The structures,mechanical properties and electronic structures of M metals(M=Ti,V,Cr,Mn and Fe)dopedβ-Si_(3)N_(4) were investigated by First-principles calculations within CASTEP.The calculated lattice parameters of...The structures,mechanical properties and electronic structures of M metals(M=Ti,V,Cr,Mn and Fe)dopedβ-Si_(3)N_(4) were investigated by First-principles calculations within CASTEP.The calculated lattice parameters ofβ-Si_(3)N_(4) were consistent with previous date.The cohesive energy and formation enthalpy show that initialβ-Si_(3)N_(4) has the highest structural stability.The calculated elastic constant and the Voigt-Reuss-Hill approximation indicate that elastic moduli ofβ-Si_(3)N_(4) are slightly reduced by M doping.Based on Poisson’s and Pugh’s ratio,β-Si_(3)N_(4) is a ductile material and the toughness ofβ-Si_(3)N_(4) increases with M doping,and Fe doping exhibited the best toughness.The results of density of states,charge distributions and overlapping populations indicate thatβ-Si_(3)N_(4) has the strong covalent and ionic bond strength between N and Si.展开更多
The microstructure of both {10(?)1} and {10(?)0} (directions perpendicular to the {10(?)1} and {10(?)0} planes) α-Si_3N_4 whiskers were investi- gated by high resolution electron microscopy (HREM).On one side of the ...The microstructure of both {10(?)1} and {10(?)0} (directions perpendicular to the {10(?)1} and {10(?)0} planes) α-Si_3N_4 whiskers were investi- gated by high resolution electron microscopy (HREM).On one side of the {10(?)1} α-Si_3N_4 whiskers many planar defects were ob- served,two kinds of micrograins on one side of the {10(?)0} whiskers were found.In one type,sepa- rated α-Si_3N_4 (28 H) micrograins had the same orientation with respect to the matrix whisker;in the other type,connected polymicrograins consisted of both α-and β-Si_3N_4 (14H).展开更多
The Si_3N_4 whisker reinforced 6061Al composite with bending strength of 790 MPa was prepared by squeeze casting process.After heat-treatment under T6 regime i.e.530℃, 1 h solutioning and 160℃,24 h aging,an incremen...The Si_3N_4 whisker reinforced 6061Al composite with bending strength of 790 MPa was prepared by squeeze casting process.After heat-treatment under T6 regime i.e.530℃, 1 h solutioning and 160℃,24 h aging,an increment in strength and microhardness may be over 20% and 28% respectively,The microstructures of Si_3N_4 whisker and Si_3N_4/Al interface were observed by meas of HRTEM.The relation between interracial structure and composite properties was discussed.展开更多
Using newly developed Cu58Ni12Ti30 alloy as brazing filler metal, this paper has carried out the joining wxperiments of Si3N4 and the joint shength tests at room temperature.The joint brazed at 1,293K for 10 min exhib...Using newly developed Cu58Ni12Ti30 alloy as brazing filler metal, this paper has carried out the joining wxperiments of Si3N4 and the joint shength tests at room temperature.The joint brazed at 1,293K for 10 min exhibited the maximum strength value of 157.2 MPa.The microstructures of the joint cross-section were observed and the elements area distributions on the interface were examined by means of scanning electron microscope with X-ray wave-dispersion spectrometer.The phases formed in the joint were determined by X-ray diffraction analysis method.The results showed that during the brazing process the active element Ti diffused to the interfaces and reacted with Si3N4,resulted in forming the reaction products TiN NiTiSi, and Ti4Si3(or TiSi)on the interfaces.Some effects on the trend to produce these compounds were attempted to explain from α thermodynalic point of view.展开更多
Cubic silicon nitride(-Si_(3)N_(4))is superhard and one of the hardest materials after diamond and cubic boron nitride(cBN),but has higher thermal stability in an oxidizing environment than diamond,making it a competi...Cubic silicon nitride(-Si_(3)N_(4))is superhard and one of the hardest materials after diamond and cubic boron nitride(cBN),but has higher thermal stability in an oxidizing environment than diamond,making it a competitive candidate for technological applications in harsh conditions(e.g.,drill head and abrasives).Here,we report the high-pressure synthesis and characterization of the structural and mechanical properties of a γ-Si_(3)N_(4)/Hf_(3)N_(4) ceramic nanocomposite derived from single-phase amorphous silicon(Si)-hafnium(Hf)-nitrogen(N)precursor.The