A crystal plasticity finite element(CPFE)model was established and 2D simulations were carried out to study the relationship between microvoids and the microplasticity deformation behavior of the dual-phase titanium a...A crystal plasticity finite element(CPFE)model was established and 2D simulations were carried out to study the relationship between microvoids and the microplasticity deformation behavior of the dual-phase titanium alloy under high cyclic loading.Results show that geometrically necessary dislocations(GND)tend to accumulate around the microvoids,leading to an increment of average GND density.The influence of curvature in the tip plastic zone(TPZ)on GND density is greater than that of the size of the microvoid.As the curvature in TPZ and the size of the microvoid increase,the cumulative shear strain(CSS)in the primaryα,secondaryα,andβphases increases.Shear deformation in the prismatic slip system is dominant in the primaryαphase.As the distance between the microvoids increases,the interactive influence of the microvoids on the cumulative shear strain decreases.展开更多
Based on the bulk free energy density and the degenerate mobility constructed by the quartic double-well potential function,a phase field model is established to simulate the evolution of intragranular microvoids due ...Based on the bulk free energy density and the degenerate mobility constructed by the quartic double-well potential function,a phase field model is established to simulate the evolution of intragranular microvoids due to surface diffusion in a stress field.The corresponding phase field governing equations are derived.The evolution of elliptical microvoids with different stressesΛ,aspect ratiosβand linewidths hˉis calculated using the mesh adaptation finite element method and the reliability of the procedure is verified.The results show that there exist critical values of the stressΛc,the aspect ratioβc and the linewidth hˉc of intragranular microvoids under equivalent biaxial tensile stress.When Λ≥Λ_(c),β≥β_(c) or h≤h_(c),the elliptical microvoids are instable with an extending crack tip.WhenΛ<Λ_(c),β<β_(c) or hˉ>h_(c),the elliptical microvoids gradually cylindricalize and remain a stable shape.The instability time decreases with increasing the stress or the aspect ratio,while increases with increasing the linewidth.In addition,for the interconnects containing two elliptical voids not far apart,the stress will promote the merging of the voids.展开更多
Tensile deformation and microvoid formation of quenched and tempered SA508 Gr.3 steel were studied using an in-situ digital image correlation technique and in-situ electron backscatter diffraction(EBSD)measurements.Th...Tensile deformation and microvoid formation of quenched and tempered SA508 Gr.3 steel were studied using an in-situ digital image correlation technique and in-situ electron backscatter diffraction(EBSD)measurements.The quenched steel with a mixture of up-per bainite and granular bainite exhibited a high ultimate tensile strength(UTS)of~795 MPa and an elongation of~25%.After temper-ing,long-rod carbides and accumulated carbide particles were formed at the interface of bainite–ferrite subunits and prior austenite grain boundaries(PAGBs),respectively.The UTS of the tempered steel decreased to~607 MPa,whereas the total elongation increased to 33.0%with a local strain of 191.0%at the necked area.In-situ EBSD results showed that strain localization in the bainite–ferrite pro-duced lattice rotation and dislocation pileup,thus leading to stress concentration at the discontinuities(e.g.,martensite–austenite islands and carbides).Consequently,the decohesion of PAGBs dotted with martensite–austenite islands was the dominant microvoid initiation mechanism in the quenched steel,whereas microvoids primarily initiated through the fracturing of long-rod carbides and the decohesion of PAGBs with carbides aggregation in the tempered steel.The fracture surfaces for both the quenched and tempered specimens featured dimples,indicating the ductile failure mechanism caused by microvoid coalescence.展开更多
The statistical evolution of microvoids under high stress triaxiality is investigated. Based on the expression for the void growth rate in a power-law viscous matrix and the balance law of microvoids’ number, the evo...The statistical evolution of microvoids under high stress triaxiality is investigated. Based on the expression for the void growth rate in a power-law viscous matrix and the balance law of microvoids’ number, the evolution of microvoids’ number density under dynamic loading is studied. Thus, the si2E distribution functions of microvoids are found from the theoretical analysis, and the effect of strain rate-sensitivity of the matrix on the evolution of microvoids is examined. The present theoretical analysis may provide a reasonable explanation for the experimental phenomena observed by previous researchers.展开更多
