查看更多>>摘要：Owing to the special layer-by-layer deposition process of directed energy deposition(DED),columnar coarse grains,produced by cyclic reheating with intrinsic directional heat flow along the building direc-tion,are difficult to avoid.These grains result in strong anisotropic characteristics with poor mechanical properties,which restrict the application of DED products.This work proposes a novel fabrication strat-egy based on the cyclic-phase-transformation behavior,which can reduce the anisotropy and improve the mechanical properties of DED-printed stainless steel.Using this fabrication strategy,316 L powder(austenitic stainless steel)and 17-4PH powder(martensitic stainless steel)were mixed in different mass ratios to fabricate five types of DED-printed stainless steels.Among the five samples,P70(mixture of 70 wt％17-4PH powder and 30 wt％316 L powder)showed the weakest anisotropy and the best mechanical properties,which can be attributed to the cyclic phase transformation under cyclic reheating treatment and the transformation-induced plasticity(TRIP)effect over a wide range of strains,respectively.Com-pared with the pure 316 L printed material,the tensile test results of P70 showed that the yield strength(YS)and ultimate tensile strength(UTS)increased by 35.4％and 34.5％,respectively,whereas the uniform elongation(UE)and total elongation(TE)were improved by 63.9％and 31.4％,respectively.In addition,the strength-ductility balance(UTS × UE)increased by as much as 120.4％.The proposed fabrication strategy is expected to reduce the anisotropy in other materials that undergo cyclic-phase-transformation phe-nomena during additive manufacturing.
查看更多>>摘要：An integrated calculated approach based on weakly coupled finite element(FEM)-viscoplastic self-consistent(VPSC)model was established to simulate the texture evolution during the variable strain path extrusion process of magnesium alloys.The spiral die extrusion(SDE)process with additional circumfer-ential shear deformation was applied to investigate the effect of path control on texture adjustment and verify the accuracy of the model.The results indicated that the additional spiral shear resulting from the overall inclined flow path effectively reduced the intensity of the {0002}//ED fiber texture by suppressing basal slip activation in the core area,while the local shear deformation along the spiral equal channel strain path led to the formation of an inclined {0002}//ND plane texture on the side.Using the modified Hall-Petch relationship,the correlation between texture and yield strength was quantified.Specifically,the weakening of the texture effectively suppressed {10-12} tensile twinning,which compensated for the deficiency of compressive yield strength without significantly sacrificing tensile yield strength,and thus improved the tension-compression asymmetry.Furthermore,the strongly inclined {0002}//ND plane tex-ture inhibited the widespread activation of basal slip during tensile yielding,thereby enhancing the yield strength.
查看更多>>摘要：This study proposes a novel strategy for the design of a new family of metastable Zr alloys.These al-loys offer improved mechanical properties for implants,particularly in applications where conventional stainless steels and Co-Cr alloys are currently used but lack suitability.The design approach is based on the controlled twinning-induced plasticity(TWIP)effect,significantly enhancing the ductility and strain-hardenability of the Zr alloys.In order to draw a"blueprint"for the compositional design of biomedical TWIP(Bio-TWIP)Zr alloys-using only non-toxic elements,the study combines D-electron phase stability calculations(specifically bond order(Bo)and mean d-orbital energy(Md))with a systematic experimental screening of active deformation mechanisms within the Zr-Nb-Sn alloy system.This research aids in ac-curately identifying the TWIP line,which signifies the mechanism shift between TWIP and classic slip as the primary deformation mechanism.To demonstrate the efficacy of the TWIP mechanism in enhancing mechanical properties,Zr-12Nb-2Sn,Zr-13Nb-1Sn,and Zr-14Nb-3Sn alloys are selected.Results indicate that the TWIP mechanism leads to a significant improvement of strain-hardening rate and a uniform elongation of～20％in Zr-12Nb-2Sn,which displays both{332}<113>mechanical twinning and disloca-tion slip as the primary deformation mechanisms.Conversely,Zr-14Nb-3Sn exhibits the typical mechan-ical properties found in stable body-centered cubic(BCC)alloys,characterized by the sole occurrence of dislocation slip.Cell viability tests confirm the superior biocompatibility of Zr-Nb-based alloys with deformation twins on the surface,in line with existing literature.Based on the whole set of results,a comprehensive design diagram is proposed.
