Nanocrystalline alloys often exhibit unusual thermal stability as a consequence of kinetic and thermodynamic barriers to grain growth.However,the physical mechanisms governing alloy stability need to be identified.In ...Nanocrystalline alloys often exhibit unusual thermal stability as a consequence of kinetic and thermodynamic barriers to grain growth.However,the physical mechanisms governing alloy stability need to be identified.In this work,we found that grain boundary(GB)relaxation renders Ni-W alloyed films relatively stable at low annealing temperature,while twinning-mediated grain growth occurs via dislocation-GB/twin boundary(TB)interactions as the annealing temperature increases.At a relatively low temperature,TB strengthening plays a dominant role in plastic deformation,whereas precipitation strengthening gradually controls the deformation mechanism with the increase of annealing temperature.Our findings provide evidence for improving mechanical property through alloying and microstructure design,and have a crucial guiding significance in material selection and miniaturized applications such as Micro Electro Mechanical Systems.展开更多
A nanotwinned 316 L austenitic stainless steel was prepared by means of surface mechanical grinding treatment.After recovery annealing,the density of dislocations decreases obviously while the average twin/matrix lame...A nanotwinned 316 L austenitic stainless steel was prepared by means of surface mechanical grinding treatment.After recovery annealing,the density of dislocations decreases obviously while the average twin/matrix lamella thickness still keeps in the nanometer scale.The annealed nanotwinned sample exhibits a high tensile yield strength of 771 MPa and a considerate uniform elongation of 8%.TEM observations showed that accommodating more dislocations and secondary twinning inside the nanotwins contribute to the enhanced ductility and work hardening rate of the annealed nanotwinned sample.展开更多
Homogeneous columnar Cu film with fully embedded nanotwins was successfully fabricated on Ti/Cu seed layer on silicon wafer. The nanotwins with thickness of tens of nanometers are generally parallel to the silicon sur...Homogeneous columnar Cu film with fully embedded nanotwins was successfully fabricated on Ti/Cu seed layer on silicon wafer. The nanotwins with thickness of tens of nanometers are generally parallel to the silicon surface, showing a strong （111 ） preferred orientation. The acid concentration was found to be important in influencing the formation of nanoscale twins. By adjusting the acid concentration, the nanotwins can be induced from the top columnar grain to middle columnar grain and reach the bottom equiaxed grain, and a microstructural transformation model was given. A theory focusing on the cathode overpotential was proposed to reveal the effect of acid concentration on the growth mechanism of nanoscale twins. An appropriate adsorption proportion of hydrogen on cathode （acid concentration 17 ml L^-1） could increase the overpotential which supplies adequate nucleation energy for nanoscale twins formation.展开更多
Two distinct regimes of strain rate sensitivity on yield strength are found in a high-strength nantwinned steel.The yield strength increases from 1410 to 1776 MPa when the strain rate increases from 10–3 to 1400 s-1....Two distinct regimes of strain rate sensitivity on yield strength are found in a high-strength nantwinned steel.The yield strength increases from 1410 to 1776 MPa when the strain rate increases from 10–3 to 1400 s-1.It is proposed from the measured small activation volume that the yielding of the nanotwinned steel at higher strain rates is governed by the dislocation bowing out from the carbon atmosphere.At lower strain rates,however,the yielding is controlled by the continuous re-pinning of dislocations due to the fast diffused carbon atoms,which leads to the relative insensitivity of yield strength to the strain rate.展开更多
The electroplated(111)-oriented nanotwinned-Cu(nt-Cu) film was utilized as substrate for Ag and Cu sinter joining to improve the weak interface connection between the metal paste and the substrate.It was found that bo...The electroplated(111)-oriented nanotwinned-Cu(nt-Cu) film was utilized as substrate for Ag and Cu sinter joining to improve the weak interface connection between the metal paste and the substrate.It was found that both Cu and Ag sinter joints using(111)-oriented nt-Cu film exhibited a higher bonding strength than that using traditional random-oriented Cu film.Especially,the joints sintered with Cu paste on(111)-oriented nt-Cu film possessed a higher bonding strength of 53.7 MPa at the sintering condition of 300 °C,0.4 MPa in N2 atmosphere,compared to that on random-oriented Cu film with a value of 31.3 MPa.The results show that as metal substrate layer,the(111)-oriented nt-Cu film can improve the connection performance of Ag and Cu sinter joints,which could further promote their application in dieattachment technology for the next-generation power semiconductors.展开更多
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 cyc...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.展开更多
Plastic-deformation behaviors of gradient nanotwinned(GNT)metallic multilayers are investigated in nanoscale via molecular dynamics simulation.The evolution law of deformation behaviors of GNT metallic multilayers wit...Plastic-deformation behaviors of gradient nanotwinned(GNT)metallic multilayers are investigated in nanoscale via molecular dynamics simulation.The evolution law of deformation behaviors of GNT metallic multilayers with different stacking fault energies(SFEs)during nanoindentation is revealed.The deformation behavior transforms from the dislocation dynamics to the twinning/detwinning in the GNT Ag,Cu,to Al with SFE increasing.In addition,it is found that the GNT Ag and GNT Cu strengthen in the case of a larger twin gradient based on more significant twin boundary(TB)strengthening and dislocation strengthening,while the GNT Al softens due to more TB migration and dislocation nucleation from TB at a larger twin gradient.The softening mechanism is further analyzed theoretically.These results not only provide an atomic insight into the plastic-deformation behaviors of certain GNT metallic multilayers with different SFEs,but also give a guideline to design the GNT metallic multilayers with required mechanical properties.展开更多
