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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2008

Moisture-Induced Delayed Spallation and Interfacial Hydrogen Embrittlement of Alumina Scales

While interfacial sulfur is the primary chemical factor affecting Al2O3 scale adhesion, moisture-induced delayed spallation appears as a secondary, but impressive, mechanistic detail. Similarities with bulk metallic phenomena suggest that hydrogen embrittlement from ambient humidity, resulting from the reaction Al(sub alloy)+3(H2O)(sub air) = Al(OH)(-) (sub 3) +3H(+) may be the operative mechanism. This proposal was tested on pre-oxidized Rene N5 by standard cathodic hydrogen charging in 1N H2SO4, as monitored by weight change, induced current, and microstructure. Cathodic polarization at -2.0 V abruptly stripped mature Al2O3 scales at the oxide-metal interface. Anodic polarization at +2.0 V, however, produced alloy dissolution. Finally, with no applied voltage, the acid electrolyte produced neither scale spallation nor alloy dissolution. Thus, hydrogen charging was detrimental to alumina scale adhesion. Moisture-induced interfacial hydrogen embrittlement is concluded to be the cause of delayed scale spallation and desktop thermal barrier coating failures.

Smialek, James L.↗
NASA NTRS2009

Moisture-Induced Delayed Alumina Scale Spallation on a Ni(Pt)Al Coating

Delayed interfacial scale failure takes place after cooling for samples of a Ni(Pt)Al-coated CMSX4 single crystal superalloy, cycled at 1150 C for up to 2000 hr. One sample exhibited premature coating grain boundary wrinkling, alumina scale spallation to bare metal, and a final weight loss of 3.3 mg/cm2 . Spallation under ambient conditions was monitored with time after cooldown and was found to continue for 24 hr. This produced up to 0.05 mg/cm2 additional loss for each hold, accumulating 0.7 mg/cm 2 (20 percent of the total) over the course of the test. After test termination, water immersion produced an additional 0.15 mg/cm2 loss. (A duplicate sample produced much less wrinkling and time dependent spalling, maintaining a net weight gain.) The results are consistent with the general phenomena of moisture-induced delayed spallation (MIDS) of mature, distressed alumina scales formed on oxidation resistant M-Al alloys. Relative ambient humidity is discussed as the factor controlling adsorbed moisture, reaction with the substrate, and hydrogen effects on interface strength.

Smialek, James L.↗
NASA NTRS2011

Moisture-Induced TBC Spallation on Turbine Blade Samples

Delayed failure of TBCs is a widely observed laboratory phenomenon, although many of the early observations went unreported. "The weekend effect" or "DeskTop Spallation" (DTS) is characterized by initial survival of a TBC after accelerated laboratory thermal cycling, then failure by exposure to ambient humidity or water. Once initiated, failure can occur quite dramatically in less than a second. To this end, the water drop test and digital video recordings have become useful techniques in studies at NASA (Smialek, Zhu, Cuy), DECHMA (Rudolphi, Renusch, Schuetze), and CNRS Toulouse/SNECMA (Deneux, Cadoret, Hervier, Monceau). In the present study the results for a commercial turbine blade, with a standard EB-PVD 7YSZ TBC top coat and Pt-aluminide diffusion bond monitored by weight change and visual appearance. Failures were distributed widely over a 5-100 hr time range, depending on temperature. At some opportune times, failure was captured by video recording, documenting the appearance and speed of the moisture-induced spallation process. Failure interfaces exhibited alumina scale grains, decorated with Ta-rich oxide particles, and alumina inclusions as islands and streamers. The phenomenon is thus rooted in moisture-induced delayed spallation (MIDS) of the alumina scale formed on the bond coat. In that regard, many studies show the susceptibility of alumina scales to moisture, as long as high strain energy and a partially exposed interface exist. The latter conditions result from severe cyclic oxidation conditions, which produce a highly stressed and partially damaged scale. In one model, it has been proposed that moisture reacts with aluminum in the bond coat to release hydrogen atoms that 'embrittle' the interface. A negative synergistic effect with interfacial sulfur is also invoked.

Smialek, James L.↗
NASA NTRS2011

Moisture-Induced TBC Spallation on Turbine Blade Samples

Delayed failure of TBCs is a widely observed laboratory phenomenon, although many of the early observations went unreported. The weekend effect or DeskTop Spallation (DTS) is characterized by initial survival of a TBC after accelerated laboratory thermal cycling, then failure by exposure to ambient humidity or water. Once initiated, failure can occur quite dramatically in less than a second. To this end, the water drop test and digital video recordings have become useful techniques in studies at NASA (Smialek, Zhu, Cuy), DECHMA (Rudolphi, Renusch, Schuetze), and CNRS Toulouse/SNECMA (Deneux, Cadoret, Hervier, Monceau). In the present study the results for a commercial turbine blade, with a standard EB-PVD 7YSZ TBC top coat and Pt-aluminide diffusion bond coat are reported. Cut sections were intermittently oxidized at 1100, 1150, and 1200 C and monitored by weight change and visual appearance. Failures were distributed widely over a 5-100 hr time range, depending on temperature. At some opportune times, failure was captured by video recording, documenting the appearance and speed of the moisture-induced spallation process. Failure interfaces exhibited alumina scale grains, decorated with Ta-rich oxide particles, and alumina inclusions as islands and streamers. The phenomenon is thus rooted in moisture-induced delayed spallation (MIDS) of the alumina scale formed on the bond coat. In that regard, many studies show the susceptibility of alumina scales to moisture, as long as high strain energy and a partially exposed interface exist. The latter conditions result from severe cyclic oxidation conditions, which produce a highly stressed and partially damaged scale. In one model, it has been proposed that moisture reacts with aluminum in the bond coat to release hydrogen atoms that embrittle the interface. A negative synergistic effect with interfacial sulfur is also invoked.

