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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Effects of Boron and Zirconium on Grain Boundary Morphology and Creep Resistance in Nickel-Based Superalloy

Grain boundaries (GBs) in nickel-based superalloys have a significant influence on the creep resistance of these alloys. In this study, the trace elements of boron and zirconium were added to the newly designed LESS 1 (Ni-22Cr-20Co-2W-2Nb-1.5Al-1.5Ti-0.03C) alloy, and their effects on the GB precipitation behaviors of carbides were observed. The combined addition of boron and zirconium generates a continuous film structure containing MC carbides and M{sub 23}(C, B){sub 6}-type boro-carbides, as well as a grain boundary γ′. Based on the change in microstructure, rupture time is significantly increased. According to the observed microstructures and simulation results, it was confirmed that boron forms M{sub 23}(C, B){sub 6}-type boro-carbides and zirconium appears to stabilize MC carbides at GBs.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of the fictive temperature on the modulus, hardness, yield strength, dynamic mechanical and creep response of Zr 44 Ti 11 Cu 10 Ni 10 Be 25 metallic glasses

Characterizing the dependence of the mechanical properties and structure of metallic glasses on processing parameters and thermal history is of significant importance to understand the nature of glass. One way of defining a specific structural state of a glass is by means of its fictive temperature T f at which the supercooled liquid falls out of a metastable equilibrium and forms a glass. Since the metastable equilibrium state before quenching is assumed to be well defined, T f is ultimately thought of as a quantity that describes a specific, reproducibly obtainable structural state. In this research, the effect of systematic variations of an alloy’s structural state, achieved through purposefully setting its T f , on a range of mechanical properties like modulus, hardness, yield strength, and creep response is studied. Towards this end, amorphous Zr 44 Ti 11 Cu 10 Ni 10 Be 25 samples with different T f ’s were produced and subsequently examined using nanoindentation, dynamic mechanical thermal analysis, and density measurements. It is observed that the lower fictive temperatures directly result in a higher the packing efficiency, i.e., a lower available (‘free’) volume in the glass. The resulting denser atomic packing manifests in increased values for hardness, modulus, and yield strength. These effects were found to be substantial despite the fact that all samples featured the exact same chemical composition and phase.

36 MATERIALS SCIENCE↗
DOE OSTI2015

A DLM/FD/IB method for simulating compound vesicle motion under creeping flow condition

In this article we present first a new distributed Lagrange multiplier/fictitious domain (DLM/FD) method for simulating fluid–particle interaction in Stokes flow. A conjugate gradient method driven by both pressure and distributed Lagrange multiplier, called one-shot method, has been developed to solve the discrete Stokes problem while enforcing the rigid body motion within the region occupied by the particle. The methodology is validated by comparing the numerical results of a neutrally buoyant particle of either a circular or elliptic shape with the associated Jeffery's solutions. We have successively combined the above methodology with an immersed boundary (IB) method and an elastic membrane modeled by a spring network to simulate the dynamics of a compound vesicle. In simple shear flow under creeping flow condition, the results are consistent with those obtained in literature. In Poiseuille flow, the compound vesicle motion is dominated by the motion of the vesicle membrane as expected and stays in the central region of the channel.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

A 3D DLM/FD method for simulating the motion of spheres and ellipsoids under creeping flow conditions

We present in this article a novel distributed Lagrange multiplier/fictitious domain (DLM/FD) method for simulating fluid-particle interaction in three-dimensional (3D) Stokes flow. The methodology is validated by comparing the numerical results for a neutrally buoyant particle, of either spherical or prolate shape, with the associated Jeffrey's solutions for a simple shear flow. The results concerning two balls, interacting under creeping flow conditions in a bounded shear flow, are consistent with those available in the literature. We will discuss also the interactions of two balls in a bounded shear flow, when these balls are very close initially. For a prolate ellipsoid rotating in a shear flow under the sole effect of the particle inertia, shear plane tumbling is stable, while log-rolling is unstable. For two prolate ellipsoids interacting in a bounded shear flow, the results are similar to those for two balls if the major axes are initially orthogonal to the shear plane (a result not at all surprising considering that the intersections of the ellipsoids with the shear pane are circular).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Creep performance and microstructure of grade 91 steel weldments with integrated welding and thermal processing

Ferritic-Martensitic steel welds typically require post weld heat treatment (PWHT) to restore toughness and high temperature performance. This off-line thermal process reduces disparities between weld and base metal, but can cause distortion, cracking, or simply be impractical due to assembly size and joint non-uniformity. Here we show integrated welding and thermal processing applied to modified 9Cr-1Mo (Grade 91) steel, favored for advanced power generation applications, performed in real time through the addition of a secondary heat source near the primary weld head. Optimal integrated processing reduces weld fusion and heat affected zone hardness by 125 HV, approaching performance of conventional 730 °C, 60 min PWHT processing. Microstructures and mechanical performance are compared for mechanized GTAW welds, with equivalent lifetimes noted in cross-weld creep rupture tests up to 234 MPa at 550 °C, and up to 104 MPa at 650 °C. The integrated process was validated on a Grade 91 pressure vessel with multipass cold wire feed GTAW. After 550 °C, 71.4 bar thermomechanical cyclic testing, the maximum weld hardness is <350 HV.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Modeling irradiation creep of graphite using rate theory

