on.Although a dozen of other elements may be added,conventional alloys

Materials genome strategy for metallic glasses

Metallic glasses(MGs)have attracted extensive attention in the past decades due to their unique chem-ical,physical and mechanical properties promising for a wide range of engineering applications.A thor-ough understanding of their structure-property relationships is the key to the development of novel MGs with desirable performance.New strategies,as proposed by Materials Genome Initiative(MGI),construct a new paradigm for high-throughput materials discovery and design,and are being increas-ingly implemented in the search of new MGs.While a few reports have summarized the application of high-throughput and/or machine learning techniques,a comprehensive assessment of materials genome strategies for developing MGs is still missing.Herein,this paper aims to present a timely overview of key advances in this fascinating subject,as well as current challenges and future opportunities.A holistic approach is used to cover the related topics,including high-throughput preparation and characterization of MGs,and data-driven machine learning strategies for accelerating the development of novel MGs.Fi-nally,future research directions and perspectives for MGI-assisted design of MGs are also proposed and surmised.

Theoretical Perspectives on Natural and Artificial Micro-swimmers

The observations of microorganisms revealed how they swim.As inspired by them,studies on artificial micro-robots with various actuation mechanisms have been thriving.To elucidate the essential concepts in understanding the dynamic behaviors of natural and artificial micro-swimmers—so as to control the latter in future applications,in this paper,we summarize the historical achievements and describe our theoretical perspectives.We first introduce the studies on microorganisms and their propulsion mechanisms.After reviewing the basic principles and the development of the understanding of them,we take the flagellated micro-swimmers as an example to elaborate on the theories of their locomotion.We review the progress in actuation strategies of artificial flagellated micro-swimmers and then propose two possible strategies to realize the turning of a flagellated micro-swimmer—the key step toward steerability through remote fields.Finally,we describe the inadequacies of the investigations on this topic and our perspectives on future developments.

A strategy to design eutectic high-entropy alloys based on binary eutectics

1.Introduction As an ancient method for manufacturing metallic materials,casting has the advantages of low cost,high process flexibility and forming complex parts.However,due to the poor as-cast mechanical properties,various processes after casting are generally necessary for traditional metallic materials to eliminate casting defects(e.g.,voids,shrinkage cavities and elemental segregation)and improve their properties,such as rolling,forging,and heat treatment.

金属玻璃的自然老化

金属玻璃本质上是亚稳态的,在高温下倾向于通过弛豫和晶化达到低能状态.然而在室温下,通常认为其无定形结构可以保持足够长的时间.本文报道了在自然环境中保存12年的一系列CuZr金属玻璃的长期稳定性.结果表明,低Zr含量的CuZr金属玻璃(低于66.7 at%)由于室温氧化引起了严重的失稳;高Zr含量的CuZr金属玻璃形成了一层致密的ZrO_(2)而保留了光泽的表面,尽管如此,其热性能(如玻璃转变温度T_(g)和晶化温度T_(x))由于氧的渗透发生了明显改变.本文的研究结果不仅为理解金属玻璃的热稳定性提供了重要的见解,而且对无序固体的工程应用具有重要意义.

Enhanced crystallization resistance and thermal stability via suppressing the metastable superlattice phase in Ni-(Pd)-P metallic glasses

Virtually,glass formation is to avoid crystallization during solidification,i.e.,a consequence of the competition between the undercooled liquid and primary crystalline phases.It is found that the crystallization resistance of the binary Ni-P system was drastically enhanced with alloying of Pd and correspondingly,the critical size for glass formation increased significantly from the micrometer to millimeter scale.Thermodynamically,the introduction of Pd could effectively increase the atomic size mismatch and heat of mixing,which are beneficial to stabilize the supercooled liquid.Kinetically,the introduction of Pd not only successfully suppresses the formation of metastable superlattice phase,which is prone to nucleation and growth in the supercooled liquid state,but also changes the crystallization mechanism from the primary to eutectic mode.The current finding sheds light on understanding glass formation of the most studied Pd-Ni-P system and the glass-forming ability in general.

Effects of trace elements on mechanical properties of the TiZrHfNb high-entropy alloy

Refractory high-entropy alloys have great potential to be utilized as high-temperature materials,and the repeatability and reproducibility of their mechanical properties are critical for practical applications.In this work,nevertheless,we found that the mechanical properties of the TiZrHfNb HEA greatly varied with the content of impurities in the samples even using high-purity raw materials.Specifically,the oxygen impurity is mainly responsible for the increment of the yield stress due to the strong interstitial hardening effect,whilst the ductility deterioration closely associates with the content of metalloid elements B,C,and Si.Our analysis reveals that the metalloid elements not only tend to segregate at grain boundaries but also enhance the aggregation of Zr and Ti.Such co-segregation induced the formation of strong(Zr,Ti)-metalloid bonds,resulting in grain boundary embrittlement and brittle fracture.Our current work demonstrates that the impurity contents in refractory HEAs need to be strictly controlled during production in order to improve their stability of mechanical performance.