查看更多>>摘要：The power-law relationship between creep rate decay and time is one of the intrinsic characteristics of metallic glasses.In the current work,a La30Ce30Ni10Al20Co10 high-entropy metallic glass was selected as the model alloy to test the influences of physical aging and cyclic loading on the power-law creep mechanism,which was probed by the dynamic mechanical analysis in terms of the stochastic activation,and contiguous interplay and permeation of shear transformation zones.It is demonstrated that a no-table discrepancy appears between thermal treatment and mechanical treatment on the power-law creep mechanism of this high-entropy metallic glass.On the one hand,physical aging below the glass transi-tion temperature introduces the annihilation of potential shear transformation zones which contribute to creep.On the other hand,cyclic loading can tailor the"forward"jump operations competing with the"backward"ones of shear transformation zones by controlling the interval time(recovery time).The cur-rent research offers a new pathway towards understanding the creep mechanism of high-entropy metallic glasses.
查看更多>>摘要：Deformation twinning is an important deformation mechanism in nickel-based superalloys.For super-alloys,deformation twins are generally observed at low or intermediate temperatures and high strain rates;however,the appearance of microtwins(MTs)at high temperatures has rarely been reported.In this study,transmission electron microscopy(TEM)was used to study MT formation in Ni-Co-based su-peralloys following compression at 1120℃/1 s-1.The deformation behavior was discussed in detail to reveal the mechanism of MT formation.The twinning mechanism at elevated temperatures was theoret-ically attributed to the low stacking fault energy(SFE)and poor dislocation-driven deformations caused by the high strain rate in specific directions.
查看更多>>摘要：Rational design of highly efficient,robust and nonprecious electrocatalysts for the oxygen reduction reac-tion(ORR),oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)is highly demanded and challenging.Here,heterostructural Co3O4@Ni2P arrays with numerous reaction sites,unique interfa-cial electronic structure and fast charge transfer kinetics are developed as electrocatalysts for rechargeable Zn-air batteries and overall water splitting.Both density functional theory calculation and X-ray absorp-tion fine structure analysis manifest that the synergistic structural and abundant electronic modulations interfaces are formed,thus simultaneously promoting the electrocatalytic kinetics,activities and stabil-ities.Specifically,it can achieve an ultralow overpotential of 270 mV and 28 mV at 10 mA cm-2 for OER and HER,respectively.The water electrolyzer delivers a current density of 10 mA cm-2 at 1.563 V;furthermore,rechargeable Zn-air batteries triggered by this heterostructure can achieve excellent cyclic stability of 177 h(2 h per cycle)at 10 mA cm-2;both devices are superior to the Pt/C+Ir/C.This work not only designs an efficient trifunctional electrocatalyst but also paves an avenue to understand the het-erostructure engineering for catalysts development and disclose the underlying relationship of interfacial electronic structures and catalytic properties.
查看更多>>摘要：In the current work,the BCC-AlCoCrFeNi bulk nanocrystalline high-entropy alloy(nc-HEA)with ultra-high hardness was formed by nanoscale diffusion-induced phase transition in a nanocomposite.First,a dual-phase Al/CoCrFeNi nanocrystalline high-entropy alloy composite(nc-HEAC)was prepared by a laser source inert gas condensation equipment(laser-IGC).The as-prepared nc-HEAC is composed of well-mixed FCC-Al and FCC-CoCrFeNi nanocrystals.Then,the heat treatment was used to trigger the interdiffusion between Al and CoCrFeNi nanocrystals and form an FCC-AlCoCrFeNi phase.With the in-crease of the annealing temperature,element diffusion intensifies,and the AlCoCrFeNi phase undergoes a phase transition from FCC to BCC structure.Finally,the BCC-AlCoCrFeNi bulk nc-HEA with high Al con-tent(up to 50 at.％)was obtained for the first time.Excitingly,the nc-HEAC(Al-40％)sample exhibits an unprecedented ultra-high hardness of 1124 HV after annealing at 500℃for 1 h.We present a systematic investigation of the relationship between the microstructure evolution and mechanical properties dur-ing annealing,and the corresponding micro-mechanisms in different annealing stages are revealed.The enhanced nanoscale thermal diffusion-induced phase transition process dominates the mechanical per-formance evolution of the nc-HEACs,which opens a new pathway for the design of high-performance nanocrystalline alloy materials.
