Effect of cerium on microstructure,texture and properties of ultrahigh-purity copper

The effects of Ce addition(310 ppm and 1500 ppm)on the microstructure,texture and properties of ultrahigh-purity copper(99.99999%)were systematically studied using scanning electron microscopy(SEM),transmission electron microscopy(TEM)and electron backscattered diffraction(EBSD)analyses,combined with the microhardness and conductivity tests.Regarding the microstructure of the as-cast and as-extruded samples,the addition of Ce refines the grain size of the ultrahigh-purity copper and the refinement effect of 310Ce alloy is greater than that of 1500Ce alloy.This is due to the stronger compone nt supercooling and the accele rated recrystallization caused by lower Ce co ntent.In addition,Ce can react with Cu to form the Cu-Ce eutectic phases,which are deformable during the hot deformation.Furthermore,the added Ce can weaken the texture,showing a variation of brass recrystallization(BR),rotated cube,copper and S texture components,which depends on the recrystallization,the particle stimulated nucleation(PSN)as well as the stacking fault energy(SFE).Most remarkably,the introduction of Ce enhances the hardness of the ultrahigh-purity copper without obviously reducing its conductivity.The major{111}orientations and the stress distributions are responsible for such a superior conductivity of the Ce-containing alloys.

纳米结构金属晶界塑性机理的原子尺度模拟研究进展

纳米结构金属中富含晶界,对材料微观结构演化与宏观力学性能调控具有重要意义.厘清纳米结构金属晶界塑性变形的原子尺度机理,将之与材料宏观力学行为相联系,是纳米结构金属力学研究关注的核心问题.近年来,我们围绕晶界塑性变形机制及其影响因素,采用原子尺度模拟方法,探究了晶界诱导孪生与界面宏微观自由度的关联规律,揭示了晶界位错往复滑移主导的纳米晶体循环塑性机制,提出了孪晶界滑移诱导纳米晶体极端剪切变形,发展了取向依赖的晶界迁移和滑移转变模型,为纳米结构金属晶界行为预测与调控提供了新思路.本文梳理了晶界塑性原子尺度模拟的研究现状,总结了本团队从原子尺度探究晶界塑性变形机理的相关进展,指出了纳米结构金属晶界调控理论与力学表征的难点和挑战.

Solvent sieving separators implement dual electrolyte for highvoltage lithium-metal batteries

Lithium-metal batteries(LMBs)based on high-voltage cathodes would deliver high specific energy density to meet the demand of future energy storage.However,developing liquid electrolytes with wide electrochemical window for high-energy LMBs is intrinsically challenging.Herein,we demonstrate metal-organic framework-functionalized separators(PE@MOF)with solvent sieving capability that implement dual electrolyte for LMBs.The capability of PE@MOF separator to block the diffusion of liquid electrolytes has been investigated.The PE@MOF separator notably suppresses solvents shuttling,enabling the independent optimization of cathode-electrolyte and anode-electrolyte interfaces.By adapting commercial carbonate and ether electrolytes on cathode and anode sides,respectively,robust cathode-electrolyte interphase(CEI)and solid electrolyte interface(SEI)have been built on both electrodes.The lifespan of LiCoO_(2)(LCO)|Li full cell has been notably extended when using dual electrolyte and the solvent-sieving PE@MOF separator.This work demonstrates a new strategy to separately optimize the local environments at electrodes and to develop high-energy LMBs using low-cost and commercially available electrolytes.

Evolution of precipitate and precipitate/matrix interface in Al-Zn-Mg-Cu(-Ag)alloys

Evolution of precipitate and precipitate/matrix interface in artificially aged Al-Zn-Mg-Cu(-Ag)alloys has been systematically studied.In the early stage of ageing,Ag,as a fast diffuser,can promote the formation of solute pairs and small clusters.Solute clusters are further demonstrated to be able to act as precursors forη’precipitates by in-situ STEM heating.With prolonged ageing time,the precipitate/matrix interface evolves from the Zn-dominated interface between early-stageη’and Al matrix to the Zn and Mg co-segregatedη’/Al andη_(2)/Al interfaces.Theη’/Al interfacial layers are shown to precede the formation ofη’,while theη_(2)/Al interfaces are found to be closely related to the thickening process ofη_(2)and the involved particular atomic movements are specified.Experimental observations and DFT calculations re-veal that forη’andη_(2),Ag can dissolve into the precipitate as well as locate at the precipitate/matrix interface without showing preference.For Cu,its dissolution in the precipitate and segregation on the interface mainly occur forη_(2)rather thanη’.The incorporation of Ag and Cu does not change the defined precipitate structure.

