Steady-state Creep Stage Research Articles

Tensile Creep Behavior of Two NiAl-Based Alloys

The tensile creep behaviors of two NiAl-based alloys (HIPed NiAl-33.5Cr-0.5Zr and DS NiAl-28Cr-5.8Mo-0.2Hf) have been investigated. The creep results indicated that the creep curves of both alloys have similar shapes, which are composed of primary creep stage, steady-state creep stage, longer accelerated creep stage, and about 25-45% creep strain. The apparent stress exponents are in the range of 4.8-7.5 and the apparent activation energies of 520-584 kJ/mol were also analyzed. The creep deformations were controlled by the sub-grain boundary formation for the HIPed NiAl-Cr(Zr) and dislocation climb for the DS NiAl-Cr(Mo,Hf). The creep rupture data of both alloys obey the Monkman-Grant relationship.

Steady state creep behavior of short fiber composites by mapping, logarithmic functions (MF) and dimensionless parameter (DP) techniques

A new mathematical insight based on logarithmic, polynomial mapping functions (MF), and dimensionless parameter (DP) models is presented for determination of some unknowns in the steady state creep stage of short fiber composites subjected to axial loading. These unknowns are displacement rate in outer surface of the unit cell, shear and equivalent stresses at interface and outer surface of the unit cell, and average axial stress in fiber. Dimensionless parameter technique is presented for determination of displacement rate and equivalent stress in outer surface of the unit cell. However, the polynomial mapping function is presented for determination of average axial stress in fiber. Most important novelty of the present research work is determination of the mentioned unknowns in steady state creep by DP and MF techniques without using the shear-lag theory unlike the previous researches. Good agreements are found among the new approaches and previous analytical results based on the shear-lag theory and also numerical solutions (FEM) for predicting the steady state creep behavior in short fiber composites.

Creep properties and permeability evolution in triaxial rheological tests of hard rock in dam foundation

Triaxial creep tests were carried out under seepage pressure by using rock servo-controlled triaxial rheology testing equipment. Based on experimental results, rock rheological properties influenced by seepage−stress coupling were studied, and variations of seepage rate with time in complete creep processes of rock were analyzed. It is shown that, when the applied stress is less than failure stress level, the creep deformation is not obvious, and its main form is steady-state creep. When applied stress level is greater than or less than but close to fracture stress, it is easier to see the increase of creep deformation and the more obvious accelerative creep characteristics. The circumferential creep deformation is obviously higher than the axial creep deformation. At the stage of steady-state creep, the average of seepage flow rate is about 4.7×10^(−9) m/s at confining pressure (σ3) of 2 MPa, and is about 3.9×10^(−9) m/s at σ3 of 6 MPa. It is seen that the seepage flow rate at σ3 of 2 MPa in this case is obviously larger than that at σ3 of 6 MPa. At the stage of creep acceleration, the seepage flow rate is markedly increased with the increase of time. The variation of rock permeability is directly connected to the growth and evolution of creep crack. It is suggested that the permeability coefficient in complete creep processes of rock is not a constant, but is a function of rock creep strain, confining pressure, damage variable and pore water pressure. The results can be considered to provide a reliable reference for the establishment of rock rheological model and parameter identification.

An Experimental and Theoretical Investigation of Creep in Ultrafine Crystalline Nickel RF-MEMS Devices

The creep behavior of ultrafine crystalline RF microelectromechanical systems (RF-MEMS) devices is investigated in this paper. The RF-MEMS devices are characterized through a highly accurate capacitance-sensing setup, under a special bi-state bias condition. In particular, the devices are kept continuously biased (on-state) for up to 1400 h, but the bias voltage is momentarily removed for 1 min every hour in order to record the off-state capacitance. Measurements at two bias voltages of 20 and 40 V are reported. The capacitance measurement uncertainty is less than 200 aF and the long-term stability is better than 4 fF throughout the measurement period. Furthermore, we report the creep behavior under the same bi-state bias condition through independent direct optical measurements conducted with a confocal microscope-based setup. The measurement uncertainty for the vertical displacement reported in these measurements is less than 50 nm. These measurements show for the first time the entire profile evolution of the deflected beams and membranes for up to 168 h. Both the capacitance and optical measurements reveal the same trends for the creep behavior of the devices. A physics-based creep model for the ultrafine crystalline nickel devices is also included in this paper. The required model parameters are obtained by fitting the simulation results to the measured creep curve under a bias voltage of 20 V. Using the same material parameters and geometry, this model correctly captures the rate of deformation during the steady-state creep stage at 40 V. The model reveals that the observed steady-state creep deformation at both 20 and 40 V is dominated by Coble creep.

