查看更多>>摘要：Discontinuously reinforced titanium matrix composites(DRTMCs)with a network structure have been extensively researched due to their superior combination of strength and ductility.However,their fatigue performance has remained unknown.In order to elucidate the fatigue behavior of DRTMCs,a tension-tension fatigue test was performed on a TiB/near α-Ti compos-ite with network structure.The results showed that the variability of fatigue lifetime increased as the stress level decreased.Fractography analysis indicated that fatigue crack initiation was associated with facet formation,while the subsequent propa-gation was hindered by the network structure comprising TiB whiskers and silicides.Crystallographic characterization further revealed that facets formed due to a combination of shear and normal stress.The reduction in fatigue lifetime was attributed to an increase in the effective slip length,which was influenced by the orientation of grains near the crack-initiation sites toward basal slip in the life-limiting specimen.Quasi in situ observation suggested that the crack initiation was facilitated by both basal and prismatic slip of α-Ti as well as fracture of TiBw.Crack propagation was found to be associated with basal and prismatic slip systems with high Schmid factors,regardless of whether the crack was intergranular or intragranular.
查看更多>>摘要：Oxygen is crucial in influencing the microstructure evolution of selective laser melted(SLMed)Ti-6Al-4V,significantly impacting its applicability in various sectors.Therefore,this study investigates the influnce of oxygen on microstructure evolution,particularly α'martensite transformation and refinement mechanisms.Four alloys,Ti-6Al-4V-xO(x=0.11,0.16,0.21,and 0.25 wt％),were fabricated by the SLM process.The martensite start temperature(Ms)of Ti-6Al-4V,as evaluated by computation,is 656.8 ℃,and oxygen was found to increase the Ms by about 10 ℃ per 0.1 wt％.The SLMed alloy samples exhibit[001]β growth texture along the build direction.Crystallographic analysis of martensite morphology suggests internal twinning on {10(1)1} planes as the lattice invariant strain,which becomes more predominant with increasing oxygen content.Refinement of α'martensite plates by oxygen is due to increased lattice distortion,reduced prior β grain size,and oxygen seg-regation to β grain boundaries.Our findings contribute to improving the understanding of the effect of oxygen on the trans-formation mechanism of α'martensite during SLM of Ti-6Al-4V.
查看更多>>摘要：Submerged friction stir processing(SFSP)with flowing water was employed to alleviate the porosities and coarse-grained structure introduced by wire-arc manufacturing.As a result,uniform and ultrafine grained(UFG)structure with average grain size of 0.83 μm was achieved with the help of sharply reduced heat input and holding time at elevated temperature.The optimized UFG structure enabled a superior combination of strength and ductility with high ultimate tensile strength and elongation of 273.17 MPa and 15.39％.Specifically,grain refinement strengthening and decentralized θ(Al2Cu)phase in the sample subjected to SFSP made great contributions to the enhanced strength.In addition,the decrease in residual stresses and removal of pores substantially enhance the ductility.High rates of cooling and low temperature cycling,which are facilitated by the water-cooling environment throughout the machining process,are vital in obtaining superior microstructures.This work provides a new method for developing a uniform and UFG structure with excellent mechanical properties.
查看更多>>摘要：Rapid advancements in the aerospace industry necessitate the development of unified,lightweight and thermally conductive structures.Integrating complex geometries,including bionic and porous structures,is paramount in thermally conductive structures to attain improved thermal conductivity.The design of two high-porosity porous lattice structures was inspired by pomelo peel structure,using Voronoi parametric design.By combining characteristic elements of two high-porostructuressity porous lattice structures designed,a novel high-porosity porous gradient structure is created.This structure is based on gradient design.Utilizing selective laser melting(SLM),fabrication comprises three.Steady-state thermal characteristics are evaluated via finite element analysis(FEA).The experimental thermal conductivity measurements correlate well with simulation results,validating the sequence of K_L as the highest,followed by D_K_L and then D_L.Heat treatment sig-nificantly improves thermal conductivity,enhancing the base material by about 45.6％and porous structured samples by approximately 43.7％.
