It is extremely difficult to strengthen bulk aluminum(Al)by twins,due to its high stacking fault energy under standard loading conditions.In this study,a simple yet effective solution was proposed for introducing twin...It is extremely difficult to strengthen bulk aluminum(Al)by twins,due to its high stacking fault energy under standard loading conditions.In this study,a simple yet effective solution was proposed for introducing twins to strengthen bulk Al.The method involves the addition of nanoparticles with high volume fraction combined with the tailoring of sintering temperature toward the melting point of Al during hot pressing.Sintering temperature plays an important role in forming twins in bulk Al containing high content nanoparticles.The twin content increases with increasing sintering temperature in the range of 590-640℃.At sintering temperature of 640℃,the twin content reaches 17%,enabling the significant improvement in the yield strength of the bulk Al from 251 to 400 MPa,compared with the sample with few or no twins.The twin strengthening may serve as a major strengthening mechanism for bulk Al,and its strengthening contribution is comparable to the dominant Orowan strengthening resulting from the added nanoparticles.展开更多
The thermal expansion behavior of aluminum matrix composites reinforced with hybrid （nanometer and micrometer） Al2O3 particles was measured between 100 and 600℃ and compared to theoretical models. The results revea...The thermal expansion behavior of aluminum matrix composites reinforced with hybrid （nanometer and micrometer） Al2O3 particles was measured between 100 and 600℃ and compared to theoretical models. The results revealed that the nanoparticle concentration had significant effect on the thermal expansion behavior of the composites. For the composites with lower nanoparticle concentration, their coefficient of thermal expansion （CTE） is determined by a stress relaxation process. While for the composites with higher nanoparticle concentration, their CTE is determined by a percolation process.展开更多
Increasing nanoparticle volume fraction has been proved to be effective in improving the strength of nanoparticle reinforced Al matrix nanocomposite. However, the underlying mechanisms for the ultrahigh strength of th...Increasing nanoparticle volume fraction has been proved to be effective in improving the strength of nanoparticle reinforced Al matrix nanocomposite. However, the underlying mechanisms for the ultrahigh strength of those nanocomposites with high volume fraction(> 10 vol.%) nanoparticles are short of experimental research. In this study, the strengthening mechanisms of high strength Al matrix nanocomposite reinforced with 15 vol.% Al_(2)O_(3) nanoparticles were investigated experimentally and analyzed theoretically. The results show that the thermal mismatch induced geometrically necessary dislocations exhibit a negligible strengthening effect, because of their low density in the nanocomposite that is contradiction to the conventional dislocation punch model. Orowan mechanism makes a major strengthening contribution in view of the deformation process dominated by nanoparticle-dislocation interactions due to the extreme pinning effect of nanoparticles on dislocation motion. In addition, the several mechanisms including grain boundary strengthening, load transfer strengthening, and elastic modulus mismatch induced dislocation strengthening contribute to the strength increase.展开更多
The heterogeneous magnesium(Mg) matrix nanocomposite with dispersed soft phase exhibits high strength and toughness. Herein, the deformation behavior and failure process were investigated to reveal the unique mechanic...The heterogeneous magnesium(Mg) matrix nanocomposite with dispersed soft phase exhibits high strength and toughness. Herein, the deformation behavior and failure process were investigated to reveal the unique mechanical behavior of the heterogeneous microstructure under compression. The extensive plastic deformation is accompanied by the flattening and tilting of the soft phase, inhibiting strain localization and leading to strain hardening. Moreover, a stable crack multiplication process is activated, which endows high damage tolerance to the heterogeneous Mg matrix nanocomposites. The final failure of the composite is caused by crack coalescence in the shear plane along a tortuous path. The presence of dispersed soft phases within the hard matrix induces a noticeable change in mechanical response. Especially,the malleability of the heterogeneous Mg matrix nanocomposite is two and ten times higher than that of pure Mg and the homogeneous Mg matrix nanocomposite, respectively. The current study provides a novel strategy to break the trade-off between strength and toughness in metal matrix nanocomposites.展开更多
