Steady-state Creep Research Articles

Synergistic effects of Si and Y on corrosion behavior of cast cladding steels by pre-laying Y powder for nuclear applications in static liquid LBE

Ferritic/martensitic (F/M) steels with (9–12)wt.% Cr are potential candidates for structural applications in Lead-cooled Fast Reactors (LFR) due to their low solubility in liquid Lead-Bismuth Eutectic (LBE) coolant. How to improve corrosion resistance of 9-12Cr F/M steels has been a challenge. Here we cast three steels by pre-laying yttrium (Y) powder on the bottom of mold to obtained Y-bearing steels. Precursory work on this family of Y-bearing steels has previously highlighted them with the longer steady state creep stage and the lower minimum creep rate at 650 °C under 120 MPa, which has motivated their study under more extreme conditions, such as corrosion behavior in liquid LBE. We find a synergistic effect of Si and Y on both optimizing structure and reducing thickness of oxide scale at various oxygen concentrations, exposing temperatures and test times. The oxide scale changed from dual layers to single spinel layer, the thickness reduced from 36 μm to 4 μm, dissolution corrosion and PbBi penetration were inhibited. Low diffusion coefficient of ions in SiO2 and Y segregation on grain boundaries of steels hinder the outward diffusion of cations, and strong affinity between Y and O leads to the formation of Y2O3 preferentially, acting as nucleation sites of SiO2 and Cr2O3, accelerating a dense and continuous oxide scale. This work enables the development of structure steels with high corrosion resistance for various high temperature application, including boiler steel, gas turbine, and heat exchanger tubes.

Quasi-in-situ investigation on microstructure degradation of a fully lamellar TiAl alloy during creep

Microstructure degradation of a fully lamellar TNM alloy during creep has been investigated by using the quasi-in-situ creep tests, which were conducted at 750 °C under 250 MPa, for (1–5.4) % creep strain. The results indicated that the fully lamellar structure exhibited gradual microstructure degradation during creep. At the primary creep region, dynamic recrystallization of the γ phase occurred at the colony boundary and a massive proportion phase transformation occurred, these caused microstructure instability and a relatively large primary strain. Within the steady-state creep region, the steady creep strain rate was 8.89E-8 s−1. The fine (α2+γ) clusters with more interface provided obstacles for the dislocation movement because of α2 parallel decomposition. Moreover, the interfacial β0 phase precipitation was suppressed the α2 → γ transformation and remained the integrity and continuity of the lamellae structure, which was attributed to the steady-state creep resistance. At the tertiary creep region, many cavities nucleated at the colony boundary or γg/β0 phase boundaries due to the local stress concentration. The dynamic recrystallized of the γg area was extended and damaged the integrity of the fully lamellar structure. Moreover, the partial dissolution of α2 lamellae caused discontinuous lamellae and reduced the creep resistance. It concluded that microstructure stability has a strong influence on creep resistance.

On the exceptional creep resistance in a die-cast Gd-containing Mg alloy with Al addition

Over the past few decades, aluminium (Al) has been considered to be beneficial for castability but detrimental for the creep resistance of magnesium (Mg) alloys. However, an excellent creep resistance has been achieved in a die-cast Mg3.5RE(La,Ce,Nd)1.5GdMnAl alloy, with a super low steady-state creep rate (SCR) of 1.35 × 10−10s−1 at 300 °C/50 MPa. Compared with the counterpart Al-free Mg3.5RE(La,Ce,Nd)1.5GdMn alloy, the SCR decreased by 71%. The synergistic effect of Al, Gd and Mn induced a novel thermally stable (TS) AlMnGd ternary short-range order (SRO, 0–2 nm)/cluster (2–10 nm) in the Mg matrix. After creep at 300 °C/50 MPa for 400 h, the AlMnGd SRO was still observed, and the AlMnGd clusters were under 10 nm and coherent with the Mg matrix. High density AlMnGd SRO/clusters were observed for pinning dislocations, which was the main reason for the improvement in the creep resistance in contrast to the counterpart Al-free alloy. The TS Mg12RE(La,Ce,Nd) network at grain boundaries (GBs) impeded dislocation mobility, which also played an important role for the creep resistance compared to the traditional die-cast Mg-Al-based alloys. Under the critical conditions of 300 °C and 50–80 MPa, the creep still satisfied the power law, and the dominant creep mechanisms were SRO/cluster drag dislocation gliding, associated with GB diffusion. This work provides clear evidence for the long-term existence of some SRO/clusters in Mg alloys under critically high temperatures and stresses. Moreover, the utilization of AlMnGd SRO/clusters can be a novel approach for designing heat-resistant Mg alloys.

