泥质粉砂岩蠕变特性及非线性蠕变模型研究

为揭示地下岩体非线性蠕变力学特性,对中风化泥质粉砂岩开展分级单轴加载蠕变试验。泥质粉砂岩典型蠕变曲线可划分为减速蠕变、稳态蠕变和加速蠕变阶段,使用给定蠕变速率阈值法求得的岩石长期强度为14.3 MPa。为了描述岩石非线性蠕变特性,引入了一个与时间应力水平相关的非线性黏塑性元件,将其与广义Kelvin体和带开关的黏性体串联,得到了改进的非线性黏弹塑性蠕变模型。使用Origin平台的Levenberg-Marquardt非线性最小二乘法反演得到模型的蠕变力学参数,通过将广义Kelvin蠕变模型、伯格斯蠕变模型和改进黏弹塑性蠕变模型与试验曲线进行比较,分析了各自的适用特点。结果表明：本研究提出的改进黏弹塑性蠕变模型可以较好地描述中风化泥质粉砂岩加速蠕变阶段特征,揭示了泥质粉砂岩的非线性蠕变力学特性。     

P92钢的蠕变损伤容许量系数及蠕变断裂机理

利用P92钢在595、610、640、670℃的高应力试验条件下的蠕变试验数据,得出其Norton应力指数,依据Norton应力指数的大小判定其蠕变机理为位错蠕变。同时结合1种新的蠕变变形及断裂模型,引入将蠕变损伤看作1个内在的阶段变量的蠕变损伤容许量系数,根据蠕变损伤容许量λ=2.94,判断其蠕变变形和断裂是位错运动控制的。微观组织的观察也表明,蠕变后的试样中位错密度大大降低,高密度位错是P92钢持久强度高的原因,伴随着位错密度的下降,P92钢持久强度降低直至断裂。     

Creep of titanium-silicon alloys

Operative creep mechanisms in laboratory melts of Ti-5Zr-0.5Si and Ti-5Al-5Zr-0.5Si have been investigated as a function of microstructure, creep stress, and temperature. From creep rate data and transmission electron microscopy results, it has been shown that an important creep strengthening mechanism at 811 K in Si bearing Ti alloys is clustering of solute atoms on dislocations. All of the alloys investigated showed anomalously high apparent activation energies and areas for creep, and a high exponent (n) in the Dorn equation. In addition, the effect of heat treatment was investigated and it is shown that the highest creep strength was obtained by using a heat treatment which retained the maximum amount of silicon in solution. This is consistent with the proposed creep strengthening mechanism. An investigation of the creep behavior of several other Si containing alloys including two commercial alloys, Ti-11 and IMI-685 indicated similar results.     

Creep of titanium-silicon alloys

Operative creep mechanisms in laboratory melts of Ti-5Zr-0.5Si and Ti-5Al-5Zr-0.5Si have been investigated as a function of microstructure, creep stress, and temperature. From creep rate data and transmission electron microscopy results, it has been shown that an important creep strengthening mechanism at 811 K in Si bearing Ti alloys is clustering of solute atoms on dislocations. All of the alloys investigated showed anomalously high apparent activation energies and areas for creep, and a high exponent (n) in the Dorn equation. In addition, the effect of heat treatment was investigated and it is shown that the highest creep strength was obtained by using a heat treatment which retained the maximum amount of silicon in solution. This is consistent with the proposed creep strengthening mechanism. An investigation of the creep behavior of several other Si containing alloys including two commercial alloys, Ti-11 and IMI-685 indicated similar results.      

Creep mechanisms in beryllium

Observations of beryllium samples which have been creep tested between 922 K and 1422 K indicate that creep behavior is controlled by the relative strengths of the grain boundaries and the matrix. Since creep deformation can occur predominantly by grain boundary sliding or entirely by deformation within the grains, the creep strength was found to be controlled by the weaker of the two features. Low melting phases containing aluminum and silicon which formed along the grain boundaries acted as stress concentrations which favored localized grain boundary deformation, and recrystallization. Creep resistance was found to drop markedly when the BeO content was reduced substantially below 1 pct.     

Creep mechanisms in beryllium

Observations of beryllium samples which have been creep tested between 922 K and 1422 K indicate that creep behavior is controlled by the relative strengths of the grain boundaries and the matrix. Since creep deformation can occur predominantly by grain boundary sliding or entirely by deformation within the grains, the creep strength was found to be controlled by the weaker of the two features. Low melting phases containing aluminum and silicon which formed along the grain boundaries acted as stress concentrations which favored localized grain boundary deformation, and recrystallization. Creep resistance was found to drop markedly when the BeO content was reduced substantially below 1 pct.     