In this paper,we reported a novel method for synthesis of non-oxide porous ceramics by using random copolymers as precursors.A silazane oligomer and styrene monomer were used as starting materials,which were copolymer...In this paper,we reported a novel method for synthesis of non-oxide porous ceramics by using random copolymers as precursors.A silazane oligomer and styrene monomer were used as starting materials,which were copolymerized at 120 ℃ to form random polysilazane-polystyrene copolymers.The copolymers were then pyrolyzed at 500 ℃ to obtain porous ceramics by completely decomposing polystyrene（PS） and converting polysilazane（PSZ） into non-oxide Si-C-N ceramics.The obtained material contained a bi-model pore-structure consisting of both micro-sized and nano-sized pores with very high surface area of more than500 m;/g.We also demonstrated that the pore structure and surface area of the materials can be tailored by changing the ratio of the two blocks.Current results suggest a promising simple method for making multiscaled porous non-oxide materials.展开更多
The nanostructured Al-based composites possess the combination of high yield strength and good ductility. In this paper, a micromechanical model is presented to simulate the mechanical response of bimodal nanostructur...The nanostructured Al-based composites possess the combination of high yield strength and good ductility. In this paper, a micromechanical model is presented to simulate the mechanical response of bimodal nanostructured A1 and the particle-reinforced aluminum matrix composite （PAMC）. The constitutive relations for different phases are addressed in the model, as well as the contribution of microcracks. Numerical results show that the model can successfully describe the enhanced strength and ductility of the bimodal nanostructured AI, and the predictions of the PAMC are in good agreement with the experimental data. It is worth noting that the strength and ductility are sensitive to the volume fraction of constituents and the distribution of rnicrocracks in both bimodal nanostructured A1 and PAMG. Therefore, the present theoretical results can be used to optimize the microstructure for improving the mechanical properties of nanostructured Al-based composites.展开更多
The effect of the applied electric field on the conductive behavior of zirconia ceramics is studied by measuring its initial current-voltage curve at various temperatures. The results show that when the field strength...The effect of the applied electric field on the conductive behavior of zirconia ceramics is studied by measuring its initial current-voltage curve at various temperatures. The results show that when the field strength is higher than the threshold for flash-sintering, the curves exhibit a nonlinear behavior by having an additional current on top of the linear current according to Ohm＇s law. Analyzing its transport behavior reveals that the additional current density is due to the extra oxygen vacancies induced by the electric field. The formation rate of the extra vacancies and associated current was related to the field strength.展开更多
The complex metallic alloy（CMA）, Al（76.8）Fe（24）, was in-situ synthesized in the Al-based hybrid composite by powder metallurgy technique. The structural analysis by X-ray diffraction, scanning electron microscop...The complex metallic alloy（CMA）, Al（76.8）Fe（24）, was in-situ synthesized in the Al-based hybrid composite by powder metallurgy technique. The structural analysis by X-ray diffraction, scanning electron microscopy,and transmission electron microscopy indicated that the Al（76.8）Fe（24） CMA phase was formed by diffusion of Fe atoms into the Al matrix during the sintering stage. The formation of the CMA phase was mainly determined by the sintering temperature which was just above the eutectic temperature of Al–Fe. Moreover,the fully dense Al-based hybrid composite was obtained and exhibited ultrahigh strength ～1100 MPa,indicating that this method is expected to be effective in producing CMA particle reinforced Al-based hybrid composite.展开更多
