查看更多>>摘要：The morphology of MAX phase powders significantly influences their microwave absorption properties.However,the traditional synthesis via solid-state reactions produces irregular powders,and the prepa-ration of MAX phase powders with specific morphology remains a challenge.Herein,(V0.8Ti0.1Cr0.1)2AlC MAX phase microrods were fabricated for the first time in NaCl/KCl molten salts using vanadium,tita-nium,chromium,aluminum,and short carbon fibers as precursors.It was found that despite acting as a carbon source,carbon fibers also acted as sacrificial templates.By adjusting the molar ratio of metal powders and short carbon fibers,a series of carbon fiber@(V0.8Ti0.1Cr0.1)2AIC microrods with core-sheath structure were also obtained.Carbon fiber@(V0.8Ti0.1Cr0.1)2AlC microrods with a molar ratio of 8:2 showed the optimum microwave absorption performance.The reflection loss(RL)value reached up to-63.26 dB at 2.40 mm,and the effective absorption bandwidth(EAB)was about 5.28 GHz with a thickness of 2.02 mm.Based on the electromagnetic parameter analysis and theoretical simulation,the enhanced mi-crowave absorption performance was attributed to the synergistic effect of different factors like dielectric loss,magnetic loss,multiple reflection,and scattering.This work offers a facile route to modulate the morphology of MAX phase powders and may accelerate its application as microwave absorbers.
查看更多>>摘要：Laser remelting(LR)has attracted widespread attention in recent years as an effective method to reduce internal defects and improve the surface quality of additively manufactured(AM)parts.In the present study,three different LR inter-layer scanning strategies(LRO,LR90 and LR45)and their effects on the porosity,microstructure,crystallographic texture and related mechanical properties of parts have been studied.Optical microscope,X-ray diffraction,and scanning electron microscope were used as charac-terization tools.In the LR90 sample,it shows obvious{111}<110>texture and strong<111>preferred orientation along the scanning direction(SD),while the 0° offset and the 45° rotation of LR scanning strategy form a finer microstructure and weak crystallographic texture.Meanwhile,the mechanical prop-erties of the LR sample are improved compared with the sample only by laser metal deposition(LMD),and a combination of higher strength and optimal uniform elongation is obtained in the LR45 sample.The overall results show that a reasonable LR scanning strategy can reduce the anisotropy of AM parts and improve their mechanical properties.
查看更多>>摘要：The layered structural parameters have been reported to be critical for tuning the tensile properties of laminated metals.Here,we investigated the effects of the thickness ratio(rc/f)of coarse-grained layers(CLs)to fine-grained layers(FLs)on the enhanced ductility of the laminated Al.The local strain evolution demonstrates that the strain delocalization ability of laminated Al is improved with the decrease of rc/f.The interfacial strain gradients,which can produce extra work hardening,gradually approach and cover the CLs with the rc/f decreasing,explaining the trend of uniform elongation in laminated Al with various rc/f.The integrated fracture morphology characterization reveals that the increase of the rc/f leads to an improvement in the tolerance of the interfacial microcracks,which is corresponding to the variation of fracture elongation in the laminated Al.Moreover,there is an evident transition of transverse propagation path of interfacial microcracks from the CLs to FLs with increasing the rc/f.Based on a geometrical cri-terion of microcracks connectivity,the preferential transverse propagation path of interfacial microcracks in these laminated Al was rationalized.The calculation based on this criterion also predicted the criti-cal rc/f corresponding to the optimal combination of strength and fracture elongation.This work deepens the understanding of the role of structural parameters of laminated metals in achieving the strength and ductility synergy.
查看更多>>摘要：We have investigated the low cycle fatigue(LCF)properties and the extent of strengthening in a dense additively manufactured stainless steel containing different volume fractions of cell structures but having all other microstructure characteristics the same.The samples were produced by laser powder bed fusion(L-PBF),and the concentration of cell structures was varied systematically by varying the annealing treat-ments.Load-controlled fatigue experiments performed on samples with a high fraction of cell structures reveal an up to 23 times increase in fatigue life compared to an essentially cell-free sample of the same grain configuration.Multiscale electron microscopy characterizations reveal that the cell structures serve as the soft barriers to the dislocation propagation and the partials are the main carrier for cyclic loading.The cell structures,stabilized by the segregated atoms and misorientation between the adjacent cells,are retained during the entire plastic deformation,hence,can continuously interact with dislocations,pro-mote the formation of nanotwins,and provide massive 3D network obstacles to the dislocation motion.The compositional micro-segregation caused by the cellular solidification features serves as another non-negligible strengthening mechanism to dislocation motion.Specifically,the cell structures with a high density of dislocation debris also appear to act as dislocation nucleation sites,very much like coherent twin boundaries.This work indicates the potential of additive manufacturing to design energy absorbent alloys with high performance by tailoring the microstructure through the printing process.
