Creep Region Research Articles

Use of the Wilshire Equations to Correlate and Extrapolate Creep Data of Inconel 617 and Nimonic 105.

Advanced power plant alloys must endure high temperatures and pressures for durations at which creep data are often not available, necessitating the extrapolation of creep life. A recently developed creep life extrapolation method is the Wilshire equations, with which multiple approaches can be used to increase the goodness of fit of available experimental data and improve the confidence level of calculating long-term creep strength at times well beyond the available experimental data. In this article, the Wilshire equation is used to extrapolate the creep life of Inconel 617 and Nimonic 105 to 100,000 h. The use of (a) different methods to determine creep activation energy, (b) region splitting, (c) heat- and processing-specific tensile strength data, and (d) short-duration test data were investigated to determine their effects on correlation and extrapolation. For Inconel 617, using the activation energy of lattice self-diffusion as resulted in a poor fit with the experimental data. Additionally, the error of calculated rupture times worsened when splitting regions. For Nimonic 105, the error was reduced when heat- and processing-specific tensile strengths were used. Extrapolating Inconel 617 creep strength to 100,000 h life gave conservative results when compared to values calculated by the European Creep Collaborative Committee.

Tensile characterization of a lead alloy: creep induced strain rate sensitivity

The aim of this work is to investigate and define the tensile properties of cable sheathing lead alloy. In particular its strain rate sensitivity due to the pronounced creep already present at room temperature, in relation to the material work hardening and processing is object of study. Therefore, a series of specimens have been manufactured using cable sheathing of three different extrusion thicknesses and tested in displacement control at different strain rates. The practical difficulties generated by the highly plastic behavior have been overcome by the use of Digital Image Correlation (DIC) technique as a tool for measuring the strain field on the specimen surface. The creep behavior has been described, modeled and calibrated using Isight and finally implemented in the FEA code Abaqus. Two different numerical models have been used for modelling the time dependent deformation, a power law and the Anand model. The first predict correctly the response of the alloy in the primary creep region while the second shows better results for secondary creep.

Investigating the Creep Properties of PET-Modified Asphalt Concrete

This study has investigated the creep properties of asphaltic concrete modified with different dosages of waste polyethylene terephthalate (PET) in two different ranges of size. Uniaxial dynamic creep test at 40°C was conducted on the cylindrical specimens of the mixtures. The load was applied in two different frequencies of 0.5 and 5Hz. Creep test results showed that the accumulated strain under dynamic loading increased with increasing PET content, with lower values for the mixtures containing finer PET particles. Moreover, it was found that the accumulated strain under the loading with higher frequency was more than that under lower frequency, with higher sensitivity to frequency for the mixtures containing finer PET. The results of dynamic creep tests were used for determination of the constants of a three stage model. The linear creep slope in the second region of the creep curve and the flow number showed that the increase of PET content and size results in decrease of permanent deformation resistance. However, the mixtures modified with 4% of fine and coarse PET particles had the highest loading cycles at the end of primary creep region, where most of the strain was recoverable.

Microstructure Evolution of a High Nb Containing TiAl Alloy with (α2 + γ) Microstructure during Elevated Temperature Deformation

In order to verify the correctness of the transition of deformation mechanism with the change in deformation parameters and to reveal the types and mechanism of dynamic recrystallization of γ grains during compression deformation, microstructure characterization of Ti-43.5Al-8Nb-0.2W-0.2B (at. %) alloy after isothermal compression deformation were performed. When the alloy was deformed at 1000 °C/10−2 s−1, the initial γ grains are elongated and significantly refined and the fraction of low angle grain boundaries (LAGB) of γ grains is obviously increased and the texture intensity remains unchanged, which indicates that the compression deformation in dislocation creep region is dominated by intragranular deformation and dynamic recrystallization (DRX) of γ grains. Besides, the lattice rotation at grain boundary serrations may be responsible for the nucleation of new recrystallized γ grains, and the following growth process may be achieved by the migration of γ grain boundaries. However, when the alloy deformed at 1050 °C/10−4 s−1 and 1000 °C/10−4 s−1, the γ grains maintain equiaxed shapes and distribute more uniformly and the fraction of LAGB of γ grains is slightly raised and the texture sharpness decreases, which indicates that the compression deformation in grain boundary sliding (GBS) region is mainly controlled by GBS of γ grains and DRX occurs simultaneously within some coarse γ grains.

