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.

Mechanical Properties and Deformation Mechanisms of Metallic Glasses Under Hydrostatic Pressure

Metallic glasses(MGs)are promising structural materials with high strength,large elastic strain limit and enhanced wear resistance.On the way to tune the mechanical performance of MGs,numerous efforts have been devoted to investigating the effect of hydrostatic pressure(σh)on the mechanical properties and deformation mechanisms of MGs.The application of sufficiently large magnitudes ofσh to MGs has been considered a feasible way to improve strength and delocalize plastic deformation in MGs.In this paper,we review recent studies on pressure-dependent thermodynamic properties,mechanical strength and hardness,as well as elevated ductility of MGs due to the pressure-induced change of deformation and failure mechanisms.Observations of intriguing mechanical behaviors of MGs,and the corresponding theoretical modeling and atomistic understanding of plastic deformation in MGs under pure hydrostatic pressure and more general stress states are discussed.These findings not only deepen the understanding of pressure-dependent mechanical behaviors of MGs,but also point out the potential of tuning mechanical behaviors of MGs through stress engineering.

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.

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.

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.

Transition from source-to stress-controlled plasticity in nanotwinned materials below a softening temperature

Nanotwinned materials have been widely studied as a promising class of nanostructured materials that exhibit an exceptional combination of high strength,good ductility,large fracture toughness,remarkable fatigue resistance,and creep stability.Recently,an apparent controversy has emerged with respect to how the strength of nanotwinned materials varies as the twin thickness is reduced.While a transition from hardening to softening was observed in nanotwinned Cu when the twin thickness is reduced below a critical value,continuous hardening was reported in nanotwinned ceramics and nanotwinned diamond.Here,by conducting atomistic simulations and developing a theoretical modeling of nanotwinned Pd and Cu systems,we discovered that there exists a softening temperature,below which the material hardens continuously as the twin thickness is reduced(as in nanotwinned ceramics and diamond),while above which the strength first increases and then decreases,exhibiting a maximum strength and a hardening to softening transition at a critical twin thickness(as in nanotwinned Cu).This important phenomenon has been attributed to a transition from source-to stress-controlled plasticity below the softening temperature,and suggests that different hardening behaviors may exist even in the same nanotwinned material depending on the temperature and that at a given temperature,different materials could exhibit different hardening behaviors depending on their softening temperature.

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.

Correlated necklace dislocations in highly oriented nanotwinned metals

In this paper, we review recent progress in the understanding of a novel dislocation mechanism, named correlated necklace dislocations(CNDs), activated in highly oriented nanotwinned(NT) metals under monotonic and cyclic loading applied parallel to the twin boundaries(TBs). This mechanism was initially revealed to be responsible for the continuous strengthening behavior of NT metals when the TB spacing(λ) is reduced to around 1 nm. It was later found that the presence of a crack-like defect could trigger the operation of CNDs at much larger TB spacings. Most recently, atomistic modeling and experiments demonstrated a history-independent and stable cyclic response of highly oriented NT metals governed by CNDs formed in the NT structure under cyclic loading. CNDs move along the twin planes without directional lattice slip resistance, thus contributing to a symmetric cyclic response of the NT structure regardless of pre-strains imposed on the sample before cyclic loading. We conclude with potential research directions in the investigation of this unique deformation mechanism in highly oriented NT metals.

Publisher Correction:Transition from source-to stresscontrolled plasticity in nanotwinned materials below a softening temperature

The original version of this Article was incorrectly labelled as a‘Review Article’.This has now been corrected to‘Article’in both the HTML and PDF versions.