Finite element analysis of steady-state uniaxial basic creep of high-performance concrete

This study aims to propose a new approach to predict the long-term thermal compressive and tensile uniaxial basic creep behavior of high-performance concrete sample at different temperature levels (20 °C, 50 °C, 80 °C), by using nonlinear finite element analysis. The new approach is based on the coupling between Burger's rheological model, two-phase composite material models and maturity concept. Burger's rheological model is employed to predict the compressive single axial BC strain. Furthermore, the maturity approach is employed to evaluate the mechanical properties of concrete. However, the thermal properties of concrete (thermal conductivity and specific heat capacity) are estimated by seven models available in literature based on two-phase composite material models. The obtained results show that the thermal shrinkage plays an important role in the estimation of the BC. In addition, increasing the temperature amplifies the rate and the magnitude of the BC and thermal stress generated by thermal shrinkage. The numerical results show that the BC is more relevant in tension than in compression when compressive and tensile creep in isothermal conditions and stress/strength ratio (30%) are equivalent.

Revealing thickness dependence of hardness, strain rate sensitivity, and creep resistance of nano-crystalline magnesium/titanium multilayers by nanoindentation

The size effect on mechanical properties of hcp/hcp multilayer has been short of understanding hitherto. In this study, we prepared Magnesium/Titanium (Mg/Ti) multilayers with various individual layer thicknesses (h = 50, 100, 150, 200 nm) by magnetron sputtering. Relying on nanoindentation, hardness and elastic modulus were little changed for the multilayers with h of 50 ∼ 150 nm, while they were evidently lower when h increased to 200 nm. The determined strain rate sensitivities were 0.029, 0.032, 0.035, and 0.062 for the samples with h = 50, 100, 150, 200 nm, respectively. According to evolution of grain size, it suggests that Hall-Petch law dominates the strengthen effect of this hcp/hcp multilayers, rather than blocking effect of interface and decrease of dislocation content by reducing individual layer thickness. On the other hand, creep resistance was gradually promoted as reducing h. It indicates that interface and dislocation content could be important for the time-dependent plastic deformation. The room-temperature creep mechanism was discussed based on the strain rate sensitivity of steady-state creep flow. It indicates that dislocation glide could be suppressed and grain boundary glide, dislocation climb, and even interface glade could be plausible when h decreased to 50 nm.

Exact analytical solutions for bending creep behavior of viscoelastic laminated arches with interlayers

In this work, an exact analytical solution is proposed in order to investigate the bending creep behavior of viscoelastic laminated arches with interlayers under a distributed load. In the analytical model, both the lamina layers and bonding interlayers are considered as viscoelastic materials, which are described by Burgers model for transient and steady-state creep stages. The stress and displacement fields in each viscoelastic lamina layer are obtained on the basis of the exact elasticity theory as well as Boltzman superposition principle. The general solution of stresses and displacements with unknown coefficients is derived by utilizing the Fourier series expansion approach. The interlayer slip effect in the laminated arches is considered in the continuity conditions. By employing analytical Laplace transformation, the coefficients are further determined according to the continuity and load conditions. The present solution is compared to the finite element solution and they show a good agreement. The influences of various parameters are investigated in detail on the bending creep behavior of the arch.

Influences of Post-Heat Treatment on the Microstructure Evolution and Creep Properties of Ni-Based Superalloy IN718 Fabricated by Electron Beam Melting

In this study, the Ni-based superalloy IN718, fabricated using an electron beam melting process, was investigated in as-built and various heat-treated conditions. The relationships between the microstructure characteristics and creep properties were elucidated. Under testing conditions of 650 °C and 650 MPa, the direct-aged specimen exhibited the lowest steady-state creep rate, at 0.15 × 10−8 s−1. The superior creep resistance can be attributed to the higher volume fraction of γ’/γ”-strengthening precipitates within the grain and fine δ precipitates along the grain boundaries. Being coherent to the γ matrix, the nano-sized γ’/γ” precipitates effectively hindered the dislocation motion in the grain interior. In addition, controlled grain boundary δ precipitates inhibited grain boundary sliding and decelerated the steady-state creep strain rate during creep deformation.