Ti-600合金蠕变性能及硅对其蠕变性能的影响

研究了Ti-600合金在3种温度(550、600、650℃)、5种应力(150、200、250、300、350 MPa)下的蠕变性能,并分析了硅化物对合金蠕变性能的影响。研究结果表明,Ti-600合金具有较小的稳态蠕变速率及较大的蠕变激活能,反映出该合金具有较好的蠕变抗力。当温度升高、应力增大时,Ti-600合金的稳态蠕变速率增大。600℃下,当蠕变应力高达350 MPa时,Ti-600合金的稳态蠕变速率低至3.72×10-7s-1。Ti-600合金的蠕变激活能最高可达574.6kJ·mol-1,最低为332.7 kJ·mol-1。在蠕变过程中,Ti-600合金内析出了S2型(TiZr)6Si3硅化物,能够钉扎位错、阻碍位错滑移,提高合金的蠕变抗力。     

Creep behavior of electron-beam-melted rhenium

Creep deformation in electron-beam-melted polycrystalline rhenium sheet was evaluated at 2200° to 4200°F (1477° to 2588°K) and 4 to 40 ksi (28 to 276 MN per sq m). Comparisons were made with powder metallurgy rhenium under similar conditions. Changes in creep-rupture behavior resulting from electron beam melting of rhenium were greater ductility, higher primary creep rate, and longer rupture life, especially at lower temperatures. The activation energy for creep was 72 kcal per mole for electron-beam-melted rhenium and 64 kcal per mole for powder metallurgy rhenium.     

Creep deformation of TD-nickel chromium

The creep behavior of thoria dispersed nickel-chromium (TD-NiCr) was examined at 1093°C (2000°F). Major emphasis was placed on 1) the effects of the material and the test related variables (grain size, temperature, stress, strain and strain rate) on the deformation characteristics, and 2) the evaluation of single crystal TD-NiCr material produced by a directional recrystallization technique. Creep activation enthalpies were found to increase with increasing grain size reaching maximum values for the single crystal TD-NiCr. Stress exponent of the steady state creep rate was also significantly higher for the single crystal material as compared to that determined for the polycrystalline TD-NiCr. The elevated temperature deformation of TD-NiCr was analyzed in terms of two parallel-concurrent processes: 1) diffusion controlled grain boundary sliding and 2) dislocation motion. The characteristics of the dislocation motion deformation mode (as observed in the single crystal TD-NiCr) suggest that strong particle-dislocation interactions are present. The relative contributions of dislocation motion and grain boundary sliding in TD-NiCr were estimated. In creep, grain boundary sliding was found to predominate for the small, equiaxed grain structures, whereas the dislocation deformation mode became significant for only the large grain TD-NiCr and the single crystal material. Formerly Graduate Assistant, Case Western Reserve University, Cleveland, Ohio 44106.     

Creep strength of magnesium-based alloys

The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Creep of laminated aluminum composites

The creep behavior of a laminate system consisting of alternate layers of pure aluminum and SAP (sintered aluminum powder) sheet has been examined in the temperature range 323 to 473 K and in the stress range 35 to 68 MN m⁻². It was observed that secondary creep strain in the laminates was greater than in elemental SAP; the secondary creep strain rate in laminates was lower than that in pure aluminum and the creep rate decreased with increasing fracture of SAP. A stress exponent (n) value of ∼20 was observed for most of the laminates and was reasonably constant for 3, 5, 7, and 9 ply laminates and volume fractionsV _f ) in the range 0.3 <V _f < 0.65. For higher volume fractions of SAP the mechanical behavior of the laminates was similar to that of SAP. The experimental activation energy for creep of 30.5 ± 5 Kcal mol⁻¹ correlates well with that for self-diffusion in aluminum. Laminating induced appreciable ductility to the SAP. Formerly Postgraduate Research Student, Department of Metallurgy, University of Manchester/UMIST     

Creep of aluminum-based closed-cell foams

Metal foams creep when loaded mechanically at high homologous temperatures. We have studied the creep behavior of closed-cell aluminum-based foams with relative densities of 0.092, 0.112, and 0.163. Compressive creep tests were performed at 300 °C at strain rates ranging from 10⁻⁹ to 10⁻⁴ s⁻¹. Special efforts were made to produce and characterize a bulk reference material exhibiting the same chemical composition. Results show that the foams exhibit a lower creep strength and a higher stress exponent than predicted by the Gibson-Ashby model for regular foams. The possible mechanisms responsible for this deviation are discussed. A semi-empirical rate equation is established which describes the experimental data well.     