Miniaturized spectrometers with high resolving power and cost-effectiveness are desirable but remain an open challenge.In this work,we repurpose a fiber generated by the catastrophic fuse effect and ingeniously harnes...Miniaturized spectrometers with high resolving power and cost-effectiveness are desirable but remain an open challenge.In this work,we repurpose a fiber generated by the catastrophic fuse effect and ingeniously harness it for a speckle-based computational spectrometer.Without complex disorder engineering,the axially random micro-cavities in the fused fiber enhance the wavelength sensitivity of multimode interference,enabling a 10 cm fiber to achieve a spectral resolution of 0.1 nm.This performance exhibits sixfold improvement over a common multimode fiber configuration of the same length.Furthermore,we develop a spectral reconstruction method that combines a weighted transmission matrix with automatic differentiation,which reduces the reconstruction error by approximately half and enhances the peak signal-to-noise ratio by 6.12 dB compared to traditional Tikhonov regularization.Spectra spanning a 40 nm range,exhibiting both sparse and dense characteristics,are accurately reconstructed.To the best of our knowledge,this represents the first application of fused fiber in computational spectrometers,demonstrating its potential for a wide range of spectral measurement scenarios.展开更多
The expansion property of cement mortar under the attack of sulfate ions is studied by experimental and theoretical methods. First, cement mortars are fabricated with the ratio of water to cement of 0.4, 0.6, and 0.8....The expansion property of cement mortar under the attack of sulfate ions is studied by experimental and theoretical methods. First, cement mortars are fabricated with the ratio of water to cement of 0.4, 0.6, and 0.8. Secondly, the expansion of specimen immerged in sulphate solution is measured at different times. Thirdly, a theoretical model of expansion of cement mortar under sulphate erosion is suggested by virtue of represent volume element method. In this model, the damage evolution due to the interaction between delayed ettringite and cement mortar is taken into account. Finally, the numerical calculation is results indicate that the model perfectly describes performed. The numerical and experimental the expansion of the cement mortar.展开更多
An improved three-dimensional （3-D） experimental visualization methodology is presented tor evaluating the fracture mechanisms of ferritic stainless steels by in-situ tensile testing with an environmental scanning e...An improved three-dimensional （3-D） experimental visualization methodology is presented tor evaluating the fracture mechanisms of ferritic stainless steels by in-situ tensile testing with an environmental scanning electron microscope （ESEM）. The samples were machined with a radial notched shape and a sloped surface. Both planar surface deformation and sloping surface deformation-induced microvoids were observed during dynamic tension experiments, where a greater amount of information could be obtained from the sloping surface. The results showed that microvoids formed at the grain boundaries of highly elongated large grains. The microvoids nucleated in the severely deformed regions grew nearly parallel to the tensile axis, predominantly along the grain boundaries. The microvoids nucleated at the interface of particles and the matrix did not propagate due to the high plasticity of the matrix. The large microvoids propagated and showed a zigzag shape along the grain boundaries,seemingly a consequence of the fracture of the slip bands caused by dislocation pile-ups. The final failure took place due to the reduction of the load-beating area.展开更多
The microcrystalline structure and microvoid structure in carbon fibers during graphitization process （2300-2700 ℃） were characterized employing laser micro-Raman scattering （Raman）, X-ray diffraction （XRD）, sm...The microcrystalline structure and microvoid structure in carbon fibers during graphitization process （2300-2700 ℃） were characterized employing laser micro-Raman scattering （Raman）, X-ray diffraction （XRD）, small angle X-ray scattering （SAXS）, and high-resolution transmission electron microscopy （HR-TEM）. The crystalline sizes （La, Lc） increased and interlayer spacing （d002） decreased with increasing heat treatment temperature （HTT）. The microvoids in the fibers grew up and contacted to the neighbors with the development of microcrystalline. In addition, the preferred orientation of graphite crystallite along fiber axis decreased and microvoids increased. The results are crucial for analyzing the evolution of microstructure of carbon fibers in the process of heat treatment and important for the preparation of high strength and high modulus carbon fibers.展开更多
Two kinds of C-Si-Mn-Cr series tested steels were designed to obtain dual phase microstructures of ferrite （F） ＋martcnsite （M） or ferrite （F）-bainite （B） with different mechanical properties. Effects of stren...Two kinds of C-Si-Mn-Cr series tested steels were designed to obtain dual phase microstructures of ferrite （F） ＋martcnsite （M） or ferrite （F）-bainite （B） with different mechanical properties. Effects of strengthening phase on yielding and fracture behaviours during uniaxial tension of dual phase steel were discussed. Compared with hot-rolled martensite dual phase steel, ferrite-bainite dual phase steel has high ratio of yield strength to tensile strength （YS/TS） and low elongation. During necking process of uniaxial tension, microvoids of ferrite-martensite steel are generated by fracture of ferrite/martensite boundary or martensite islands with irregular shape. But ferrite matrix elongated remarkably along deformation direction, and strengthening phase also coordinated with ferrite matrix. Compatible de formation between ferrite and bainite is distinct. Ferrite-bainite dual phase steel has fine and less microvoid, and phase boundary of ferrite and bainite is beneficial for restraining generation and extending of microvoid.展开更多
