共查询到20条相似文献,搜索用时 31 毫秒
1.
The steady-state creep behaviour of Ti3Al and Ti3Al+10 wt% Nb was studied in the temperature range 550 to 825° C and in the stress range 69 to 312 MN m–2. The temperature and stress dependences of the steady-state creep rates were determined for both intermetallics, and the activation energy and stress-exponent were measured. At temperatures above 700° C, the stress dependence of the steady-state creep rate indicated two distinct creep regimes: at stresses above 138 MN m–2, the creep was controlled most probably by dislocation climb; at stresses below 138 MN m–2, a transition regime with a lower stress-exponent value was obtained. 相似文献
2.
Mechanical damping and dynamic modulus measurements in alumina and tungsten fibre-reinforced aluminium composites 总被引:2,自引:0,他引:2
Simultaneous measurements of mechanical damping, or internal friction (Q
–1 ), and dynamic Young's modulus (E) were made near 80 kHz and at strain amplitudes () in the range 10–8 to 10–4 on small specimens of continuous or chopped fibre-reinforced metal matrix composites (MMCs): 6061 aluminium reinforced with alumina (Al/Al2O3) and 6061 aluminium reinforced with tungsten (Al/W). Baseline experiments were also done on 99.999% aluminium (pure Al). The strain amplitude dependence of damping and the temperature dependence of dynamic modulus were of particular interest in this study. The temperature (T) dependence of the modulus from room temperature up to 475° C was determined for the Al/Al2O3 and pure Al specimens and a highly linear decrease in modulus with increasing temperature was observed. The rate of modulus loss (dE/dT –80 M Pa° C–1 ) was the same for both materials and the reduction in modulus of the Al/Al2O3 was attributed to the reduction in modulus of the alu minium matrix, not the alumina fibres. The size, type, and amount of fibre reinforcement were found to have a significant effect on the strain amplitude dependence of the damping in both MMCs. Unreinforced aluminium exhibited classical dislocation damping trends with a region of strain amplitude independent damping at low strains (less than 10–5) followed by a non linear, strain amplitude dependent region at higher strains. The addition of alumina fibres (chopped or continuous), while increasing stiffness, resulted in a significant reduction in damping capacity for the MMC relative to that for aluminium and near complete suppression of the amplitude dependent response. The damping levels increased as the volume fraction of fibre, and therefore, the amount of fibre/matrix (FM) interface decreased, indicating that the matrix, not factors such as increased dislocation densities at the FM interface, was the dominant influence on the damping. Analysis of the Al/Al2O3 results by Granato-Lücke (GL) theory indicated that dislocation densities were increased relative to those in aluminium, but the dis locations were well pinned and unable to increase damping levels effectively. Analysis of the Al/W results by GL theory also revealed high dislocation densities, but, unlike the Al/Al2O3 specimens, the Al/W specimens (continuous fibres) exhibited strong amplitude dependent damping (starting near strain levels of 2 × 10–6) with damping levels approximately twice those of pure aluminium. Trends showed increased damping with increased fibre diameter, not with increased FM interface area. There was some evidence that it was the tungsten fibre itself that dominated the damping behaviour in Al/W composites, not the aluminium matrix or the FM interface. 相似文献
3.
The mechanical properties of commercially available SiC-based ceramic fibres were measured in the temperature range from 400–1300°C. The measurements were performed in air and in inert gas atmospheres, respectively. The Nicalon and Tyranno fibres were tested as filament bundles and the decrease in strength occurring at temperatures above 600 °C was found in both atmospheres. To obtain a well-defined gauge length at the testing temperature, a furnace with very steep temperature gradients at both ends was built. To eliminate grip-induced damage in the heating zone the fibre bundles were fixed outside the furnace with cold grip units. These grips guaranteed the uniformity of load distribution imposed on to each of the individual filaments in the fibre bundle. A significant shrinkage of the fibres occurring during the creep test performed under low loads indicates a change in the microstructure of the fibres at high temperatures. 相似文献
4.