查看更多>>摘要：Additively manufactured(AM)metallic materials commonly possess substantial tensile surface residual stress,which is detrimental to the load-bearing service behavior.Recently,we demonstrated that deep cryogenic treatment(DCT)is an effective method for improving the tensile properties of CoCrFeMnNi high-entropy alloy(HEA)samples fabricated by laser melting deposition(LMD),by introducing high com-pressive residual stress and deformation microstructures without destroying the AM shape.However,car-rying out the DCT in a single-step mode does not improve the residual stress gradients inherent from the LMD process,which are undesirable as the mechanical properties will not be homogeneous within the sample.In this work,we show that carrying out the DCT in a cyclic mode with repeated cryogenic cooling and reheating can significantly homogenize the residual stress in LMD-fabricated CoCrFeMnNi HEA,and improve tensile strength and ductility,compared with single-step DCT of the same cryogenic soaking du-ration.Under cyclic DCT,the thermal stress is re-elevated to a high value at each cryogenic cooling step,leading to the formation of denser and more intersecting reinforcing crystalline defects and hcp phase transformation,compared to single-step DCT of the same total cryogenic soaking duration in which the thermal stress relaxes towards a low value over time.The enhancement of defect formation in the cyclic mode of DCT also leads to more uniform residual stress distribution in the sample after the DCT.The results here provide important insights on optimizing DCT processes for post-fabrication improvement of mechanical properties of AM metallic net shapes.
Jeong In KimJu Yeon KimSung-Ho KookJeong-Chae Lee...
52-70页
查看更多>>摘要：Numerous studies highlight advantages of electrospun scaffolds in bone tissue engineering,in which cellular behavior is tightly affected by fiber topographical cues of scaffolds.However,the classic elec-trospinning setup limits a desired presentation of biomimetic fibrous microenvironments that sense mechanosignaling and regulate stem cell behavior.The aims of this study were to fabricate advanced as-spun scaffolds presenting tree-like microfiber/nanonet networks and to evaluate their regulatory poten-tials on behavior of human mesenchymal stem cells(hMSCs)and bone regeneration.Here we developed a novel electrospinning setup that allowed the presentation of patterned Trunk microfibers(TMF)and/or branched nanonet fibers(BNnFs)in biomimetic fibrous scaffolds.As the cellular mechanisms,anisotropic-hierarchical topography of TMF controlled behavior of hMSCs through focal adhesion formation and Yes-associated protein(YAP)induction,whereas BNnF disturbed such mechanosensing responses in the cells.The fiber microenvironment-related expression and nuclear localization of YAP were.also correlated with the potentials of as-spun scaffolds to enhance osteogenic differentiation of the hMSCs and alveolar bone defect healing in an animal model.Collectively,this study provides an advanced approach of the modi-fied electrospinning setup for presentation of biomimetic fibrillar microenvironments in as-spun scaffolds along with their application in stem cell behavior regulation and regenerative tissue engineering.
查看更多>>摘要：Metal-organic-frameworks(MOFs)derived carbon-based composites with balanced impedance matching and synergistic dielectric/magnetic loss are considered as promising microwave absorbers.With the aim to promote interfacial polarization,herein,heterogeneous junctions composed of magnetic Ni core and binary dielectric shells(C and PEDOT)are synthesized by annealing Ni-MOFs precursors and an in-situ polymerization strategy,forming Ni@C@PEDOT spheres with multilayer heterogeneous interfaces.The re-sults indicate that the final absorption attenuation is sensitive to the thickness of the dielectric PEDOT layer,when the thickness of the PEDOT layer is 224 nm,an optimal reflection loss of-72.4 dB is achieved at 2 mm and the effective absorption bandwidth reaches 6.4 GHz with a thickness of only 1.85 mm,the excellent absorption attenuation is accredited to the promoted impedance matching,enhanced conduc-tion loss as well as the synergistic interfacial polarization induced by magnetic core and binary dielectric shells.Meanwhile,this work offers a simple and significant strategy in preparation for ideal microwave absorbers by rational design of multilayer heterogeneous interfaces.
