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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Creep Properties of Directionally Solidified Nb-Modified Ni-Base Superalloy, Mar-M247

The Ni-base superalloy Mar-M247 is widely used for elevated-temperature applications in both equiaxed and directionally solidified forms. The alloy contains about 3 wt.% Ta. Due to the cost and density of Ta, an atom-for-atom substitution of Nb for Ta was investigated. The creep properties in the temperature range of 750-950 °C and stresses ranging from 200 to 1000 MPa were determined. In general, the creep properties of the Nb-modified Mar-M247 material were very similar to those of Mar-M247 samples at 750 and 850 °C. However, the Mar-M247 samples exhibited superior creep properties at 950 °C. The lower creep properties of the Nb-modified Mar-M247 are likely due to the finer starting microstructure, more rapid coarsening rate of the γ′ and reduced γ′ strengthening effect (i.e., APB energy) of Nb, compared to Ta. No evidence of TCP phases was observed in either alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Creep Deformation and Rupture Behavior of P92 Steel Weld Joint Fabricated by NG-TIG Welding Process

The creep deformation and rupture behavior of P92 steel weld joint fabricated by narrow-gap TIG (NG-TIG) welding process have been investigated at 923 K over a stress range of 80-140 MPa. The prior-austenite grain size, M{sub 23}C{sub 6} precipitate size and hardness have been found to vary significantly in the weld joint. The reduction in hardness from weld metal to base metal with a trough at the outer edge of heat-affected zone (HAZ) has been observed. Coarsening of M{sub 23}C{sub 6} precipitate, recovery of martensite lath dislocation structure and formation of subgrain structure led to lower hardness in the intercritical HAZ. The creep rupture life of NG-TIG weld joint was lower than the base metal, the difference in creep rupture life between base metal and weld joint has been increased significantly with decrease in applied stress. The fracture location in the joint changed from base metal at high-stress regime to the fine grain (FG) HAZ (Type IV cracking) under lower stress level. Fracture in the FGHAZ evidenced the significant reduction in ductility, localized deformation and extensive localized cavitation. Extensive Laves phase formation with significant loss of solution strengthening contribution from tungsten and coarsening of M{sub 23}C{sub 6} precipitate with prolonged creep exposure led to reduction in hardness and more extensive cavitation in the FGHAZ resulting in premature Type IV failure of the weld joint. Weld strength reduction factor about 0.59 has been evaluated for 10{sup 5} h at 923 K.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Fatigue Performance of an Improved Creep Strength 10%Cr Steel

The deployment of 9-12% Cr steels for elevated temperature applications up to 650 °C presents a cost-effective alternative to more expensive nickel-based alloys in steam turbine power generation. To enhance creep resistance at this temperature range, a novel ferritic-martensitic steel, designated CPJ7, was developed and fabricated at the National Energy Technology Laboratory. The alloy design aimed to mitigate the transformation of strengthening carbides into deleterious phases that degrade creep performance. Results have demonstrated that CPJ7 exhibits favorable creep and oxidation resistance at 650 °C. However, its fatigue performance remains unexplored. This study builds upon prior research by evaluating the low cycle fatigue behavior of CPJ7 and verifying that modifications beneficial to creep performance were not detrimental to the fatigue performance. The alloy was tested at both 650 °C and ambient temperature under fully reversed bending conditions (R = − 1) and a load ratio of 0.05. Furthermore, the alloy exhibits cyclic softening, a behavior consistent with other 9-10 wt.% Cr steels. Analysis of the microstructure and hysteresis loops further corroborate cyclic softening mechanisms typical of ferritic-martensitic steels. Overall, the fatigue performance of CPJ7 meets or exceeds that of P91 steel, demonstrating its potential for high-temperature structural applications.