In this work we examined irradiation induced creep of graphite in the framework of transition state rate theory. Experimental data for two grades of nuclear graphite (H-337 and AGOT) were analyzed to determine the stress exponent (n) and activation energy (Q) for plastic flow under irradiation. Here we show that the mean activation energy lies between 0.14 and 0.32 eV with a mean stress-exponent of 1.0 ± 0.2. A stress exponent of unity and the unusually low activation energies strongly indicate a diffusive defect transport mechanism for neutron doses in the range of 3-4 x 10 22 n/cm 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2016

Creep study of mechanisms involved in low-temperature superplasticity of UFG Ti-6Al-4V processed by SPD

The deformation kinetics of ultrafine-grained Ti-6Al-4V with mean (sub)grain size about 150 nm (produced by isothermal multiaxial forging) and superplastic properties at the relatively low temperature of 873 K was investigated in compression and tension over a large range of strain rates from 10{sup −7} to 10{sup −2} s{sup −1}. Electron microscopic observations showed that the grains coarsen during deformation towards the quasi-stationary spacing w{sub qs} of strain induced boundaries. In spite of the grain coarsening the grains were generally smaller than w{sub qs} allowing high-angle boundaries to dominate the quasi-stationary strength. Texture measurements indicate that dislocation glide plays a large role in deformation. Glide in this alloy is significantly influenced by solid solution strengthening leading to a stress sensitivity of strain rate of n = 3. The present ultrafine-grained Ti alloy displays a stress sensitivity exponent n = 2 over an extended stress range where its superplastic behavior is optimal. While the deformation kinetics of present ultrafine-grained Ti alloy can be roughly explained by the traditional formula for superplastic flow, the significant discrepancy to the measured values suggests that solid solution strengthening must be taken into account to get a complete insight. - Highlights: • The UFG Ti-6Al-4V alloy behaves superplastically at low temperature of 873 K. • Grain coarsening at low stresses limits superplasticity of UFG Ti alloy. • Solute strengthening plays an important role in low-temperature superplasticity. • Acceleration of creep in UFG Ti alloy is caused by processes related to hab.

36 MATERIALS SCIENCE↗
DOE OSTI2018

High temperature irradiation induced creep in Ag nanopillars measured via in situ transmission electron microscopy

Irradiation induced creep (IIC) rates are measured in compression on Ag nanopillar (square) beams in the sink-limited regime. The IIC rate increases linearly with stress at lower stresses, i.e. below ≈2/3 the high temperature yield stress and parabolically with pillar width, L, for L less than ≈300 nm. Here, the data are obtained by combining in situ transmission electron imaging with simultaneous ion irradiation, laser heating, and nanopillar compression. Results in the larger width regime are consistent with prior literature.

36 MATERIALS SCIENCE↗
DOE OSTI2018

In-situ observation of an irradiation creep deformation mechanism in zirconium alloys

We report an original experimental study on a zirconium alloy deformed in situ under ion irradiation and applied stress inside a Transmission Electron Microscope. We observe that dislocations initially pinned on irradiation defects can be unpinned and glide at a lower stress under the effect of the irradiation. It is proposed that unpinning occurs by a local effect of the displacement cascade created by the incoming ion in the vicinity of the pinning point. This novel mechanism of dislocation glide assisted by irradiation is thought to play a crucial role on in reactor irradiation creep of zirconium alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Measurement of Activation Volume for Creep of Dry Olivine at Upper-Mantle Conditions

Olivine is the most abundant and among the weakest phases in Earth's upper mantle, and thus, its rheological properties play a critical role in governing thermal structure and convective flow in the upper mantle. A persistent obstacle to constraining the in situ flow properties of olivine by laboratory experiment has been the difficulty in resolving the effect of pressure, which is weak within the 0- to ~2-GPa pressure range of conventional laboratory deformation instruments but potentially strong over the 1- to ~14-GPa range of the upper mantle. Using a deformation-DIA, one of a new generation of bonafide deformation devices designed for operation to ≥10 GPa, we have deformed dry, polycrystalline San Carlos olivine in high-temperature creep with the singular intent of providing the best achievable measurement of activation volume V* and a comprehensive statement of uncertainty. Under strictly dry conditions, at constant temperature (1,373 K) and strain rate (1 × 10 -5 s -1 ), varying only pressure (1.8 to 8.8 GPa), we measure V* = 15 ± 5 cm 3 /mol. We have reproduced the well-known mechanism change from [100]-slip to [001]-slip near 5 GPa and determined that, whatever the change in V* associated with the change in slip system, the effective value of 15 ± 5 cm3/mol is still accurate for modeling purposes in the 2- to 9-GPa pressure range. This is a substantial pressure effect, which in the absence of a temperature gradient would represent a viscosity increase from the top to bottom of the upper mantle of 5 ± 2 orders of magnitude.