The three-dimensional(3D)graphene-based materials have raised significant interest due to excellent catalytic performance and unique electronic properties,while the preparation of uniform and stable 3D graphene struct...The three-dimensional(3D)graphene-based materials have raised significant interest due to excellent catalytic performance and unique electronic properties,while the preparation of uniform and stable 3D graphene structures remains a challenge.In this paper,using molecular dynamics simulations,we found that the nanotwinned copper(nt-Cu)matrix with small twin spacing can induce the wave-shaped wrinkling and sawtooth-shaped buckling graphene structures under uniaxial compression.The nt-Cu matrix possesses a symmetrical lattice structure for the lattice rotation with the dislocation annihilation,resulting in the transition of sandwiched graphene from 2D to 3D structures with good uniformity.The newly formed twin boundaries(TBs)in the nt-Cu matrix improve the resistance of graphene against the out-of-plane deformation so that graphene can maintain a stable wrinkling or buckling morphology in a wide strain range.These 3D texturing structures show great flexibility and their micro parameters can be controlled by applying different compressive strains.Furthermore,we propose a simple sliding method for decoupling graphene from the nt-Cu matrix without any damage.This work provides a novel strategy to induce and transfer the uniform wrinkling and buckling of graphene,which may expand the application of graphene in energy storage and catalysts.展开更多
Based on molecular dynamics(MD)simulation,the mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with{111}texture during tensile deformation were systematically studied from the aspects of Schmid f...Based on molecular dynamics(MD)simulation,the mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with{111}texture during tensile deformation were systematically studied from the aspects of Schmid factor of the dominant slip system and the dislocation mechanism.The results show that the Schmid factor of dominated slip system is altered by changing the inclining angle of the twin boundaries(TBs),while the yield stress or flow stress does not strictly follow the Schmid law.There exist hard and soft orientations involving different dislocation mechanisms during the tensile deformation.The strengthening mechanism of hard orientation lies in the fact that there exist interactions between the dislocations and the TBs during plastic deformation,which leads to the dislocation blocking and reactions.The softening mechanism of soft orientation lies in the fact that there is no interaction between the dislocations and the TBs because only the slip systems parallel to the TBs are activated and the dislocations slip on the planes parallel to the TBs.It is concluded that the plastic anisotropy in the nanotwinned polycrystalline copper with{111}texture is aroused by the combination effect of the Schmid factor of dominated slip system and the dislocation mechanism.展开更多
Molecular dynamics simulations are performed to investigate the mechanical behavior of nanotwinned NiCo-based alloys containing coherent L1_(2) nano-precipitates at different temperatures,as well as the interactions b...Molecular dynamics simulations are performed to investigate the mechanical behavior of nanotwinned NiCo-based alloys containing coherent L1_(2) nano-precipitates at different temperatures,as well as the interactions between the dislocations and nano-precipitates within the nanotwins.The simulation results demonstrate that both the yield stress and flow stress in the nanotwinned NiCo-based alloys with nano-precipitates decrease as the temperature rises,because the higher temperatures lead to the generation of more defects during yielding and lower dislocation density during plastic deformation.Moreover,the coherent L1_(2) phase exhibits excellent thermal stability,which enables the hinderance of dislocation motion at elevated temperatures via the wrapping and cutting mechanisms of dislocations.The synergistic effect of nanotwins and nano-precipitates results in more significant strengthening behavior in the nanotwinned NiCo-based alloys under high temperatures.In addition,the high-temperature mechanical behavior of nanotwinned NiCo-based alloys with nano-precipitates is sensitive to the size and volume fraction of the microstructures.These findings could be helpful for the design of nanotwins and nano-precipitates to improve the high-temperature mechanical properties of NiCo-based alloys.展开更多
Nanotwinned polycrystals exhibit an excellent strength-ductility combination due to nanoscale twins and grains. However, nanotwin-assisted grain coarsening under mechanical loading reported in recent experiments may r...Nanotwinned polycrystals exhibit an excellent strength-ductility combination due to nanoscale twins and grains. However, nanotwin-assisted grain coarsening under mechanical loading reported in recent experiments may result in strength drop based on the Hall-Petch law. In this paper, a phase-field model is developed to investigate the effect of coupled evolutions of twin and grain boundaries on nanotwin-assisted grain growth. The simulation result demonstrates that there are three pathways for coupled motions of twin and grain boundaries in a bicrystal under the applied loading, dependent on the amplitude of applied loading and misorientation of the bicrystal. It reveals that a large misorientation angle and a large applied stress promote the twinning-driven grain boundary migration. The resultant twin-assisted grain coarsening is confirmed in the simulations for the microstructural evolutions in twinned and un-twinned polycrystals under a high applied stress.展开更多
Nanotwinned materials possess exceptional properties and represent a promising class of metastable materials.However,their controlled synthesis remains challenging.Here,we report the synthesis of twinned PtCo nanopart...Nanotwinned materials possess exceptional properties and represent a promising class of metastable materials.However,their controlled synthesis remains challenging.Here,we report the synthesis of twinned PtCo nanoparticles via a laser ablation approach,which promotes the formation of unique nanostructures.By optimizing laser parameters such as power and exposure time,we achieved a nanotwinning yield up to 61%,significantly higher than 12%in commercial PtCo.In proton-exchange membrane fuel cell tests,the nanotwinned PtCo catalyst demonstrated a mass activity of 0.56 A·mgPt^(-1) with 82.1%retention after accelerated stress testing.Notably,the voltage loss at 1 A·cm^(-2) was only 24.3 mV.Particle agglomeration and the reduction of stacking fault energy through alloying were identified as possible mechanisms for nanotwin formation.This work provides an efficient and rapid route for the controlled synthesis of nanotwinned particles,and the approach could potentially enable scalable production of high-performance catalysts.展开更多