The Creep-Damage Model of Salt Rock Based on Fractional Derivative

The use of salt rock for underground radioactive waste disposal facilities requires a comprehensive analysis of the creep-damage process in salt rock. A computer-controlled creep setup was employed to carry out a creep test of salt rock that lasted as long as 359 days under a constant uniaxial stress. The acoustic emission (AE) space-time evolution and energy-releasing characteristics during the creep test were studied in the meantime. A new creep-damage model is proposed on the basis of a fractional derivative by combining the AE statistical regularity. It indicates that the AE data in the non-decay creep process of salt rock can be divided into three stages. Furthermore, the authors propose a new creep-damage model of salt rock based on a fractional derivative. The parameters in the model were determined by the Quasi-Newton method. The fitting analysis suggests that the new creep-damage model provides a precise description of full creep regions in salt rock.

Effect of Cr atoms in vortex dynamics of NbSe2 superconductor and study of second magnetization peak effect

We report on the high quality single crystal of CrxNbSe2 growth to study the effect of Cr impurity on the vortex dynamics of NbSe2 superconductor using M–H curves with different relaxation rates. The field dependence nature of critical current density (Jc) is well investigated using Larkin-Ovchinnikov collective pinning model and Anderson-Kim’s plastic creep model. We have observed significant improvement in the field dependence nature of Jc with increasing Cr impurities in NbSe2. This reveals the crossover of vortex lattice from elastic (E) to plastic (P) creep region causing second magnetization peak (SMP) or fishtail effect at highest Cr concentration, which has not been yet observed. Also addition of Cr impurity enhances the Jc up to 4 × 105 A/cm2. However, the interaction between core region of vortices and pinning centres ensure the δl pinning in Cr0.0005NbSe2 and significantly improved with increment of Cr impurity, which allowing both δl and δTc mechanism. Both, δl and δTc core pinning strongly enhance the coupling between vortices in pinning potential of nonstatistical distribution of Cr impurity, which makes 3D magnetic fluxes into 2D nature. Furthermore, the point pining and surface pinning mechanisms are responsible for pinning at low magnetic field region in Cr0.0005NbSe2. The appearance of prominent peak in pinning force density near upper critical field (Hc2) indicates the lattice softening at higher Cr impurity. These studies not only provide fundamental study of vortex dynamic but also open the door for further study in low Tc superconducting materials.

Size effect on tensile creep behavior of micrometer-sized single-crystal gold

In order to clarify the size effect on the tensile creep properties of micrometer-sized single-crystal Au at room temperature, we conducted long-term (up to 14 h) creep experiments for two sets of specimens of approximately 0.5 and 1.5 μm in size, and they were observed with in situ field-emission scanning electron microscopy (FESEM). We directly measured the specimen elongation from the FESEM images to eliminate the measurement error in the displacement sensor owing to thermal drift, which ensured accurate creep strain evaluation. On one hand, at high stress range, where the stress σ was close to the yield stress σY (i.e., σ/σY ≈ 1), the creep strain continuously increased, and the creep curve consisted of typical transient and steady-state creep regions for both specimens of sizes ∼0.5 and ∼1.5 μm. On the other hand, at low stress range (σ/σY ≈ 0.8), the creep was mainly induced by intermittent strain bursts, which were not observed in bulk metal. Thus, the creep behavior transitioned from continuous to discrete as the applied stress decreased. The smaller specimens required higher stress to reach a strain rate on the order of 10−7–10−6 s−1, indicating that the resistance to creep deformation increased with the decrease of specimen size. The “smaller is stronger” trend presented in the long-term creep deformation in the size range of ∼0.5–1.5 μm. This finding implies that such small crystals can sustain high stress over a long period and can be used as elements of micro devices.