The conductivity of polymer-derived SiOCN ceramics exhibited an Arrhenius dependence on pyrolysis temperature, with the activation energy of ~3.95 eV. The formation and structure change of the free carbon phase were d...The conductivity of polymer-derived SiOCN ceramics exhibited an Arrhenius dependence on pyrolysis temperature, with the activation energy of ~3.95 eV. The formation and structure change of the free carbon phase were detected by means of electron spin resonance spectroscopy and X-ray photoelectron spectroscopy. It reveals that the number of dangling bonds on the free carbon is increased as pyrolysis temperature increases, with the activation energy of ~3.87 eV. So it is demonstrated that the pyrolysis-temperature induced increase in the conductivity is mainly attributed to the increase of dangling on the graphite-like carbon.展开更多
Clinically,fractures are the main cause of computer-aided design and computer-aided manufacturing(CAD/CAM)3 mol%-yttria-stabilized tetragonal zirconia polycrystal(Y-TZP)all-ceramic dental restorations failure because ...Clinically,fractures are the main cause of computer-aided design and computer-aided manufacturing(CAD/CAM)3 mol%-yttria-stabilized tetragonal zirconia polycrystal(Y-TZP)all-ceramic dental restorations failure because of repetitive occlusal loading.The goal of this work is to study the effect of test methods and specimen’s size on the flexural strength of five ceramic products.Both biaxial flexure test(BI)and uni-axial flexure tests(UNI),including three-point flexure test(3PF)and four-point flexure test(4PF),are used in this study.For all five products,the flexural strength is as follows:BI>3PF>4PF.Furthermore,specimens with smaller size(3PF-s)have higher values than the bigger ones(3PF).The difference between BI and UNI resulted from the edge flaws in ceramic specimens.The relationship between different UNI(including 3PF-s,3PF and 4PF)can be explained according to Weibull statistical fracture theory.BI is recommended to evaluate the flexural strength of CAD/CAM Y-TZP dental ceramics.展开更多
Lead-free(K0.5-x/2Na0.5-x/2Lix)(Nb0.8Ta0.2)O3(KNLNT)and(K0.49-x/2Na0.49-x/2-LixCa0.01)(Nb0.8Ta0.2)O3(KNLNT-Ca)ceramics were prepared by a conventional ceramic processing.Structural analysis shows that the Ca^2+ doping...Lead-free(K0.5-x/2Na0.5-x/2Lix)(Nb0.8Ta0.2)O3(KNLNT)and(K0.49-x/2Na0.49-x/2-LixCa0.01)(Nb0.8Ta0.2)O3(KNLNT-Ca)ceramics were prepared by a conventional ceramic processing.Structural analysis shows that the Ca^2+ doping takes the A site of ABO3 perovskite and decreases the phase transition temperature.Property measurements reveal that as a donor dopant,the Ca^2+ doping results in higher room-temperature dielectric constant,lower dielectric loss,and lower mechanical quality factor.In addition,the Ca^2+ doping does not change the positive piezoelectric coefficient d33,but increases the converse piezoelectric coefficient d 33*significantly.This is likely due to the increase in the relaxation,as well as the appearance of(CaNa/K·-VNa/K’)defect dipoles.展开更多
基金supported by the Key Research and Development Project in Sichuan Province(Grant No.2020YFG0140)the China Postdoctoral Science Foundation(Grant No.2020M683349).
文摘It is extremely difficult to strengthen bulk aluminum(Al)by twins,due to its high stacking fault energy under standard loading conditions.In this study,a simple yet effective solution was proposed for introducing twins to strengthen bulk Al.The method involves the addition of nanoparticles with high volume fraction combined with the tailoring of sintering temperature toward the melting point of Al during hot pressing.Sintering temperature plays an important role in forming twins in bulk Al containing high content nanoparticles.The twin content increases with increasing sintering temperature in the range of 590-640℃.At sintering temperature of 640℃,the twin content reaches 17%,enabling the significant improvement in the yield strength of the bulk Al from 251 to 400 MPa,compared with the sample with few or no twins.The twin strengthening may serve as a major strengthening mechanism for bulk Al,and its strengthening contribution is comparable to the dominant Orowan strengthening resulting from the added nanoparticles.
文摘The thermal expansion behavior of aluminum matrix composites reinforced with hybrid （nanometer and micrometer） Al2O3 particles was measured between 100 and 600℃ and compared to theoretical models. The results revealed that the nanoparticle concentration had significant effect on the thermal expansion behavior of the composites. For the composites with lower nanoparticle concentration, their coefficient of thermal expansion （CTE） is determined by a stress relaxation process. While for the composites with higher nanoparticle concentration, their CTE is determined by a percolation process.
