查看更多>>摘要：In this study,functionally graded foam made of Inconel 625 superalloy was successfully produced using the template replication method,with open-cell polyurethane foams as a precursor.The products have a similar pore morphology as the templates and adjacent layers were successfully sintered together by par-ticle bonding.Sound absorption experiments on graded metallic foams reveal that the sound absorption at particular frequency ranges can be improved by various permutations of foam layers.For graded foam of two distinct pore sizes,a mathematical equation was proposed to predict the location of the intersec-tion point of the sound absorption curves,thereby aiding in graded foam design.An increase in sound absorption coefficients by resonance-like effects can be introduced before the intersection points by plac-ing the foam layer of smaller pore size nearer to the sound source.The sound absorption performances can be further customized when the thickness proportion of the pore sizes is changed and when the number of distinct pore sizes used is increased.The sound absorption performance at lower frequencies is generally boosted by resonance-like effects when the layer of foam with the largest pore size is placed furthest from the sound source.Given the same composition of foam with a fixed thickness proportion of pore sizes,one can introduce resonance-like effects to improve the sound absorption performance compared to other permutations while possibly satisfying weight requirements in practical applications.This study provides valuable insights and mathematical guidelines in the design and manufacturing of functionally graded metallic foam for specific applications.
查看更多>>摘要：In this perspective,we have highlighted the current literature and explained the synthesis,structure,mor-phology,modification strategies,and photocatalytic applications of emerging BiPO4-based photocatalysts.Since BiPO4 is a large bandgap photocatalyst,it uses UV light for the excitation of electrons,and also,the recombination of charge carriers is an issue in BiPO4.Various novel modification strategies of BiPO4 photocatalysts viz.defect modifications,heterojunction formation,phase-junctions,surface plasmon res-onance,Schottky junction have been successfully proposed and highlighted.These modifications enhance the light absorption and inhibit the recombination of charge carriers BiPO4 photocatalyst.Finally,future aspects for further research on BiPO4-based photocatalysts are also explored.It expects that BiPO4-based photocatalysts represent a promising strategy for developing practical photocatalysts for energy and en-vironmental remediation applications.
查看更多>>摘要：For additive manufactured aluminum alloys,the inferior mechanical properties along the building direc-tion have been a serious weakness.In this study,an optimized heat treatment was developed as a simple and effective solution.The effects of direct aging on microstructure and mechanical properties along the building direction of AISi10Mg samples produced via selective laser melting(SLM)were investigated.The results showed that,compared with the conventional heat treatment at elevated temperatures,direct aging at temperatures of 130-190℃could retain the fine grain microstructure of SLM samples and pro-mote further precipitation of Si phase,however,the growth of pores occurred during direct aging.With increasing aging temperature,while finer cell structures were obtained,more and larger pores were de-veloped,resulting in decreased density of the samples.Two types of pore formation mechanisms were identified.Considering the balance between the refinement of cell structure and the growth of pores,aging at 130℃was determined as the optimized heat treatment for SLM AISi10Mg samples.The tensile strength along the building direction of the 130℃aged sample was increased from 403 MPa to 451 MPa,with relatively high elongation of 6.5％.
查看更多>>摘要：Metal-organic framework-derived composites have been widely used in electromagnetic wave(EMW)ab-sorption,but the traditional synthetic strategy greatly limits the structure and species of MOFs.This re-search provided a solvent-free method to synthesize Co-MOF and its derivatives.Using CoSnO3 as the precursor,the preparation of Co-MOF is achieved by bridging the cobalt(Ⅱ)ion of CoSnO3 and the 2-methylimidazole skeleton.The CoSn/N-doped carbon(CoSn/NC)composites derived from CoSnO3-MOF(Co-MOF with CoSnO3 as Co source)retain the original morphology of CoSnO3.Besides,the polarization effect produced by the N-doped carbon layers also benefits the excellent EMW absorption performance of the CoSn/NC composites.It is reflected in the minimum reflection loss(RL)of-48.2 dB at 2.2 mm and the effective bandwidth(EBA)of 5.84 GHz.This work provides a new channel to the construction of Co-MOFs,which could be extended to other Co-based oxides and vastly expand the species of MOFs based on metallic Co.