Characterization of the elevated temperature compressive deformation behavior of high Nb containing TiAl alloys with two microstructures

The elevated temperature compressive deformation behavior of typical β solidifying γ-TiAl alloys with two microstructures was investigated. Combining with the microstructure characterization, the variation of the stress exponent n and the apparent activation energy Q with isothermal compression parameters indicates that a transition of deformation mechanism from dislocation creep to grain boundary sliding (GBS) occurred in both two alloys, but for the (β + γ) alloy, the domination of the compressive deformation by GBS began at a lower temperature and a higher strain rate than (α2 +γ) alloy. The compressive deformation of (α2 +γ) alloy might be mainly controlled by the γ phase lattice diffusion. But for the (β + γ) alloy, in the dislocation creep region it is because of the introduction of β/βo phase lattice diffusion that the Q value of the (β + γ) alloy is below that of the (α2 +γ) alloy. However, in the GBS region the compressive deformation may also be only controlled by the γ lattice diffusion similar with (α2 +γ) alloy, which leads to the rise of the Q value. The constitutive equations considering the compensation of the strain predicts well the flow curves. The power dissipation maps were developed on the basis of the above equations and the dynamic material model (DMM).

Indentation Creep Behavior of Nugget Zone of Friction Stir Welded 2014 Aluminum Alloy

The present study is aimed at evaluating the creep behavior of the nugget zone of friction welded 2014 Aluminum alloy by indentation creep tests. Impression creep testing was carried out at different temperatures of 300°C, 350°C and 400 °C with stress 124.77MPa, 187.16MPa, 249.55 MPa using a 1.0 mm diameter WC indenter. Experiments were conducted till the curve enters the steady state creep region. Constitutive modeling of creep behavior was carried out considering the temperature, stress and steady state creep rate. Microstructural investigation of the crept specimen at 400°C temperature and 187.16 MPa load was carried out and found that the small precipitates accumulate along the grain boundaries at the favorable conditions of the creep temperature and stress, new precipitates evolve due to the ageing. The grains are broken and deformed due to the creep phenomena.

Investigating the creep properties of asphaltic concrete containing nano-silica

This study was aimed at investigating the creep behaviour of a typical asphalt concrete containing different percentages of nano-silica. The penetration grade of 60/70 asphalt cement was modified with different percentages of nano-silica (i.e., 1, 3 and 5%, by the weight) and was used for making the asphalt concrete specimens. The asphalt concrete specimens were subjected to dynamic creep tests. Dynamic creep tests were conducted at different stress levels and temperatures. A three-stage model, developed was fitted to the dynamic creep test results to capture the primary, secondary and the tertiary creep regions, and calculate the flow number and the steady-state strain rate in the secondary creep region. The results showed that, the flow number increased, and the steady-state strain rate decreased with increasing nano-silica content, indicating the increase of resistance against permanent deformation.

Effect of Tungsten on Creep Behavior of 9%Cr–3%Co Martensitic Steels

The effect of increasing tungsten content from 2 to 3 wt % on the creep rupture strength of a 3 wt % Co-modified P92-type steel was studied. Creep tests were carried out at a temperature of 650 °C under applied stresses ranging from 100 to 220 MPa with a step of 20 MPa. It was found that an increase in W content from 2 to 3 wt % resulted in a +15% and +14% increase in the creep rupture strength in the short-term region (up to 103 h) and long-term one (up to 104 h), respectively. On the other hand, in the long-term creep region, the effect of W on creep strength diminished with increasing rupture time, up to complete disappearance at 105 h, because of depletion of excess W from the solid solution in the form of precipitation of the Laves phase particles. An increase in W content led to the increased amount of Laves phase and rapid coarsening of these particles under long-term creep. The contribution of W to the enhancement of creep resistance has short-term character.