Silicon or titanium was deposited on the filaments of carbon fibres by chemical vapour depositions and the reactions between the deposited silicon or titanium and the carbon fibres were investigated below 1300° C. Between the silicon and the carbon fibres, -SiC layers formed at rates of 1.5 to 3 nm in 3 h at 1300° C. These rates were 10–4 times that of the TiC formation by the reaction of titanium with carbon fibre. Furthermore, the effect of the reaction on fibre strength was investigated. By reaction with silicon, the carbon fibre at a carbonized stage decreased in strength at the beginning of the reaction, but afterwards it recovered to the original level. The carbon fibre at a graphitized stage maintained its original strength after heat treatment for several hours at 1300° C. With the TiC-coated carbon fibres, the carbon fibres decreased in strength following the relation
m d
–1/2, where d is the thickness of the TiC layer. 相似文献
5.
J. W. Holmes 《Journal of Materials Science》1991,26(7):1808-1814
The tensile creep behaviour of a SiC-fibre-Si3N4-matrix composite was investigated in air at 1350 C. The unidirectional composite, containing 30 vol % SCS-6 SiC fibres, was prepared by hot pressing at 1700 C. Creep testing was conducted at stress levels of 70, 110, 150 and 190 MPa. An apparent steady-state creep rate was observed at stress levels between 70 and 150 MPa; at 190 MPa, only tertiary creep was observed. For an applied stress of 70 MPa, the steady-state creep rate was approximately 2.5×10–10 s–1 with failure times in excess of 790 h. At 150 MPa, the steady-state creep rate increased to an average of 5.6×10–8 s–1 with failure times under 40 h. The creep rate of the composite is compared with published data for the steady-state creep rate of monolithic Si3N4. 相似文献
6.
The high-temperature mechanical behaviour and microstructural evolution of experimental SiC fibres (Hi-Nicalon) with a low oxygen content (<0.5 wt%) have been examined up to 1600 °C. Comparisons have been made with a commercial Si-C-O fibre (Nicalon Ceramic Grade). Their initial microstructure consists of -SiC crystallites averaging 5–10 nm in diameter, with important amounts of graphitic carbon into wrinkled sheet structures of very small sizes between the SiC grains. The fall in strength above 800 °C in air is related to fibre surface degradation involving free carbon. Crystallization of SiC and carbon further develops in both fibres subject to either creep or heat treatment at 1300 °C and above for long periods. The fibres are characterized by steady state creep and greater creep resistance (one order of magnitude) compared to the commercial Nicalon fibre. The experimental fibre has been found to creep above 1280 °C under low applied stresses (0.15 GPa) in air. Significant deformations (up to 14%) have been observed, both in air and argon above 1400 °C. The stress exponents and the apparent activation energies for creep have been found to fall in the range 2–3, both in air and argon, and in the range 200–300 kJ mol–1 in argon and 340–420 kJ mol–1 in air. The dewrinkling of carbon layer packets into a position more nearly aligned with the tensile axis, their sliding, and the collapse of pores have been proposed as the mechanisms which control the fibre creep behaviour. 相似文献
7.
The nucleation, growth and coalescence of grain-boundary cavities is the primary damage mechanism observed during creep of structural ceramics. Furthermore, grain-boundary sliding (GBS) has been identified as the driving force process. Although the creep characteristics of structural ceramics have been extensively studied, very little is known about the details of GBS during creep and how GBS relates to cavitation kinetics. This paper presents the results of a study using a machine vision system to measure Mode II GBS displacements in a Lucalox Al2O3. Specifically, sliding displacements as large as 0.4 m were measured. The measured displacements indicate that some grain boundaries experienced shear strains and strain rates of 4200% and 2.3×10–2 s–1, respectively. The techniques utilized for these measurements are described in detail, and data gathered during a 2 1/2 h compressive creep test under a stress of 138 MPa at 1600 °C are presented and discussed. 相似文献
8.