Secondary Creep Analysis of FG Rotating Cylinder with Exponential, Linear and Quadratic Volume Reinforcement.

Creep is an irreversible time-dependent deformation in which a material under constant mechanical stress and elevated temperature for a considerably prolonged period of time, starts to undergo permanent deformation. Creep deformation occurs in three stages namely, primary, secondary and tertiary. Out of these three stages, secondary or steady state creep is particularly an area of engineering interest as it has almost a constant creep rate. Creep deformation plays a significant role in understanding effective service life of an engineering component working under high temperature conditions as such components such as super-heater and re-heater tubes and headers in a boiler, jet engines operating at temperature as high as 1200 C, usually experience a failure or rupture due to creep phenomenon. Design engineers keep a close attention on working stress conditions and elevated temperature under which an engineering component is expected to work as these conditions determine the onset of creep behavior in an engineering component. By recognizing the parameters of material response to creep behavior, engineers can analyse the useful service life and hazardous working conditions for an engineering components. Recognizing the creep phenomenon as high temperature design limitation, ASME Boiler and Pressure Vessel Code have provided guidelines on maximum allowable stresses for materials to be used in creep range. One of the criteria for determination of allowable stresses is 1% creep deformation of material in 100,000 h of service. Thus, the study of creep behavior in engineering components pertaining to high stress and temperature working conditions is very important as it affects the reliability and performance of the engineering components. The aim of our study is to understand the behavior of secondary creep deformation so that an advanced reinforced functionally graded material with better creep resistance, can be designed. In this paper, a secondary creep analysis of functionally graded (FG) thick-walled rotating cylinder under internal and external pressure is conducted. The novelty of the model intends to specify secondary creep stresses and strains by employing exponential, linear and quadratic volume reinforcement for ceramic in metal matrix in radial direction. This will help us to understand the effect of volume reinforcement in FG cylinder under internal/external pressure and rotating centrifugal body force by obtaining secondary creep stresses and strains. The response of the FG cylinder with isotropic material is analyzed and the solution for stress–strain rates in radial and tangential directions are obtained in closed form. Comparison of steady state creep stresses and strains under exponential, linear and quadratic volume reinforcement profiles are discussed and presented graphically.

Influence of Cryo-Processing and Post-SPD Annealing on Creep Behavior of CP Titanium.

The commercial purity of VT1-0 titanium was processed by the rolling process and executed at elevated, room, and cryo-temperatures. These processings led to the formation of an ultrafine-grained microstructure, with the mean grain size at a nanometer level. Some of these materials were statically annealed at a temperature of 823 K for 1 h, which led to significant subgrains and grain coarsening. The constant load creep tests in tension were carried out in argon on all states of materials, at temperatures of 648–723 K and different ranges of applied stresses. From the value of the steady-state creep rate, the control creep mechanisms were determined. The microstructure analyses were carried out via SEM and TEM. It was found that titanium prepared at elevated and room temperatures have a higher creep strength than titanium prepared at cryo-temperatures. Furthermore, the post-SPD —annealing led to a significant decrease in the creep properties. The influence of the preparation temperature on the difference of the creep behavior were discussed and explained using the microstructure analyses of the tests’ samples.