The Effect of Eutectic Structure on the Creep Properties of Sn-3.0Ag-0.5Cu and Sn-8.0Sb-3.0Ag Solders

Solder joints are the main weak points of power modules used in harsh environments. For the power module of electric vehicles, the maximum operating temperature of a chip can reach 175 °C under driving conditions. Therefore, it is necessary to study the high-temperature reliability of solder joints. This study investigated the creep properties of Sn-3.0Ag-0.5Cu (SAC305) and Sn-8.0Sb-3.0Ag (SSA8030) solder joints. The creep test was conducted at 175 and 190 °C with the application of 2.45 MPa. The SAC305 solder had superior creep properties to those of SSA8030 solder at 175 °C and at largely the same homologous temperature ( T H ~0.91 for SAC305 and T H ~0.92 for SSA8030). Both solders had primary β-Sn and a eutectic mixture of β-Sn and Ag3Sn. Compared to SSA8030, the SAC305 solder contained ~10% more eutectic structure and contained Ag3Sn that was 3 times smaller and more round in shape. Furthermore, the SSA8030 solder precipitated SnSb in an elongated fiber shape (1–50 μm in size) after the creep test. Coarse and elongated Ag3Sn and SnSb of the SSA8030 solder negatively affected crack propagation in the dislocation creep region and decreased the creep resistance.

Nano-scale simulation based study of creep behavior of bimodal nanocrystalline face centered cubic metal.

In this paper, the creep behavior of nanocrystalline Ni having bimodal grain structure is investigated using molecular dynamics simulation. Analysis of structural evolution during the creep process has also been performed. It is observed that an increase in size of coarse grain causes improvement in creep properties of bimodal nanocrystalline Ni. Influence of bimodality (i.e., size difference between coarse and fine grains) on creep properties are found to be reduced with increasing creep temperature. The dislocation density is observed to decrease exponentially with progress of creep deformation. Grain boundary diffusion controlled creep mechanism is found to be dominant at the primary creep region and the initial part of the secondary creep region. After that shear diffusion transformation mechanism is found to be significantly responsible for deformation as bimodal nanocrystalline Ni transforms to amorphous structure with further progress of the creep process. The presence of <0, 2, 8, 5>, <0, 2, 8, 2>,and <0, 1, 10, 2>distorted icosahedra has a significant influence on creep rate in the tertiary creep regime according to Voronoi cluster analysis.

Investigating the Performance Characteristics of Asphaltic Concrete Containing Nano-Silica

Using nano-technology materials in the asphalt pavement industry is new compared with Portland cement concrete. The main objective of this study is to investigate the effects of nano-silica modification on some properties of a penetration grade asphalt cement and a typical asphalt concrete. 60/70 penetration grade bitumen was modified with different percentages of nano-silica (i.e. 1, 3 and 5%, by weight) and was used for making the asphalt concrete specimens. After evaluating the basic properties of the modified binder, the asphalt concrete specimens were evaluated based on the stiffness and resistance against fatigue cracking, moisture damage and permanent deformation. The fatigue life of the modified mixtures was also calculated using an already developed regression model. The results showed that penetration grade and ductility increase and softening point decreases with increasing nano-silica content. Furthermore, results showed that the addition of nano-silica results in the increase of stiffness, tensile strength, resilient modulus, fatigue life and resistance against permanent deformation and moisture damage. The reduction of indirect tensile strength in wet the condition decreases with increasing nano-silica content. Dynamic creep test results showed that, the flow number of the control mixture and the mixture containing 1% of nano-silica is much lower than 10000 loading cycles. However, the mixtures containing 3 and 5% of nano-silica do not reach to the tertiary creep region after 10000 loading cycles.

Creep Behaviors of CrMnFeCoNi High Entropy Alloy at Intermediate Temperatures

In this study, creep properties and fracture behavior of CrMnFeCoNi high entropy alloy (HEA) were studied at intermediate temperatures. The invert-type transient primary creep behaviors were observed in CrMnFeCoNi high entropy alloy. Creep behaviors of HEA are similar to those of class I solid solution alloys. The transient creep curves upon increase of stress by 5MPa in the steady state creep region did not change much except the sudden strain increase. And, no decrease of creep rate was observed upon increase of stress. Instead, the slightly invert transient creep or almost straight creep curves were observed, supporting the high friction stress. CrMnFeCoNi high entropy alloy has a stress exponent of 3.75 and the creep activation energy was calculated to be 278KJ/mole. The fracture strain increased from 1.3 to 1.6 with the decrease of stress from 96 MPa to 48MPa. The lower stress exponent along with the invert type primary creep curves strongly suggest that the creep of CrMnFeCoNi high entropy alloy at 600°C~650°C occurs by a glide controlled process.