查看更多>>摘要：Higher manganese silicide(HMS)is a P-type medium temperature thermoelectric(TE)material,which has attracted widespread attention over the past few decades due to its remarkable mechanical prop-erties,excellent chemical and thermal stability,as well as the non-toxicity,abundance and competitive price.The peak power factor(PF)of HMS is as high as～1.50×10-3 W m-1 K-2 because of its intrinsic high electrical conductivity and Seebeck coefficient.However,the thermal conductivity of HMS is also high,resulting in relatively low zT values.Introducing nano-dispersion in the matrix is one of the most effective methods to enhance the TE properties via reducing the lattice thermal conductivity significantly without drastic changes on the other parameters.In this study,CsPbBr3 QDs with uniform size were syn-thesized and introduced into HMS bulks.The PF(at 823 K)was enhanced to 1.71×10-3 W m-1 K-2,which is improved 14.0％approximately compared with that of pure HMS owing to the combined effect of element doping and energy filtering.The lattice thermal conductivity(at 823 K)decreased from 2.56 W m-1 K-1 to 1.99 W m-1 K-1 synchronously(～22.0％)due to the intensive phonon scattering caused by Cs doping,and the embedding of Pb riched CsPbBr3 QDs and Pb QDs.A maximum zT value of 0.57(823 K)is achieved in CsPbBr3 QDs/HMS composites,which is 36.0％higher than that of pure HMS.Pre-dictably,for other TE materials,it is also feasible to improve the TE properties via introducing metastable quantum dots.
查看更多>>摘要：Pure(pristine)anatase and mono-doped and co-doped derivatives of TiO2 having nitrogen(N)and cobalt(Co)as dopants with respective fixed doping concentrations of 0.7 mol.％and 1.0 mol.％were synthesized using auto-combustion sol-gel technique.The doping effects at corresponding non-metal and transition metal sites of TiO2 on the basis of the structural,optical and electrical properties have been investi-gated.X-ray diffraction(XRD)measurement confirms the formation of pure anatase phase of TiO2 for all samples having 141/amd space group of tetragonal structure which has been also verified by the Raman spectroscopy measurement.Various crystallographic parameters have been calculated by performing Ri-etveld refinement of XRD data including average crystallite size that has been observed in the range of 10-15 nm.Pure anatase phase indicates the incorporation of Co2+into TiO2 lattice which assists the sub-stitution of N in place of oxygen in co-doped TiO2.The band gap tuning towards the visible region from 3.2 to 2.1 eV has been achieved with mono-doping and co-doping of the N and Co in TiO2 lattice.This can be described in terms of the formation of localized levels of N-2p and Co-3d states in mono-doping and an isolated intermediate band formation in co-doping case.Electrical properties have been inves-tigated in details and explained as the synergetic effects of structural and inherent ionic characters of various dopants.The observed band gaps of all doped samples lie within the visible region which makes them pertinent as the solar energy harnessing materials for photocatalytic and photovoltaic applications.
查看更多>>摘要：As a typical dual-phase eutectic high entropy alloy(EHEA),AlCoCrFeNi2.1 can achieve the fair match-ing of strength and ductility,which has attracted wide attention.However,the engineering applications of as-cast AlCoCrFeNi2.1 EHEAs still face challenges,such as coarse grain and low yield strength result-ing from low solidification rate and temperature gradient.In this study,selective laser melting(SLM)was introduced into the preparation of AlCoCrFeNi2.1 EHEA to realize unique strength-ductility balance,with emphasis on investigating the effects of processing parameters on its eutectic microstructure and properties.The results show that the SLM-ed samples exhibit a completely eutectic structure consisting of ultra-fine face-centered cubic(FCC)and ordered body-centered cubic(B2)phases,and the duplex mi-crostructure undergoes a morphological evolution from lamellar structure to cellular structure as laser en-ergy input reducing.The SLM-ed AlCoCrFeNi2.1 EHEA presents an excellent match of high tensile strength(1271 MPa),yield strength(966 MPa),and good ductility(22.5％)at room temperature,which are signifi-cantly enhanced by the ultra-fine grains and heterogeneous structure due to rapid solidification rate and high temperature gradient during SLM.Especially,the yield strength increment of～50％is realized with no loss in ductility as compared with the as-cast samples with the same composition.On this basis,the precise complex component with excellent mechanical properties is well achieved.This work paves the way for the performance improvement and complex parts preparation of EHEA by microstructural design using laser additive manufacturing.
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