Multi-scale Crystal Viscoplasticity Approach for Estimating Anisotropic Steady-State Creep Properties of Single-Crystal SnAgCu Alloys

Creep can have a significant impact on the reliability of Sn-based solder alloys even at room temperature because of their relatively low melting temperatures. SnAgCu (SAC) solder joints used in electronic packaging typically consist of only a few highly anisotropic grains, which consist of Sn dendrites embedded in a near-eutectic Sn-Ag component. The unique grain structure of each joint leads to stochastic variations in the thermomechanical response of such joint. Therefore, grain-scale testing and modeling are recommended to better characterize the anisotropic behavior and to estimate the influence of stochastic variability of grain structure on the viscoplastic response of the joint. This work aims to investigate the anisotropic steady-state creep behavior of single-crystal SAC solder joints. The orientation-dependent viscoplastic behavior of individual SAC grains is modeled with a multi-scale crystal-viscoplasticity approach for representing the relevant dislocation mechanics. The response predicted by the crystal viscoplasticity model is then captured in an equivalent homogenized continuous-scale constitutive model (Hill-Garofalo formulation).This modeling strategy was implemented in numerical simulations, as an illustrative example, to analyze the behavior of single-grain solder joints subjected to combined steady compression (from heat-sink clamping force) and thermal cyclic loading. The cyclic ratcheting in the presence of the steady compressive force causes: (i) transverse expansion of the solder ball, potentially leading to eventual short circuits; and (ii) cyclic fatigue damage leading to eventual failure of the solder joint. The proposed grain-scale modeling approach is shown to be able to address the stochastic variability of both these damage mechanisms, as a function of grain orientation. The present methodology can be used to predict the realistic behavior of solder joints, based on their microstructure, and provide valuable insights for their reliability analysis.

Construction of nonlinear creep damage model of frozen sandstone based on fractional-order theory

The frozen wall of a coal mine vertical shaft undergoes creep deformation under long-term load, and excessive deformation induces damage to the frozen wall. The creep behaviour of frozen rock is key in controlling the long-term stability of frozen walls. In this study, Cretaceous saturated frozen sandstone was used as the research object to conduct creep mechanics tests under different confining pressure conditions. Based on the Riemann-Liouville-type integral function, a 6-element frozen sandstone nonlinear creep constitutive equation considering temperature–damage–stress coupling was established and verified by experimental data. The results show that: compared with the rock creep test at 20 °C, the creep process of saturated frozen sandstone also presents an attenuation creep stage, a steady-state creep stage, and an accelerated creep stage. A nonlinear creep damage equation is established based on the fractional-order theory. Comparing the theoretical formula with the experimental data, it was found that the agreement was higher. The research results can provide a theoretical and experimental basis for evaluating creep-induced instability and failure of frozen walls.

High-temperature creep behavior and hot-pressing consolidation of NiAl

AbstractThe steady-state creep behavior of NiAl was studied by constant strain-rate tests in the strain-rate regime 1.5 × 10−4≤ ɛ̇ ≤ 1.0 × 10−2s−1.Primary creep of NiAl was investigated at different stresses (10 MPa−40 MPa) in the temperature range from 1200 to 1400°C. The steady-state creep rate was fitted to a temperature-compensated power-law expression ɛ̇ =B(σ/μ)nexp(−Q/RT), from which a stress exponent n≈3.5 and an activation energy Q ≈ 398 kJ/mol were derived, respectively. The obtained activation parameters indicate that dislocation creep dominates the creep behavior of NiAl at temperatures above 1200°C up to 1400°C. The transient creep behavior at 40 MPa could be fitted to a power-law equationε=ε0+btmwithbα σ3:5and 0.52<m<0.68. The creep data of NiAl obtained in this study were utilized for modeling and simulation of the consolidation process of NiAl composites during hot pressing by finite-element analysis.

Study on the Creep and Fracture Evolution Mechanism of Rock Mass with Weak Interlayers

To study the influence of weak interlayers on the creep failure characteristics of rock masses, based on the continuous-discontinuous method (CDEM), the uniaxial compression creep experiments of rock masses containing weak layers were numerically simulated; and the weakened rock masses under different conditions were analyzed in detail. We focused on the final failure mode and creep curve of the rock mass with a weak interlayer (θ = 30°, d = 20 mm, c = 1) as examples by selecting the crack distribution state of the model during compression at different time steps. We analyzed the propagation and convergence mode of cracks in a rock mass with weak layers. The research results show that the existence of weak interlayers affects the integrity of the rock mass and the creep failure mode. With the increase in the inclination of the weak interlayer, the failure mode of the rock mass changes from shear failure through the weak layer to slip along the weak layer. For shear failure, the total strain and steady-state creep rate of the rock mass first decrease and then increase, showing a U-shaped distribution; as the thickness of the weak interlayer increases, the rock mass always follows the shear in the weak layer. Creep failure occurs on the fracture surface, and the total strain and steady-state creep rate of the rock mass are positively correlated with the thickness. If the thickness continues to increase, there is no significant difference in the creep characteristics of the rock mass; the volume occupied by the soft rock in the body increases, the overall rigidity of the rock mass decreases, and the plastic deformation increases. The form of creep failure of the rock mass changes from sliding shear failure along the weak layer to sliding shear failure through the weak interlayer. The total strain and steady-state creep rate of the rock mass increase with the increase in the number of weak layers; the greater the distance between the weak layers, the smaller the total strain and steady-state creep rate of the rock mass. The slower the crack growth rate, the less likely the rock mass to undergo creep damage.