查看更多>>摘要：In this work,accelerated sintering of Y2O3 dispersion strengthened tungsten alloy with a well-regulated structure was achieved by a novel dissolution-precipitation strategy.As indicated,yttrium oxide was firstly dissolved into the lattices of W powder precursor during the thermal plasma synthesis process in a one-step and ultra-fast way,and then homogeneously precipitated out within W grains during sintering.The theoretical calculation reveals that the formation process of Y2O3 dispersoids enhanced the driving force of densification by increasing the sintering stress and declining the macroscopic viscosity,resulting in improved diffusion ability for the W skeleton.The microstructural investigation further confirmed the occurrence of mass inter-diffusion at the W-Y2O3 interface,which provides a fast diffusion pathway for W atoms,and is responsible for the accelerated densification kinetics.Being sintered at 1600 ℃ for 1 h,the as-obtained alloy possesses a high relative density of 98.26％,together with a refined grain size of 970 nm for W and 50 nm for intragranular Y2O3,respectively.
查看更多>>摘要：Magnetic resonance imaging(MRI)is a non-invasive medical imaging technique that has been widely applied in the clinical diagnosis of diseases across various fields.Currently,there is a dearth of high-performance dual-mode contrast agents that provide precise diagnostic information for complex diseases.In this study,iron oxide-gadolinium nanoparticles(IO-Gd NPs)bridged by hydrophilic ligand ethylenedi-amine tetramethylenephosphonic acid(EDTMP)are designed inspired by Solomon-Bloembergen-Morgan(SBM)theory,where T1 and T2 relaxivity increase with a reduction in Gd content.In particular,the NPs with minimum Gd content exhibit excellent dual-mode contrast performance(r1=94.42 mM-1 s-1,r2=343.62 mM-1 s-1).The underlying mechanism is that the bridged EDTMP coordinate limits Gd tum-bling and forces Gd3+to expose itself to the surface of the NPs.This provides more opportunities for water molecules to coordinate with Gd3+and significantly enhances proton relaxivity.Moreover,the hy-drophilicity of the ligand enhances the water dispersion stability of the NPs and increases the exchange rate of water protons near the NPs,further enhancing the dual-mode contrast effect.Finally,the biocom-patibility and in vitro/vivo imaging performances of the IO@EDTMP-Gd NPs are systematically evaluated,and the results demonstrate their potential as dual-mode contrast agents.This study provides a new strategy for developing dual-mode MRI contrast agents that can further improve the accuracy of MRI in the diagnosis of complex diseases.
查看更多>>摘要：Pressure-assisted sintering(PAS)utilizing bimetallic core-shell nanoparticles(CS NPs)has garnered widespread attention for its ability to produce advanced components with excellent thermal and elec-trical conductivities,even at low temperatures.This study presents the first investigation into the PAS of bimetallic CS NPs,using Ag-coated Cu material as a representative system,through all-atom molecu-lar dynamics(MD)simulations.The study provides detailed insights into the atomic-scale processes and mechanisms that govern pressure-assisted sintering,elucidating the coalescence phenomena and reveal-ing the significant influence of external pressure.By comparing atomistic behaviors,such as diffusion,migration,and microstructure evolution,between PAS and thermal sintering,a deeper understanding of the underlying mechanisms is achieved.Furthermore,the results highlight the importance of considering microstructural characteristics in addition to void fraction assessment for accurately estimating mechani-cal properties.This study enhances our understanding of sintering methods and offers a new perspective on designing process conditions in the PAS process.
查看更多>>摘要：Intervertebral disc herniation(IVDH)is a common manifestation of intervertebral disc degeneration(IVDD)characterized by inflammation that results in the rupture of the annulus fibrosus(AF)and her-niation of the nucleus pulposus(NP).While current clinical research primarily focuses on regulating the degenerative NP,the crucial role of the AF in maintaining the mechanical stability and metabolic balance of the intervertebral disc(IVD)has been overlooked.Resolving immunoregulation and AF repair is im-perative to effectively prevent recurrent herniation.Therefore,this study introduces a bioactive sealant(OD/GM/QCS-sEVs),which combines gelatin methacryloyl(GM)and oxidized dextran(OD)with quater-nized chitosan(QCS)and incorporates small extracellular vesicles(sEVs).The developed sealant possesses injectability,self-healing capabilities,tissue adhesiveness,and mechanical stability,with an average ad-hesive strength of 109.63 kPa.In vitro experiments demonstrate that OD/GM/QCS-sEVs effectively seal AF defects while preserving mechanical properties comparable to those of a normal IVD.Additionally,the sealant releases sEVs through a pH-responsive mechanism,thereby modulating macrophage polarization to the M2 phenotype via the NF-κB signaling pathway.This mechanism facilitates immunoregulation and anti-inflammatory effects,and promotes stem cell differentiation into fibrocartilage.Animal experiments confirm the ability of OD/GM/QCS-sEVs to seal defects,prevent proteoglycan loss,inhibit IVDD develop-ment,and promote AF regeneration.Overall,OD/GM/QCS-sEVs hold promise as an innovative bioactive sealant for recurrent herniation by resolving immunoregulation and AF regeneration.
