首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 109 毫秒
1.
The volume fractions of stress-induced martensite formed by certain plastic strains were determined by X-ray diffraction and quantitative metallography in an Fe–Mn–Si–Cr–N alloy at room temperature. The results are fitted by least square method and are well consistent with an exponential function fM=1−exp{−β[1−exp(−η)]n} deduced by Olson and Cohen, who used it to fit with experimental data for AISI304 stainless steel. The similarity of and β, as well as the difference in n for these two alloys are discussed in relation to their nucleation mechanisms.  相似文献   

2.
A series of Cr–Ni alloys were overlaid on a Fe3Al surface by tungsten inert gas arc welding (TIG) technology. The microstructure of the Cr–Ni surface layers were analysed by means of optical metallography, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that when the appropriate TIG parameters were used and Cr25–Ni13 and Cr25–Ni20 alloys were used for the overlaid materials, the Cr–Ni surface layers were crack-free. The matrix of the surface layer was austenite (A), pro-eutectoid ferrite (PF), acicular ferrite (AF), carbide-free bainite (CFB) and lath martensite (LM), distributed on the austenitic grain boundaries as well as inside the grains. The phase constituents of the Cr25–Ni13 surface layer were γ-Fe, Fe3Al, FeAl, NiAl, an Fe–C compound and an Fe–C–Cr compound. The microhardness of the fusion zone was lower than that of the Fe3Al base metal and Cr25–Ni13 surface layer.  相似文献   

3.
The relation between the γ/γ + boundary temperature, T, and the equivalent values of [Cr] and [Ni], as well as the variation of the ferrite volume, Vf, with the temperature in + γ dual-phase steels have been studied. With the aid of a computer, the regressive expressions derived from the experimental results are: TC) = T3 + 21.2 [Cr] − 15.8 [Ni] + 223; Vf (%) = 0.715 exp [0.015(TTδ)] − exp[0.015(TcTδ)] + 1.85 exp [0.0083(TTc]).  相似文献   

4.
Various methods have been presented to obtain the effective damping of a symmetric laminated composite. In this paper, modified classical lamination theory based upon the elastic–viscoelastic correspondence principle was utilized by developing the basic damping of Poisson's ratio for accurately predicting the damping of laminated composite beams. The analysis involves an extension of the elastic–viscoelastic approach. Futhermore, Ni and Adams' theory was used for verifying the modified classical lamination theory. Six typical laminated composites with [±θ]s,[0/θ]s,[0/±θ/90]s,[90/±θ/0]s,[0/90/±θ]s and [90/0/±θ]s, stacking sequences were employed for this study. Numerical results have shown that damping values were in some difference among prediction methods over the particular range of fiber orientation.  相似文献   

5.
We consider a Ginzburg-Landau model free energy F(ε, e1, e2) for a (2D) martensitic transition, that provides a unified understanding of varied twin/tweed textures. Here F is a triple well potential in the rectangular strain (ε) order parameter and quadratic e12, e22 in the compressional and shear strains, respectively. Random compositional fluctuations η(r) (e.g. in an alloy) are gradient-coupled to ε, ˜ − ∑rε(r)[(Δx2 − Δy2)η(r)] in a “local-stress” model. We find that the compatibility condition (linking tensor components ε(r) and e1(r), e2(r)), together with local variations such as interfaces or η(r) fluctuations, can drive the formation of global elastic textures, through long-range and anisotropic effective ε-ε interactions. We have carried out extensive relaxational computer simulations using the time-dependent Ginzburg-Landau (TDGL) equation that supports our analytic work and shows the spontaneous formation of parallel twins, and chequer-board tweed. The observed microstructure in NiAl and FexPd1 − x alloys can be explained on the basis of our analysis and simulations.  相似文献   

6.
The microwave dielectric properties and microstructures of Ba(Mg1/3Ta2/3)O3 (BMT) ceramics sintered at low temperatures with 2–3 wt.% NaF additives were investigated. BMT ceramics sintered at 1340 °C for 3–12 h showed dielectric constants (r) of 25.5–25.7, Qf values of 41 500–50 400 GHz and temperature coefficients of the resonator frequency (τf) of 10.9–21.4 ppm °C−1. The variation of sintering time almost had no effect on the dielectric constant. The Qf value increased and the τf decreased with increasing sintering time. The ordering degree of Mg2+ and Ta5+ at B-sites increased with increasing sintering time.  相似文献   