Highly nanotwinned(NT) metals have advantages such as high strength,good ductility,favorable corrosion resistance,and thermal stability.It has been demonstrated that the introduction of high density NT microstructures...Highly nanotwinned(NT) metals have advantages such as high strength,good ductility,favorable corrosion resistance,and thermal stability.It has been demonstrated that the introduction of high density NT microstructures can enhance the tribological properties of metals.However,the influence of the microstructure and the composition of NT alloys on the tribological behavior are not clear.In this work,the sliding wear behavior of fully NT materials,specifically Cu-Al and Cu-Ni alloys,are studied by a nanoscratch technique using a nanoindenter.The effects of microstructure and chemical composition on the wear properties are also studied.The results show that the chemical composition has an obvious influence on the wear resistance and microstructural deformation.For NT Cu-Al alloys,the hardness and sliding wear resistance improve with increased Al content from Cu-2 wt.%Al to Cu-6 wt.%Al.NT Cu-10 wt.%Ni alloy shows even better wear resistance than Cu-6 wt.%Al.The microstructural analysis shows that NT Cu alloys with higher wear resistance correspond to a smaller deformation-affected zone.The improvement of sliding wear properties of Cu-Al alloys with higher Al content may be ascribed to their decreased stacking fault energy.NT Cu-Ni alloy shows better wear resistance than Cu-Al alloy,this may be related to the formation of intermetallic compounds in Cu-Al system.This study broadens the knowledge about tribological properties of NT materials and provides a potential method to optimize their sliding wear resistance by altering the chemical composition of NT Cu alloys.展开更多
Binderless nanotwinned cubic boron nitride(nt-cBN) synthesized from onion-structured BN precursors under high pressure and high temperature shows a very fine microstructure consisting of densely lamellar nanotwins(ave...Binderless nanotwinned cubic boron nitride(nt-cBN) synthesized from onion-structured BN precursors under high pressure and high temperature shows a very fine microstructure consisting of densely lamellar nanotwins(average thickness of 4 nm) within nanograins. The unique nanotwinned microstructure offers high hardness, wear resistance, fracture toughness, and thermal stability which are essential for advanced cBN tool materials. Thus, a circular micro tool of nt-cBN was fabricated using femtosecond laser contour machining followed by focused ion beam precision milling. Thereafter turning tests were performed on hardened steel using the studied micro tool. To evaluate the cutting performance, the machined surface quality and subsurface damage of the hardened steel were characterized. The wear mechanism of the nt-cBN micro tool was also investigated. It is found that the fabricated nt-cBN micro tool can generate high quality surface with surface roughness less than 7 nm and nanograin subsurface of about 500 nm deep. In addition, abrasive wear is found to be the dominant wear mechanism of the nt-cBN micro tool in turning hardened steel. These results indicate that nt-cBN has outstanding potential for ultra-precision cutting hardened steel.展开更多
Nanotwinned diamond（nt-diamond）,which demonstrates unprecedented hardness and stability,is synthesized through the martensitic transformation of onion carbons at high pressure and high temperature（HPHT）.Its hardne...Nanotwinned diamond（nt-diamond）,which demonstrates unprecedented hardness and stability,is synthesized through the martensitic transformation of onion carbons at high pressure and high temperature（HPHT）.Its hardness and stability increase with decreasing twin thickness at the nanoscale.However,the formation mechanism of nanotwinning substructures within diamond nanograins is not well established.Here,we characterize the nanotwins in nt-diamonds synthesized under different HPHT conditions.Our observation shows that the nanotwin thickness reaches a minimum at ~20 GPa,below which phase-transformation twins and deformation twins coexist.Then,we use the density-functional-based tight-binding method and kinetic dislocation theory to investigate the subsequent plastic deformation mechanism in these pre-existing phase-transformation diamond twins.Our results suggest that pressure-dependent conversion of the plastic deformation mechanism occurs at a critical synthetic pressure for nt-diamond,which explains the existence of the minimum twin thickness.Our findings provide guidance on optimizing the synthetic conditions for fabricating nt-diamond with higher hardness and stability.展开更多
When a metal is subject to cyclic loading with cyclic stress values much lower than its tensile strength,fatigue occurs due to accumulative and irreversible damages developed in the microstructure,causing service fail...When a metal is subject to cyclic loading with cyclic stress values much lower than its tensile strength,fatigue occurs due to accumulative and irreversible damages developed in the microstructure,causing service failure of the metal workpiece or even fatal disaster in the worst cases.To develop materials with higher fatigue limit and longer fatigue life relies on reducing or suppressing展开更多
Ordered structures with functional units offer the potential for enhanced performance in metallic materials.Among these structures,gradient nanotwinned(GNT)microstructures demonstrate excellent controllability.This pa...Ordered structures with functional units offer the potential for enhanced performance in metallic materials.Among these structures,gradient nanotwinned(GNT)microstructures demonstrate excellent controllability.This paper provides a comprehensive review of the current state-of-the-art studies on GNT structures,encompassing various aspects such as design strategies,mechanical properties characterization,spatially gradient strain evolution analysis,and the significant role of geometrically necessary dislocations(GNDs).The primary objective is to systematically unravel the fundamental strengthening mechanisms by gaining an in-depth understanding of the deformation behavior of nanotwinned units.Through this work,we aim to contribute to the broader field of materials science by consolidating knowledge and providing insights for the development of novel metallic materials with enhanced properties and tailored performance characteristics.展开更多