The polyurethane/polyacrylonitrile (PU/PAN) and polyurethane/cellulose acetate (PU/CA) blend ultra filtration membranes were prepared based on Loeb-Sourirajan phase transition method. The change of the structures and ...The polyurethane/polyacrylonitrile (PU/PAN) and polyurethane/cellulose acetate (PU/CA) blend ultra filtration membranes were prepared based on Loeb-Sourirajan phase transition method. The change of the structures and properties of the PU/PAN and PU/CA membranes with the heat treatment process was studied. The results showed: the water flux decreased and retention increased with the increase of heat treatment temperature of PU/PAN blend membrane, but the water flux of PU/CA blend membrane got the maximum with heat treatment temperature of 60℃ and decreased rapidly with the heat treatment temperature of 100 ℃. The interfacial microvoid structure and its influence on the properties of PU/PAN and PU/CA blend membranes were studied.展开更多
The combined effects of void size and void shape on the void growth are studied by using the classical spectrum method. An infinite solid containing an isolated prolate spheroidal void is considered to depict the void...The combined effects of void size and void shape on the void growth are studied by using the classical spectrum method. An infinite solid containing an isolated prolate spheroidal void is considered to depict the void shape effect and the Fleck-Hutchinson phenomenological strain gradient plasticity theory is employed to capture the size effects. It is found that the combined effects of void size and void shape are mainly controlled by the remote stress triaxiality. Based on this, a new size-dependent void growth model similar to the Rice-Tracey model is proposed and an important conclusion about the size-dependent void growth is drawn： the growth rate of the void with radius smaller than a critical radius rc may be ignored. It is interesting that rc. is a material constant independent of the initial void shape and the remote stress triaxiality.展开更多
The origin of low ductility at elevated tempera- ture in the cryogenic and non-magnetic steel Fe-23Mn-4Al-5Cr-0.3C was investigated by means of impact test at elevated temperatures,opti- cal microscope,SEM and TEM.The...The origin of low ductility at elevated tempera- ture in the cryogenic and non-magnetic steel Fe-23Mn-4Al-5Cr-0.3C was investigated by means of impact test at elevated temperatures,opti- cal microscope,SEM and TEM.The experimental results indicate that the impact toughness lowers with increasing testing temperature,and exhibits a low trough of ductility from 865℃ to 1050℃ and a peak value at 1150℃.The intergranular fracture occurs in the temperature region of low ductility and the transgranular ductile rupture below this re- gion principally.There is a process of precipitating, gathering,growing,and dissolution of carbide (FeMnCr)_(23)C_6 and AlN between 500℃ and 1000℃.The coalescence of microvoids caused by the process at grain boundaries results in the intergranular fracture and the low ductility at ele- vated temperatures.展开更多
The effect of externally applied mechanical pre-conditioning, i.e. pre-impact treatment, on the fracture resistance was investigated for polypropylene （PP）. Impact strength was obtained via notched and/or unnotched ...The effect of externally applied mechanical pre-conditioning, i.e. pre-impact treatment, on the fracture resistance was investigated for polypropylene （PP）. Impact strength was obtained via notched and/or unnotched samples. It has been shown that the pre-impact treatment is favorable to the improvement of the fracture resistance. The impact strength increases linearly with the applied pre-impact energy. Both optical and SEM results show that there are at least two possible mechanisms for the improvement of the fracture resistance. One is the crack blunting effect which is introduced to notched sample by pre-impact treatment, reducing the sensitivity of PP to the applied notch. The other is the formation of large amount of microvoids induced by pre-impact treatment, which changes the stress distribution and induces intensive plastic deformation of PP at the second impact measurement, leading to the improvement of the fracture resistance.展开更多
In this paper,we report the variation of defective structure of nanocrystalline Fe-Mo-Si-B alloys with the increase of grain size.Results suggest that for the average grain size D=55 nm,microvoid-sized defects exhibit...In this paper,we report the variation of defective structure of nanocrystalline Fe-Mo-Si-B alloys with the increase of grain size.Results suggest that for the average grain size D=55 nm,microvoid-sized defects exhibit maximum volume and minimum concentration.展开更多