synthesis of the-Si_(3)N_(4)/Hf_(3)N_(4) nanocomposite is performed at~20 GPa and ca.1500 ℃ in a large volume multi anvil press.The structural evolution of the amorphous precursor and its crystallization to-Si_(3)N_(4)/Hf_(3)N_(4) nanocomposites under high pressures is assessed by the in situ synchrotron energy-dispersive X-ray diffraction(ED-XRD)measurements at~19.5 GPa in the temperature range of ca.1000-1900℃.The fracture toughness(K_(IC))of the two-phase nanocomposite amounts~6/6.9 MPa·m^(1/2) and is about 2 times that of single-phaseγ-Si_(3)N_(4),while its hardness of ca.30 GPa remains high.This work provides a reliable and feasible route for the synthesis of advanced hard and tough-Si_(3)N_(4)-based nanocomposites with excellent thermal stabililty.展开更多
β-Si3N4 whisker reinforced aluminum composites was fabricated by squeeze casting before extrusion and an effect of content of Mg on the High Strain Rate Superplastic- ity (HSRS) were investigated, The optimum tempera...β-Si3N4 whisker reinforced aluminum composites was fabricated by squeeze casting before extrusion and an effect of content of Mg on the High Strain Rate Superplastic- ity (HSRS) were investigated, The optimum temperature of the composites at which maximum total elongation is obtained decreases according to magnesium content and the β-Si_3N_4w/Al-3Mg exhibits the total elongation of about 200% at the strain rate of 10^(-1) s^(-1) and at 853-858 K, although the β-Si_3N_4w/Al-0Mg composite shows about 100% elongation at the strain rate of about 1×10^(-1) s^(-1) at 903-913 K. Optimum strain rate of the composites fabricated by squeeze casting was about 1×10^(-1) s^(-1) but TEM observation indicates that the β-Si_3N_4w/Al-Mg has a fine grain of about 2- 3μm and that the whisker might no react with Mg at the interfaces, although the β-Si_3N_4 whisker react with aluminum matrix.展开更多
As the dominated composition of Si_(3)N_(4)ceramics,α-silicon nitride(α-Si_(3)N_(4))can satisfy the strength and fracture toughness demand in the applications.However,α-Si_(3)N_(4)is oxygen-sensitive at high temper...As the dominated composition of Si_(3)N_(4)ceramics,α-silicon nitride(α-Si_(3)N_(4))can satisfy the strength and fracture toughness demand in the applications.However,α-Si_(3)N_(4)is oxygen-sensitive at high temperatures,which limits its high-temperature performance.To improve the oxidation resistance ofα-Si_(3)N_(4)ceramics,it is necessary to shed light on the oxidation mechanism.Herein,the initial oxidation ofα-Si_(3)N_(4)was systematically studied at the atomic and molecular levels.The density functional theory(DFT)calculation denotes that the(001)surface ofα-Si_(3)N_(4)has the best stability at both room temperature and high temperature.Besides,the oxidation process of theα-Si_(3)N_(4)(001)surface consists of O adsorption and N desorption,and the consequent formation of nitrogen-vacancy(VN)is the key step for further oxidation.Moreover,the molecular dynamics(MD)simulation indicates that the oxidation rate ofα-Si_(3)N_(4)(100)surface is slower than that ofα-Si_(3)N_(4)(001)surface due to the lower N concentration at the outermost layer.Therefore,the oxidation resistance ofα-Si_(3)N_(4)can be improved by regulating the(100)surface as the dominant exposure surface.In addition,reducing the concentration of N on the final exposed surface ofα-Si_(3)N_(4)by mean of constructing the homojunction of the Si-terminal(100)surface and other N-containing surfaces(such as(001)surface)should be also a feasible approach.展开更多
文摘The structures,mechanical properties and electronic structures of M metals(M=Ti,V,Cr,Mn and Fe)dopedβ-Si_(3)N_(4) were investigated by First-principles calculations within CASTEP.The calculated lattice parameters ofβ-Si_(3)N_(4) were consistent with previous date.The cohesive energy and formation enthalpy show that initialβ-Si_(3)N_(4) has the highest structural stability.The calculated elastic constant and the Voigt-Reuss-Hill approximation indicate that elastic moduli ofβ-Si_(3)N_(4) are slightly reduced by M doping.Based on Poisson’s and Pugh’s ratio,β-Si_(3)N_(4) is a ductile material and the toughness ofβ-Si_(3)N_(4) increases with M doping,and Fe doping exhibited the best toughness.The results of density of states,charge distributions and overlapping populations indicate thatβ-Si_(3)N_(4) has the strong covalent and ionic bond strength between N and Si.