查看更多>>摘要：Dwell fatigue effect is a long-standing problem threatening the long-term service reliability for fan blades and fan disks of an aircraft engine.To understand the basic mechanism of dwell fatigue damage,pure fa-tigue and 60 s dwell fatigue properties of bimodal Ti-6Al-4V alloys with different volume fractions of the primary α'(αp)phase were examined comparatively.The results showed that both pure fatigue and dwell fatigue life decreased with increasing the volume fraction of the αp phase and the dwell fatigue life was lower than the pure fatigue one.The quasi-in-situ test results and the quantitative characterization of damage behaviors of the local microstructure units defined by the αp-secondary α(αs)combination reveal that the αs phase close to the αp phase with extensively slip activities was gradually damaged under dwell fatigue loading,while that under pure fatigue loading was undamaged,demonstrating that the dwell loading induced the damage of the αs phase,and further reduced the fatigue life.A stress relaxation-based model is proposed to describe the physical mechanism on dwell fatigue damage of the bimodal Ti-6Al-4V alloy,i.e.the elastic deformation of the αs phase caused by the strain incompatibil-ity would be gradually transformed into plastic deformation during the dwell stage,and thus promotes fatigue damage.The model provides new insights into the microscopic process of stress/strain transfer between the soft and hard microstructure units under dwell fatigue loading.
查看更多>>摘要：We systematically compared the mechanical properties of CrCoNi,a recently emerged prototypical medium-entropy alloy(MEA)with face-centered-cubic(FCC)structure,with hallmark FCC alloys,in par-ticular,the well-known austenitic 316L and 316LN stainless steels,which are also concentrated single-phase FCC solid solutions and arguably next-of-kin to the MEAs.The tensile and impact properties,across the temperatures range from 373 K to 4.2 K,as well as fracture toughness at 298 K and 77 K,were documented.From room temperature to cryogenic temperature,all three alloys exhibited similarly good mechanical properties;CrCoNi increased its tensile uniform elongation and fracture toughness,which was different from the decreasing trend of the 316L and 316LN.On the other hand,the stainless steels showed higher fracture toughness than CrCoNi at all temperatures.To explain the differences in macro-scopic mechanical properties of the three alloys,microstructural hardening mechanisms were surveyed.CrCoNi MEA relied on abundant mechanical twinning on the nanoscale,while martensitic transformation was dominant in 316L at low temperatures.The deformation mechanisms in the plastic zone ahead of the propagating crack in impact and fracture toughness tests were also analyzed and compared for the three alloys.
Hyun ChungDae Woong KimWoo Jin ChoHeung Nam Han...
270-280页
查看更多>>摘要：High-and medium-entropy alloys(HEAs and MEAs)possess high solid-solution strength.Numerous in-vestigations have been conducted on its impact on yield strength,however,there are limited reports regarding the relation between solid-solution strengthening and strain-hardening rate.In addition,no attempt has been made to account for the dislocation-mediated plasticity;most works focused on twinning-or transformation-induced plasticity(TWIP or TRIP).In this work we reveal the role of solid-solution strengthening on the strain-hardening rate via systematically investigating evolutions of defor-mation structures by controlling the Cr/V ratio in prototypical V1-xCrxCoNi alloys.Comparing the TWIP of CrCoNi with the dislocation slip of V0.4Cr0.6CoNi,the hardening rate of CrCoNi was superior to slip-band refinements of V0.4Cr0.6CoNi due to the dynamic Hall-Petch effect.However,as V content increased fur-ther to V0.7Cr0.3CoNi and VCoNi,their rate of slip-band refinement in V0.7Cr0.3CoNi and VCoNi with high solid-solution strength surpassed that of CrCoNi.Although it is generally accepted in conventional alloys that deformation twinning results in a higher strain-hardening rate than dislocation-mediated plasticity,we observed that the latter can be predominant in the former under an activated huge solid-solution strengthening effect.The high solid-solution strength lowered the cross-slip activation and consequently retarded the dislocation rearrangement rate,i.e.,the dynamic recovery.This delay in the hardening rate decrease,therefore,increased the strain-hardening rate,results in an overall higher strain-hardening rate of V-rich alloys.
查看更多>>摘要：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 scanning-transmission-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 of η1 precipitates takes place concurrently with η'.A novel type of metastable phase ηi'is identified as the precursor of η1,which can lower the lattice misfit between η1 and Al matrix along the direction of[10(1)0]η1//[001]Al.Accordingly,a pathway for the formation of η1 via η1'is demonstrated.Precipitates η'together with η1 and η1'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.
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