In this paper,morphological structure,thermodynamic compatibility and relationship be-tween porosities and blend ratios of the wet coagulated PU/PVA sheets were studied by meansof observation of scanning electronic mi...In this paper,morphological structure,thermodynamic compatibility and relationship be-tween porosities and blend ratios of the wet coagulated PU/PVA sheets were studied by meansof observation of scanning electronic micrographies,tests of dynamic mechanical properties,moisture regain and vapor permeability.Furthermore,the formation of microvoids at interfacesof separated phases was discussed and a microvoid formation mechanism at phase interfaces ofhydrophobic-hydrophilic blends was suggested and compared with that ofhydrophobic-hydrophobic blends.展开更多
Spalling is a typical tensile failure that results from the coupling evolution of microstructure and microdamage under high strain-rate loading.To understand the spalling damage behavior of polycrystalline materials a...Spalling is a typical tensile failure that results from the coupling evolution of microstructure and microdamage under high strain-rate loading.To understand the spalling damage behavior of polycrystalline materials at mesoscale,this paper develops a spalling model by integrating the crystal plasticity theory and the microvoid growth theory.The model is implemented in ABAQUS simulation via the VUMAT subroutine to simulate a planar impact process of copper,and the results are compared with experimental data.Due to the inhomogeneity of crystal plastic slip,the local stress fluctuates severely near the grain boundary.Therefore,without introducing the fluctuation in the threshold stress for microdamage evolution,this model can simulate the heterogeneous feature of microvoid nucleation,growth,and coalescence in materials.The results show that microvoids tend to nucleate at 25°–50°misorientation angle grain boundaries,which undergo a high probability of stress fluctuation.展开更多
As a virtual experimental device for analysis and calculation of grown-in microdefects formation in undoped silicon dislocation-free single crystals the software is proposed. The software is built on the basis on diff...As a virtual experimental device for analysis and calculation of grown-in microdefects formation in undoped silicon dislocation-free single crystals the software is proposed. The software is built on the basis on diffusion model of formation, growth and coalescence of grown-in microdefects. Diffusion model describes kinetics of defect structure changes during cooling after growth on crystallization temperature to room temperature. The software allows the use of personal computer to investigate the defect structure of dislocation-free silicon single crystals with a diameter on 30 mm to 400 mm grown by floating-zone and Czochralski methods.展开更多
Hydrogenated amorphous silicon(a-Si:H) films were deposited by reactive facing target sputtering(FTS) technique with a mixture of Ar and H2 reaction gas.Fourier transform infrared(FTIR) absorption,Raman scattering and...Hydrogenated amorphous silicon(a-Si:H) films were deposited by reactive facing target sputtering(FTS) technique with a mixture of Ar and H2 reaction gas.Fourier transform infrared(FTIR) absorption,Raman scattering and ultraviolet-visible optical absorption are used to investigate the microstructure and optical properties of the deposited films.The decrease of the concentration of bonded hydrogen,especially that of(Si-H2)n with increasing substrate temperature(Ts),was observed in FTIR spectra,suggesting the atomic density increases and the concentration of microvoids decrease in a-Si:H films.The increase of both the short range order and the intermediate range order of amorphous network for a-Si:H films were verified by Raman scattering spectra,in which increasing Ts decreasing the band width of TO and the scattering intensity ratio ITA/ITO were obtained.All above results clarify the effect of increasing Ts on the microstructure amelioration for a-Si:H films.The reduction of disordered domains is correlated with the film growing process,where the increased surface diffusion mobility and etching of weak bonds is induced by increasing Ts.Furthermore,analysis of optical absorption indicates that the films with a lower optical band gap and a narrower band edge can be obtained by this FTS technique.展开更多
Superplastic behaviors of quasicrystal phase containing Mg-5.8Zn-1Y-0.48Zr alloy sheets fabricated by combination of extrusion and hot-rolling processes have been investigated at temperature ranging from 623 to 753 K ...Superplastic behaviors of quasicrystal phase containing Mg-5.8Zn-1Y-0.48Zr alloy sheets fabricated by combination of extrusion and hot-rolling processes have been investigated at temperature ranging from 623 to 753 K and at the strain rates ranging from 10-4 to 10-2 s-1 by uniaxial tensile tests. An excellent superplasticity with the maximum elongation to failure of 1020% was obtained at 753 K and the strain rate of 1.04×10-3 s-1 and its strain rate sensitivity, m, is as high as up to 0.75. The microstructure was stable during superplastic deformation due to the uniformly distributed fine quasicrystal particles. In addition, micro-cavities and their coalescences were observed in the superplastic deformation of the ZW61 magnesium alloy. Grain boundary sliding (GBS) was considered to be the main deformation mechanism during the superplastic deformation. Dislocation creep controlled by atom diffusion through grain boundaries or interior grains is suggested mainly to accommodate the GBS in super-plastic deformation.展开更多
基金the National Key Research and Development Program of China(No.2021YFB3702603).