基金financially supported by the Key Research and Development Project in Sichuan Province(Grant No.2020YFG0140)the Fundamental Research Funds for the Central Universities(Grant No.2682020CX47)+1 种基金the China Postdoctoral Science Foundation(Grant No.2020M683349)。
文摘Increasing nanoparticle volume fraction has been proved to be effective in improving the strength of nanoparticle reinforced Al matrix nanocomposite. However, the underlying mechanisms for the ultrahigh strength of those nanocomposites with high volume fraction(> 10 vol.%) nanoparticles are short of experimental research. In this study, the strengthening mechanisms of high strength Al matrix nanocomposite reinforced with 15 vol.% Al_(2)O_(3) nanoparticles were investigated experimentally and analyzed theoretically. The results show that the thermal mismatch induced geometrically necessary dislocations exhibit a negligible strengthening effect, because of their low density in the nanocomposite that is contradiction to the conventional dislocation punch model. Orowan mechanism makes a major strengthening contribution in view of the deformation process dominated by nanoparticle-dislocation interactions due to the extreme pinning effect of nanoparticles on dislocation motion. In addition, the several mechanisms including grain boundary strengthening, load transfer strengthening, and elastic modulus mismatch induced dislocation strengthening contribute to the strength increase.
基金financially supported by the National Natural Science Foundation of China (Grant No. 51871187)Department of Science and Technology of Sichuan Province (Grant No. 2020YFG0140)Fundamental Research Funds for the Central Universities (Grant No. 2682019LK04)。
文摘The heterogeneous magnesium(Mg) matrix nanocomposite with dispersed soft phase exhibits high strength and toughness. Herein, the deformation behavior and failure process were investigated to reveal the unique mechanical behavior of the heterogeneous microstructure under compression. The extensive plastic deformation is accompanied by the flattening and tilting of the soft phase, inhibiting strain localization and leading to strain hardening. Moreover, a stable crack multiplication process is activated, which endows high damage tolerance to the heterogeneous Mg matrix nanocomposites. The final failure of the composite is caused by crack coalescence in the shear plane along a tortuous path. The presence of dispersed soft phases within the hard matrix induces a noticeable change in mechanical response. Especially,the malleability of the heterogeneous Mg matrix nanocomposite is two and ten times higher than that of pure Mg and the homogeneous Mg matrix nanocomposite, respectively. The current study provides a novel strategy to break the trade-off between strength and toughness in metal matrix nanocomposites.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.21174112 and 51242009)the Research Fund of State Key Laboratory of Solidification Processing(Grant No.82-TZ-2013)+1 种基金the project "111"(B08040)the grant of the New Century Excellent Talents of Education Ministry of China(NCET-110817)
文摘In this paper,we reported a novel method for synthesis of non-oxide porous ceramics by using random copolymers as precursors.A silazane oligomer and styrene monomer were used as starting materials,which were copolymerized at 120 ℃ to form random polysilazane-polystyrene copolymers.The copolymers were then pyrolyzed at 500 ℃ to obtain porous ceramics by completely decomposing polystyrene（PS） and converting polysilazane（PSZ） into non-oxide Si-C-N ceramics.The obtained material contained a bi-model pore-structure consisting of both micro-sized and nano-sized pores with very high surface area of more than500 m;/g.We also demonstrated that the pore structure and surface area of the materials can be tailored by changing the ratio of the two blocks.Current results suggest a promising simple method for making multiscaled porous non-oxide materials.
基金financially supported by State Key Laboratory of Traction Power (Grant No.2015TPL_Z01)Fundamental Research Funds for the Central Universities (Grant No.2682015RC07)+1 种基金support received from the National Natural Science Foundation of China (Grant Nos.11472243,and 11621062)Doctoral Fund of Ministry of Education of China (20130101120175)
文摘The nanostructured Al-based composites possess the combination of high yield strength and good ductility. In this paper, a micromechanical model is presented to simulate the mechanical response of bimodal nanostructured A1 and the particle-reinforced aluminum matrix composite （PAMC）. The constitutive relations for different phases are addressed in the model, as well as the contribution of microcracks. Numerical results show that the model can successfully describe the enhanced strength and ductility of the bimodal nanostructured AI, and the predictions of the PAMC are in good agreement with the experimental data. It is worth noting that the strength and ductility are sensitive to the volume fraction of constituents and the distribution of rnicrocracks in both bimodal nanostructured A1 and PAMG. Therefore, the present theoretical results can be used to optimize the microstructure for improving the mechanical properties of nanostructured Al-based composites.