SiC continuous fibre (15 vol%)/AlN composite was fabricated using a sintering additive of 4Ca(OH)2 · Al2O3 by hot-pressing at 1650 °C and 17.6 MPa in vacuum. Analytical transmission electron microscopy and scanning electron microscopy were used to investigate the microstructure of as-fabricated and crept SiC fibre/AlN composites. The room-temperature mechanical and high-temperature creep properties of the composite were investigated by four-point bending. The incorporation of SiC fibre into AlN matrix improved significantly the room-temperature mechanical properties. This improvement could result from the crack deflections around the SiC fibres. However, the incorporation degraded severely the high-temperature creep properties under oxidizing atmosphere. This could be attributed to the development of the pores and various oxides at the matrix grain boundary and matrix/fibre interface during creep test. 相似文献
9.
Bi(Al)-Ca-Sr Cu-O glass-ceramic fibres over 100 cm in length were successfully drawn from a glass preform above the crystallization temperature,T
x. The diameters of the uniformly drawn fibres with circular cross-section could be controlled in the range from 25–200 m and the drawing speed was as high as 200 cm min–1. In this work Al2O3 was used to modify the properties of the glass. It increased the glass transition and crystallization temperatures but did not significantly increase the glass working range. Shrinkage and increase of density during heat treatment of the glass fibres were observed. The annealed (825°C/12 h in air) Bi4Al0.1Ca3Sr3Cu4Oy glass-ceramic fibre showed aT
c(onset) of 82 K and aT
c(zero) of 71 K. 相似文献
10.
T. Ota T. Takebayashi M. Takahashi Y. Hikichi H. Suzuki 《Journal of Materials Science》1996,31(6):1431-1433
Orthorhombic kalsilite (KAlSiO4) was prepared by solid-state reaction from K2CO3, Al2O3, and SiO2. The axial thermal expansion coefficients of the orthorhombic kalsilite were 1.6×10–5°C–1 for the a-axis, 1.6×10–5°C–1 for the b-axis, 2.8×10–5°C–1 for the c-axis, and 2.0×10–5°C–1 for the average from room temperature to 1000°C. A high thermal expansion ceramic consisting of the orthorhombic kalsilite was prepared by sintering. The densification was promoted by adding Li2CO3. The KAlSiO4 ceramic sintered at 1200°C for 2 h with 5 wt% Li2CO3 had a bending strength of 65 MPa and linear thermal expansion coefficient of 2.2×10–5 °C–1 from room temperature to 600°C. 相似文献
11.
Mechanical and thermal properties of silicon-carbide composites fabricated with short Tyranno® Si-Zr-C-O fibre 总被引:1,自引:0,他引:1
K. Itatani K. Hattori D. Harima M. Aizawa I. Okada I. J. Davies H. Suemasu A. Nozue 《Journal of Materials Science》2001,36(15):3679-3686
Silicon carbide (SiC) composites reinforced with 10–50 mass% (10.5–51.2 vol%) of short Tyranno® Si-Zr-C-O fibre (average length 0.5 mm) and 0–10 mol% of Al4C3as a sintering aid were fabricated using the hot-pressing technique. Firstly, the effect of Si-Zr-C-O fibre addition on the relative density (bulk density/true density) of the SiC composite hot-pressed at 1800 °C for 30 min was examined by fixing the amount of Al4C3to be 5 mol%. Although the relative density was reduced to 87.4% for 10 mass% of Si-Zr-C-O addition, further increases in the amount of Si-Zr-C-O fibre increased density to a maximum of 92.8% at 40 mass% of fibre addition. Secondly, the effect of varying the amount of Al4C3addition on the relative density was examined by fixing the amount of Si-Zr-C-O fibre to be 40 mass%. The optimum amount of Al4C3addition for the fabrication of dense SiC composite was found to be 5 mol%. The fracture toughness of the hot-pressed SiC composites with 20–40 mass% of Si-Zr-C-O fibre addition (amount of Al4C3: 5 mol%) was 3.2–3.4 MPa · m1/2and approximately 1.5 times higher than that (2.39 MPa · m1/2) of the hot-pressed SiC composite with no Si-Zr-C-O fibre addition. SEM observation showed evidence of Si-Zr-C-O fibre debonding and pull-out at the fracture surfaces. The hot-pressed SiC composite with 5 mol% of Al4C3and 40 mass% of Si-Zr-C-O fibre additions showed excellent heat-resistance at 1300 °C in air due to the formation of a SiO2layer at and near exposed surfaces. 相似文献
12.
Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness 总被引:1,自引:0,他引:1
Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO2 as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10–5 h–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si3N4. 相似文献
13.
Raw, dewaxed and oxidized jute fibres and those chemically modified with phenol and formaldehyde (treatment done for 3 h at 95° C and pH 8) before or subsequent to oxidation, were subjected to graft copolymerization with methyl methacrylate (MMA) in the presence or absence of some other monomers such as maleic anhydride (MA) or methacrylic acid (MAA) in limited aqueous system using K2S2O8 as the initiator under photoconditions with the objective of inducing improved rot resistance and dyeability without loss in tensile strength of the fibre. For preparing oxy-jute, dewaxed and preswollen (dewaxed jute fibre swollen with 10% aqueous urea at 90° C for 2h) jute fibre were separately subjected to mild oxidation or bleaching using aqueous H2O2 and NalO4 and non-aqueous chlorine (Cl2 in CCl4) under specified conditions. Optimum conditions for graft copolymerization have been established by examination of the effect of such variables as monomer concentration, time of polymerization and nature of chemical modification of jute fibre prior to vinyl grafting. Percentage grafting, tenacity (g denier–1), dye fixation (%) and rot resistance (expressed as percentage retention of tensile strength of the fibre after a standard soil burial test) were evaluated and analysed. High rot resistance (80–90% retention of tensile strength after soil burial test) and dye fixation (%) of as high as 86% were readily obtained for grafted jute fibres. Washing fastness and light fastness properties of the dyed fibres (grafted and ungrafted) were also examined and compared. 相似文献
14.
Y. Waku N. Nakagawa H. Ohtsubo A. Mitani K. Shimizu 《Journal of Materials Science》2001,36(7):1585-1594
Unidirectionally solidified Al2O3/Y3Al5O12 (YAG) or Al2O3/Er3Al5O12 (EAG) eutectic composites, which are named as Melt Growth Composites (MGCs) has recently been fabricated by unidirectional solidification. The MGCs have a new microstructure, in which continuous networks of single-crystal Al2O3 phases and single-crystal oxide compounds (YAG or EAG) interpenetrate without grain boundaries. The MGCs fabricated are thermally stable and has the following properties: 1) the flexural strength at room temperature can be maintained up to 2073 K (just below its melting point), 2) a fracture manner from room temperature to 2073 K is an intergranular fracture different from a transgranular fracture of sintered composite with the same composition, 3) the compressive creep strength at 1873 K and a strain rate of 10–4/sec is 7–13 times higher than that of sintered composites, 4) the mechanism of creep deformation is based on the dislocation creep models completely different from the Nabarro-Herring or Coble creep models of the sintered composites, and 5) it shows neither weight gain nor grain growth, even upon heating at 1973 K in an air atmosphere for 1000 hours. The above superior high-temperature characteristics are caused by such factor as the MGCs having a single-crystal Al2O3/single-cryatal oxide compounds without grain boundaries and colonies, and the formation of the thermodynamically stable and compatible interface without amorphous phase. 相似文献
15.
A particulate precursor of superconductive Bi2Sr2CaCu2 oxide dispersed in an aqueous poly(vinyl alcohol) solution, was shaped into fibres. The fibres fired at a T
max of 850 °C were very porous. Although they exhibited a Meissner on-set transition at 85 K, electrical resistance at 0.02 Acm–2 at 77 K was still not zero. Exposing the porous fibres to a temperature slightly above their melting point densified them. The treated fibres had a sharp Meissner transition and could carry critical current (J
c) as high as 331 A cm–2 at 77 K. They were also more resistant to the effect of a magnetic field than the fibre fired at 850 °C. Eighty percent of the treated fibre was found to be the superconductive 2 2 1 2 phase. 相似文献
16.