Microscale metallic structures enhanced by additive manufacturing technology have attracted extensive attention especially in microelectronics and electromechanical devices.Meniscus-confined electrodeposition(MCED)adv...Microscale metallic structures enhanced by additive manufacturing technology have attracted extensive attention especially in microelectronics and electromechanical devices.Meniscus-confined electrodeposition(MCED)advances microscale 3D metal printing,enabling simpler fabrication of superior metallic microstructures in air without complex equipment or post-processing.However,accurately predicting growth rates with current MCED techniques remain challenging,which is essential for precise structure fabrication and preventing nozzle clogging.In this work,we present a novel approach to electrochemical 3D printing that utilizes a self-adjusting,voxelated method for fabricating metallic microstructures.Diverging from conventional voxelated printing which focuses on monitoring voxel thickness for structure control,this technique adopts a holistic strategy.It ensures each voxel’s position is in alignment with the final structure by synchronizing the micropipette’s trajectory during deposition with the intended design,thus facilitating self-regulation of voxel position and reducing errors associated with environmental fluctuations in deposition parameters.The method’s ability to print micropillars with various tilt angles,high density,and helical arrays demonstrates its refined control over the deposition process.Transmission electron microscopy analysis reveals that the deposited structures,which are fabricated through layer-by-layer(voxel)printing,contain nanotwins that are widely known to enhance the material’s mechanical and electrical properties.Correspondingly,in situ scanning electron microscopy(SEM)microcompression tests confirm this enhancement,showing these structures exhibit a compressive yield strength exceeding 1 GPa.The indentation tests provided an average hardness of 3.71 GPa,which is the highest value reported in previous work using MCED.The resistivity measured by the four-point probe method was(1.95±0.01)×10^(−7)Ω·m,nearly 11 times that of bulk copper.These findings demonstrate the considerable advantage of this technique in fabricating complex metallic microstructures with enhanced mechanical properties,making it suitable for advanced applications in microsensors,microelectronics,and micro-electromechanical systems.展开更多
It is extremely difficult to introduce high-density nano twins during the solidification process of TiAl alloy.In this study,high-density nanotwins are inducted in the as-cast Ti48Al2Cr alloyed by adding Re element.Ph...It is extremely difficult to introduce high-density nano twins during the solidification process of TiAl alloy.In this study,high-density nanotwins are inducted in the as-cast Ti48Al2Cr alloyed by adding Re element.Phase transformation,morphology characteristics of nano twins,compressive and tensile proper-ties,and the related mechanisms have been studied.Results show that B2 phase enriched with Re tends to precipitate along theα_(2)/γinterface within lamellar colony.The stacking fault energy(SFE)ofγphase decreases from 43 mJ/m^(2) to 16 mJ/m^(2) as Re content increases from 0 at.%to 0.6 at.%,decreasing the crit-ical shear stress for twin formation.Compared to the mismatch value ofα_(2)/γinterface(0.004),which of B2/α_(2) and B2/γinterfaces increase to 0.247 and 0.149,respectively.Driven by high interfacial stress,high-density dislocations are generated at the B2/α_(2) interface,providing the dislocation slip channel for the formation of stacking faults(SFs)and nanotwins at the B2/γinterface.Therefore,the mechanism of inducting high-density nanotwins is to reduce the stacking fault energy ofγphase by Re and form highly mismatched B2/α_(2) interface.Compressive strength and the strain increase from 1723 MPa to 2398 MPa and 29%to 39%as Re content increases from 0 at.%to 0.6 at.%,respectively.Tensile strength increases from 356 MPa to 452 MPa without sacrificing plasticity.The improvement in strength and plasticity are attributed to the nano-twinning strengthening and interfacial thermal mismatch strengthening.Forming nanotwins during solidification process serve as the nucleation sites for newly formed twins during de-formation process,increasing the deformation tolerance of TiAl alloy.展开更多
Electric current heat treatment is an innovative technique to improve microstructures and mechanical properties of metallic materials.The microstructures and mechanical properties of a powder metallurgy high-speed ste...Electric current heat treatment is an innovative technique to improve microstructures and mechanical properties of metallic materials.The microstructures and mechanical properties of a powder metallurgy high-speed steel(PM-HSS)treated by electric current heat treatment and traditional heat treatment are comparatively investigated.Results showed that after austenitizing at 1130°C,the structure of PM-HSS sample composed of ferrite matrix,M_(6)C,M_(23)C_(6),and MC carbides,transformed into a martensite matrix accompanied by M_(6)C and MC carbides.Compared to the traditional austenitizing at 1130℃ for 30 min,the electric current austenitizing at 1130℃ for 5 min dissolved more carbides,resulting in a greater solid solution of alloying elements in the matrix.Further traditional triple tempering led to carbide coarsening,whereas electric current triple tempering promoted the carbide dissolution.Notably,the dissolution of more carbides resulted in a higher C content in the martensite matrix of HSS treated by electric current,significantly promoting the formation of nanotwins(5-20 nm in width).The electric current triple tempering sample exhibited a yield strength of 3097 MPa,compressive strength of 5016 MPa,and a fracture strain of 30.0%,outperforming the traditional triple tempering sample by nearly 600 MPa in yield strength.Analysis revealed that this significant strengthening was primarily attributed to nanotwin formation and solid solution strengthening caused by carbide dissolution.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.92163201,52431006,52441407,U23A6013,92360301 and U2330203)the Shaanxi Province Innovation Team Project(2024RS-CXTD-58)+2 种基金the Shaanxi Province Youth Innovation Team Project(22JP042)the Natural Science Basic Research Plan in Shaanxi Province(2022JQ-460)the Fundamental Research Funds for the Central Universities(xtr062024006,xtr022019004 and xzy022022024).