9% Cr ferritic-martensitic steel↗
DOE OSTI2022

A creep-resistant additively manufactured Al-Ce-Ni-Mn alloy

Existing additively manufactured aluminum alloys exhibit poor creep resistance due to coarsening of their strengthening phases and refined grain structures. In this paper, we report on a novel additively manufactured Al-10.5Ce-3.1Ni-1.2Mn wt.% alloy which displays excellent creep resistance relative to cast high-temperature aluminum alloys at 300–400 °C. In this work, the creep resistance of this alloy is attributed to a high volume fraction (~35%) of submicron intermetallic strengthening phases which are coarsening-resistant for hundreds of hours at 350 °C. The results herein demonstrate that additive manufacturing provides opportunities for development of creep-resistant aluminum alloys that may be used in bulk form in the 250–400 °C temperature range. Pathways for further development of such alloys are identified.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Creep and failure at metal-oxide interfaces

Small-scale bicrystal creep experiments were performed on contacts formed via in situ high-temperature diffusion bonding of metal-oxide interfaces including Ag-ZrO 2 , Pd-ZrO 2 , Pt-ZrO 2 , and Ag-high entropy oxide. This work characterizes deformation and failure at metal-oxide interfaces during mechanical loading. Interfacial sliding can be activated easily, while tensile interfacial creep was not observed at any condition of stress or temperature measured. Plastic strain, instead, localizes within the metal under tensile loading. A variety of mechanisms for plastic strain occur in the metal including lattice dislocation-mediated plasticity, twinning, low-angle grain boundary formation, and low-angle grain boundary creep. Surface and low-angle grain boundary diffusion occur under conditions where no metal-oxide tensile creep is observed, highlighting the significant differences in their interfacial mechanical response. High-temperature interfacial failure occurs when the mean curvature at the contact neck is approximately zero and the applied stresses comparable to brittle fracture stresses. The brittle fracture stresses were measured to be σ ƒ = 180 ± 90 MPa at the Ag-ZrO 2 interface at 225 °C, σ ƒ = 460 ± 160 MPa at the Pd-ZrO 2 interface at 680 °C, and σ ƒ = 640 ± 440 MPa at the Pt-ZrO 2 interface at 1010 °C.

Creep↗
DOE OSTI2025

Step-loaded creep testing of Zircaloy-4 cladding at higher temperatures in the α-phase

A refined understanding of zirconium-based cladding thermomechanical performance during rapid transients is essential for enhancing the safety and operation of light-water reactors. Traditional models for zirconium alloys under accident conditions generally assume that creep dominates fuel cladding performance. Here, these historic models have largely remained unchanged and serve as the basis for safety criteria development. As the U.S. nuclear industry pursues higher burnup levels, the increased release of fission gases during transients raises the risk of cladding failure in the low-temperature hcp α-phase, making the fidelity of these models of greater importance. Creep testing was conducted from 550–700°C with 25–120 MPa applied hoop stresses to investigate Zircaloy-4 deformation at accident-relevant temperatures in the α-phase. Step-loading was employed to capture creep behavior across a wide stress range from a single sample. The stress-strain rate data at higher temperatures (650 and 700°C) were well-described by isotropic versions of the Erbacher and Kaddour models, while the lower temperature data (550 and 600°C) were underpredicted by both anisotropic and isotropic model variants. Greater strain rates during the initial loading step at 650 and 700°C were attributed to recrystallization and grain growth of sub-micron crystallites. Yet, texture analysis revealed the basal split texture remained after testing. These observations produced results suggesting Zircaloy-4 claddings experience higher creep rates across the α-phase than previously thought, possibly related to dynamic anisotropy due to temperature dependent activation of deformation mechanisms, effects of biaxial loading, and variation in material condition between the current testing used in previous model development.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Nucleation rate controlled grain boundary and lattice creep

Nucleation versus diffusion rate-limited bicrystal and single crystal creep exhibit different scaling dependencies that enable the mechanisms to be isolated when measured as a function of sample size. It has recently been suggested that nucleation rate-limited kinetic models generally describe the non-Newtonian portion of the creep response well, but more direct evidence is required. This work analyzes the grain boundary creep response of UO 2 , a pyrochlore high entropy oxide, silver, and palladium, along with the lattice creep of silver using small-scale in situ loading in the transmission electron microscope. At small sizes, each system exhibits scale dependence associated with nucleation rate-limited kinetics. Fits of the data produce activation volumes on the order of a few Burgers vectors cubed with positive temperature coefficients as expected for nucleation kinetics. The activation enthalpies fall in the range of about 0.4 eV to 1.7 eV, being lower for the metals and higher for the oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Shrinkage-induced deformations and creep of structural concrete: 1-year measurements and numerical prediction