Effects of Sc on the vacancy and solute behaviours in aluminium

Element Sc is a promising candidate for optimizing the high-temperature mechanical properties of Al alloys.In this study,the Sc-solute,Al_(3) Sc-vacancy and Al_(3) Sc-solute interactions in aluminium are inves-tigated extensively by using first-principles calculations.The correlation between the various interaction energies and the solute atomic size,and the Sc-solute compound formation energy has been evaluated.A negative correlation between the first nearest neighbour Sc-solute binding energies and the lowest Sc-solute compound formation energies has been identified,while the second nearest neighbour Sc-solute binding energies increase monotonically with the solute atomic size.Al_(3) Sc precipitates can bind vacancy strongly at the specific atomic site,but their relatively low number density limits their influence on va-cancy behaviours during the ageing period shortly after quenching.Compared to the atomic size,the trend for solute segregating at the interface between Al_(3) Sc precipitate and Al bulk is more strongly re-lated to the Sc-solute binding energy.The calculated results can clarify the available experimental obser-vations for Al-Sc,Al-Cu,Al-Mg-Si and Al-Zn-Mg-Cu alloys,and it is hoped to guide the design of high-performance Al alloys.

层状TiAl原位透射电镜拉伸揭示界面主导的塑性各向异性

基于功能基元和序构范式将功能基元序构成特定取向组织,可以最大限度的利用层状结构材料的力学性能各向异性.然而,其各向异性变形行为的微观起源尚不清楚.本文以全片层的γ-TiAl/α_(2)-Ti_(3)Al双相单晶为模型,通过沿特征取向的原位透射电镜拉伸试验和多尺度结构表征,揭示了界面功能基元主导的塑性各向异性及相应的变形机制.相比于片层内部本征的位错行为,跨γ/α2界面取向序构主导的滑移系连续性和界面强度在塑性各向异性中起决定性作用.因此,当平行或垂直于片层方向拉伸时,层状材料发生穿片层的应力传递或界面分层,进而导致强度和延展性的各向异性.这些机理对理解层状结构材料的塑性各向异性具有普遍意义,有利于高性能合金的结构设计.

Atomistic Evolution of Hf_(2)S/γ′Interfaces in a Hf‑containing Ni‑based Single‑Crystal Superalloy

The evolution of interfacial structures of needle-like sulfides in a Hf-containing Ni-based single-crystal superalloy has been studied with a combination of the state-of-the-art spherical aberration-corrected scanning transmission electron microscope and three-dimensional electron diffraction methods.The Hf_(2)S precipitates possess an ABACBC stacking sequence with layered structure of Hf-S-Hf-Hf-S-Hf.The Hf_(2)S/γ′interfaces exhibit different types of metastable interfacial structures,including the sharp interface with segregated Hf atom columns,the one with indistinct transition region and the one with ordered transition region.These metastable structures represent the different stages of Hf_(2)S growth during the aging process,based on which a sequential layer-by-layer growth mechanism of Hf_(2)S is proposed.

Atomic-scale mechanism of the θ’’→θ’ phase transformation in Al-Cu alloys

The phase transformation of θ’’→θ’ in an Al-5.7 Cu alloy was investigated by aberration-corrected scanning transmission electron microscopy, and the tranformation mode of θ’’→θ’ during aging treatment was clarified. In the presence of the θ’ phases, θ’ was found to be formed by in-situ transformation fromθ’’ with the same plate shape, size and broad faces. The transformation starts from multiple sites within the θ’ precipitate and the whole θ’ phase finally forms as the preferential θ’ sections grow and connect with each other. Antiphase domain boundaries are also found in some θ’ precipitates when the disregistry exists between different θ’ sections.