查看更多>>摘要：Fe is the most detrimental impurity element in recycled Al-Si alloys due to the formation of brittle,pri-mary,Fe-containing,intermetallic particles during solidification.Their removal from the Al-Si melts e.g.by filtration can reduce the Fe content.New active filter materials can facilitate the formation of these particles for their removal and contribute to the production of high-quality,recycled Al-Si alloys.The interaction of the alloy with the filter material can lead to modification of the thermodynamics of the alloy or of the kinetics of the particle formation.Time-resolved,three-dimensional microstructural inves-tigations have been carried out to study the formation of primary intermetallic particles in Al7.1Si1.5Fe and Al7.1Si0.75Fe0.75Mn alloy melts in contact with Al2O3 and Al2O3-C filter substrate material during a melt conditioning treatment at 620 ℃.The microstructures,in particular the primary intermetallic par-ticles αc and αh,have been characterized by computed tomography(CT)and supplementary scanning electron microscopy(SEM).As expected by thermodynamics,the total volume fraction of primary par-ticles remains unchanged by the interaction with the substrate materials.However,kinetic advantages for Fe-removal efficiency can be achieved by an accelerated and preferred selective particle formation in contact with the Al2O3-C material.Furthermore,particle formation is discussed in view of its different stages:nucleation,growth,and ripening.
查看更多>>摘要：Porous ultra-high temperature ceramics(UHTCs)are recognized as novel candidates for fulfilling the requirements of thermal protection systems of hypersonic aircrafts,as they possess excellent high-temperature resistance and low thermal conductivity.Currently,the reported porous UHTCs predomi-nantly exhibit an open pore structure.By contrast,closed-cell UHTCs,formed by employing ceramic hol-low microspheres(HMs)as pore-forming agents,hold great potential for achieving superior thermal insu-lation performance.Unfortunately,the implementation of this strategy has been hindered by the scarcity of raw materials and preparation techniques.In this paper,ZrC-SiC closed-cell ceramics were first successfully prepared through a combination of tape casting and chemical vapor infiltration(CVI)techniques,utilizing the self-developed ZrC HMs as the primary raw material.The morphology,microstructure,and thermal insulation properties of the obtained ZrC-SiC closed-cell ceramics were investigated.The results indicate that when the content of ZrC HMs is 30 vol.％,the density of the prepared porous ceramics is 2.09 g cm-3,with a closed porosity of 14.05％and a thermal conductivity of 1.69 W(m K)-1.The results clearly prove that the CVI process can success-fully convert ZrC HMs into closed pore structures within porous ceramics.The introduction of ZrC HMs suppresses the contribution of free electrons to thermal conductivity and brings about a large number of solid-gas interfaces,which increases the interfacial thermal resistance and significantly reduces the phonon thermal conductivity.Consequently,the as-prepared ZrC-SiC closed-cell ceramics show excellent thermal insulation properties.This study provides a new idea and method for the development of porous UHTCs and offers a more reliable material choice for thermal protection systems.
查看更多>>摘要：In addition to the three well-known Ag-related precipitates(Ω,X'and Z)in the Al-Cu-Mg-Ag alloys,Ag can also be involved in the formation of the as-cast second phases.However,the effect of Ag ad-dition in Al-Cu-Mg-Ag alloys has not been completely studied and even the structure of the as-cast Ag-containing phases is still controversial.By employing the focused ion beam(FIB)combined with transmis-sion electron microscopy(TEM)techniques and density functional theory(DFT)calculations,the forma-tion mechanisms of the Ag-containing phases in the as-cast Al-Cu-Mg-Ag alloys have been investigated.The Ag-containing phases are a series of hexagonal C14-type Laves phases with continuously varying Ag concentrations,described as(AlxCuyAg1-x-y)2Mg.Moreover,the specific occupancy sites of the atoms in(AlxCuyAg1-x-y)2Mg were determined.The formation of the(AlxCuyAg1-x-y)2Mg can be attributed to the stronger Ag-induced aggregation of solute atoms in the initial stage and the establishment of strong Ag-X(X=Al,Mg and Ag)bonding in the Ag-containing phases.Furthermore,our experiments have revealed the solidification sequence of Al-Cu-Mg-Ag alloys,and pointed out that(AlxCuyAg1-x-y)2Mg is formed at a lower temperature(493.9 ℃)through the reaction L⇌Al2CuMg+(AlxCuyAg1-x-y)2Mg.The study could have positive implications for refinement of the Al-Cu-Mg-Ag quaternary phase diagram and promote the composition-property design of novel aluminum alloys based on(AlxCuyAg1-x-y)2Mg in the future.
NETL
NSTL
万方数据