Highlights: • Extensive experimental study on drying shrinkage, creep and microcracking of concrete • All specimens prepared from a single batch of ordinary-strength structural concrete • 1st year of measurements of (not only) non-uniformly drying beams with span up to 3-m • The database is downloadable from free-to-use research data repository • Modified MPS model for concrete creep used in blind prediction of all experiments The material models for creep and shrinkage operating on the material point level in FEM are usually intended for challenging complex applications and structures, where the average cross-sectional approach does not suffice. The identification of the growing number of material parameters induced by increasing model capabilities relies on very specific and narrow-oriented yet interconnected experiments which are scarce. The presented comprehensive experiments aim to provide a clearer image of the complicated interaction among the basic phenomena: drying, shrinkage, creep, and microcracking. The cornerstone of this ongoing research is a unique set of 30 partially-sealed unreinforced concrete beams with span 1.75–3.0 m subjected to drying. To minimize material variation, all specimens in this study were cast from a single batch of ordinary strength structural concrete with slag-blended binder. The resulting experimental database will be suitable both for validation and development of the constitutive models.

36 MATERIALS SCIENCE↗
DOE OSTI2012

New Ferritic Steels with Combined Optimal Creep Resistance and Ductility by Coupling Thermodynamic Calculations with Focused Experiments

Two critical issues restricting the applications of NiAl precipitate-strengthened ferritic steels are their poor room temperature ductility and insufficient creep resistance at temperatures higher than 600 C. In this study, a thermodynamic modeling approach is integrated with experiments focused on investigating the ductility and creep resistance of steel alloys based on the Fe-Ni-Al-Cr-Mo multi-component system. The mechanical property studies showed that the creep resistance increases with increasing the volume fraction of B2-ordered precipitates, while the opposite trend was observed for the ductility. Low solubility of Al in the {alpha}-Fe matrix was found to favor a ductility increase. Thermodynamic calculations were used to predict the volume fraction of B2-ordered precipitate and the elemental partitioning to guide the selection of alloy compositions that might exhibit the balanced creep resistance and ductility. Key experiments were then conducted to validate the prediction. This integrated approach was found to be very effective in the alloy development.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Age hardening and creep resistance of cast Al–Cu alloy modified by praseodymium

The effects of praseodymium on age hardening behavior and creep resistance of cast Al–Cu alloy were investigated. The results indicated that praseodymium facilitated the formation of the θ′ precipitates during the age process and improved the hardness of the Al–Cu alloy. Besides, praseodymium resulted in the formation of the Al{sub 11}Pr{sub 3} phase in the grain boundaries and among the dendrites of the modified alloy. Because of the good thermal stability of Al{sub 11}Pr{sub 3} phase, it inhibits grain boundary migration and dislocation movement during the creep process, which contributes to the improvement in the creep resistance of the modified alloy at elevated temperatures. - Highlights: • Pr addition enhances the hardness and creep resistance of the Al–Cu alloy. • Pr addition facilitates the formation of the θ′ precipitates. • Pr addition results in the formation of the Al11Pr3 phase in the Al–Cu alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Creep-induced microstructural evolution in a nickel-based superalloy designed for advanced ultra-supercritical boilers

Highlights: • Microstructural evolution of a nickel-based superalloy during long-term creep was investigated. • Precipitate-free zones (PFZs) widely arose near the grain boundary under stress. • The coarsened and elongated particles in the PFZs were identified as γ′ precipitates. • The boundary movement plays a role in evolution of grain boundary precipitates. - Abstract: Long-term microstructural stability is a major concern for candidate materials used in advanced ultra-supercritical (A-USC) power plants. In the present work, precipitation behavior of a nickel based superalloy designed for A-USC boilers, with a base composition of Ni-25Cr-20Co, was investigated during long-term thermal exposure under different stresses at 1043 K. The results showed that the coarsening of γ′ precipitates in the grain interior had occurred during thermal exposure in the presence and absence of stress. The microstructure around grain boundary was altered remarkably under stress. The precipitate-free zones (PFZs) commonly existed near the grain boundary in the gauge sections of specimens after long-term creep. Coarsened and elongated particles in the PFZs were identified as γ′-Ni{sub 3} (Ti, Al) precipitates by chemical composition analysis and diffraction pattern. Furthermore, the morphology and distribution of MC-type carbides have no significant changes observed due to their good stability, but discontinuous M{sub 23}C{sub 6} appeared under stress, due to formation of PFZs resulting in dissolution of the adjacent M{sub 23}C{sub 6}. Besides, the creep-rupture mechanism was discussed to explore a potential approach for improving creep properties.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Precipitation behavior during aging and creep in 18Cr–9Ni–3Cu–Nb–N steel

