Silicon-carbide-fiber-reinforced silicon-carbide-ceramic-based matrix(SiC/SiC)composites possess excellent properties such as low density,high strength and high temperature resistance,showing a potential application f...Silicon-carbide-fiber-reinforced silicon-carbide-ceramic-based matrix(SiC/SiC)composites possess excellent properties such as low density,high strength and high temperature resistance,showing a potential application for structural components in the aerospace field,but their oxidation behavior remains largely unknown.In this study,Yb_(2)Si_(2)O_(7)modified SiC/SiC(SiC/SiC-Yb_(2)Si_(2)O_(7))mini-composites were prepared by introducing Yb_(2)Si_(2)O_(7)as anti-oxidation phase into SiC fiber bundles via Sol-Gel and depositing SiC matrix by chemical vapor deposition(CVD).Influence of Yb_(2)Si_(2)O_(7)on microstructure,mechanical property and oxidation behavior of SiC/SiC mini-composites was investigated.The results showed that after oxidation in air at 1200 and 1400℃for 50 h,the tensile strength retentions of SiC/SiC mini-composites were 77%and 69%,respectively,and the fracture morphology exhibited flat.The Yb_(2)Si_(2)O_(7)introduced by Sol-Gel partially distributed in layers,contributing to the toughening of the material.On the fracture surface,there was interlayer debonding,which extended energy dissipation mechanism of SiC/SiC mini-composites.Tensile strength of SiC/SiC-Yb_(2)Si_(2)O_(7)mini-composites at room temperature was 484 MPa.After oxidation in air at 1200 and 1400℃for 50 h,the tensile strengths decreased to 425 and 374 MPa,resulting in retention rates of 88%and 77%,respectively.It displayed typical non-brittle fracture characteristics.The interface oxygen content of SiC/SiC mini-composites at the fracture surface was higher than that of SiC/SiC-Yb_(2)Si_(2)O_(7)mini-composites,indicating that introduction of Yb_(2)Si_(2)O_(7)could alleviate oxygen diffusion towards the interface,and therefore improve the oxidation resistance of SiC/SiC-Yb_(2)Si_(2)O_(7)mini-composites.展开更多
ZrB_(2)-based ceramics typically necessitate high temperature and pressure for sintering,whereas ZrB_(2)-SiC ceramics can be fabricated at 1500℃using the process of reactive melt infiltration with Si.In comparison to...ZrB_(2)-based ceramics typically necessitate high temperature and pressure for sintering,whereas ZrB_(2)-SiC ceramics can be fabricated at 1500℃using the process of reactive melt infiltration with Si.In comparison to the conventional preparation method,reactive synthesis allows for the more facile production of ultra-high temperature ceramics with fine particle size and homogeneous composition.In this work,ZrSi_(2),B4C,and C were used as raw materials to prepare ZrB_(2)-SiC via combination of tape casting and reactive melt infiltration herein referred to as ZBC ceramics.Control sample of ZrB_(2)-SiC was also prepared using ZrB_(2)and SiC as raw materials through an identical process designated as ZS ceramics.Microscopic analysis of both ceramic groups revealed smaller and more uniformly distributed particles of the ZrB_(2)phase in ZBC ceramics compared to the larger particles in ZS ceramics.Both sets of ceramics underwent cyclic oxidation testing in the air at 1600℃for a cumulative duration of 5 cycles,each cycle lasting 2 h.Analysis of the oxidation behavior showed that both ZBC ceramics and ZS ceramics developed a glassy SiO_(2)-ZrO_(2)oxide layer on their surfaces during the oxidation.This layer severed as a barrier against oxygen.In ZBC ceramics,ZrO_(2)is finely distributed in SiO_(2),whereas in ZS ceramics,larger ZrO_(2)particles coexist with glassy SiO_(2).The surface oxide layer of ZBC ceramics maintains a dense structure because the well-dispersed ZrO_(2)increases the viscosity of glassy SiO_(2),preventing its crystallization during the cooling.Conversely,some SiO_(2)in the oxide layer of ZS ceramics may crystallize and form a eutectic with ZrO_(2),leading to the formation of ZrSiO_(4).This leads to cracking of the oxide layer due to differences in thermal expansion coefficients,weakening its barrier effect.An analysis of the oxidation resistance shows that ZBC ceramics exhibit less increase in oxide layer thickness and mass compared to ZS ceramics,suggesting superior oxidation resistance of ZBC ceramics.展开更多