Creep strength of magnesium-based alloys

The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Creep at very low rates

The creep rate in a land-based power station must be less than 10⁻¹¹ s⁻¹. At these low rates of deformation the transport of matter occurs by the migration of vacancies rather than by the glide of dislocations. A quantitative understanding of these diffusional processes is, therefore, important. First type of diffusional creep (Nabarro-Herring (N-H)): the sources and sinks of vacancies are grain boundaries. The vacancies may diffuse through the bulk of the grain or along the grain boundaries (Coble (C)). Second type (Harper-Dorn (H-D)): the vacancies diffuse from edge dislocations with their Burgers vectors parallel to the major tensile axis to those with Burgers vectors perpendicular to this axis. The coherence of the polycrystalline aggregate is maintained by sliding along the grain boundaries. The three mechanisms of vacancy migration, grain boundary sliding, and dislocation glide may all interact. The theories of N-H and C creep in pure metals are established and confirmed, but H-D creep and grain boundary sliding are less well understood. Practical engineering materials are usually strengthened by precipitates that accumulate on grain boundaries and slow down creep in complicated ways. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in Dan Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

Creep rupture under non-proportional loading

The quasi-empirical metallurgical theory of Dyson and McLean have been used to predict the behaviour of virgin Nimonic 80A tubes tested at 750°C under reverse torsion creep. The same theory has been modified to describe the behaviour of pre-strained Nimonic 80A tubes tested under conditions of constant and reverse torque. In all cases nucleation appears to be a scalar quantity and creep damage or cavity volume fraction is vectorial in character.     

Creep of Polymer Matrix Composites. I: Norton/Bailey Creep Law for Transverse Isotropy

This research concerns polymer matrix composite (PMC) materials having long or continuous reinforcement fibers embedded in a polymer matrix. The objective is to develop comparatively simple, designer friendly constitutive equations intended to serve as the basis of a structural design methodology for this class of PMC. Here (Part I), the focus is on extending the deformation model of an anisotropic deformation/damage theory presented earlier. The resulting model is a generalization of the simple Norton/Bailey creep law to transverse isotropy. A companion paper (Part II) by the writers deals with damage and failure of the same class of PMC. An important feature of the proposed deformation model is its dependence on hydrostatic stress. Characterization tests on thin-walled tubular specimens are defined and conducted on a model PMC material. Additional exploratory tests are identified and carried out for assessing the fundamental forms of the multiaxial creep law.     

Effect of Tungsten on Primary Creep Deformation and Minimum Creep Rate of Reduced Activation Ferritic-Martensitic Steel

Effect of tungsten on transient creep deformation and minimum creep rate of reduced activation ferritic-martensitic (RAFM) steel has been assessed. Tungsten content in the 9Cr-RAFM steel has been varied between 1 and 2 wt pct, and creep tests were carried out over the stress range of 180 and 260 MPa at 823 K (550 °C). The tempered martensitic steel exhibited primary creep followed by tertiary stage of creep deformation with a minimum in creep deformation rate. The primary creep behavior has been assessed based on the Garofalo relationship, \( \varepsilon = \varepsilon_{\text{o}} + \varepsilon_{\text{T}} [1-\exp (-r^{\prime} \cdot t)] + \dot{\varepsilon }_{\text{m}} \cdot t \) , considering minimum creep rate \( \dot{\varepsilon }_{\text{m}} \) instead of steady-state creep rate \( \dot{\varepsilon }_{\text{s}} \) . The relationships between (i) rate of exhaustion of transient creep r′ with minimum creep rate, (ii) rate of exhaustion of transient creep r′ with time to reach minimum creep rate, and (iii) initial creep rate \( \dot{\varepsilon }_{\text{i}} \) with minimum creep rate revealed that the first-order reaction-rate theory has prevailed throughout the transient region of the RAFM steel having different tungsten contents. The rate of exhaustion of transient creep r′ and minimum creep rate \( \dot{\varepsilon }_{\text{m}} \) decreased, whereas the transient strain ? _T increased with increase in tungsten content. A master transient creep curve of the steels has been developed considering the variation of \( \frac{{\left( {\varepsilon - \varepsilon_{\text{o}} } \right)}}{{\varepsilon_{\text{T}} }} \) with \( \frac{{\dot{\varepsilon }_{\text{m}} \cdot t}}{{\varepsilon_{\text{T}} }} \) . The effect of tungsten on the variation of minimum creep rate with applied stress has been rationalized by invoking the back-stress concept.     

Creep deformation of ductile two-phase alloys

A continuum mechanics model is developed to explain the creep deformation of ductile two-phase alloys. The model predicts that the transient creep is caused by the internal stresses in second phase and matrix resulting from the difference in creep strain between two phases induced by the strength difference, even if the inherent transient creep in both phases is not taken into account. The difference in creep strain between two phases in steady-state creep is analytically obtained for the alloys in which both second phase and matrix exhibit the exponential law, the power-law or the hyperbolic sine law creep. The continuum mechanics model gives the same values of steady-state creep rate as the constant creep rate model by McDanels and co-workers. The results of analysis based on the continuum mechanics model are compared with the experimental results.