Microstructural changes occurring during sliding wear of self-mated Al2O3-SiC whiskerreinforced composites were studied using optical, scanning electron microscopy and transmission electron microscopy. Pin-on-disc specimens were slid in air at 2.7 m s–1 sliding velocity under a 26.5 N load for 1 h. Wear tests were conducted at 23, 600, 800 and 1200°C. Mild wear with a wear factor of 2.4 x 10–7–1.5 x 10–6 mm3 N–1 m–1 was experienced at all test temperatures. The composite showed evidence of wear by fatigue mechanisms at 800°C and below. Tribochemical reaction (SiC oxidation and reaction of SiO2 and Al2O3) leads to intergranular failure at 1200°C. Distinct microstructural differences existing at each test temperature are reported.Resident Research Associate at NASA Lewis Research Center. 相似文献
17.
Because silica refractory has good volume stability and creep properties at high temperature, it has been used in several furnaces. However, silica has three polymorphs (quartz, tridymite and cristobalite) and each polymorph has an- type transformation. It is known that cristobalite is the stable phase of silica between 1470° C and the melting point of silica refractories. However, sometimes cristobalite was found in silica refractories used in the stable temperature region of tridymite. Therefore, the cause and mechanism of this tridymite-to-cristobalite transformation below 1470° C was studied. Although the transformation temperature of tridymite to cristobalite was also 1470° C in the sample used in this research, it decreased on addition of Al2O3. The apparent activation energy of the tridymite-to-cristobalite transformation was found to be 787 kJ mol–1 above 1470° C and 176 kJ mol–1 below 1470° C with Al2O3 by measuring the transformation rate. It was also observed using EPIVIA that the tridymite included CaO; however, CaO and Al2O3 were located on the outside of the cristobalite which was produced below 1470°C. Therefore, it is supposed that the liquid phase was produced by the penetration of Al2O3, and impurities in the tridymite crystal diffused outside and then silica was precipitated as cristobalite. 相似文献
18.
CaO-Al2O3-MgO (CAM) ceramic fibre produced via inviscid melt spinning (IMS) was investigated for phase transformation. Differential thermal analysis (DTA) on the as-spun CAM fibre gave two transformation peaks, one for exothermic peak at around 927°C and the other for endothermic one at around 1100°C. In order to identify each phase transformation x-ray diffraction (XRD) analysis was performed on the CAM fibres heat-treated to each phase transformation completion temperature. The exothermic peak was determined to represent crystallization of remaining amorphous phase in the as-spun CAM fibre. The endothermic peak was determined to correspond to transformation of non-equilibrium CaO · Al2O3 phase to equilibrium 3CaO· 5Al2O3 phase. 相似文献
19.
C. -H. Hsu H. W. Jacobson J. T. Schwartz J. D. Connolly Jr R. F. Tietz 《Journal of Materials Science》1995,30(13):3467-3470
A particulate precursor of YBa2Cu3O7–x
was dispersed in a concentrated acetic/formic acid solution of yttrium, barium and copper acetates to form an extensible, viscous mixture. This dispersion was stable, unlike one prepared from superconductive YBa2Cu3O7–x
itself. Fibres extruded from the viscous dispersion were fired at 935 °C in the presence of oxygen.Although the fired fibres were not fully densified, they had a sharp Meissner transition temperature at 90 K and an apparent critical current density (J
c) of 900 A cm–2 (1800 A cm–2 corrected for porosity) at 77 K. The superconducting fibre was sensitive to magnetic field, which is evidence of weak coupling between the superconducting grains. 相似文献
20.
W. A. Groen P. F. Van Hal M. J. Kraan N. Sweegers G. De With 《Journal of Materials Science》1995,30(19):4775-4780
The synthesis of Al28C6O21N6 powder (ALCON), starting from the binary compounds is described. The powder is resistant to oxidation in air up to 760°C. From the prepared powder, fully dense ceramics have successfully been prepared using hot pressing. The as-prepared ceramics had a thermal conductivity of 20 W m–1 K–1. Experiments showed that it is also possible to prepare ALCON ceramics by reactive hot-pressing, starting from Al2O3, AlN and Al4C3. Further optimization is expected to raise the thermal conductivity significantly. The strength, about 300 MPa, is similar to that of AlN. The thermal expansion coefficient of 4.8 × 10–6K–1 closely matches that of silicon, making application of ALCON ceramics as heat sinks an interesting possibility. 相似文献