7.
A series in this journal on high-temperature properties of “fracture-resistant ultralloys for space-power systems” preceded the present paper: the antecedent publications covered tungsten(W), rhenium(Re) alloys with and without thoria(ThO2) (W, 23Re; W, 27Re; W, 30Re and W, 30Re, 1ThO2). This paper reports radiative and thermionic effects of hafnium carbide(HfC) and Re variation in W alloys: normal spectral emissivity(ελ) is used in pyrometry to determine the true temperature of a surface. Effective work function (φe) is an important consideration in the selection of the electrode materials for high-temperature thermionic energy converters in space-power applications. The 0.535μ, ε0.65μ and φe trends of W, Re, 0.35HfC with 5–20% Re were measured in the range of 1700–2500K. The results indicate that ελ decreases with increasing temperatures and Re contents. The presence of HfC produced higher ελ values than those of sintered materials with comparable W,Re alloy contents. The results also indicate that φe increases with rhenium contents. This can be explained as growth of the potential barrier at the metal, vacuum boundary associated with a volume effect—the decrease in the lattice constant of W.  相似文献   

8.
In this study the effect of copper addition on the structure, precipitation kinetics and hardness in the Al–Li and Al–Li–Cu alloys aged at 200°C was investigated. The structures of precipitates were studied using X-ray-small-angle-scattering (XSAS) and transmission electron microscopy (TEM) methods. The changes in the structure parameter (Rg) of both alloys was calculated using two methods, the Guinier approximation and correlation function γ(r). By use of a plot of r γ(r) the distribution law of the T1 disc thickness was obtained and the coexisting spherical particles of δ′ were estimated. Two types of δ′ precipitates of approximately 2 nm size and above 8 nm and the T1 precipitates of thickness between 3 and 4 nm were observed.  相似文献   

9.
The change of magnetoelastic properties after thermal treatments has been investigated for two groups of metallic glasses. (Fe79Co21)75+xSi15−1.4xB10+0.4x (x (at.%)=0, 2, 4, 6, 8, 10) has been studied both in the as-prepared state and after thermal annealing in an applied magnetic field, to achieve a particular domain structure, at temperatures well below the crystallization temperatures. Changes in the ΔE effect, magnetomechanical coupling (k) and internal friction coefficient (Q−1) are reported, reaching values of about 60% of the saturation value ES. Fe64Ni10Nb3Cu1Si13B9 alloys annealed in vacuum for 1 h in the temperature range 350–550 °C showed maximum values of the ΔE effect and k of 61% and 0.85, respectively, accompanied by a minimum value of Q of around 2 for the sample annealed at 460 °C. These variations are related to the progress of nanocrystalization. The properties achieved are among the best reported for magnetomechanical applications.  相似文献   

10.
A fracture criterion for three-dimensional crack problems   总被引:5,自引:0,他引:5  
A criterion for predicting the growth of three-dimensional cracks is developed on the basis of the strain energy density concept which has been used successfully for treating two-dimensional crack problems. Fracture is assumed to initiate from the nearest neighbor element located by a set of spherical coordinates (r, θ, φ) attached to the crack border. The new fracture surface is described by a locus of these elements whose locations correspond to the strain energy function, dW/dV, being a minimum. The function dW/dV is found to be singular of the type 1/r and is of quadratic form in the three stress intensity factors k1, k2 and k3 expressed through the strain energy density factor S. It is postulated that unstable crack propagation initiates from a region where S reaches a critical value Scr = r0(dW/dV)cr. The locations of failure lying on the fracture surface is determined by holding (dW/dV)cr = Smin/r0 constant. The quantity Smin stands for the value of S minimized with respect to θ and φ and r0 is a radial distance measured from the crack border.

An example of failure prediction for an embedded elliptical crack subjected to both normal and shear loads is presented. According to the S-criterion, fracture initiation takes place at the ends of the minor axis. An unexpected result is that for a narrow elliptical crack and Poisson's ratio of 1/3 the lowest failure load occurs when the uniaxial tensile load makes an angle of approximately 60° with the crack surface and is in the plane of the major axis. This is in contrast to the expectation that the lowest critical load occurs when the uniaxial tension is perpendicular to the crack surface. In the limit as the elliptical crack becomes increasingly narrower, the result reduces to the two dimensional line crack case of Mode I and III loading. The S-criterion is also applied to the failure prediction of three dimensional cracks under compressive loads.  相似文献   


11.
Physical and electrical properties of hafnium silicon oxynitride (HfSixOyNz) dielectric films prepared by UV ozone oxidation of hafnium silicon nitride (HfSiN) followed by annealing to 450 °C are reported. Interfacial layer growth was minimized through room temperature deposition and subsequent ultraviolet/ozone oxidation. The capacitance–voltage (CV) and current–voltage (IV) characteristics of the as-deposited and annealed HfSixOyNz are presented. These 4 nm thick films have a dielectric constant of 8–9 with 12 at.% Hf composition, with a leakage current density of 3×10−5 A/cm2 at Vfb+1 V. The films have a breakdown field strength >10 MV/cm.  相似文献   