文摘Nanocrystalline alloys often exhibit unusual thermal stability as a consequence of kinetic and thermodynamic barriers to grain growth.However,the physical mechanisms governing alloy stability need to be identified.In this work,we found that grain boundary(GB)relaxation renders Ni-W alloyed films relatively stable at low annealing temperature,while twinning-mediated grain growth occurs via dislocation-GB/twin boundary(TB)interactions as the annealing temperature increases.At a relatively low temperature,TB strengthening plays a dominant role in plastic deformation,whereas precipitation strengthening gradually controls the deformation mechanism with the increase of annealing temperature.Our findings provide evidence for improving mechanical property through alloying and microstructure design,and have a crucial guiding significance in material selection and miniaturized applications such as Micro Electro Mechanical Systems.
基金Financial support from the National Natural Science Foundation of China(Grant No.51771196)the National Key R&D Program of China(Grant No.2017YFA0204401 and No.2017YFA0204402)the Key Research Program of Frontier Science,Chinese Academy of Sciences。
文摘A nanotwinned 316 L austenitic stainless steel was prepared by means of surface mechanical grinding treatment.After recovery annealing,the density of dislocations decreases obviously while the average twin/matrix lamella thickness still keeps in the nanometer scale.The annealed nanotwinned sample exhibits a high tensile yield strength of 771 MPa and a considerate uniform elongation of 8%.TEM observations showed that accommodating more dislocations and secondary twinning inside the nanotwins contribute to the enhanced ductility and work hardening rate of the annealed nanotwinned sample.
基金financially supported by the National Natural Science Foundation of China(No.51401218)the Osaka University Visiting Scholar Program(No.J135104902)
文摘Homogeneous columnar Cu film with fully embedded nanotwins was successfully fabricated on Ti/Cu seed layer on silicon wafer. The nanotwins with thickness of tens of nanometers are generally parallel to the silicon surface, showing a strong （111 ） preferred orientation. The acid concentration was found to be important in influencing the formation of nanoscale twins. By adjusting the acid concentration, the nanotwins can be induced from the top columnar grain to middle columnar grain and reach the bottom equiaxed grain, and a microstructural transformation model was given. A theory focusing on the cathode overpotential was proposed to reveal the effect of acid concentration on the growth mechanism of nanoscale twins. An appropriate adsorption proportion of hydrogen on cathode （acid concentration 17 ml L^-1） could increase the overpotential which supplies adequate nucleation energy for nanoscale twins formation.
文摘Two distinct regimes of strain rate sensitivity on yield strength are found in a high-strength nantwinned steel.The yield strength increases from 1410 to 1776 MPa when the strain rate increases from 10–3 to 1400 s-1.It is proposed from the measured small activation volume that the yielding of the nanotwinned steel at higher strain rates is governed by the dislocation bowing out from the carbon atmosphere.At lower strain rates,however,the yielding is controlled by the continuous re-pinning of dislocations due to the fast diffused carbon atoms,which leads to the relative insensitivity of yield strength to the strain rate.
文摘The electroplated(111)-oriented nanotwinned-Cu(nt-Cu) film was utilized as substrate for Ag and Cu sinter joining to improve the weak interface connection between the metal paste and the substrate.It was found that both Cu and Ag sinter joints using(111)-oriented nt-Cu film exhibited a higher bonding strength than that using traditional random-oriented Cu film.Especially,the joints sintered with Cu paste on(111)-oriented nt-Cu film possessed a higher bonding strength of 53.7 MPa at the sintering condition of 300 °C,0.4 MPa in N2 atmosphere,compared to that on random-oriented Cu film with a value of 31.3 MPa.The results show that as metal substrate layer,the(111)-oriented nt-Cu film can improve the connection performance of Ag and Cu sinter joints,which could further promote their application in dieattachment technology for the next-generation power semiconductors.
基金Project supported by the National Natural Science Foundation of China(No.11902289)the Hundred Talents Program of Zhejiang University,China。
文摘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.
基金the National Natural Science Foundation of China(Grant Nos.51621004,11572118,51871092,and 11772122)the National Key Research and Development Program of China(Grant No.2016YFB0700300)。
文摘Plastic-deformation behaviors of gradient nanotwinned(GNT)metallic multilayers are investigated in nanoscale via molecular dynamics simulation.The evolution law of deformation behaviors of GNT metallic multilayers with different stacking fault energies(SFEs)during nanoindentation is revealed.The deformation behavior transforms from the dislocation dynamics to the twinning/detwinning in the GNT Ag,Cu,to Al with SFE increasing.In addition,it is found that the GNT Ag and GNT Cu strengthen in the case of a larger twin gradient based on more significant twin boundary(TB)strengthening and dislocation strengthening,while the GNT Al softens due to more TB migration and dislocation nucleation from TB at a larger twin gradient.The softening mechanism is further analyzed theoretically.These results not only provide an atomic insight into the plastic-deformation behaviors of certain GNT metallic multilayers with different SFEs,but also give a guideline to design the GNT metallic multilayers with required mechanical properties.
基金Australia Research Council Discovery Project(DP170103092)National Natural Science Foundation of China(NSFC51701030).