Transient minimum creep of a γ′ strengthened Co-base single-crystal superalloy at 900 °C

A characteristic creep behavior, with a transient minimum creep rate region, was exhibited in a Co-Al-W-Ta-Ti single-crystal superalloy at 900°C and 420MPa. Interrupted creep tests were performed at various creep regions, the investigation of substructural evolution indicated the configurations of stacking faults were different during various creep regions. The extension of stacking faults and the formation of Lomer-Cottrell locks are hypothesized to be responsible for the accelerating region and steady-state region, respectively.

Determination of Creep Properties from Small Punch Test with Reverse Algorithm

In the early 1980’s, the small punch technique was developed in USA and Japan. This technique has been applied to nuclear reactors, electric power plants for safety assessments. European researchers have carried out the pioneer work on SP creep testing in the 1990’s. European Code of Practice (CoP) documents have become available in 2006, which provides a guide line to perform small punch tests for metallic materials, and introduces methods for estimation of tensile properties, fracture toughness and creep properties. Since then several conferences and symposiums have been held to exchange ideas and experiences, and several efforts have been made to improve the approaches for interpreting the material properties from test results. However, due to the complexity of the small punch test itself and lack of systematic test data for verification, especially in the creep region, up to now there is no common acceptable method to interpret creep properties. This paper discusses some of the problems commonly encountered in the small punch creep test (SPCT) analysis, such as the uniqueness in reverse creep analysis for identification of material parameters, how to consider effects of large deformation and strain hardening, how to separate deflection into a part caused by loading and a part caused by creep, and how to estimate the elastic-plastic properties of a material in the creep temperature region. Based on these considerations, several existing approaches for interpreting creep properties have been re-evaluated. In addition, a software package, based on the optimization toolbox of Matlab has been developed for identification of material parameters automatically. Verifications are performed by checking the agreement between properties derived by small punch test and uni-axial tests. Discussions on problems of existing approaches and how to improve them further, are described in the paper. Experimental data are provided by JRC Petten, Netherlands, and Henan Electric Power Research Institute, China. Recently a working group led by JRC Petten is engaged in upgrading the CoP to a European standard. It is expected that in the future more data will become available for further verification.

An investigation on the dynamics of domain switching of Bi0.5Na0.5TiO3-based ferroelectric ceramics

The dynamics of domain switching of Bi0.5Na0.5TiO3-based ferroelectric ceramic was investigated. With the electric field increasing, the domain switch experienced three sections of no domain switch region, creep region and flow region. In the creep region, the domain switch is dominated by the thermally activated domain wall movement. In flow region, the domain wall experience the viscous flow motion. For the BNT-BA-Zn ceramic, the activation energy of creep domain switch is much higher than it in flow region. With temperature increasing, the sensitivity of domain wall motion to the frequency is decreasing. This result demonstrated the domain switch of different ferroelectric materials could be quantitatively analyzed and compared based on the simple polarization - current - electric field data.

Proposal of Local Failure Evaluation Method in Creep Region and Study of Applicability

Proposal of Local Failure Evaluation Method in Creep Region and Study of Applicability

A fractal derivative constitutive model for three stages in granite creep

In this paper, by replacing the Newtonian dashpot with the fractal dashpot and considering damage effect, a new constitutive model is proposed in terms of time fractal derivative to describe the full creep regions of granite. The analytic solutions of the fractal derivative creep constitutive equation are derived via scaling transform. The conventional triaxial compression creep tests are performed on MTS 815 rock mechanics test system to verify the efficiency of the new model. The granite specimen is taken from Beishan site, the most potential area for the China’s high-level radioactive waste repository. It is shown that the proposed fractal model can characterize the creep behavior of granite especially in accelerating stage which the classical models cannot predict. The parametric sensitivity analysis is also conducted to investigate the effects of model parameters on the creep strain of granite.