Inconsistent creep between dendrite core and interdendritic region under different degrees of elemental inhomogeneity in nickel-based single crystal superalloys

The creep inconsistency between dendrite core and interdendritic region is investigated in a nickel-based single crystal superalloy under 1373 K and 137 MPa.Two specimens with higher and lower degree of elemental inhomogeneity on dendritic structures are compared.For specimen with higher inhomogeneity,stronger segregation of refractory elements reinforces the local strength in dendrite core,but damages the strength in interdendritic region.Creep strain is accumulated faster in interdendritic region giving rise to promoted dislocation shearing inγphase,faster degradation of dislocation networks and facilitated topological inversion of rated structures.Although the segregation of refractory elements produces a high density of topologically close-packed(TCP)phase in dendrite core,faster accumulation of creep strain forms microcracks prior in interdendritic region that gives rise to final rupture of the specimen.In another specimen,increased solid solution time gives rise to overall reduced inhomogeneity.Creep inconsistency is relieved to show more uniform evolution of dislocation substructures and rafting between dendrite core and interdendritic region.The second specimen is ruptured by formation and extension of microcracks along TCP phase although the precipitation of TCP phase is relatively restricted under reduced inhomogeneity.Importantly,the balance of local strength between dendrite core and interdendritic region results in over 40%increase of creep rupture life of the second specimen.

Discrete twinning dynamics and size-dependent dislocation-to twin transition in body-centred cubic tungsten

Body-centred cubic(BCC) metals are known to have unstable intrinsic stacking faults and high resistance to deformation twinning, which can strongly influence their twinning behaviour. Though twinning mechanisms of BCC metals have been investigated for more than 60 years, the atomistic level dynamics of twinning remains under debate, especially regarding its impact on competition between twinning and slip. Here, we investigate the atomistic level dynamics of twinning in BCC tungsten(W) nanowires using in situ nanomechanical testing. Quantitative experimental studies directly visualize that deformation twins in W nanowires have a minimum size of six-layers and grow in increments of approximately three-layers at a time, in contrast to the layer-by-layer growth of deformation twins in face-centred cubic metals. These unique twinning dynamics induces a strong competition with ordinary dislocation slip,as exhibited by a size-dependent dislocation-to-twin transition in W nanowires, with a transition size of ~40 nm. Our work provides physical insight into the dynamics of twinning at the atomic level, as well as a size-dependent dislocation-twinning competition, which have important implications for the plastic deformation in a broad class of BCC metals and alloys.

Atomistic dynamics of disconnection-mediated grain boundary plasticity:A case study of gold nanocrystals

Grain boundary(GB)mediated deformation is a vital contributor to the plasticity of polycrystalline materials,where the disconnection model has become a widely recognized approach to depict the GB dynamics.However,experimental understanding of the atomistic disconnection dynamics remains scarce.In this case study of gold nanocrystals,atomistic disconnection dynamics governing the shear-coupled migration of flat GBs have been systematically investigated via in situ transmission electron microscopy nanomechanical testing supported by molecular dynamics simulations.Specifically,the site-dependent nucleation,shear-driven propagation,and diverse interactions associated with distinct GB disconnections are systematically elucidated and quantitatively compared.Moreover,the disconnection-mediated GB plasticity proves to prevail among different tilt and mixed GBs in gold.Eventually,a conceptual map of disconnection-mediated GB dynamics is established,which would furnish a unified understanding of GB plasticity in metallic materials.