基金financially supported by National Natural Science Foundation of China(Grant Nos.51372202,51402237,51532003,51602264)State Key Laboratory of Traction Power(Grand No.2015TPL Z01)
文摘The effect of the applied electric field on the conductive behavior of zirconia ceramics is studied by measuring its initial current-voltage curve at various temperatures. The results show that when the field strength is higher than the threshold for flash-sintering, the curves exhibit a nonlinear behavior by having an additional current on top of the linear current according to Ohm＇s law. Analyzing its transport behavior reveals that the additional current density is due to the extra oxygen vacancies induced by the electric field. The formation rate of the extra vacancies and associated current was related to the field strength.
基金the financial support from the State Key Laboratory of Traction Power (Grant No. 2015TPL Z01)the State Key Laboratory of Solidification Processing (Grant Nos. 82TZ-2013 and SKLSP201609)+1 种基金the Fundamental Research Funds for the Central Universities (Grant No. 2682015RC07)the “111” Project (Grant No. B08040)
文摘The complex metallic alloy（CMA）, Al（76.8）Fe（24）, was in-situ synthesized in the Al-based hybrid composite by powder metallurgy technique. The structural analysis by X-ray diffraction, scanning electron microscopy,and transmission electron microscopy indicated that the Al（76.8）Fe（24） CMA phase was formed by diffusion of Fe atoms into the Al matrix during the sintering stage. The formation of the CMA phase was mainly determined by the sintering temperature which was just above the eutectic temperature of Al–Fe. Moreover,the fully dense Al-based hybrid composite was obtained and exhibited ultrahigh strength ～1100 MPa,indicating that this method is expected to be effective in producing CMA particle reinforced Al-based hybrid composite.
文摘The conductivity of polymer-derived SiOCN ceramics exhibited an Arrhenius dependence on pyrolysis temperature, with the activation energy of ~3.95 eV. The formation and structure change of the free carbon phase were detected by means of electron spin resonance spectroscopy and X-ray photoelectron spectroscopy. It reveals that the number of dangling bonds on the free carbon is increased as pyrolysis temperature increases, with the activation energy of ~3.87 eV. So it is demonstrated that the pyrolysis-temperature induced increase in the conductivity is mainly attributed to the increase of dangling on the graphite-like carbon.
基金This work was supported by grants from Natural Science Foundation of China(81200814)National Key Technology R&D Program of China(no.2012BAI22B03).
文摘Clinically,fractures are the main cause of computer-aided design and computer-aided manufacturing(CAD/CAM)3 mol%-yttria-stabilized tetragonal zirconia polycrystal(Y-TZP)all-ceramic dental restorations failure because of repetitive occlusal loading.The goal of this work is to study the effect of test methods and specimen’s size on the flexural strength of five ceramic products.Both biaxial flexure test(BI)and uni-axial flexure tests(UNI),including three-point flexure test(3PF)and four-point flexure test(4PF),are used in this study.For all five products,the flexural strength is as follows:BI>3PF>4PF.Furthermore,specimens with smaller size(3PF-s)have higher values than the bigger ones(3PF).The difference between BI and UNI resulted from the edge flaws in ceramic specimens.The relationship between different UNI(including 3PF-s,3PF and 4PF)can be explained according to Weibull statistical fracture theory.BI is recommended to evaluate the flexural strength of CAD/CAM Y-TZP dental ceramics.
文摘Lead-free(K0.5-x/2Na0.5-x/2Lix)(Nb0.8Ta0.2)O3(KNLNT)and(K0.49-x/2Na0.49-x/2-LixCa0.01)(Nb0.8Ta0.2)O3(KNLNT-Ca)ceramics were prepared by a conventional ceramic processing.Structural analysis shows that the Ca^2+ doping takes the A site of ABO3 perovskite and decreases the phase transition temperature.Property measurements reveal that as a donor dopant,the Ca^2+ doping results in higher room-temperature dielectric constant,lower dielectric loss,and lower mechanical quality factor.In addition,the Ca^2+ doping does not change the positive piezoelectric coefficient d33,but increases the converse piezoelectric coefficient d 33*significantly.This is likely due to the increase in the relaxation,as well as the appearance of(CaNa/K·-VNa/K’)defect dipoles.