查看更多>>摘要：As a type of austenitic stainless steel,316L stainless steel has excellent plasticity,corrosion resistance,and biocompatibil-ity,making it widely used in industries,especially in the marine environments.However,its lower yield strength and wear resistance are the obvious disadvantages that restrict its application in more fields.In this work,an Fe-based amorphous alloy(Feam)was selected as reinforcement to enhance the 316L stainless steel prepared by selective laser melting(SLM),and microstructure evolution,mechanical properties,tribological and corrosion performance of the SLMed samples were investigated in detail.The relative density values of both 316L stainless steel and Feam-reinforced samples are above 99％,which suggests that Feam-reinforced samples also have outstanding formability.In the as-etched micrograph,all of the SLMed samples exhibit cellular structure.Feam-reinforced samples have thicker sub-grain boundaries,and retained amorphous phase can be observed in the samples reinforced with 10 wt％and 15 wt％Feam.As the addition of Feam increases,the microhard-ness and compression strength of the Feam-reinforced samples gradually improve and reach 449.2 HV and 2181.9 MPa,respectively.The wear morphologies show that the 316L stainless steel and Feam-reinforced samples both experience abrasive wear and corrosion wear in a 3.5 wt％NaCl solution.Meanwhile,as the amount of Feam added increases,the coefficient of friction and wear rate of SLMed samples gradually decrease.Compared to the unreinforced sample,Feam-reinforced samples have lower corrosion current density and higher pitting potential according to the potentiodynamic polarization curves and also exhibit superior corrosion resistance in the salt spray environment.This work suggests that the addition of Fe-based amorphous alloy can improve the mechanical properties and wear resistance of 316L stainless steel,as well as its ability to withstand salt spray corrosion.
查看更多>>摘要：The surface spinning strengthening(3S)mechanism and fatigue life extension mechanism of 316L stainless steel welded joint were systematically elucidated by microstructural analyses and mechanical tests.Results indicate that surface gradient hardening layer of approximately 1 mm is formed in the base material through grain fragmentation and deformation twin strengthening,as well as in the welding zone composed of deformed 6-phases and nanotwins.The fatigue strength of welded joint after 3S significantly rises by 32％(from 190 to 250 MPa),which is attributed to the effective elimination of surface geometric defects,discrete refinement of 6-Fe phases and the appropriate improvement in the surface strength,collectively mitigating strain localization and surface fatigue damage within the gradient strengthening layer.The redistributed fine δ-Fe phases benefited by strong stress transfer of 3S reduce the risk of surface weak phase cracking,causing the fatigue fracture to transition from microstructure defects to crystal defects dominated by slip,further suppressing the initiation and early propagation of fatigue cracks.
查看更多>>摘要：The investigation concentrates on friction stir welded(FSW)Al-Cu-Li alloy concerning its local microstructural evolution and mechanical properties.The grain features were characterized by electron back scattered diffraction(EBSD)technology,while precipitate characterization was conducted by using transmission electron microscopy(TEM)aligned along[011]Al and[001]Al zone axes.The mechanical properties are evaluated through micro-hardness and tensile testing.It can be found that nugget zones exhibit finely equiaxed grains evolved through complete dynamic recrystallization(DRX),primarily occurring in continuous dynamic recrystallization(CDRX)and discontinuous dynamic recrystallization(DDRX).In the thermal-mechanically affected zone(TMAZ),numerous sub-structured grains,exhibiting an elongated morphology,were created due to partial DRX,signifying the dominance of CDRX,DDRX,and geometric dynamic recrystallization(GDRX)in this region.T1 completely dissolves in the nugget zone(NZ)leading to the formation of Guinier-Preston zones and increase of δ',β'and S'.Conversely,T1 partially solubilizes in TMAZ,the lowest hardness zone(LHZ)and heat affected zone(HAZ),and the residual T1 undergoes marked coarsening,revealing various T1 variants.The solubilization and coarsening of T1 are primary contributors to the degradation of hardness and strength.θ'primarily dissolves and coarsens in NZ and TMAZ,whilst this precipitate largely coarsens in HAZ and LHZ.σ,TB,grain boundary phases(GBPs)and precipitate-free zone(PFZ)are newly generated during FSW.o exists in the TMAZ,LHZ and HAZ,whereas TB nucleates in NZ.GBPs and PFZ mostly develop in LHZ and HAZ,which can cause strain localization during tensile deformation,potentially leading to LHZ joint fracture.