Atomic-scale study on the precipitation behavior of an Al-Zn-Mg-Cu alloy during isochronal aging

The precipitation behavior of a 7075 Al alloy during isochronal heat treatments at three different heating rates has been studied using differential scanning calorimetry,high-angle-annular-dark-field scanningtransmission-electron microscope and density functional theory calculation.In the early stage of aging,GPI and GPII zones form sequentially and cause two characteristic DSC peaks.Subsequently,the formation ofη_(1) precipitates takes place concurrently withη’.A novel type of metastable phaseη_(1)’is identified as the precursor ofη_(1),which can lower the lattice misfit betweenη_(1) and Al matrix along the direction of[1010]_(η_(1))//[001]_(Al).Accordingly,a pathway for the formation ofη_(1) viaη_(1)’is demonstrated.Precipitatesη’together withη_(1) andη_(1)’contribute to the third DSC peak.With the further increase of temperature,ηprecipitates become prevailing.Based on the quantitative analyses,the influence of the heating rate and ending temperature on the cross section and number density of phases formed is discussed.

Coordinated grain boundary deformation governed nanograin annihilation in shear cycling

Grain growth and shrinkage are essential to the thermal and mechanical stability of nanocrystalline metals,which are assumed to be governed by the coordinated deformation between neighboring grain boundaries(GBs)in the nanosized grains.However,the dynamics of such coordination has rarely been reported,especially in experiments.In this work,we systematically investigate the atomistic mechanism of coordinated GB deformation during grain shrinkage in an Au nanocrystal film through combined stateof-the-art in situ shear testing and atomistic simulations.We demonstrate that an embedded nanograin experiences shrinkage and eventually annihilation during a typical shear loading cycle.The continuous grain shrinkage is accommodated by the coordinated evolution of the surrounding GB network via dislocation-mediated migration,while the final grain annihilation proceeds through the sequential dislocation-annihilation-induced grain rotation and merging of opposite GBs.Both experiments and simulations show that stress distribution and GB structure play important roles in the coordinated deformation of different GBs and control the grain shrinkage/annihilation under shear loading.Our findings establish a mechanistic relation between coordinated GB deformation and grain shrinkage,which reveals a general deformation phenomenon in nanocrystalline metals and enriches our understanding on the atomistic origin of structural stability in nanocrystalline metals under mechanical loading.

In situ atomistic observation of the deformation mechanism of Au nanowires with twin–twin intersection

Twin–twin intersections are often observed in face-centered cubic(FCC)metallic nanostructures,which have important contributions to the plastic deformation and strengthening of FCC metals with low stacking fault energies.However,a deep insight into the underlying mechanism involved in the formation and evolution of twin–twin intersections remains largely lacking,especially in experiments.Here,by conducting the in situ straining experiments under high resolution transmission electron microscope(TEM),we directly visualize the dynamic evolution of a twin–twin intersection in Au nanowire at the nanoscale.It shows that dislocations in the incoming twin can either glide onto or transmit across the barrier twin via dislocation interaction with the twin boundary,resulting in the twin–twin intersection.Dynamic twinning and de-twinning of the twin–twin intersection govern the whole deformation of the nanowire.These findings reveal the dynamic behaviors of twin–twin intersection under mechanical loading,which benefits further exploration of FCC metals and engineering alloys with twin–twin intersection structures.

Surface Effect on Vibration of Timoshenko Nanobeam Based on Generalized Differential Quadrature Method and Molecular Dynamics Simulation

Nanobeams have promising applications in areas such as sensors,actuators,and resonators in nanoelectromechanical systems(NEMS).Considering the effects of gyration inertia,surface layer mass,surface residual stress,and surface Young's modulus,this study develops the vibration equations of the Timoshenko nanobeam.The generalized differential quadrature(GDQ)method and molecular dynamics(MD)simulation are used to study the surface effect on vibration.For a rectangular cross section,surface residual stress and surface Young's modulus are all affected by the height of the cross section rather than by the length-height ratio.If surface layer mass is considered,then the first three natural frequencies all decrease relative to their counterparts in the case in which surface layer mass is ignored.Results show that the effect of gyration inertia on resonance frequency is negligible.Longitudinal vibration does not easily occur relative to the bending and rotation vibrations of nanobeams.In addition,the results obtained by the GDQ method fit those obtained by MD simulation for beams with length-height ratios of 4-8.This study provides insights into the mechanism of the vibration of short and deep nanobeams and sheds light on the quantitative design of the elements in NEMSs.