文摘A crystal plasticity finite element(CPFE)model was established and 2D simulations were carried out to study the relationship between microvoids and the microplasticity deformation behavior of the dual-phase titanium alloy under high cyclic loading.Results show that geometrically necessary dislocations(GND)tend to accumulate around the microvoids,leading to an increment of average GND density.The influence of curvature in the tip plastic zone(TPZ)on GND density is greater than that of the size of the microvoid.As the curvature in TPZ and the size of the microvoid increase,the cumulative shear strain(CSS)in the primaryα,secondaryα,andβphases increases.Shear deformation in the prismatic slip system is dominant in the primaryαphase.As the distance between the microvoids increases,the interactive influence of the microvoids on the cumulative shear strain decreases.
基金supported by the Natural Science Foundation of Jiangsu Province of China (No. BK20141407)the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions。
文摘Based on the bulk free energy density and the degenerate mobility constructed by the quartic double-well potential function,a phase field model is established to simulate the evolution of intragranular microvoids due to surface diffusion in a stress field.The corresponding phase field governing equations are derived.The evolution of elliptical microvoids with different stressesΛ,aspect ratiosβand linewidths hˉis calculated using the mesh adaptation finite element method and the reliability of the procedure is verified.The results show that there exist critical values of the stressΛc,the aspect ratioβc and the linewidth hˉc of intragranular microvoids under equivalent biaxial tensile stress.When Λ≥Λ_(c),β≥β_(c) or h≤h_(c),the elliptical microvoids are instable with an extending crack tip.WhenΛ<Λ_(c),β<β_(c) or hˉ>h_(c),the elliptical microvoids gradually cylindricalize and remain a stable shape.The instability time decreases with increasing the stress or the aspect ratio,while increases with increasing the linewidth.In addition,for the interconnects containing two elliptical voids not far apart,the stress will promote the merging of the voids.
基金financially supported by the National Natural Science Foundation of China(No.52171042),the National Key Research and Development Program of China(No.2023YFB3406804)the“Jianbing”R&D Program of Zhejiang Province,China(No.2023C01081)the Na-tional Engineering Research Center for Advanced Manufac-turing Technology and Equipment of Heavy Castings and Forgings(Erzhong(Deyang)Heavy Equipment Co.,Ltd.).
文摘Tensile deformation and microvoid formation of quenched and tempered SA508 Gr.3 steel were studied using an in-situ digital image correlation technique and in-situ electron backscatter diffraction(EBSD)measurements.The quenched steel with a mixture of up-per bainite and granular bainite exhibited a high ultimate tensile strength(UTS)of~795 MPa and an elongation of~25%.After temper-ing,long-rod carbides and accumulated carbide particles were formed at the interface of bainite–ferrite subunits and prior austenite grain boundaries(PAGBs),respectively.The UTS of the tempered steel decreased to~607 MPa,whereas the total elongation increased to 33.0%with a local strain of 191.0%at the necked area.In-situ EBSD results showed that strain localization in the bainite–ferrite pro-duced lattice rotation and dislocation pileup,thus leading to stress concentration at the discontinuities(e.g.,martensite–austenite islands and carbides).Consequently,the decohesion of PAGBs dotted with martensite–austenite islands was the dominant microvoid initiation mechanism in the quenched steel,whereas microvoids primarily initiated through the fracturing of long-rod carbides and the decohesion of PAGBs with carbides aggregation in the tempered steel.The fracture surfaces for both the quenched and tempered specimens featured dimples,indicating the ductile failure mechanism caused by microvoid coalescence.
基金Laboratory for Nonlinear Mechanics of Continuous Media(Institute of Mechanics,Chinese Academy of Sciences)Doctoral Program Foundation of the State Education Commission of China.