文摘The three-dimensional(3D)graphene-based materials have raised significant interest due to excellent catalytic performance and unique electronic properties,while the preparation of uniform and stable 3D graphene structures remains a challenge.In this paper,using molecular dynamics simulations,we found that the nanotwinned copper(nt-Cu)matrix with small twin spacing can induce the wave-shaped wrinkling and sawtooth-shaped buckling graphene structures under uniaxial compression.The nt-Cu matrix possesses a symmetrical lattice structure for the lattice rotation with the dislocation annihilation,resulting in the transition of sandwiched graphene from 2D to 3D structures with good uniformity.The newly formed twin boundaries(TBs)in the nt-Cu matrix improve the resistance of graphene against the out-of-plane deformation so that graphene can maintain a stable wrinkling or buckling morphology in a wide strain range.These 3D texturing structures show great flexibility and their micro parameters can be controlled by applying different compressive strains.Furthermore,we propose a simple sliding method for decoupling graphene from the nt-Cu matrix without any damage.This work provides a novel strategy to induce and transfer the uniform wrinkling and buckling of graphene,which may expand the application of graphene in energy storage and catalysts.
基金the National Natural Science Foundation of China(No.51871070).
文摘Based on molecular dynamics(MD)simulation,the mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with{111}texture during tensile deformation were systematically studied from the aspects of Schmid factor of the dominant slip system and the dislocation mechanism.The results show that the Schmid factor of dominated slip system is altered by changing the inclining angle of the twin boundaries(TBs),while the yield stress or flow stress does not strictly follow the Schmid law.There exist hard and soft orientations involving different dislocation mechanisms during the tensile deformation.The strengthening mechanism of hard orientation lies in the fact that there exist interactions between the dislocations and the TBs during plastic deformation,which leads to the dislocation blocking and reactions.The softening mechanism of soft orientation lies in the fact that there is no interaction between the dislocations and the TBs because only the slip systems parallel to the TBs are activated and the dislocations slip on the planes parallel to the TBs.It is concluded that the plastic anisotropy in the nanotwinned polycrystalline copper with{111}texture is aroused by the combination effect of the Schmid factor of dominated slip system and the dislocation mechanism.
基金Project supported by the National Natural Science Foundation of China(Grant No.12072317)the Natural Science Foundation of Zhejiang Province(Grant No.LZ21A020002)+2 种基金Ligang Sun gratefully acknowledges the support received from the Guangdong Basic and Applied Basic Research Foundation(Grant No.22022A1515011402)the Science,Technology and Innovation Commission of Shenzhen Municipality(Grant No.GXWD20231130102735001)Development and Reform Commission of Shenzhen(Grant No.XMHT20220103004).
文摘Molecular dynamics simulations are performed to investigate the mechanical behavior of nanotwinned NiCo-based alloys containing coherent L1_(2) nano-precipitates at different temperatures,as well as the interactions between the dislocations and nano-precipitates within the nanotwins.The simulation results demonstrate that both the yield stress and flow stress in the nanotwinned NiCo-based alloys with nano-precipitates decrease as the temperature rises,because the higher temperatures lead to the generation of more defects during yielding and lower dislocation density during plastic deformation.Moreover,the coherent L1_(2) phase exhibits excellent thermal stability,which enables the hinderance of dislocation motion at elevated temperatures via the wrapping and cutting mechanisms of dislocations.The synergistic effect of nanotwins and nano-precipitates results in more significant strengthening behavior in the nanotwinned NiCo-based alloys under high temperatures.In addition,the high-temperature mechanical behavior of nanotwinned NiCo-based alloys with nano-precipitates is sensitive to the size and volume fraction of the microstructures.These findings could be helpful for the design of nanotwins and nano-precipitates to improve the high-temperature mechanical properties of NiCo-based alloys.
基金Project supported by the National Natural Science Foundation of China(No.11672285)the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDB22040502)+1 种基金the Collaborative Innovation Center of Suzhou Nano Science and Technologythe Fundamental Research Funds for the Central Universities
文摘Nanotwinned polycrystals exhibit an excellent strength-ductility combination due to nanoscale twins and grains. However, nanotwin-assisted grain coarsening under mechanical loading reported in recent experiments may result in strength drop based on the Hall-Petch law. In this paper, a phase-field model is developed to investigate the effect of coupled evolutions of twin and grain boundaries on nanotwin-assisted grain growth. The simulation result demonstrates that there are three pathways for coupled motions of twin and grain boundaries in a bicrystal under the applied loading, dependent on the amplitude of applied loading and misorientation of the bicrystal. It reveals that a large misorientation angle and a large applied stress promote the twinning-driven grain boundary migration. The resultant twin-assisted grain coarsening is confirmed in the simulations for the microstructural evolutions in twinned and un-twinned polycrystals under a high applied stress.
文摘Nanotwinned materials possess exceptional properties and represent a promising class of metastable materials.However,their controlled synthesis remains challenging.Here,we report the synthesis of twinned PtCo nanoparticles via a laser ablation approach,which promotes the formation of unique nanostructures.By optimizing laser parameters such as power and exposure time,we achieved a nanotwinning yield up to 61%,significantly higher than 12%in commercial PtCo.In proton-exchange membrane fuel cell tests,the nanotwinned PtCo catalyst demonstrated a mass activity of 0.56 A·mgPt^(-1) with 82.1%retention after accelerated stress testing.Notably,the voltage loss at 1 A·cm^(-2) was only 24.3 mV.Particle agglomeration and the reduction of stacking fault energy through alloying were identified as possible mechanisms for nanotwin formation.This work provides an efficient and rapid route for the controlled synthesis of nanotwinned particles,and the approach could potentially enable scalable production of high-performance catalysts.
