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Lamellar aspect-ratio and thickness-dependent strength-ductility synergy in pure nickel during in-situ micro-tensile loading

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Journal of Materials Science & Technology 2023年第26期157卷 89-97页

作者： Zi-Meng Wang Yun-Fei Jia Kai-Shang Li Yong Zhang Jia-Dong Cai Xian-Cheng Zhang Hiroyuki Hirakata Shan-Tung Tu Key Laboratory of Pressure Systems and Safety Ministry of EducationSchool of Mechanical and Power EngineeringEast China University of Science and TechnologyShanghai 200237China Department of Mechanical Engineering and Science Kyoto UniversityKyoto daigaku-katsuraNishikyo-kuKyoto 615-8540Japan

In order to overcome the trade-offbetween strength and ductility in traditional metallic materials,the gradient lamellar structure was fabricated through an ultrasound-aided deep rolling technique in pure Ni with high stacking fault energy after heat treatment.The gradient lamellar Ni was successively di-vided into three regions.In-situ micro-tensile tests were performed in different regions to reveal the corresponding microscopic mechanical behaviors.Microscopic characterization techniques were adopted to explore the effects of microstructural parameters and defects on mechanical properties.This work demonstrates that the micro-tensile sample with small lamellar thickness and large aspect ratio possesses excellent strength and ductility when the loading direction is parallel to the long side of lamellar grain boundaries.The finding is helpful to the design of metallic material microstructure with superior com-prehensive properties.On one hand,the reason for high strength is that the strength increases with the decrease of lamellar thickness according to the Hall-Petch effect.Besides,initial dislocation density also participates in the strengthening mechanism.On the other hand,the deformation mechanisms include dislocation slip,grain rotation,and the effects of grain boundaries on dislocations,jointly contributing to good ductility.

关键词： Lamella In-situ micro-tensile test Strength-ductility synergy Aspect ratio grain rotation

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Microstructural evolution and oxidation inα/βtitanium alloy under fretting fatigue loading

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Friction 2023年第10期11卷 1906-1921页

作者： Hanqing LIU Xiaohong SHAO Kai TAN Zhenjie TENG Yaohan DU Lang LI Qingyuan WANG Qiang CHEN Failure Mechanics and Engineering Disaster Prevention Key Laboratory of Sichuan Province Sichuan UniversityChengdu 610065China Department of Mechanical Engineering Kyushu UniversityFukuoka 819-0395Japan Shenyang National Laboratory for Materials Science Institute of Metal ResearchChinese Academy of SciencesShenyang 110016China Institute of Physical Chemistry University of MünsterMünster 48149Germany

Coupling effects of fretting wear and cyclic stress could result in significant fatigue strength degradation,thus potentially causing unanticipated catastrophic fractures.The underlying mechanism of microstructural evolutions caused by fretting wear is ambiguous,which obstructs the understanding of fretting fatigue issues,and is unable to guarantee the reliability of structures for long-term operation.Here,fretting wear studies were performed to understand the microstructural evolution and oxidation behavior of anα/βtitanium alloy up to 108 cycles.Contact surface degradation is mainly caused by surface oxidation and the generation of wear debris during fretting wear within the slip zone.The grain size in the topmost nanostructured layer could be refined to~40 nm.The grain refinement process involves the initial grain rotation,the formation of low angle grain boundary(LAGB;2°–5°),the in-situ increments of the misorientation angle,and the final subdivision,which have been unraveled to feature the evolution in dislocation morphologies from slip lines to tangles and arrays.The formation of hetero microstructures regarding the nonequilibrium high angle grain boundary(HAGB)and dislocation arrays gives rise to more oxygen diffusion pathways in the topmost nanostructured layer,thus resulting in the formation of cracking interface to separate the oxidation zone and the adjoining nanostructured domain driven by tribological fatigue stress.Eventually,it facilitates surface degradation and the formation of catastrophic fractures.

关键词： fretting wear oxygen pick-up dynamic recrystallization grain rotation low angle grain boundary(LAGB) grain refinement

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Synergetic deformation mechanism in hierarchical twinned high-entropy alloys

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Journal of Materials Science & Technology 2022年第7期102卷 80-88页

作者： Wenjun Lu Jianjun Li Department of Mechanical and Energy Engineering Southern University of Science and TechnologyShenzhen 518055China Max-Planck-Institut für Eisenforschung Max-Planck-Straße 1Düsseldorf 40237Germany College of Mechanical and Electrical Engineering Central South UniversityChangsha 410083China

The mechanical properties of crystalline materials can be efficiently optimized using a hierarchical twinned structure.Conventional deformation mechanisms for coherentΣ3 boundaries generally involve three basic models:cross-slip,partial dislocation step,and full dislocation step.In this study,we report a novel deformation mechanism that allows the co-existence of twin-separation,phase transformations,grain rotation,and cracking,around a triple junction of twin boundaries in a hierarchical twinned highentropy alloy.The deformation mechanisms in the reference high-entropy alloy(Fe-30 Mn-10 Co-10 Cr at.%)were investigated using LAADF-STEM.The triple junction of the hierarchical twinned structure gradually deformed during in-situ strain and showed mechanisms significantly different from that observed in the purely twinned structures.These new mechanisms are referred to as“novel synergetic deformation mechanisms of hierarchical twin boundaries”.Understanding the fundamental mechanisms of the hierarchical twin boundaries under deformation could assist the design of strong and ductile bulk materials with hierarchical twinned structure.

关键词： In-situ LAADF-STEM Hierarchical twin boundaries Twin-separation Phase transformations grain rotation Cracking

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Coordinated grain boundary deformation governed nanograin annihilation in shear cycling

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Journal of Materials Science & Technology 2021年第27期86卷 180-191页

作者： Yingbin Chen Qishan Huang Qi Zhu Kexing Song Yanjun Zhou Haofei Zhou Jiangwei Wang Center of Electron Microscopy and State Key Laboratory of Silicon Materials School of Materials Science and EngineeringZhejiang UniversityHangzhou310027PR China Center for X-Mechanics Department of Engineering MechanicsZhejiang UniversityHangzhou310027PR China School of Materials Science&Engineering Henan University of Science and TechnologyLuoyang471023PR China

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.

关键词： Nanocrystalline metals grain shrinkage and annihilation grain boundary migration grain rotation Coordinated deformation

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NUMERICAL SIMULATION FOR DEFORMATION OF NANO-grainED METALS

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Acta Mechanica Sinica 2002年第5期18卷 506-515页

作者：杨卫洪伟 Department of Engineering Mechanics Tsinghua UniversityBeijing 100084China

Electro-deposition technique is capable of producing nano-grained bulk copper specimens that exhibit superplastic extensibility at room temperature. Metals of such small grain sizes deform by grains sliding,with little distortion occur- ring in the grain cores.Accommodation mechanisms such as grain boundary diffusion, sliding and grain rotation control the kinetics of the process.Actual deformation min- imizes the plastic dissipation and stored strain energy for representative steps of grain neighbor switching.Numerical simulations based on these principles are discussed in this paper.

关键词： nano-grained metals grain boundary sliding grain rotation stress strain relation

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