Effect of an electric potential on the formation of a dislocation structure during creep of aluminum

The effect of an electric potential on the formation of a dislocation substructure at the stage of steady-state creep in commercial-purity aluminum is analyzed using the theoretical concepts of steady-state creep and the equations describing a dislocation ensemble with allowance for an applied electric potential. The effect of an electric potential on the creep rate and acceleration is considered through the mechanism of the effect of the potential on the surface energy of aluminum.

High temperature creep behavior and mechanism of a TiAl-based intermetallic

The creep mechanism and behavior of a lower plastic lamellar structure TiAl-based intermetallic at 760 °C were investigated. The research results show that the alloy has distinct creep behavior, such as the stages of creep being not obvious and the creep behavior being sensitive to stress. There is a stress critical value, about 200 MPa in experimental alloy. While the stress is lower than the critical value, there is the secondary stage after the primary stage in creep curves, and with the increase of the stress, primary creep stage becomes short and directly follows the tertiary creep stage in the creep curves. When the stress is over the critical level, there is a shorter steady-state creep stage in the curve of strain rate in intermetallic, then strain rate increases rapidly. The deformation mechanism at 760 °C is main due to the slip of dislocation.

Intermediate temperature creep of directionally solidified Ni-based superalloy containing local recrystallization

Intermediate temperature creep of directionally solidified Ni-based superalloys containing local recrystallization (RX) was studied. A sharp reduction of the creep rupture life was observed with increasing the transverse RX area fraction on the cross section of the specimen. Creep curves of specimens containing local RX showed that the steady state creep stage was much shorter than that of the “good” specimens. The creep rates were also slightly higher when there was local RX. The creep and failure mechanism was discussed based on the dislocation morphology in the vicinity of the RX grain boundaries and the formation and propagation of cracks in the RX region according to the microstructure observation of the interrupted creep specimens.

Effect of cyclic stress reduction on high temperature creep characteristics of solid solution Al−2·9 wt-%Mg alloy

Creep characteristics of solid solution Al−2·9 wt-%Mg alloy have been investigated under the effect of cyclic stress reduction of different amplitudes (12·47−26·18 MPa) and frequencies (0·20−0·44 Hz) at different working temperatures ranging from 533 to 593 K. It was found that increasing both amplitude and frequency of cyclic stress reduction resulted in an increase in both transient and steady state creep stages. The enhancement of creep rate was observed above either a threshold stress or a threshold frequency. The activation energy values of the mechanisms operating in both transient and steady state creep stages were found to be 130 and 171 kJ mol−1 respectively.

Triaxial Creep Behavior of Coal Containing Gas in Laboratory

Based on laboratory results of time-dependent mechanical behavior tests, the creep property of coal containing gas is investigated under different load conditions, using servo-controlled triaxial rheology testing machine at room temperature. The effect of effective axial stress, effective confining pressure and gas pressure on the creep behavior of coal containing gas is discussed respectively in detail. At primary creep stage, creep strain rate of coal samples, which can be described by a power function, decreases gradually with testing time. At steady-state creep stage, effective axial stress, effective confining pressure and gas pressure are important factors which affect the creep strain rate of coal samples. An exponential function is proposed to model the effect of effective axial tress on creep strain rate, at the same time, a linear equation can be used to describe the effects of effective confining pressure on creep strain rate. While, gas pressure influences indirectly the creep behavior of coal containing gas through making a change of the value of effective stresses. Accelerating creep stage is the last stage of non-attenuating creep and the beginning of coal samples failure, as well. Macro-cracks will be produced and lead to the failure of coal samples finally as far as accelerating creep stage appears.

Creep of replicated microcellular aluminium

Open-pore replicated microcellular 99.99% pure aluminium is tested in tensile creep, varying the temperature from 150 to 450 °C, the applied stress from 0.15 to 0.5 MPa, and the relative density from 0.14 to 0.28. Tensile creep curves are of classical shape, with a well-defined secondary stage of steady-state creep, for all except a few samples that were tested at higher temperature and lower stress; these display signs of extensive oxidation along the pore surface. All other samples crept at 250 °C or above exhibit a steady-state creep rate with an activation energy of 141 kJ mol −1, a stress exponent of 7.2 ± 0.4, and a dependence on relative density to a power near −21. The substructure of microcellular aluminium crept in this regime consists in subgrains that straddle, as in a bamboo structure, individual struts making the foam. This observation, coupled with the stress exponent near 7.5, shows that fine-scale microcellular pure aluminium can creep >250 °C under substructure-invariant conditions. At 150 °C, creep data are more scattered and show a steeper dependence on applied stress, suggesting power-law breakdown. The Monkman–Grant correlation describes well the tensile failure of this material.