12.
The microstructures and mechanical properties of binary and ternary Cr-based alloys containing Nb, Zr, or both Nb and Zr, have been studied in both the as-cast and annealed conditions. The level of alloying in each instance was targeted to lie below, or approximately at, the maximum solubility in chromium. The as-cast microstructures of these alloys consisted of Cr-rich solid solution surrounded by small amounts of interdendritic Cr–Cr2X eutectic structure. Annealing at 1473 K resulted in solid-state precipitation of the Cr2X Laves phase in the Cr–Nb and Cr–Nb–Zr alloys, but not in the Cr–Zr alloys. The binary Cr2Nb phase consisted of an extensively twinned ({111}<112> twins) C15 structure whereas the presence of Zr modifies its appearance substantially; the twinned C15 structure persists. Oxides were occasionally present and their compositions were qualitatively determined. Vickers hardness primarily depended upon the volume fraction of the Cr2X Laves phase present. Age hardening due to solid-state precipitation of Cr2X Laves phase within the Cr-rich matrix was observed in the Nb-containing alloys. The room temperature bend strength of the alloys was strongly affected by the presence of grain-boundary Cr2X phase. It is considered that porosity as well as oxides in the alloys also lowers their bend strength.  相似文献   

13.
Solid solutions of Bi3(Nb1−xTax)O7 (x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h)−1 between 0 ≤ x ≤ 1 and Bi3(Nb1−xTax)O7 ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi3(Nb1−xTax)O7 ceramics increased from 8.2 to 9.1 g cm−3, microwave permittivity decreased from 95 to 65 while Qf values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τf from −113 ppm °C−1 of Bi3NbO7 to −70 ppm °C−1 of Bi3TaO7. Microwave permittivity, Qf values and τf values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi3(Nb1−xTax)O7 ceramics could be a good candidate for low temperature co-fired ceramics application.  相似文献   

14.
Based on thermodynamic considerations together with measurement of the stacking fault probability (Psf) by X-ray diffraction profile analysis, the stacking fault energy (SFE, γ) of austenite in Fe–Mn–Si shaped memory alloys can be estimated. For instance, the stacking fault energy of an fcc(γ) phase in an Fe–30.3Mn–6.06Si was calculated as 7.8 mJ/m2. Compositional dependence of stacking fault energy in these alloys with certain composition range has also been derived as SFE(γ)=180.54+7.923 wt.% Mn–46.38 wt.% Si (J/mol), showing that the stacking fault energy increases with the addition of Mn and decreases with the addition of Si.  相似文献   

15.
The BaxSr1−xTiO3 (BST)/Pb1−xLaxTiO3 (PLT) composite thick films (20 μm) with 12 mol% amount of xPbO–(1 − x)B2O3 glass additives (x = 0.2, 0.35, 0.5, 0.65 and 0.8) have been prepared by screen-printing the paste onto the alumina substrates with silver bottom electrode. X-ray diffraction (XRD), scanning electron microscope (SEM) and an impedance analyzer and an electrometer were used to analyze the phase structures, morphologies and dielectric and pyroelectric properties of the composite thick films, respectively. The wetting and infiltration of the liquid phase on the particles results in the densification of the composite thick films sintered at 750 °C. Nice porous structure formed in the composite thick films with xPbO–(1 − x)B2O3 glass as the PbO content (x) is 0.5 ≥ x ≥ 0.35, while dense structure formed in these thick films as the PbO content (x) is 0.8 ≥ x ≥ 0.65. The volatilization of the PbO in PLT and the interdiffusion between the PLT and the glass lead to the reduction of the c-axis of the PLT phase. The operating temperature range of our composite thick films is 0–200 °C. At room temperature (20 °C), the BST/PLT composite thick films with 0.35PbO–0.65B2O3 glass additives provided low heat capacity and good pyroelectric figure-of-merit because of their porous structure. The pyroelectric coefficient and figure-of-merit FD are 364 μC/(m2 K) and 14.3 μPa−1/2, respectively. These good pyroelectric properties as well as being able to produce low-cost devices make this kind of thick films a promising candidate for high-performance pyroelectric applications.  相似文献   

16.
In2O3 thin films have been prepared from commercially available pure In2O3 powders by high vacuum thermal evaporation (HVTE) and from indium iso-propoxide solutions by sol-gel techniques (SG). The films have been deposited on sapphire substrates provided with platinum interdigital sputtered electrodes. The as-deposited HVTE and SG films have been annealed at 500°C for 24 and 1 h, respectively. The film morphology, crystalline phase and chemical composition have been characterised by SEM, glancing angle XRD and XPS techniques. After annealing at 500°C the films’ microstructure turns from amorphous to crystalline with the development of highly crystalline cubic In2O3−x (JCPDS card 6-0416). XPS characterisation has revealed the formation of stoichiometric In2O3 (HVTE) and nearly stoichiometric In2O3−x (SG) after annealing. SEM characterisation has highlighted substantial morphological differences between the SG (highly porous microstructure) and HVTE (denser) films. All the films show the highest sensitivity to NO2 gas (0.7–7 ppm concentration range), at 250°C working temperature. At this temperature and 0.7 ppm NO2 the calculated sensitivities (S=Rg/Ra) yield S=10 and S=7 for SG and HVTE, respectively. No cross sensitivity have been found by exposing the In2O3 films to CO and CH4. Negligible H2O cross has resulted in the 40–80% relative humidity range, as well as to 1 ppm Cl2 and 10 ppm NO. Only 1000 ppm C2H5OH has resulted to have a significant cross to the NO2 response.  相似文献   