Highlights: • The creep strength of 18Cr–9Ni–3Cu–Nb–N steel degraded in the long term. • Nb rich MX particles precipitated in grain interior after aging for just one hour at 650 to 800 °C. • The time required for precipitation of modified Z phase was about 2500 h at 650 °C and 1050 h at 700 °C. • Fine Cu particles were detected in grain interior after aging for one hour. • Large Cu particles were located at the interface between the σ phase and the matrix after long-term aging at 650 °C and 700 °C. - Abstract: Microstructural changes during aging and creep were investigated in 18Cr–9Ni–3Cu–Nb–N steel (KA-SUS304J1HTB, ASME Code Case 2328), focusing on copper phase, MX carbonitride and Z phase. The creep strength of 18Cr–9Ni–3Cu–Nb–N steel was much higher than that of conventional SUS304 steel in the short term. However, the difference of these two steels was small in the long term, because the creep strength of 18Cr–9Ni–3Cu–Nb–N steel degraded in the long term. Niobium rich MX particles precipitated in grain interior after aging for just one hour at 650 to 800 °C. Modified Z phase [Cr(Nb, V)N] particles were observed after aging accompanied by the disappearance of MX particles. The time required for precipitation of modified Z phase was about 2500 h at 650 °C and 1050 h at 700 °C. Fine copper particles were detected in grain interior after aging for one hour. Large copper particles were located at the interface between the σ phase and the matrix after long-term aging at 650 °C and 700 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Fatigue and Creep-Fatigue Deformation of an Ultra-Fine Precipitate Strengthened Advanced Austenitic Alloy

An advanced austenitic alloy, HT-UPS (high-temperature ultrafine-precipitation-strengthened), has been identified as an ideal candidate material for the structural components of fast reactors and energy-conversion systems. HT-UPS alloys demonstrate improved creep resistance relative to 316 stainless steel (SS) through additions of Ti and Nb, which precipitate to form a widespread dispersion of stable nanoscale metallic carbide (MC) particles in the austenitic matrix. The low-cycle fatigue and creep-fatigue behavior of an HT-UPS alloy have been investigated at 650 °C and a 1.0% total strain, with an R-ratio of -1 and hold times at peak tensile strain as long as 150 min. The cyclic deformation response of HT-UPS is directly compared to that of standard 316 SS. The measured values for total cycles to failure are similar, despite differences in peak stress profiles and in qualitative observations of the deformed microstructures. Crack propagation is primarily transgranular in fatigue and creep-fatigue of both alloys at the investigated conditions. Internal grain boundary damage in the form of fine cracks resulting from the tensile hold is present for hold times of 60 min and longer, and substantially more internal cracks are quantifiable in 316 SS than in HT-UPS. The dislocation substructures observed in the deformed material differ significantly; an equiaxed cellular structure is observed in 316 SS, whereas in HT-UPS the microstructure takes the form of widespread and relatively homogenous tangles of dislocations pinned by the nanoscale MC precipitates. The significant effect of the fine distribution of precipitates on observed fatigue and creep-fatigue response is described in three distinct behavioral regions as it evolves with continued cycling.

36 MATERIALS SCIENCE↗
DOE OSTI2015

The tensile and tensile-creep deformation behavior of Ti–8Al–1Mo–1V(wt%)

The deformation behavior of titanium (Ti) can be complex due to the lower crystal symmetry of the hexagonal α phase compared with metals based on cubic phases. The tensile and tensile-creep deformation behavior of a Ti–8Al–1Mo–1V(wt%) alloy with a weak crystallographic texture was investigated in-situ straining scanning electron microscopy. Tensile experiments were performed at room temperature (RT), 260°C, 455°C, and 650°C, while tensile-creep experiments were performed at 370°C and 455°C. Electron backscattered diffraction (EBSD) was performed both before and after the deformation, and slip trace analysis was used to identify the activation of the slip modes as a function of temperature and the associated global-stress-state Schmid factors. While extensive slip was characterized during the tensile tests, no twinning was observed. During the RT tensile tests the majority of the slip observed was on prismatic planes, while the relative activity of basal slip increased with temperature, such that at 455°C, basal slip and prismatic slip were equally active. Some grain boundary and triple point cracking occurred during the elevated-temperature tensile deformation, while less slip activity and more grain boundary ledge formation were observed during the tensile-creep deformation. In conclusion, during tensile-creep, basal slip was more prevalent than prismatic slip and some transgranular cracks formed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of heterogeneous microstructure on the tensile and creep performances of cast Haynes 282 alloy

