Part II. The creep behavior of Ti-Al-Nb O+bcc orthorhombic alloys

The intermediate-temperature (650 °C to 760 °C) creep behavior of orthorhombic (O)+bcc alloys containing 50 at. pct Ti was studied. Ti-25A1-25Nb, Ti-23Al-27Nb, and Ti-12Al-38Nb ingots were processed and heat treated to obtain a wide variety of microstructures. Creep deformation mechanisms and the effects of grain size, phase volume fraction, tension vs compression and aging on creep rates were examined. Unaged microstructures, which transformed during the creep experiments, exhibited larger primary creep strains than transformed microstructures, which were crept after long-term aging. The deformation observations and calculated creep exponents and activation energies suggested that separate creep mechanisms, dependent on the applied stress level, were dominating the secondary creep behavior. Coble creep characteristics, including relatively low activation energies and dislocation densities as well as stress exponents close to unity, were exhibited at low applied stresses. Experiments on fiducially marked specimens indicated that grain-boundary sliding was occurring for intermediate applied stresses. In this regime, the minimum creep rates were proportional to the applied stress squared and inversely proportional to the grain size. Dislocation-controlled creep characteristics, including stress exponents of greater than or equal to 3.5 and relatively high activation energies and dislocation densities, were exhibited at high stresses. Overall, the minimum creep rates were dependent on microstructure and stress. Within the low-to-intermediate stress regimes, subtransus processed and heat-treated microstructures, which contained much finer grain sizes than supertransus microstructures, exhibited the poorest creep resistance. The influence of grain size was not as significant within the high-stress regime. It is shown that for low-to-intermediate stress levels, grain size is the dominant microstructural feature influencing the creep behavior of O+bcc alloys.     

Viscoelastic Analysis of Constant Creep Tests on Silicate-Grouted Sands at Low Stress Levels

This paper presents the mechanical properties of silicate-grouted sands subjected to creep loadings at low stress levels. Uniaxial compressive tests were performed in order to determine the stress levels of constant creep tests. The uniaxial compressive strength rapidly increased with time over the first 7 days of curing and then approached a constant level. A series of creep tests were performed for three stress levels and viscoelastic theory was employed to assess the creep behavior. During the loading process elastic, plastic, and viscoelastic strains existed together. The recoverable portions contained elastic and time-dependent viscoelastic strains, and both were approximately linear. Test results showed that the magnitude of the instantaneous recoverable strains was independent of the unloading time. A constitutive model to predict the permanent deformation was developed.     

44Si2CrV钢弹簧的室温蠕变

试验结果得出,44Si2CrV钢弹簧经淬火+中温回火处理并除掉表面脱碳层后,在室温下所受应力超过临界蠕变应力值时即发生蠕变;钢的抗拉(或剪切)强度越高,临界蠕变应力越高。44Si2CrV钢900℃淬火,500℃回火,临界蠕变应力 τ_cc =700 MPa;900℃淬火,380℃回火, τ_cc =800 MPa。蠕变开始时的塑性变形速率较大,之后急剧减小,在一定应力下保持一定时间后蠕变停止。经长时间压缩,蠕变变形已经停止的弹簧,于200℃保温2 h后空冷,弹簧的自由高度伸长,并且在原来的应力下加载,弹簧蠕变又重新开始。     

Strain hardening during low temperature creep of 304 stainless steel

A study has been done of the strain hardening of 304 stainless steel during low temperature creep. Hardening is measured by additional loading (above the creep stress) after various creep times. Both the level and the shape of the strain versus stress curve are altered by creep; higher creep strain is linked to a higher offset yield stress and to a longer inelastic transient during the early stages of deformation during reloading. A theory is described in which the mobile dislocation density is determined by a competition between stress rate dependent injection and velocity dependent trapping. This theory predicts both the creep curve and the hardening effects of creep with good quantitative accuracy.     

Prediction of creep-rupture life of unidirectional titanium matrix composites subjected to transverse loading

Titanium matrix composites (TMCs) incorporating unidirectional fiber reinforcement are considered as enabling materials technology for advanced engines which require high specific strength and elevated temperature capability. The resistance of unidirectional TMCs to deformation under longitudinally applied sustained loading at elevated temperatures has been well documented. Many investigators have shown that the primary weakness of the unidirectional TMC is its susceptibility to failure under very low transverse loads, especially under sustained loading. Hence, a reliable model is required to predict the creep-rupture life of TMCs subjected to different transverse stress levels over a wide range of temperatures. In this article, we propose a model to predict the creep-rupture life of unidirectional TMC subjected to transverse loading based on the creep-rupture life of unidirectional TMC subjected to transverse loading based on the creep-rupture behavior of the corresponding fiberless matrix. The model assumes that during transverse loading, the effective load-carrying matrix ligament along a row of fibers controls the creep-rupture strength and the fibers do not contribute to the creep resistance of the composite. The proposed model was verified using data obtained from different TMC fabricated using three matrix compositions, which exhibited distinctly different types of creep behavior. The results show that the creep-rupture life of the transverse TMC decreases linearly with increasing ratio of the fiber diameter to the ply thickness. The creeprupture life is also predicted to be independent of fiber spacing along the length of the specimen.     