Self-Actuating Isothermal Nanomechanical Test Platform for Tensile Creep Measurement of Freestanding Thin Films

Microelectromechanical systems (MEMS) enable potent methods for nanomechanical testing. In many MEMS-based test platforms, high force polycrystalline silicon chevron-type thermal actuators are used as on chip actuators. Here, we implement thermal actuators using the refractory metal tantalum (Ta) as a new micromachined structural material in such a way that it can be integrated to test mechanical properties of a wide variety of metals. Because Ta’s coefficient of thermal expansion is more than double that of silicon, actuation is accomplished simply by raising the temperature. An isothermal condition is attained, while no force is lost to a heat sink. The platform is batch fabricated by conventional surface micromachining processes and is especially well suited to measure creep of thin films because the load spring is soft compared to the specimen uniaxially loaded in tension. This in turn gives rise to a slow stress relaxation and a large displacement range. Specimen alignment is excellent and handling procedures for thin films are simple and robust. As a validation of the apparatus, uniaxial Au tensile specimens of 110 nm thickness are fabricated and tested. Young’s modulus is measured and compares well with literature values, while creep rate is lower than other reported values for nanocrystalline Au, indicating that steady state creep is approached. This work demonstrates a powerful drift-free nanomechanical test platform for mechanical property measurement of freestanding thin films subjected to uniaxial tension. [2021-0171]

The effect of aspect ratio on the mechanical behavior of Li metal in solid-state cells

The Li metal anode is necessary for solid-state batteries to dramatically exceed the energy density of state-of-the-art conventional Li-ion batteries. To enable the Li metal anode, the mechanical properties of Li metal under compression in a commercially relevant cell configuration must be determined. This work examines the compression creep behavior of Li metal coupled with a relevant model solid-state electrolyte (Li 6.5 La 3 Ta 0.5 Zr 1.5 O 12 ) and current collector (Ni) as a function of applied stress (1–10 MPa) and Li foil thickness (100–700 μm). At 1 MPa applied stress, the strain rate of all Li foil thicknesses under compression was almost four orders of magnitude lower than what is predicted from tensile creep tests. Steady-state creep behavior was observed for Li foil ≥350 μm at 5 MPa and ≥250 μm at 10 MPa. Conversely, sustained creep deformation was not observed for 100 and 150 μm Li foils at any applied stress. It is suggested that friction at the Li/LLZO and Li/Ni interfaces leads to the formation of hydrostatic stress that controls Li deformation at low aspect ratios. These results have implications for battery pack stack pressure requirements, particularly relating to void formation during stripping and Li metal filament penetration during plating. • Compressive creep measurements of Li metal foils in solid-state cell configuration. • Strain and strain rate decreased with decreasing Li foil thickness. • The strain rate of Li measured in compression was lower than predicted from tension. • Friction at Li interfaces prevents substantial creep deformation for thin Li foils.