Based on the pseudo potential plane-wave method of density functional theory （DFT）, Ti1-xNbxAk （x=0, 0.062 5, 0.083 3, 0.125, 0.250） crystals＇ geometry structure, elastic constants, electronic structure and Mulli...Based on the pseudo potential plane-wave method of density functional theory （DFT）, Ti1-xNbxAk （x=0, 0.062 5, 0.083 3, 0.125, 0.250） crystals＇ geometry structure, elastic constants, electronic structure and Mulliken populations were calculated, and the effects of doping on the geometric structure, electronic structure and bond strength were systematically analyzed. The results show that the influence of Nb on the geometric structure is little in terms of the plasticity, and with the increase of Nb content, the covalent bond strength remarkably reduces, and Ti-Al, Nb-M （M=Ti, Al） and other hybrid bonds enhance; meanwhile, the peak district increases and the pseudo-energy gap first decreases and then increases, the overall band structure narrows, the covalent bond and direction of bonds reduce. The population analysis also shows that the results are consistent with the electronic structure analysis. The density of states of TiAINb shows that Nb doping can enhance the activity of Al and benefit the form of Al2O3 film. All the calculations reveal that the room temperature plasticity and the antioxidation properties of the compounds can be improved with the Nb content of 8.33%-12.5% （mole fraction）.展开更多
The effect of surface finish and annealing treatment on the oxidation behavior of Ti-48Al-8Cr-2Ag (molar fraction, %) alloy was investigated at 900 and 1 000 ℃, respectively in air. Thermal gravimetric analysis (TGA)...The effect of surface finish and annealing treatment on the oxidation behavior of Ti-48Al-8Cr-2Ag (molar fraction, %) alloy was investigated at 900 and 1 000 ℃, respectively in air. Thermal gravimetric analysis (TGA) was conducted for the characterization of oxidation kinetics. The microstructures of oxide scales were studied by scanning electron microscopy (SEM) and transmission election microscopy (TEM) techniques. Unfavorable effect of the annealing treatment on the oxidation behavior of the coating was also investigated. The results indicate that the oxidation behavior of the alloy is influenced by surface finish and annealing treatment. The oxidation rate of ground sample is lower than that of the polished alloy at 1 000 ℃ in air. The former forms a scale of merely Al2O3, and the latter forms a scale of the mixture of Al2O3 and TiO2. Annealing can improve the formation of TiO2.展开更多
基金National Natural Science Foundation of China(52222202)National Key R&D Program of China(2022YFB3707700)+2 种基金Project of Shanghai Science and Technology Innovation Action Plan(21511104800)Shanghai Pilot Program for Basic Research-Chinese Academy of Science,Shanghai Branch(JCYJ-SHFY-2021-001)Science Center for Gas Turbine Project(P2022-B-Ⅳ-001-001)。
文摘Silicon-carbide-fiber-reinforced silicon-carbide-ceramic-based matrix(SiC/SiC)composites possess excellent properties such as low density,high strength and high temperature resistance,showing a potential application for structural components in the aerospace field,but their oxidation behavior remains largely unknown.In this study,Yb_(2)Si_(2)O_(7)modified SiC/SiC(SiC/SiC-Yb_(2)Si_(2)O_(7))mini-composites were prepared by introducing Yb_(2)Si_(2)O_(7)as anti-oxidation phase into SiC fiber bundles via Sol-Gel and depositing SiC matrix by chemical vapor deposition(CVD).Influence of Yb_(2)Si_(2)O_(7)on microstructure,mechanical property and oxidation behavior of SiC/SiC mini-composites was investigated.The results showed that after oxidation in air at 1200 and 1400℃for 50 h,the tensile strength retentions of SiC/SiC mini-composites were 77%and 69%,respectively,and the fracture morphology exhibited flat.The Yb_(2)Si_(2)O_(7)introduced by Sol-Gel partially distributed in layers,contributing to the toughening of the material.On the fracture surface,there was interlayer debonding,which extended energy dissipation mechanism of SiC/SiC mini-composites.Tensile strength of SiC/SiC-Yb_(2)Si_(2)O_(7)mini-composites at room temperature was 484 MPa.After oxidation in air at 1200 and 1400℃for 50 h,the tensile strengths decreased to 425 and 374 MPa,resulting in retention rates of 88%and 77%,respectively.It displayed typical non-brittle fracture characteristics.The interface oxygen content of SiC/SiC mini-composites at the fracture surface was higher than that of SiC/SiC-Yb_(2)Si_(2)O_(7)mini-composites,indicating that introduction of Yb_(2)Si_(2)O_(7)could alleviate oxygen diffusion towards the interface,and therefore improve the oxidation resistance of SiC/SiC-Yb_(2)Si_(2)O_(7)mini-composites.