Factors Affecting Mechanical and Creep Properties of Silicate-Grouted Sands

Factors affecting the strength, modulus, stress-strain, and time-to-failure relationships of moist-cured silicate-grouted sands were investigated from short-term and creep tests. Variables included in the short-term tests were curing time, sand gradation and mineralogy, rate of loading, curing time, and confining pressure. Confining pressure was varied up to 550 kPa, and the stress and strain loading rates were varied from 0.05 to 5.0 Pa∕min and from 0.01 to 1.0%∕min, respectively. The shear strength and failure strain of moist-cured grouted sands were independent of the confining pressure, but they were affected by all other variables investigated. Compressive failure strains for silicate-grouted sands were less than 0.4% and the limitation in improving the compressive strength of sand has been quantified. Grouted limestone sand had the highest strength. The creep behavior of grouted sand was also investigated. Stress-strain and time-to-failure relationships for grouted sands have been developed.     

Creep deformation properties of creep strength enhanced ferritic steels

Creep deformation properties of creep strength enhanced ferritic steels were investigated. Good linear relationships between creep strain vs. time and creep rate vs. time were observed within a transient stage in a double logarithmic plot. It was appropriately expressed by a power law rather than exponential law, logarithmic law and Blackburn’s equation. With decrease in stress, a magnitude of creep strain at the onset of accelerating creep stage decreased from about 2% in the short-term to less than 1% in the long-term. Life fraction of the time to specific strain of 1% creep strain and 1% total strain, to time to rupture tended to increase with decrease in stress. A time to 1% total strain, that is a parameter for design of high temperature components, was observed in the transient creep stage in the short-term regime, however, it shifted to the accelerating creep stage in the long-term regime. Difference in stress dependence of the minimum creep rate was observed in the high- and low-stress regimes with a boundary condition of 50% of 0.2% offset yield stress. Stress dependence of the minimum creep rate in the high stress regime was equivalent to a strain rate dependence of flow stress observed in tensile test, and a magnitude of stress exponent, n, in the high stress regime decreased with increase in temperature from 20 at 550°C to 10 at 700°C. On the other hand, n value in the low stress regime was about 5, and creep deformation in the low stress regime was considered to be controlled by dislocation climb.     

<Emphasis Type="Italic">In Situ</Emphasis> Neutron-Diffraction Studies on the Creep Behavior of a Ferritic Superalloy

Precipitate strengthening effects toward the improved creep behavior have been investigated in a ferritic superalloy with B2-type (Ni,Fe)Al precipitates. In situ neutron diffraction has been employed to study the evolution of the average phase strains, (hkl) plane-specific lattice strains, interphase lattice misfit, and grain-orientation texture during creep deformation of the ferritic superalloy at 973 K (700 °C). The creep mechanisms and particle-dislocation interactions have been studied from the macroscopic creep behavior. At a low stress level of 107 MPa, the dislocation-climb-controlled power-law creep is dominant in the matrix phase, and the load partition between the matrix and the precipitate phases remains constant. However, intergranular stresses develop progressively during the primary creep regime with the load transferred to 200 and 310 oriented grains along the axial loading direction. At a high stress level of 150 MPa, deformation is governed by the thermally activated dislocation glide (power-law breakdown) accompanied by the accelerated texture evolution. Furthermore, an increase in stress level also leads to load transfer from the plastically deformed matrix to the elastically deformed precipitates in the axial direction, along with an increase in the lattice misfit between the matrix and the precipitate phases.     

High-Temperature Creep Deformation and Fracture Behavior of a Directionally Solidified Ni-Base Superalloy DZ951

The high-temperature creep deformation and fracture behavior of a directionally solidified Ni-base superalloy DZ951 have been investigated over a wide stress range of 110 to 880 MPa at high temperatures (700 °C to 1000 °C). In this article, the detailed creep deformation and fracture mechanism have been studied. The results show that the creep curves exhibit strong temperature dependence. From transmission election microscopy (TEM) observations, it is suggested that the deformation mechanism is temperature dependent and mainly consists of three dislocation-controlling mechanisms: stacking faults and dislocation-pair shearing, dislocation bowing, and dislocation climbing. It is found that the fracture mode of DZ951 alloy changes from cleavagelike fracture at low temperature to ductile fracture at high temperature. At 700 °C, the creep cracks mainly initiate at the surface and propagate along the cleavagelike facets. With increasing temperature, cracks can initiate at the surface, carbide/matrix interface, and cast pore. The growth of microcrack has a direction perpendicular to the stress direction. The creep-rupture data follow the Monkman–Grant relationship in different temperature regions.     