基金the National Science Foundation(Grant Nos.NSF-DMR-0955338 and NSF-OISE-1460006)
文摘Highly nanotwinned(NT) metals have advantages such as high strength,good ductility,favorable corrosion resistance,and thermal stability.It has been demonstrated that the introduction of high density NT microstructures can enhance the tribological properties of metals.However,the influence of the microstructure and the composition of NT alloys on the tribological behavior are not clear.In this work,the sliding wear behavior of fully NT materials,specifically Cu-Al and Cu-Ni alloys,are studied by a nanoscratch technique using a nanoindenter.The effects of microstructure and chemical composition on the wear properties are also studied.The results show that the chemical composition has an obvious influence on the wear resistance and microstructural deformation.For NT Cu-Al alloys,the hardness and sliding wear resistance improve with increased Al content from Cu-2 wt.%Al to Cu-6 wt.%Al.NT Cu-10 wt.%Ni alloy shows even better wear resistance than Cu-6 wt.%Al.The microstructural analysis shows that NT Cu alloys with higher wear resistance correspond to a smaller deformation-affected zone.The improvement of sliding wear properties of Cu-Al alloys with higher Al content may be ascribed to their decreased stacking fault energy.NT Cu-Ni alloy shows better wear resistance than Cu-Al alloy,this may be related to the formation of intermetallic compounds in Cu-Al system.This study broadens the knowledge about tribological properties of NT materials and provides a potential method to optimize their sliding wear resistance by altering the chemical composition of NT Cu alloys.
基金supported by the National Natural Science Foundation of China(Grant Nos.51205343,51332005&51421091)Hebei Provincial Science Foundation of China(Grant No.E2016203372)
文摘Binderless nanotwinned cubic boron nitride(nt-cBN) synthesized from onion-structured BN precursors under high pressure and high temperature shows a very fine microstructure consisting of densely lamellar nanotwins(average thickness of 4 nm) within nanograins. The unique nanotwinned microstructure offers high hardness, wear resistance, fracture toughness, and thermal stability which are essential for advanced cBN tool materials. Thus, a circular micro tool of nt-cBN was fabricated using femtosecond laser contour machining followed by focused ion beam precision milling. Thereafter turning tests were performed on hardened steel using the studied micro tool. To evaluate the cutting performance, the machined surface quality and subsurface damage of the hardened steel were characterized. The wear mechanism of the nt-cBN micro tool was also investigated. It is found that the fabricated nt-cBN micro tool can generate high quality surface with surface roughness less than 7 nm and nanograin subsurface of about 500 nm deep. In addition, abrasive wear is found to be the dominant wear mechanism of the nt-cBN micro tool in turning hardened steel. These results indicate that nt-cBN has outstanding potential for ultra-precision cutting hardened steel.
基金supported by the National Natural Science Foundation of China(51421091,51332005,51572225,51272227, 51172197,51525205 and 51672239)the US National Science Foundation(EAR-1361276)
文摘Nanotwinned diamond（nt-diamond）,which demonstrates unprecedented hardness and stability,is synthesized through the martensitic transformation of onion carbons at high pressure and high temperature（HPHT）.Its hardness and stability increase with decreasing twin thickness at the nanoscale.However,the formation mechanism of nanotwinning substructures within diamond nanograins is not well established.Here,we characterize the nanotwins in nt-diamonds synthesized under different HPHT conditions.Our observation shows that the nanotwin thickness reaches a minimum at ~20 GPa,below which phase-transformation twins and deformation twins coexist.Then,we use the density-functional-based tight-binding method and kinetic dislocation theory to investigate the subsequent plastic deformation mechanism in these pre-existing phase-transformation diamond twins.Our results suggest that pressure-dependent conversion of the plastic deformation mechanism occurs at a critical synthetic pressure for nt-diamond,which explains the existence of the minimum twin thickness.Our findings provide guidance on optimizing the synthetic conditions for fabricating nt-diamond with higher hardness and stability.
文摘When a metal is subject to cyclic loading with cyclic stress values much lower than its tensile strength,fatigue occurs due to accumulative and irreversible damages developed in the microstructure,causing service failure of the metal workpiece or even fatal disaster in the worst cases.To develop materials with higher fatigue limit and longer fatigue life relies on reducing or suppressing
基金support from the National Natural Science Foundation of China(Nos.51931010 and 92163202)and the Key Research Program of Frontier Science and International partnership program(No.GJHZ2029)+1 种基金Z.C.acknowledges financial support from the National Natural Science Foundation of China(No.52001312)Youth Innovation Promotion Association,Chinese Academy of Sciences(CAS).
文摘Ordered structures with functional units offer the potential for enhanced performance in metallic materials.Among these structures,gradient nanotwinned(GNT)microstructures demonstrate excellent controllability.This paper provides a comprehensive review of the current state-of-the-art studies on GNT structures,encompassing various aspects such as design strategies,mechanical properties characterization,spatially gradient strain evolution analysis,and the significant role of geometrically necessary dislocations(GNDs).The primary objective is to systematically unravel the fundamental strengthening mechanisms by gaining an in-depth understanding of the deformation behavior of nanotwinned units.Through this work,we aim to contribute to the broader field of materials science by consolidating knowledge and providing insights for the development of novel metallic materials with enhanced properties and tailored performance characteristics.