Study on the Mechanical and Creep Properties of the Bamboo Fiber Under Thermal or Hygro Conditions

ABSTRACT The aim of the study is to investigate the mechanical and creep properties of the bamboo fiber under thermal or hygro conditions. Bamboo fibers were extracted mechanically, and then undergone alkali treatment. The bamboo fiber in this study is actually thin bamboo sticks instead of the real bamboo fibers. This bamboo stick will retain the structure of the original. Tensile tests of the bamboo fiber were. The yield and the tensile strength were found to decrease with the increase of the moisture content or temperature. Creep tests of the bamboo fibers were conducted to investigate the viscoelastic behavior. Results show that the strain creep curve can be divided into three stages. In primary creep phase, strain increases at a decreasing rate. The secondary phase exhibits a linear response with time, also called steady-state creep. The tertiary stage is the stage in which strain response almost increases exponentially, resulting in the failure of the fiber. In the creep test, the applied force was maintained 25 N, and the fracture strain was about 0.7%–0.8%. A mathematical constitutive model was constructed to fit the creep curves. The creep fitting curve only includes the primary and secondary phases, and the fitted curve can roughly show the creep behavior of the bamboo fiber.

Strain rate effect on the mechanical properties of ZrB2-SiC ceramics characterized by nanoindentation

ZrB2-SiC ceramics are one of the most promising materials for high temperature applications, and their mechanical properties are key indicators that determine whether they can long-time serve safely. In this work, the mechanical properties of ZrB2-SiC ceramics are determined by nanoindentation under six different strain rates. The relative density of ZrB2-SiC ceramics prepared by spark plasma sintering reaches 99.1%. The effects of strain rate on the elastic modulus, hardness, creep and pop-in behaviors are discussed in detail. Results show that the effect of strain rate on the elastic modulus can be ignored. The hardness increases with the increase of strain rate, and the increase gradually slows down. The relationship between the natural logarithms of hardness and strain rate can be accurately described as a linear function. The creep strain rate under indentation declines rapidly in the initial holding stage, and then gradually transitions into a constant in the steady-state creep stage under different strain rates. The pop-in load and displacement also showed obvious strain rate-dependence, based on which the shear strength of ZrB2 gains was estimated and compared with the theoretical value.

Long-term tensile creep behavior of a family of FCC-structured multi-component equiatomic solid solution alloys

Long-term tensile creep experiments were performed at 973 K on a family of multicomponent equiatomic solid solution alloys with face-centered cubic crystal structures, including quinary CoCrFeMnNi alloy and quaternary CoCrFeNi alloy, together with our previous report on ternary CoCrNi alloy. Analyzing the steady-state and transient creep properties and characterizing the precipitate evolution, it is suggested that dislocation creep be the dominant deformation mechanism for all these alloys. Although CoCrNi shows the highest room-temperature strength and the lowest creep rate at 973 K, the creep lifetime data for all three alloys are similar and can be described by the Monkman-Grant relationship.

Molecular dynamics simulation of rhenium effects on creep behavior of Ni-based single crystal superalloys

The tensile creep behavior of Ni-based single crystal superalloys was investigated by molecular dynamics simulations. The effects of rhenium (Re) on the creep behavior and microstructural evolution mechanism during nanoscale creep tests were discussed. The results indicate that the addition of Re can effectively enhance the creep resistance of the alloys. The reason is that Re plays an important role in the evolution of microstructure at each stage of the creep. Firstly, Re has a pinning effect to delay the failure of the dislocation network at the primary creep stage. And then, the addition of Re decreases dislocation density and increases steady creep time at the steady-state creep. Finally, Re segregates at the dislocations in the γ′ precipitates phase and prevents the dislocation from cutting into the γ′ phase by dragging dislocations at the tertiary creep stage, which enhances effectively the creep resistance of superalloys. These mechanisms help to further understand the creep behavior and derive an atomistic description of the Re effects on creep behavior and creep mechanisms in Ni-based single crystal superalloys.

High temperature creep deformation of nanocrystalline diamond films

Abstract Diamond displays a combination of unique properties, including the highest hardness among materials, chemical inertness and high thermal conductivity. Therefore, nanocrystalline diamond films offer a huge potential for industrial applications. In fine-grained ceramics as well as metallic materials, high temperature creep deformation is dominated by grain-boundary-deformation mechanisms that become increasingly important with decreasing grain size. In this work we demonstrate that it is possible to inelastically deform nanocrystalline diamond films at elevated temperatures and stresses that are significantly lower than those reported for single-crystal diamond. The initial, isothermal, transient creep flow exhibits a logarithmic character, typical of creep in general. The isothermal steady state creep deformation, which follows transient creep, is analyzed using a physics-based model for grain boundary sliding rate controlled flow.