文摘The statistical evolution of microvoids under high stress triaxiality is investigated. Based on the expression for the void growth rate in a power-law viscous matrix and the balance law of microvoids’ number, the evolution of microvoids’ number density under dynamic loading is studied. Thus, the si2E distribution functions of microvoids are found from the theoretical analysis, and the effect of strain rate-sensitivity of the matrix on the evolution of microvoids is examined. The present theoretical analysis may provide a reasonable explanation for the experimental phenomena observed by previous researchers.
基金National Natural Science Foundation of China(62305391)Scientific Fund of National University of Defense Technology(22-061,BC-03)Postgraduate Scientific Research Innovation Project of Hunan Province(XJJC2024016)。
文摘Miniaturized spectrometers with high resolving power and cost-effectiveness are desirable but remain an open challenge.In this work,we repurpose a fiber generated by the catastrophic fuse effect and ingeniously harness it for a speckle-based computational spectrometer.Without complex disorder engineering,the axially random micro-cavities in the fused fiber enhance the wavelength sensitivity of multimode interference,enabling a 10 cm fiber to achieve a spectral resolution of 0.1 nm.This performance exhibits sixfold improvement over a common multimode fiber configuration of the same length.Furthermore,we develop a spectral reconstruction method that combines a weighted transmission matrix with automatic differentiation,which reduces the reconstruction error by approximately half and enhances the peak signal-to-noise ratio by 6.12 dB compared to traditional Tikhonov regularization.Spectra spanning a 40 nm range,exhibiting both sparse and dense characteristics,are accurately reconstructed.To the best of our knowledge,this represents the first application of fused fiber in computational spectrometers,demonstrating its potential for a wide range of spectral measurement scenarios.
基金supported by the National Natural Science Foundation of China(No.10572064)the National Basic Research Program of China(973 Program,2009CD623203)+1 种基金K.C.Wong Magna Fund in Ningbo Universitythe Natural Science Foundation of Zhejiang Province(No.Y107780).
文摘The expansion property of cement mortar under the attack of sulfate ions is studied by experimental and theoretical methods. First, cement mortars are fabricated with the ratio of water to cement of 0.4, 0.6, and 0.8. Secondly, the expansion of specimen immerged in sulphate solution is measured at different times. Thirdly, a theoretical model of expansion of cement mortar under sulphate erosion is suggested by virtue of represent volume element method. In this model, the damage evolution due to the interaction between delayed ettringite and cement mortar is taken into account. Finally, the numerical calculation is results indicate that the model perfectly describes performed. The numerical and experimental the expansion of the cement mortar.
文摘An improved three-dimensional （3-D） experimental visualization methodology is presented tor evaluating the fracture mechanisms of ferritic stainless steels by in-situ tensile testing with an environmental scanning electron microscope （ESEM）. The samples were machined with a radial notched shape and a sloped surface. Both planar surface deformation and sloping surface deformation-induced microvoids were observed during dynamic tension experiments, where a greater amount of information could be obtained from the sloping surface. The results showed that microvoids formed at the grain boundaries of highly elongated large grains. The microvoids nucleated in the severely deformed regions grew nearly parallel to the tensile axis, predominantly along the grain boundaries. The microvoids nucleated at the interface of particles and the matrix did not propagate due to the high plasticity of the matrix. The large microvoids propagated and showed a zigzag shape along the grain boundaries,seemingly a consequence of the fracture of the slip bands caused by dislocation pile-ups. The final failure took place due to the reduction of the load-beating area.
基金financially supported by the National High Technology Research and Development Program of China(863 Program,No.2015AA03A204)
文摘The microcrystalline structure and microvoid structure in carbon fibers during graphitization process （2300-2700 ℃） were characterized employing laser micro-Raman scattering （Raman）, X-ray diffraction （XRD）, small angle X-ray scattering （SAXS）, and high-resolution transmission electron microscopy （HR-TEM）. The crystalline sizes （La, Lc） increased and interlayer spacing （d002） decreased with increasing heat treatment temperature （HTT）. The microvoids in the fibers grew up and contacted to the neighbors with the development of microcrystalline. In addition, the preferred orientation of graphite crystallite along fiber axis decreased and microvoids increased. The results are crucial for analyzing the evolution of microstructure of carbon fibers in the process of heat treatment and important for the preparation of high strength and high modulus carbon fibers.