基金National Key R&D Program of China(2022YFB3707700)Shanghai Science and Technology Innovation Action Plan(21511104800)+3 种基金National Natural Science Foundation of China(52172111)National Science and Technology Major Project(2017-IV-0005-0042)Key Research Program of the Chinese Academy of Sciences(ZDRW-CN-2021-2-2)Science Center for Gas Turbine Project(P2022-B-IV-001-001)。
文摘ZrB_(2)-based ceramics typically necessitate high temperature and pressure for sintering,whereas ZrB_(2)-SiC ceramics can be fabricated at 1500℃using the process of reactive melt infiltration with Si.In comparison to the conventional preparation method,reactive synthesis allows for the more facile production of ultra-high temperature ceramics with fine particle size and homogeneous composition.In this work,ZrSi_(2),B4C,and C were used as raw materials to prepare ZrB_(2)-SiC via combination of tape casting and reactive melt infiltration herein referred to as ZBC ceramics.Control sample of ZrB_(2)-SiC was also prepared using ZrB_(2)and SiC as raw materials through an identical process designated as ZS ceramics.Microscopic analysis of both ceramic groups revealed smaller and more uniformly distributed particles of the ZrB_(2)phase in ZBC ceramics compared to the larger particles in ZS ceramics.Both sets of ceramics underwent cyclic oxidation testing in the air at 1600℃for a cumulative duration of 5 cycles,each cycle lasting 2 h.Analysis of the oxidation behavior showed that both ZBC ceramics and ZS ceramics developed a glassy SiO_(2)-ZrO_(2)oxide layer on their surfaces during the oxidation.This layer severed as a barrier against oxygen.In ZBC ceramics,ZrO_(2)is finely distributed in SiO_(2),whereas in ZS ceramics,larger ZrO_(2)particles coexist with glassy SiO_(2).The surface oxide layer of ZBC ceramics maintains a dense structure because the well-dispersed ZrO_(2)increases the viscosity of glassy SiO_(2),preventing its crystallization during the cooling.Conversely,some SiO_(2)in the oxide layer of ZS ceramics may crystallize and form a eutectic with ZrO_(2),leading to the formation of ZrSiO_(4).This leads to cracking of the oxide layer due to differences in thermal expansion coefficients,weakening its barrier effect.An analysis of the oxidation resistance shows that ZBC ceramics exhibit less increase in oxide layer thickness and mass compared to ZS ceramics,suggesting superior oxidation resistance of ZBC ceramics.
基金Project(07JJ3102) supported by Hunan Provincial Natural Science Foundation,ChinaProject(k0902132-11) supported by Changsha Municipal Science and Technology,China
文摘Based on the pseudo potential plane-wave method of density functional theory （DFT）, Ti1-xNbxAk （x=0, 0.062 5, 0.083 3, 0.125, 0.250） crystals＇ geometry structure, elastic constants, electronic structure and Mulliken populations were calculated, and the effects of doping on the geometric structure, electronic structure and bond strength were systematically analyzed. The results show that the influence of Nb on the geometric structure is little in terms of the plasticity, and with the increase of Nb content, the covalent bond strength remarkably reduces, and Ti-Al, Nb-M （M=Ti, Al） and other hybrid bonds enhance; meanwhile, the peak district increases and the pseudo-energy gap first decreases and then increases, the overall band structure narrows, the covalent bond and direction of bonds reduce. The population analysis also shows that the results are consistent with the electronic structure analysis. The density of states of TiAINb shows that Nb doping can enhance the activity of Al and benefit the form of Al2O3 film. All the calculations reveal that the room temperature plasticity and the antioxidation properties of the compounds can be improved with the Nb content of 8.33%-12.5% （mole fraction）.
基金Project(2007430028) supported by the Science and Technique Foundation of Henan Educational Committee, China
文摘The effect of surface finish and annealing treatment on the oxidation behavior of Ti-48Al-8Cr-2Ag (molar fraction, %) alloy was investigated at 900 and 1 000 ℃, respectively in air. Thermal gravimetric analysis (TGA) was conducted for the characterization of oxidation kinetics. The microstructures of oxide scales were studied by scanning electron microscopy (SEM) and transmission election microscopy (TEM) techniques. Unfavorable effect of the annealing treatment on the oxidation behavior of the coating was also investigated. The results indicate that the oxidation behavior of the alloy is influenced by surface finish and annealing treatment. The oxidation rate of ground sample is lower than that of the polished alloy at 1 000 ℃ in air. The former forms a scale of merely Al2O3, and the latter forms a scale of the mixture of Al2O3 and TiO2. Annealing can improve the formation of TiO2.