查看更多>>摘要：This research studies the effect of Mn-Co-La2O3 coating synthesized by the electrodeposition method on the oxidation resistance and electrical conductivity of the Crofer 22 APU stainless steel interconnect plates in solid oxide fuel cells.The test samples were characterized by a field emission scanning electron microscope(FESEM)equipped with energy dispersive spectroscopy(EDS),X-ray diffraction(XRD),and X-ray photoelectron spectroscopy(XPS).The oxidation kinetics of the coated and uncoated samples were studied by tracking their weight changes over time at 800 ℃,showing that the oxidation mechanism for all samples follows the parabolic law.Lower oxidation rate constant(kp)values of the coated sample com-pared with that of the uncoated one indicated a reduction in the oxidation rate of the steel substrate in the presence of the Mn-Co-La2O3 coating.The examination of the cross-section of different samples after the isothermal oxidation for 500 h at 800 ℃ exhibited that applying the composite coating leads to a decrease in the thickness of the chromia layer formed on the steel surface.Furthermore,under these conditions,the area-specific resistance(ASR)of the coated sample(13.11 mΩ cm2)is significantly lower than that of the uncoated one(41.45 mΩ cm2).
查看更多>>摘要：The impurity iron in silicon material will seriously affect the photoelectric conversion efficiency of silicon solar cells.How-ever,the traditional silicon purification method has the disadvantages of long cycle,high energy consumption and serious pollution.In this study,an efficient and green pulsed electric current purification technology is proposed.The electromigra-tion effect of iron elements,the current density gradient driving of iron phase,and the gravity of iron phase all affect the migration behavior of iron phase in silicon melt under pulsed electric current.Regardless of the depth of electrode insertion into the silicon melt,the solubility of iron in silicon decreases under the pulsed electric current,which helps to form the iron phase.At the same time,the iron phase tends to sink toward the bottom under the influence of gravity.When the electrode is shallowly inserted,a non-uniform electric field is formed in the silicon melt,and the iron phase is mainly driven by the current density gradient to accelerate sink toward the bottom.When the electrode is fully inserted,an approximately uniform electric field is formed in the silicon melt,and iron elements are preferentially migrated to the cathode by electromigration,forming iron phase sinking at the cathode.The study of impurity iron migration behavior in silicon melt under pulsed electric current provides a new approach for the purification of polycrystalline silicon.
查看更多>>摘要：The magnetic field is an effective means to control the solidification structure and the defects of metal and semiconductor crystals.This work investigates the effects of Cusp magnetic field(CMF)and longitudinal magnetic field(LMF)on the stray-crystal formation in the platform regions during the directional solidification of single-crystal superalloy with the different cross section sizes.The application of CMF reduces the formation of platform stray-crystal,while LMF increases its genera-tion.As the platform size increases,the stray-crystal ratio increases regardless of whether the magnetic fields are applied or not,the effectiveness of CMF increases,while that of LMF decreases.The reason that the effects of CMF and LMF on the platform stray-crystal formation could be attributed to the change of flow structure from the distribution characteristics of the thermoelectric magnetic force and the magnetic damping force near the liquid-solid interface.
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