Precipitation-strengthened Ni-based superalloys are leading candidate materials for advanced ultra-supercritical (A-USC) plants with steam conditions up to 760 °C (1400 °F) and 35 MPa (5 ksi). This study evaluates representative specimens from a large casting of Haynes 282 to study the effect of microstructural heterogeneity on the mechanical behavior of this alloy. The tensile test results of cast Haynes 282 over the temperature range 20–816 °C exhibited lower tensile strength and ductility in comparison with the reference wrought Haynes 282. However, the creep rupture tests of the cast alloy below 704–788 °C and 190–431 MPa presented a similar stress-Larson-Miller parameter to that of the wrought material. Microstructural and dislocation characterizations using scanning electron microscopy, conventional transmission electron microscopy, and scanning transmission electron microscopy in conjunction with energy dispersive X-ray spectroscopy were performed to understand the microstructural evolution before and after the mechanical tests. The heterogeneous microstructures of the cast Haynes 282 material, including the coarse grains, potential casting defects, and a bimodal size distribution of γ' precipitates, were detrimental to the tensile behavior, whereas the coarse-scale grains had a positive effect on the creep performance because diffusional creep was the dominant creep mechanism.

tensile testing↗
DOE OSTI2020

Tensile creep properties of a CrMnFeCoNi high-entropy alloy

Tensile creep tests were performed on a CrMnFeCoNi high-entropy alloy at temperatures from 1023 K to 1173 K. A uniform stress exponent 3.7 ± 0.1 was found across all temperatures. The apparent activation energies of creep under various applied stresses were determined to be around 230 kJ/mol and decrease with increasing stress, indicating a stress-assisted, thermally activated behavior. Steady-state creep microstructures feature no subgrain formation and high dislocation density within grains. Based on our results, the creep rate of CrMnFeCoNi is believed to be controlled by both dislocation-dislocation interactions and dislocation-lattice interactions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Superplastic Creep of Metal Nanowires From Rate-Dependent Plasticity Transition

Understanding the time-dependent mechanical behavior of nanomaterials such as nanowires is essential to predict their reliability in nanomechanical devices. This understanding is typically obtained using creep tests, which are the most fundamental loading mechanism by which the time dependent deformation of materials is characterized. However, due to existing challenges facing both experimentalists and theorists, the time dependent mechanical response of nanowires is not well-understood. Here, we use atomistic simulations that can access experimental time scales to examine the creep of single-crystal face-centered cubic metal (Cu, Ag, Pt) nanowires. Here, we report that both Cu and Ag nanowires show significantly increased ductility and superplasticity under low creep stresses, where the superplasticity is driven by a rate-dependent transition in defect nucleation from twinning to trailing partial dislocations at the micro- or millisecond time scale. The transition in the deformation mechanism also governs a corresponding transition in the stress-dependent creep time at the microsecond (Ag) and millisecond (Cu) time scales. Overall, this work demonstrates the necessity of accessing time scales that far exceed those seen in conventional atomistic modeling for accurate insights into the time-dependent mechanical behavior and properties of nanomaterials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Electrochemical control of creep in nanoporous gold

We have investigated the mechanical stability of nanoporous gold (npg) in an electrochemical environment, using in situ dilatometry and compression experiments. It is demonstrated that the gold nano-ligaments creep under the action of surface stress which leads to spontaneous volume contractions in macroscopic npg samples. The creep of npg, under or without external forces, can be controlled electrochemically. The creep rate increases with increasing potential in double-layer potential region, and deceases to almost zero when the gold surface is adsorbed with oxygen. Surprisingly, we also noticed a correlation between creep and surface diffusivity, which links the deformation of nanocrystals to mobility of surface atoms.

77 NANOSCIENCE AND NANOTECHNOLOGY↗