文摘The microstructure of both {10(?)1} and {10(?)0} (directions perpendicular to the {10(?)1} and {10(?)0} planes) α-Si_3N_4 whiskers were investi- gated by high resolution electron microscopy (HREM).On one side of the {10(?)1} α-Si_3N_4 whiskers many planar defects were ob- served,two kinds of micrograins on one side of the {10(?)0} whiskers were found.In one type,sepa- rated α-Si_3N_4 (28 H) micrograins had the same orientation with respect to the matrix whisker;in the other type,connected polymicrograins consisted of both α-and β-Si_3N_4 (14H).
文摘The Si_3N_4 whisker reinforced 6061Al composite with bending strength of 790 MPa was prepared by squeeze casting process.After heat-treatment under T6 regime i.e.530℃, 1 h solutioning and 160℃,24 h aging,an increment in strength and microhardness may be over 20% and 28% respectively,The microstructures of Si_3N_4 whisker and Si_3N_4/Al interface were observed by meas of HRTEM.The relation between interracial structure and composite properties was discussed.
文摘Using newly developed Cu58Ni12Ti30 alloy as brazing filler metal, this paper has carried out the joining wxperiments of Si3N4 and the joint shength tests at room temperature.The joint brazed at 1,293K for 10 min exhibited the maximum strength value of 157.2 MPa.The microstructures of the joint cross-section were observed and the elements area distributions on the interface were examined by means of scanning electron microscope with X-ray wave-dispersion spectrometer.The phases formed in the joint were determined by X-ray diffraction analysis method.The results showed that during the brazing process the active element Ti diffused to the interfaces and reacted with Si3N4,resulted in forming the reaction products TiN NiTiSi, and Ti4Si3(or TiSi)on the interfaces.Some effects on the trend to produce these compounds were attempted to explain from α thermodynalic point of view.
基金Part of this research was carried out at PETRA III LVP at beamline P61B(beamtime I-20200434)and P02.1Shrikant Bhat and Robert Farla acknowedge the support from the Federal Ministry of Education and Research,Germany(BMBF,Nos.05K16WC2 and 05K13WC2)+2 种基金Wei Li and Leonore Wiehl also acknowledge the travel support from DESY.Zhaoju Yu thanks the National Natural Science Foundation of China(Nos.51872246 and 52061135102)for financial supportMarc Widenmeyer and Anke Weidenkaff are grateful for the financial support by the German Ministry of Education and Research(No.03SF0618B)Wei Li acknowledges the financial support from China Scholarship Council(No.201907040060).