Creep investigations of alumina-based all-oxide ceramic matrix composites

The long-term tensile creep behavior of all-oxide ceramic matrix composites (CMCs) was investigated. The accompanying fiber bundle tests revealed a strong influence of processing conditions on the creep resistance. CMCs with perpendicular fiber orientations were tested under two different load directions, moreover CMCs with nearly unidirectional fiber texture were investigated. Depending on the conditions, the absence of a steady-state creep stage was observed in most cases and due to an early onset of damage. As the majority of the samples was tested up to high strains and rupture, different failure mechanisms could be evaluated. An attempt was made to estimate fiber controlled creep rates via bundle data, but this approach also revealed problems when elastic data from unidirectional composites are transferred to CMCs with more complex fiber architecture.

High temperature creep and low cycle fatigue of a nickel-base superalloy

The creep and low cycle fatigue behaviour of a nickel-base superalloy was investigated. The creep curves show an obvious primary creep stage followed by a short steady-state creep stage and then an accelerating creep stage leading to failure at 700 °C. The 900 °C creep curves demonstrate a shorter primary stage, and a longer accelerating creep stage without steady-state creep stage. The creep and low cycle fatigue properties simultaneously degenerate with increasing temperature. The longer primary creep stage corresponds with the relatively long cyclic hardening during low cycle fatigue at 700 °C. The slow cyclic softening is consistent with the prolonged acceleration creep stage at 900 °C. The low cycle fatigue deformation is controlled by dislocations moving through γ matrix channels controlled by the Orowan mechanism and dislocation loops shearing γ′ precipitates at 700 °C, which is similar to the primary creep stage deformation mechanism. Furthermore, dislocation climb, slip and interaction between dislocations and carbides are the dominant creep deformation mechanism as well as the main low cycle deformation mechanism at 900 °C.

Steady state creep during transformation in Al–1 wt.%Cu alloy

The steady state creep of Al–1 wt.%Cu alloy was studied under constant stresses ranging from 10.2 to 12.74 MPa in the temperature range from 553 to 673 K. The strain rate sensitivity parameter ( m) varied from 0.2 to 0.42 in the tested temperature range, characterizing the dislocation climb along grain boundaries as the rate controlling mechanism. The activation energy 125.4 kJ/mol obtained in the temperature range from 633 to 653 K characterizes the grain boundary diffusion of Al atoms as the mechanism operating in this temperature range. The variations of the microstructural parameters support the suggested mechanisms to operate in the investigated steady state creep stage.

Long-term creep deformation property of modified 9Cr–1Mo steel

The first volume of “Atlas of Creep Deformation Properties” was published on modified 9Cr–1Mo steels in March 2007, as a part of the NIMS (National Institute for Materials Science) Creep Data Sheet series. Creep deformation properties up to about 70,000 h have been investigated. No clear steady-state creep stage has been observed, and creep deformation of the steel consists of transient and accelerating creep stages. Good linear relationships between creep strain vs. time and creep rate vs. time were observed within a transient stage in a log–log plot. It was appropriately expressed by a power law rather than an exponential law, logarithmic law and Blackburn's equation. With decrease in stress, the 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 region. 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. The time to 1% total strain, that is an important parameter for design of high temperature components, was observed to lie in the transient creep stage in the short-term regime, however, it shifted to the accelerating creep stage in the long-term regime. For evaluation of long-term creep strength properties, an experimental creep test data should be extrapolated in consideration of the stress dependence of creep deformation properties.