17.
We present the results of ab initio calculations of the spin-polarized electronic structure of disordered bcc and fcc Fe1 − xSix alloys from dilute solid solutions of Si on iron up to FeSi intermetallic compound composition (x=0.01..0.5) and ordered intermetallic phases FeSi in B20 structure and Fe3Si in DO3 structure. The self-consistent calculations were performed with the coherent potential approximation within the Korringa–Kohn–Rostoker method (KKR–CPA) for disordered case and by using the tight-binding linear muffin–tin orbital (TB LMTO) method for intermetallic compounds. In the last case the supercell approach has been utilized in order to take into account the structural defects for this ordered phase. In particular, we have performed a series of calculations in the BCC Fe1 − xSix alloys with x=0.01, 0.015, 0.1, 0.15, 0.25.  相似文献   

18.
The heat-pumping scheme based on the 1st order antiferromagnetism–ferromagnetism transition induced in FeRh alloy by one-dimensional tensile stress is proposed. Using the model S–T diagram for this alloy, the heat-pump cycles are drawn up based both on the transition latent heat absorption and emission when the transition is induced isothermally and on the change in alloy's temperature when the transition is induced adiabatically by applying tensile stress. The calculated values of heat coefficient φ for the cycles are 30 at ΔT=5 К and 20 at ΔT=10 К, where ΔT is the difference between the temperature surrounding and that of the heat receiver. These values are achieved using the tensile stress of 1·109 Pa. The high values of φ make it possible to consider Fe–Rh alloys near the equiatomic composition as an effective refrigerant for elastocaloric heat-pumping near the room temperature.  相似文献   

19.
A systematic review of the crystal chemical properties of the σ phase is presented, with special emphasis on the atomic order, i.e. the distribution of the atoms on the different sites of the crystal structure. The data available in the literature have been systematically assessed, and are complemented by an experimental investigation in the following systems: Al–Nb, Al–Ta, Cr–Mn, Cr–Os, Cr–Re, Cr–Ru, Co–Mo, Fe–Mo, Fe–Re, Mn–Mo, Mn–Re, Mn–V, Mo–Re, Nb–Pt, Nb–Re, Ni–V, Pd–Ta, Re–V, Rh–Ta and Ru–W. The properties are analyzed as a function of composition and the nature and atomic size of the elements involved. The possibility of an order–disorder transition has also been reviewed and completed by diffraction experiments in two systems (Cr–Mn and Ni–V). First-principles calculations on the σ phase are reviewed in line with the Calphad approach. An analysis of the literature data concerning the Calphad modeling of systems involving the σ phase has been made. The different models used are presented and discussed. The conclusions of crystal structure data analysis are used to make some recommendations about the choice of a model in the frame of a Calphad assessment.  相似文献   

20.
A general method is presented for determining mixed-mode stress intensity factors KI and KII from isochromatic fringes near the crack tip. The method accounts for the effects of the far-field, non-singular stress, σox. A non-linear equation is developed which relates the stress field in terms of KI, KII, and σox to the co-ordinates, r and θ, defining the location of a point on an isochromatic fringe of order N.

Four different approaches for the solution of the non-linear equation are given. These include: a selected line approach in which data analysis is limited to the line θ = π and the K---N relation can be linearized and simplified, the classical approach in which two data points at (rm, θm) are selected where rm/θ = 0; a deterministic method where three arbitrarily located data points are used; and an over-deterministic approach where m (>3) arbitrarily located points are selected from the fringe field.

Except for the selected line approach, the method of solution involves an iteractive numerical procedure based on the Newton-Raphson technique. For the over-deterministic approach, the method of least squares was employed to fit the K-N relation to the field data.

All four methods provide solutions to 0.1% providing that the input parameters r, θ, and N describing the isochromatic field are exact. Convergence of the iterative methods is rapid (3–5 iterations) and computer costs are nominal. When experimental errors in the measurements of r and θ are taken into consideration, the over-deterministic approach which utilizes the method of least squares has a significant advantage. The method is global in nature and the use of multiple-point data available from the full-field fringe patterns permits a significant improvement in accuracy of KI, KII, and σox determinations.  相似文献   


设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号