58 GEOSCIENCES↗
DOE OSTI2020

Preseismic Fault Creep and Elastic Wave Amplitude Precursors Scale With Lab Earthquake Magnitude for the Continuum of Tectonic Failure Modes

Tectonic faults fail in a continuum of modes from slow earthquakes to elastodynamic rupture. Precursory variations in elastic wavespeed and amplitude, interpreted as indicators of imminent failure, have been observed in limited natural settings and lab experiments where they are thought to arise from contact rejuvenation and microcracking within and around the fault zone. However, the physical mechanisms and connections to fault creep are poorly understood. Here we vary loading stiffness during frictional shear to generate a range of slip modes and measure fault zone properties using transmitted elastic waves. We find that elastic wave amplitudes show clear changes before fault failure. The temporal onset of amplitude reduction scales with lab earthquake magnitude and the magnitude of this reduction varies with fault slip. Our data provide clear evidence of precursors to lab earthquakes and suggest that continuous seismic monitoring could be useful for assessing fault state and seismic hazard potential.

58 GEOSCIENCES↗
DOE OSTI2015

Experimental study of cyclic creep and high-cycle fatigue of welded joints of St3 steel by the DIC technique

In the paper the mechanisms of plastic deformation and fracture of welded joints of steel St3 were investigated at high-cycle fatigue and cyclic creep by the digital image correlation (DIC) technique. The evolution of strain rate is studied for the following regions: base metal, HAZ, and fusion zone. This strain rate evolution can be considered as a mechanical response of material. Three stages of deformation evolution are shown: deformation hardening (I), fatigue crack initiation (II), and the last stage is related to main crack (III). Two criteria are offered to evaluate the current mechanical state of welded joints.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Understanding the mechanisms of amorphous creep through molecular simulation

Significance The individual and collective molecular displacements in an amorphous solid undergoing plastic deformation are simulated by an atomistic method that allows incremental motions to be observed over a time window of fractions of seconds. Because the timescale matches well with the conditions of experimental measurements, simulation details provide dynamical evidence for the fundamental mechanisms of amorphous creep. In particular, knowledge of the interplay between diffusion (flow) and mechanical deformation processes enables us to explain the stress and temperature behavior of the experimental data as well as the validity of model descriptions of molecular mechanisms in terms of spatially and temporally heterogeneous fluctuations.

42 ENGINEERING↗
DOE OSTI2024

Using in situ UO 2 bicrystal sintering to understand grain boundary dislocation nucleation kinetics and creep

Capillary evolution at bicrystal UO 2 grain boundaries is characterized using in situ transmission electron microscopy. The discontinuous nature of the densification process, both particle rotation and axial strain, along with the large activation stress for densification support a hypothesis that grain boundary strain in UO 2 follows nucleation rate limited kinetics at low to intermediate stresses, that is, less than ≈ 10 8 PA. Further, the temperature dependence of the average activation stress for sintering agrees well with analysis of bulk sintering data and creep data reported within the literature when analyzed in the context of a grain boundary dislocation nucleation rate limited kinetic model.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Destruction of the Aluminum–Magnesium Alloy under the Influence of Pulsed Laser Radiation against Creep Conditions

Abstract—Using high-speed video recording, the stages of predestruction and development of a main crack initiated by pulsed infrared laser radiation on the surface of the AlMg6 aluminum–magnesium alloy deformed by the uniaxial tensile testing under creep conditions are studied. The role of macroscopic bands of plastic strain in the development of the last deformation jump with an amplitude of about 10%, and the generation and propagation of the main crack is revealed. A possible mechanism of fracture is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

A Variable Load LVDT-Based Creep Test Rig for Use in ATR Loop 2A

Understanding the creep behavior of materials in a radiation environment is essential in evaluating safety issues associated with nuclear power plant operation, especially for plants applying for life extension. In US materials testing reactors (MTRs), this understanding has been achieved through a “cook and look” approach (where specimens are irradiated under load for some period of time and then removed from an MTR for post-irradiation examination). A number of “cook and look” cycles can be required for each evaluated material, which makes this approach expensive. In addition, examinations are limited to end-state conditions, which may or may not be sufficient to reveal all phenomena of interest (because examinations are not performed at prototypic conditions relative to temperature, pressure, stress, etc.).

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