查看更多>>摘要：Ti-Mo alloys/composites are expected to be the next-generation implant material with low moduli but without toxic/allergic elements.However,synthesis mechanisms of the Ti-Mo biomaterials in Selective Laser Melting(SLM)vary according to raw materials and fundamentally influence material performance,due to inhomogeneous chemical compositions and stability.Therefore,this work provides a comparative study on microstructure,mechanical and wear performance,and underlying thermal mechanisms of two promising Ti-Mo biomaterials prepared by SLM but through different synthesis mechanisms to offer sci-entific understanding for creation of ideal metal implants.They are(i)Ti-7.5Mo alloys,prepared from a conventional Ti/Mo powder mixture,and(ii)Ti-7.5Mo-2.4TiC composites,in-situ prepared from Ti/Mo2C powder mixture.Results reveal that the in-situ Ti-7.5Mo-2.4TiC composites made from Ti/Mo2C powder mixture by SLM can produce 61.4％more β phase and extra TiC precipitates(diameter below 229.6 nm)than the Ti-7.5Mo alloys.The fine TiC not only contributes to thinner and shorter β columnar grains under a large temperature gradient of 51.2 K/μm but also benefits material performance.The in-situ Ti-7.5Mo-2.4TiC composites produce higher yield strength(980.1±29.8 MPa)and ultimate compressive strength(1561.4±39 MPa)than the Ti-7.5Mo alloys,increasing by up to 12.1％.However,the fine TiC with an aspect ratio of 2.71 dominates an unfavourable rise of elastic modulus to 91.9±2 GPa,44.7％higher than the Ti-7.5Mo alloys,which,nevertheless,is still lower than the modulus of traditional Ti-6A1-4V.While,TiC and its homogeneous distribution benefit wear resistance,decreasing the wear rate of the in-situ Ti-7.5Mo-2.4TiC composites to 6.98 x 10-4 mm3 N-1 m-1,which is 36％lower than that of the Ti-7.5Mo alloys.Therefore,although with higher modulus than the Ti-7.5Mo alloys,the SLM-fabricated in-situ Ti-7.5Mo-2.4TiC composites can expect to provide good biomedical application potential in cases where combined good strength and wear resistance are required.
查看更多>>摘要：The intermetallic compound such as Ni2Si has a brittle nature.Therefore,monolithic intermetallic com-pounds have not yet been prepared by mechanical downsizing.During mechanical drawing of bulk Cu-Ni2 Si alloy at room temperature,we observed more than 400％plastic elongation of hard and brittle Ni2 Si intermetallic nano-fibers.The calculation based on the density functional theory reveals that the fully co-herent interface induces strain on the intermetallic compound surrounded by the matrix,and lowers the intrinsic stacking fault energy below the level required to break an interatomic bond.The new interface between the Ni2Si intermetallic and Cu matrix formed by the plastic deformation is as stable as the origi-nal coherent interface formed by precipitation,and the activation energy of the newly formed interface to slip is similar to that of the Cu matrix.All of these make plastic deformation of brittle Ni2Si intermetallic possible by slip without failure.
查看更多>>摘要：Aiming at serious catastrophic oxidation problem of super-austenitic stainless steel S32654,the influence of different rare earth elements on its oxidation behavior was comparatively investigated at 1200℃.The mechanism of Y significantly improving high-temperature oxidation resistance of S32654 was unveiled.The results demonstrated that Y played much better beneficial roles than Ce and La in the initial forma-tion of oxide layer:(1)Y promoted Cr segregation to steel surface to combine with O;(2)its preferential oxidation provided nucleation cores for Cr2O3.Both roles jointly promoted the selective oxidation of Cr and then the formation of protective Cr-rich oxide layer.This provided good prerequisites for inhibiting the formation and volatilization of MoO3.Additionally,Y cation segregation to oxide grain boundaries fur-ther promoted the selective oxidation of Cr and Si to form more protective oxide layer.These beneficial roles of Y essentially eliminated the synergistic effects of MoO3 volatilization and lamellar Cr2N precipi-tation on catastrophic oxidation.Accordingly,the oxidation resistance of Y-bearing S32654 was improved by 22％-45％.
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