Plastic and viscous deformation of metals

Deformation characteristics of tensile specimens of several alloys, including electrolytic copper, α-brass, and 304 stainless steel, have been studied by application of stress and measurement of change of length in a soft tensile machine. By means of experiments in which the stress rate is reduced suddenly from a positive value to zero and the strain rate measured, both during loading and during creep, it is found that permanent deformation consists of two components, a plastic component for which the strain rate is a function of stress and stress rate, and a viscous component which is functionally dependent on stress and temperature. Plastic deformation is relatively more evident at increasing stress rate but declines in importance through the series copper, a-brass, and stainless steel. As a consequence, for a fixed strain rate during loading, the initial creep rate is low in copper and little creep occurs; in stainless steel, however, the initial creep rate is nearly equal to the loading strain rate and creep is pronounced. The theory is not fully developed but is based on a competition between thermal and mechanical release of dislocation segments from obstacles or sources. Release produces a strain increment which may be small or large depending on the relative values of stress and structural resistance. Plastic deformation occurs when the applied stress is close to the mechanical threshold, mechanical release is relatively easy, and the strain consists, at a given strain rate, of a few large strain increments per unit time. For viscous flow the relative stress is low, thermal release easy, and the strain rate is composed of many small strain increments in each unit of time.     

Dislocation mechanisms in creep of Ni3Al at intermediate temperature

The creep behaviour and dislocation structure of polycrystalline Ni₃Al have been investigated at intermediate temperatures. At a temperature just below the peak strength temperature T_p different creep responses occurred with relatively low and high stress levels respectively. Under high stress, creep exhibited a normal primary creep regime and then an inverse creep regime. At low stress, however, no visible inverse creep was displayed. With variation in temperature, an anomalous temperature-dependence of creep strength was confirmed. Microstructural observation indicates that superdislocations slip on the cube cross-slip plane more easily at lower temperatures in the temperature regime investigated in the present study, which is considered to be one of the reasons for the creep strength varying anomalously with temperature. Moreover, although inverse creep is produced by the operation of cube cross-slip, the operation of cube cross-slip does not necessarily lead to inverse creep.     

Transverse and cyclic thermal loading of the fiber reinforced metal-matrix composite SCS6/Ti-15-3

The transverse properties of a SiC fiber reinforced Ti alloy matrix composite subjected to transverse mechanical and cyclic thermal loading have been investigated. Fibers and matric have a mismatch in the coefficients of thermal expansion that induces thermal stresses in addition to those caused by mechanical loading. When fluctuations occur in the operating temperature the thermal stresses change and this could cause an incremental accumulation of plastic strain or increase in creep rate. The composite under consideration has a modest mismatch and it was found that the strain accumulation is caused by creep deformation in the matrix at the high temperature portion of the thermal cycles. In the early stages of the deformation for low transverse loading the interface is in compressive contact and the creep rate is accelerated by the cyclic thermal stresses. After debonding has occurred the cyclic thermal stress component is diminished and the creep rate is given by a matrix with holes.     

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。     

C晶界偏析对α-Fe纳米晶高温力学性能的影响

纳米晶在常温下具有很高的强度,然而其高温力学性能往往低于其对应粗晶.C是钢中的重要成分,由于其原子半径较小,容易在纳米晶中发生晶界偏析.通过分子动力学模拟,探讨了利用C的晶界偏析来提升纳米晶高温力学性能的可能性.在不同温度和应力水平下模拟了Fe-C与纯Fe纳米晶的拉伸蠕变试验,得到对应的应变速率,并根据Mukherje...     

An in vitro model of traumatic neuronal injury: loading rate-dependent changes in acute cytosolic calcium and lactate dehydrogenase release

We developed a new in vitro model of neuronal injury using NT2-N cells to examine the effects of hydrodynamic loading rate on intraneuronal calcium dynamics and lactate dehydrogenase (LDH) release. Our apparatus consisted of a parallel disk viscometer which induced fluid shear stress with well-defined magnitudes and loading rates to cultured cells. We found that the deformation response of the cells was dependent on the severity of the insult, with increased cellular strains generated for higher shear stresses at a constant loading rate. Peak intracellular free calcium concentration correlated with strain, suggesting that mechanical deformation may regulate calcium response. Slowly applied fluid shear stress elicited no response, whereas high loading rates resulted in peak calcium increases 2.9 to 3.6 times baseline values as injury severity was increased. LDH release measured within 5 min after the insult correlated with loading rate. In addition, LDH release continued to increase out to 24 h following high loading rate conditions, demonstrating that the application of fluid shear stress led to prolonged cell damage. The acute response in NT2-N cells subjected to an insult with the CSID is dependent on the loading rate, and these results suggest that initial membrane deformation may trigger subsequent events.     