基金Sponsored by Fundamental Research Funds for Central Universities of China(FRF-TP-12-046A)Beijing Higher Education Young Elite Teacher Project of China(YETP0355)
文摘Two kinds of C-Si-Mn-Cr series tested steels were designed to obtain dual phase microstructures of ferrite （F） ＋martcnsite （M） or ferrite （F）-bainite （B） with different mechanical properties. Effects of strengthening phase on yielding and fracture behaviours during uniaxial tension of dual phase steel were discussed. Compared with hot-rolled martensite dual phase steel, ferrite-bainite dual phase steel has high ratio of yield strength to tensile strength （YS/TS） and low elongation. During necking process of uniaxial tension, microvoids of ferrite-martensite steel are generated by fracture of ferrite/martensite boundary or martensite islands with irregular shape. But ferrite matrix elongated remarkably along deformation direction, and strengthening phase also coordinated with ferrite matrix. Compatible de formation between ferrite and bainite is distinct. Ferrite-bainite dual phase steel has fine and less microvoid, and phase boundary of ferrite and bainite is beneficial for restraining generation and extending of microvoid.
文摘The polyurethane/polyacrylonitrile (PU/PAN) and polyurethane/cellulose acetate (PU/CA) blend ultra filtration membranes were prepared based on Loeb-Sourirajan phase transition method. The change of the structures and properties of the PU/PAN and PU/CA membranes with the heat treatment process was studied. The results showed: the water flux decreased and retention increased with the increase of heat treatment temperature of PU/PAN blend membrane, but the water flux of PU/CA blend membrane got the maximum with heat treatment temperature of 60℃ and decreased rapidly with the heat treatment temperature of 100 ℃. The interfacial microvoid structure and its influence on the properties of PU/PAN and PU/CA blend membranes were studied.
基金The project supported by the National Natural Science Foundation of China(A10102006)the New Century Excellent Talents in Universities of China.
文摘The combined effects of void size and void shape on the void growth are studied by using the classical spectrum method. An infinite solid containing an isolated prolate spheroidal void is considered to depict the void shape effect and the Fleck-Hutchinson phenomenological strain gradient plasticity theory is employed to capture the size effects. It is found that the combined effects of void size and void shape are mainly controlled by the remote stress triaxiality. Based on this, a new size-dependent void growth model similar to the Rice-Tracey model is proposed and an important conclusion about the size-dependent void growth is drawn： the growth rate of the void with radius smaller than a critical radius rc may be ignored. It is interesting that rc. is a material constant independent of the initial void shape and the remote stress triaxiality.
基金The project was supported by the National Natural Science Foundation of China:Grant No.5870006
文摘The origin of low ductility at elevated tempera- ture in the cryogenic and non-magnetic steel Fe-23Mn-4Al-5Cr-0.3C was investigated by means of impact test at elevated temperatures,opti- cal microscope,SEM and TEM.The experimental results indicate that the impact toughness lowers with increasing testing temperature,and exhibits a low trough of ductility from 865℃ to 1050℃ and a peak value at 1150℃.The intergranular fracture occurs in the temperature region of low ductility and the transgranular ductile rupture below this re- gion principally.There is a process of precipitating, gathering,growing,and dissolution of carbide (FeMnCr)_(23)C_6 and AlN between 500℃ and 1000℃.The coalescence of microvoids caused by the process at grain boundaries results in the intergranular fracture and the low ductility at ele- vated temperatures.
基金supported by the National Natural Science Foundation(No.50973090)Program for New Century Excellent Talents in Universities(NCET-08-0823)Sichuan Youthful Science and Technology Foundation (07ZQ026-003)
文摘The effect of externally applied mechanical pre-conditioning, i.e. pre-impact treatment, on the fracture resistance was investigated for polypropylene （PP）. Impact strength was obtained via notched and/or unnotched samples. It has been shown that the pre-impact treatment is favorable to the improvement of the fracture resistance. The impact strength increases linearly with the applied pre-impact energy. Both optical and SEM results show that there are at least two possible mechanisms for the improvement of the fracture resistance. One is the crack blunting effect which is introduced to notched sample by pre-impact treatment, reducing the sensitivity of PP to the applied notch. The other is the formation of large amount of microvoids induced by pre-impact treatment, which changes the stress distribution and induces intensive plastic deformation of PP at the second impact measurement, leading to the improvement of the fracture resistance.
文摘In this paper,we report the variation of defective structure of nanocrystalline Fe-Mo-Si-B alloys with the increase of grain size.Results suggest that for the average grain size D=55 nm,microvoid-sized defects exhibit maximum volume and minimum concentration.