基金supported in part by National Key R&D Program of China under Grant 2023YFB4705600in part by the National Natural Science Foundation of China under Grants 61925304,62127810 and 62203138+1 种基金in part by the National Postdoctoral Program for Innovative Talents under Grant BX20200107in part by the Self-Planned Task(No.SKLRS202205C)of State Key Laboratory of Robotics and System(HIT).
文摘Microscale metallic structures enhanced by additive manufacturing technology have attracted extensive attention especially in microelectronics and electromechanical devices.Meniscus-confined electrodeposition(MCED)advances microscale 3D metal printing,enabling simpler fabrication of superior metallic microstructures in air without complex equipment or post-processing.However,accurately predicting growth rates with current MCED techniques remain challenging,which is essential for precise structure fabrication and preventing nozzle clogging.In this work,we present a novel approach to electrochemical 3D printing that utilizes a self-adjusting,voxelated method for fabricating metallic microstructures.Diverging from conventional voxelated printing which focuses on monitoring voxel thickness for structure control,this technique adopts a holistic strategy.It ensures each voxel’s position is in alignment with the final structure by synchronizing the micropipette’s trajectory during deposition with the intended design,thus facilitating self-regulation of voxel position and reducing errors associated with environmental fluctuations in deposition parameters.The method’s ability to print micropillars with various tilt angles,high density,and helical arrays demonstrates its refined control over the deposition process.Transmission electron microscopy analysis reveals that the deposited structures,which are fabricated through layer-by-layer(voxel)printing,contain nanotwins that are widely known to enhance the material’s mechanical and electrical properties.Correspondingly,in situ scanning electron microscopy(SEM)microcompression tests confirm this enhancement,showing these structures exhibit a compressive yield strength exceeding 1 GPa.The indentation tests provided an average hardness of 3.71 GPa,which is the highest value reported in previous work using MCED.The resistivity measured by the four-point probe method was(1.95±0.01)×10^(−7)Ω·m,nearly 11 times that of bulk copper.These findings demonstrate the considerable advantage of this technique in fabricating complex metallic microstructures with enhanced mechanical properties,making it suitable for advanced applications in microsensors,microelectronics,and micro-electromechanical systems.
基金supported by the National Natural Science Foundation of China(No.U21A2042)the National Nature Fund Youth Fund Project of China(No.52101038)Young Elite Scientists Sponsorship Program by CAST(No.2021QNRC001).
文摘It is extremely difficult to introduce high-density nano twins during the solidification process of TiAl alloy.In this study,high-density nanotwins are inducted in the as-cast Ti48Al2Cr alloyed by adding Re element.Phase transformation,morphology characteristics of nano twins,compressive and tensile proper-ties,and the related mechanisms have been studied.Results show that B2 phase enriched with Re tends to precipitate along theα_(2)/γinterface within lamellar colony.The stacking fault energy(SFE)ofγphase decreases from 43 mJ/m^(2) to 16 mJ/m^(2) as Re content increases from 0 at.%to 0.6 at.%,decreasing the crit-ical shear stress for twin formation.Compared to the mismatch value ofα_(2)/γinterface(0.004),which of B2/α_(2) and B2/γinterfaces increase to 0.247 and 0.149,respectively.Driven by high interfacial stress,high-density dislocations are generated at the B2/α_(2) interface,providing the dislocation slip channel for the formation of stacking faults(SFs)and nanotwins at the B2/γinterface.Therefore,the mechanism of inducting high-density nanotwins is to reduce the stacking fault energy ofγphase by Re and form highly mismatched B2/α_(2) interface.Compressive strength and the strain increase from 1723 MPa to 2398 MPa and 29%to 39%as Re content increases from 0 at.%to 0.6 at.%,respectively.Tensile strength increases from 356 MPa to 452 MPa without sacrificing plasticity.The improvement in strength and plasticity are attributed to the nano-twinning strengthening and interfacial thermal mismatch strengthening.Forming nanotwins during solidification process serve as the nucleation sites for newly formed twins during de-formation process,increasing the deformation tolerance of TiAl alloy.
基金financially supported by the National Natural Science Foundation of China(Nos.52271034,52301058 and 52471042)China Postdoctoral Science Foundation(No.2023M732183)Postdoctoral Fellowship Program of CPSF(No.GZB20230399).
文摘Electric current heat treatment is an innovative technique to improve microstructures and mechanical properties of metallic materials.The microstructures and mechanical properties of a powder metallurgy high-speed steel(PM-HSS)treated by electric current heat treatment and traditional heat treatment are comparatively investigated.Results showed that after austenitizing at 1130°C,the structure of PM-HSS sample composed of ferrite matrix,M_(6)C,M_(23)C_(6),and MC carbides,transformed into a martensite matrix accompanied by M_(6)C and MC carbides.Compared to the traditional austenitizing at 1130℃ for 30 min,the electric current austenitizing at 1130℃ for 5 min dissolved more carbides,resulting in a greater solid solution of alloying elements in the matrix.Further traditional triple tempering led to carbide coarsening,whereas electric current triple tempering promoted the carbide dissolution.Notably,the dissolution of more carbides resulted in a higher C content in the martensite matrix of HSS treated by electric current,significantly promoting the formation of nanotwins(5-20 nm in width).The electric current triple tempering sample exhibited a yield strength of 3097 MPa,compressive strength of 5016 MPa,and a fracture strain of 30.0%,outperforming the traditional triple tempering sample by nearly 600 MPa in yield strength.Analysis revealed that this significant strengthening was primarily attributed to nanotwin formation and solid solution strengthening caused by carbide dissolution.