Numerical models of creep and boundary sliding mechanisms in single-phase, dual-phase, and fully lamellar titanium aluminide

Finite element simulations of the high-temperature behavior of single-phase γ, dual-phase α₂+γ, and fully lamellar (FL) α₂+γTiAl intermetallic alloy microstructures have been performed. Nonlinear viscous primary creep deformation is modeled in each phase based on published creep data. Models were also developed that incorporate grain boundary and lath boundary sliding in addition to the dislocation creep flow within each phase. Overall strain rates are compared to gain an understanding of the relative influence each of these localized deformation mechanisms has on the creep strength of the microstructures considered. Facet stress enhancement factors were also determined for the transverse grain facets in each model to examine the relative susceptibility to creep damage. The results indicate that a mechanism for unrestricted sliding of γ lath boundaries theorized by Hazzledine and co-workers leads to unrealistically high strain rates. However, the results also suggest that the greater creep strength observed experimentally for the lamellar microstructure is primarily due to inhibited former grain boundary sliding (GBS) in this microstructure compared to relatively unimpeded GBS in the equiaxed microstructures. The serrated nature of the former grain boundaries generally observed for lamellar TiAl alloys is consistent with this finding.     

44Si2CrV 钢弹簧弹性减退机理的研究

研究了影响44Si2CrV钢弹簧在低于屈服强度的应力下长时间加载引起自由高度变化的因素。结果表明,44Si2CrV钢弹簧在900 MPa 的应力下压缩72 h,自由高度的缩短由两部分组成：表面脱碳层的塑性变形,造成对内部材料的牵制而使部分弹性变形不能恢复,由此所造成的弹簧自由高度缩短占总变形量的84%;钢的室温蠕变,占自由高度缩短的16%。     

The compressive creep behavior of a Pu-1 wt pct Ga δ-stabilized alloy at elevated temperatures

Constant stress compression creep tests were performed in vacuum on a high-purity Pu-1 wt pct Ga ö-stabilized alloy over the temperature range from 252° to 382°C for stresses from 700 to 2500 psi. Although the primary creep behavior could not be correlated by established techniques, the creep rates developed after true creep strains of about 0.15 provided good agreement with the temperature and stress dependence of creep for pure metals and dilute alloys. A power stress law for steady-state creep of the alloy was found forδ/E values less than 5 x 10′4, with the stress exponent being 4.0, and it was concluded that the alloy exhibits Class I solid solution behavior. For higher stress, exponential stress dependence was observed. The true activation energy for creep was found to be 38,900 cal per mole which is in good agreement with the value for self-diffus ion of plutonium in the ô-stabilized alloy. The primary creep behavior could be divided into three types: 1) at low strain rates, the creep rate gradually increases to a nearly steady-state; 2) at intermediate strain rates, the creep rate first decreases and then increases to steady-state; and 3) at high strain rates, the creep rate decreases gradually to steady-state. It was concluded that the failure of established creep correlations for primary creep of Pu-1 wt pct Ga was the result of some temperature-dependent component of creep structure, possibly resulting from radiation damage byα particles.     

Features and effect factors of creep of single-crystal nickel-base superalloys

The creep behavior of two single-crystal nickel-base superalloys with [001] orientation has been studied by measuring the creep curves, internal friction stress of dislocation motion, transmission electron microscopy (TEM) observation and energy-dispersive X-ray analysis (EDAX) composition analysis. The results show that over the stress and temperature range, there are different creep activation energies, time exponents, and effective stress exponents in two alloys at different creep stages. The size and volume fraction of the γ′ phase in the tantalum-free alloy is obviously decreased with the elevated temperature. This results in the decrease of the internal friction stress during steady-state creep. Higher levels of tungsten in the alloy result in a smaller strain value and lower directional-coarsening rate during primary creep. During steady-state creep, the primary reason for the better creep resistance of the other alloy is that it contains more Al and also Ta, which maintains a high volume fraction of γ′ phase. The dislocation climb over the γ′ rafts is the major deformation mechanisms during steady-state tensile creep. The fact that the strain rate is decreased with the increase of the size and volume fraction of the γ′ rafts may be described by a modified constitutive equation that is based on a model of the rate of dislocation motion.