文摘Cubic silicon nitride(-Si_(3)N_(4))is superhard and one of the hardest materials after diamond and cubic boron nitride(cBN),but has higher thermal stability in an oxidizing environment than diamond,making it a competitive candidate for technological applications in harsh conditions(e.g.,drill head and abrasives).Here,we report the high-pressure synthesis and characterization of the structural and mechanical properties of a γ-Si_(3)N_(4)/Hf_(3)N_(4) ceramic nanocomposite derived from single-phase amorphous silicon(Si)-hafnium(Hf)-nitrogen(N)precursor.The synthesis of the-Si_(3)N_(4)/Hf_(3)N_(4) nanocomposite is performed at~20 GPa and ca.1500 ℃ in a large volume multi anvil press.The structural evolution of the amorphous precursor and its crystallization to-Si_(3)N_(4)/Hf_(3)N_(4) nanocomposites under high pressures is assessed by the in situ synchrotron energy-dispersive X-ray diffraction(ED-XRD)measurements at~19.5 GPa in the temperature range of ca.1000-1900℃.The fracture toughness(K_(IC))of the two-phase nanocomposite amounts~6/6.9 MPa·m^(1/2) and is about 2 times that of single-phaseγ-Si_(3)N_(4),while its hardness of ca.30 GPa remains high.This work provides a reliable and feasible route for the synthesis of advanced hard and tough-Si_(3)N_(4)-based nanocomposites with excellent thermal stabililty.
文摘β-Si3N4 whisker reinforced aluminum composites was fabricated by squeeze casting before extrusion and an effect of content of Mg on the High Strain Rate Superplastic- ity (HSRS) were investigated, The optimum temperature of the composites at which maximum total elongation is obtained decreases according to magnesium content and the β-Si_3N_4w/Al-3Mg exhibits the total elongation of about 200% at the strain rate of 10^(-1) s^(-1) and at 853-858 K, although the β-Si_3N_4w/Al-0Mg composite shows about 100% elongation at the strain rate of about 1×10^(-1) s^(-1) at 903-913 K. Optimum strain rate of the composites fabricated by squeeze casting was about 1×10^(-1) s^(-1) but TEM observation indicates that the β-Si_3N_4w/Al-Mg has a fine grain of about 2- 3μm and that the whisker might no react with Mg at the interfaces, although the β-Si_3N_4 whisker react with aluminum matrix.
基金financially supported by the National Science Fund for Distinguished Young Scholars(No.52025041)the National Natural Science Foundation of China(Nos.51904021,51974021,51902020)+1 种基金the Fundamental Research Funds for the Central Universities(No.FRF-TP-19-008A1)the State Key Laboratory of Refractories and Metallurgy,Wuhan University of Science and Technology(No.G202003)。
文摘As the dominated composition of Si_(3)N_(4)ceramics,α-silicon nitride(α-Si_(3)N_(4))can satisfy the strength and fracture toughness demand in the applications.However,α-Si_(3)N_(4)is oxygen-sensitive at high temperatures,which limits its high-temperature performance.To improve the oxidation resistance ofα-Si_(3)N_(4)ceramics,it is necessary to shed light on the oxidation mechanism.Herein,the initial oxidation ofα-Si_(3)N_(4)was systematically studied at the atomic and molecular levels.The density functional theory(DFT)calculation denotes that the(001)surface ofα-Si_(3)N_(4)has the best stability at both room temperature and high temperature.Besides,the oxidation process of theα-Si_(3)N_(4)(001)surface consists of O adsorption and N desorption,and the consequent formation of nitrogen-vacancy(VN)is the key step for further oxidation.Moreover,the molecular dynamics(MD)simulation indicates that the oxidation rate ofα-Si_(3)N_(4)(100)surface is slower than that ofα-Si_(3)N_(4)(001)surface due to the lower N concentration at the outermost layer.Therefore,the oxidation resistance ofα-Si_(3)N_(4)can be improved by regulating the(100)surface as the dominant exposure surface.In addition,reducing the concentration of N on the final exposed surface ofα-Si_(3)N_(4)by mean of constructing the homojunction of the Si-terminal(100)surface and other N-containing surfaces(such as(001)surface)should be also a feasible approach.