文摘In this paper,morphological structure,thermodynamic compatibility and relationship be-tween porosities and blend ratios of the wet coagulated PU/PVA sheets were studied by meansof observation of scanning electronic micrographies,tests of dynamic mechanical properties,moisture regain and vapor permeability.Furthermore,the formation of microvoids at interfacesof separated phases was discussed and a microvoid formation mechanism at phase interfaces ofhydrophobic-hydrophilic blends was suggested and compared with that ofhydrophobic-hydrophobic blends.
基金supported by the NSFC(Nos.12172367,11790292,11988102,and U2141204)the Strategic Priority Research Program(Nos.XDB22040302 and XDB22040303).
文摘Spalling is a typical tensile failure that results from the coupling evolution of microstructure and microdamage under high strain-rate loading.To understand the spalling damage behavior of polycrystalline materials at mesoscale,this paper develops a spalling model by integrating the crystal plasticity theory and the microvoid growth theory.The model is implemented in ABAQUS simulation via the VUMAT subroutine to simulate a planar impact process of copper,and the results are compared with experimental data.Due to the inhomogeneity of crystal plastic slip,the local stress fluctuates severely near the grain boundary.Therefore,without introducing the fluctuation in the threshold stress for microdamage evolution,this model can simulate the heterogeneous feature of microvoid nucleation,growth,and coalescence in materials.The results show that microvoids tend to nucleate at 25°–50°misorientation angle grain boundaries,which undergo a high probability of stress fluctuation.
文摘As a virtual experimental device for analysis and calculation of grown-in microdefects formation in undoped silicon dislocation-free single crystals the software is proposed. The software is built on the basis on diffusion model of formation, growth and coalescence of grown-in microdefects. Diffusion model describes kinetics of defect structure changes during cooling after growth on crystallization temperature to room temperature. The software allows the use of personal computer to investigate the defect structure of dislocation-free silicon single crystals with a diameter on 30 mm to 400 mm grown by floating-zone and Czochralski methods.
基金supported by the National Natural Science Foundation of China (Grant No. 60940020)the Natural Foundation of Hebei Province (Grant Nos. E2008000619 and E2009000208)
文摘Hydrogenated amorphous silicon(a-Si:H) films were deposited by reactive facing target sputtering(FTS) technique with a mixture of Ar and H2 reaction gas.Fourier transform infrared(FTIR) absorption,Raman scattering and ultraviolet-visible optical absorption are used to investigate the microstructure and optical properties of the deposited films.The decrease of the concentration of bonded hydrogen,especially that of(Si-H2)n with increasing substrate temperature(Ts),was observed in FTIR spectra,suggesting the atomic density increases and the concentration of microvoids decrease in a-Si:H films.The increase of both the short range order and the intermediate range order of amorphous network for a-Si:H films were verified by Raman scattering spectra,in which increasing Ts decreasing the band width of TO and the scattering intensity ratio ITA/ITO were obtained.All above results clarify the effect of increasing Ts on the microstructure amelioration for a-Si:H films.The reduction of disordered domains is correlated with the film growing process,where the increased surface diffusion mobility and etching of weak bonds is induced by increasing Ts.Furthermore,analysis of optical absorption indicates that the films with a lower optical band gap and a narrower band edge can be obtained by this FTS technique.
基金Supported by the National Basic Research Program of China ("973") (Grant No. 2007CB613704)
文摘Superplastic behaviors of quasicrystal phase containing Mg-5.8Zn-1Y-0.48Zr alloy sheets fabricated by combination of extrusion and hot-rolling processes have been investigated at temperature ranging from 623 to 753 K and at the strain rates ranging from 10-4 to 10-2 s-1 by uniaxial tensile tests. An excellent superplasticity with the maximum elongation to failure of 1020% was obtained at 753 K and the strain rate of 1.04×10-3 s-1 and its strain rate sensitivity, m, is as high as up to 0.75. The microstructure was stable during superplastic deformation due to the uniformly distributed fine quasicrystal particles. In addition, micro-cavities and their coalescences were observed in the superplastic deformation of the ZW61 magnesium alloy. Grain boundary sliding (GBS) was considered to be the main deformation mechanism during the superplastic deformation. Dislocation creep controlled by atom diffusion through grain boundaries or interior grains is suggested mainly to accommodate the GBS in super-plastic deformation.