Numerical simulation of creep characteristics of soft roadway with bolt-grouting support

Based on the engineering background of a soft rock roadway in Qinan Coal Mine 82 Area, Huaibei Mining Group, three creep models with different support patterns in soft rock roadway were established by using geotechnical software of FLAC2D. According to the calculation results of different models, the change law of mechanical properties with the time of bolt-grouting support structure was obtained. Furthermore, for the test bolt-grouting support roadway, the deformation law of surrounding rock got by underground industrial experiment and field observation accords with the creep law got by numerical calculation. The results of numerical calculation and field observation show that, compared with other supports, the creep of bolt-grouting support roadway enters the steady-state creep stage from tertiary creep stage ahead, the deformations of roof, floor and two sides are decreased greatly, the plastically deforming area in surrounding rock is reduced obviously, and the distribution ranges of maximum and minimum principal stress are shrank obviously. All those fully show that the bolt-grouting support has its remarkable advantages in controlling surrounding rock creep and improving the whole strength of surrounding rock and self-bearing capacity.

Effect of initial temper on the creep behavior of a Mg–Gd–Nd–Zr alloy

The effect of initial temper on the tensile creep behavior of a cast Mg–Gd–Nd–Zr alloy has been investigated. Specimens in unaged, underaged and peak-aged conditions exhibit a sigmoidal creep stage between the primary and steady-state creep stage, while the overaged specimens have no such creep stage. Transmission electron microscope observations revealed that sigmoidal creep stage was induced by the dynamic precipitation in the microstructure, and the rapid formation of β 1-phase and β-phase plates takes responsibility for the softening of material in this stage. Comparative evaluation of creep properties of the specimens showed that alloy in overaged condition had creep resistance superior to those in other conditions. Stress and temperature dependence of the steady-state creep rate were studied over a temperature range of 250–300 °C and stress range of 50–100 MPa, and a dislocation creep mechanism was proposed for the alloy.

Effect Factors of Internal Stress and Deformation Mechanism for Single Crystal Nickel-Base Superalloys during Creep

By means of the measurement of internal friction stress of dislocation movement during steady state creep stage, and observation of microstructure, an investigation has been made into the effect factors of the internal friction stress of single crystal nickel-base superalloys during creep. Results show that the values of internal friction stress of superalloys are enhanced with the increase of the elements Ta and W concentration. The super-dislocations shearing into the y' phase are supposed to be the deformation mechanism of the superalloy 4 in the period of creep later. The fact that the coarsening y' rafted phase and the finer cubic y' phase are precipitated in the y matrix phase may effectively block the movement of dislocations, and enhance the creep resistance of the alloy, this is a main reason for resulting in the superalloy possessing a longer rupture lifetimes.

High temperature creep flow and damage properties of 9Cr1MoNbV steels: Base metal and weldment

High temperature creep flow and damage properties of 9Cr1MoNbV steel and weldment are investigated in the present study. First, an experimental database is built to compare both creep flow and damage properties of the base metal and the weldment. Metallurgical investigations before and after creep tests revealed that the microstructural state is responsible for the lower creep strength in the weldment. A simple analysis based on the description of the steady-state creep stage and on use of empirical lifetime prediction relationships is performed. It allows to predict the weldment creep lifetime without performing an extensive number of experiments.

Macrolocalization of plastic strain in creep of fine-grain aluminum

The evolution of the plastic strain macrolocalization pattern in low-temperature creep of commercial purity aluminum is studied. The localization pattern depends on a stage in the creep curve. At the stage of steady-state creep, localization zones propagate in the form of a wave traveling with a velocity proportional to the rate of buildup of the total strain. It is found that the volumes where the creep and strain localization wave propagation are activated equal each other. Based on estimates of the activation volumes, it is shown that the velocity of plastic strain localization waves is governed by thermally activated dislocation movement.

Influence of Environment on Creep Properties of MC2 Single Crystal Superalloy at 1150°C

In order to reveal the effect of oxidation on thin blade walls, a new machine allowing tests up to 1250°C under controlled atmosphere has been designed. Creep tests were performed on MC2 single crystal superalloy at 1150°C, under hydrogenated argon and dry air, but also with a switch from one atmosphere to the other after reaching the steady state creep stage. The results point out the decrease of the minimum creep rate in case of tests performed or at least started under hydrogenated argon, compared with the value obtained under synthetic dry air. This effect of oxidation was attributed to the protective oxide scale formed under hydrogenated argon. The low growth rate of alumina layer leads to a thinner zone affected by metal consumption, which is assumed to be non bearing, and prevents from vacancy flux toward the alloy. The second point results in slowing down creep mechanisms controlled by diffusion and therefore dislocation motion and microstructure evolution. Thermogravimetric tests confirm the difference of oxidation kinetics regards to environment (hydrogenated argon and dry air). However, oxide scales have different microstructures on thermogravimetric and creep samples when tested under air.