Smialek, James↗
NASA NTRS2009

Moisture-Induced Alumina Scale Spallation: The Hydrogen Factor

For some time our community has been concerned with interfacial spallation of protective alumina scales, not just upon immediate cooldown, but as a time-delayed phenomenon. Moisture-induced delayed spallation (MIDS) and desktop spallation (DTS) of TBC's refer to this process. It is most apparent for relatively adherent alumina scales that have survived cool down in a dry environment, built up considerable thickness and strain energy, and have been somewhat damaged, such as by cyclic oxidation cracking. Indeed, a "sweet zone" can be defined that maximizes the observed effect as a function of all the relevant factors. Moisture has been postulated to serve as a source of interfacial hydrogen embrittlement derived from reaction with aluminum in the alloy at an exposed interface. The purpose of this monograph is to trace the close analogy of this phenomenon to other hydrogen effects, such as embrittlement of aluminides and blistering of alloys and anodic alumina films. A formalized, top-down, logic tree structure is presented as a guide to this discussion. A theoretical basis for interfacial weakening by hydrogen is first cited, as are demonstrations of hydrogen as a reaction product or detected interfacial species. Further support is provided by critical experiments that produce the same moisture effect, but by isolating hydrogen from other potential causative factors. These experiments include tests in H2-containing atmospheres or cathodic hydrogen charging.

Smialek, James L.↗
DOE OSTI2021

Lessons Learned in Employing Data Analytics to Predict Oxidation Kinetics and Spallation Behavior of High-Temperature NiCr-Based Alloys

Machine learning (ML) can offer many advantages in predicting material properties over traditional materials development methods based solely on limited experimental investigations or physical-based simulations with the capability to reduce development cost, risk, and time. However, so far, limited efforts have been made to predict alloy oxidation kinetics and spallation behavior via ML due to the lack of consistently measured and sufficient experimental data and the inherent complexity in oxidation behavior of multicomponent high-temperature alloys. A previous study reported the ability of ML to predict oxidation kinetics of NiCr-based alloys as a function of alloy composition and operating conditions. Here, the performance of a ML model in predicting rate constants and spallation probability was evaluated in light of the roles of the data distribution of the experimental dataset (data analytics), the alloy composition, the exposure environment and the chosen oxidation approach to extracting kinetic values from the measured mass changes (but using either a simple parabolic law or a statistical cyclic oxidation model). Potential strategies to improve the predictions and enhance the extrapolative capability of the previously trained model will be discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Vertically oriented ZrO 2 –TiO 2 –Nb 2 O 5 –Al 2 O 3 mixed nanopatterned bioceramics on Ti6Al7Nb implant assessed by laser spallation technique

Recently, reports on the fabrication of nonstructural configuration have generated scientific interest due to increased awareness of the oxide nanotubes in the biomedical implant field and other industrial approaches. In this paper, highly ordered ZrO 2 –TiO 2 –Nb 2 O 5 –Al 2 O 3 nanotube arrays were grown by physical vapor deposition (PVD) of a zirconium layer on titanium-niobium alloy implant (IMPTi67) and succeeding anodic oxidation at a constant voltage (60 V) with exposure times ranging from 30 to 300 min. To crystallize the resulting mixed oxide nanotubes, heat treatment was also applied. The nanotubes were placed in an atmospheric furnace at temperatures ranging from 450 to 800 °C for 2 h. This treatment provided a morphological evolution was strongly influenced by the anodization time, where highly aligned bamboo-shaped oxide nanotubes (161 ± 44 nm in inside diameter and ~1 μm in length) were formed after 300 min of anodization. After annealing at temperature below 500 °C, the nano-array configuration remained stable and the average inner diameter decreased to around 40 nm. The effects of anodically fabrication time on the adhesion strength of four-phase multilayered mixed oxide nanotubes on anodized Zr/IMPTi67 surfaces assed by microscratch analysis. From the results, increasing the annealing temperature further to 800 °C fully destroyed the nanotubes and consequently the tubular arrangement was converted to a coarse grain structure. It was revealed that annealing at 450 °C for 2 h was the optimum conditions for stable nanotubular array generation, where the highest adhesion strength (shifted from 2595.59 to 2640.12 mN), microhardness (372 HV) and hydrophilicity (15 ± 1°) were recorded. It is notify that the adhesion strength of highly ordered mixed nanotubes was remarkably decreased after thermal treatment at 800 °C (1817.51 mN). Through the laser spallation analysis, the intrinsic adhesion strength of the Zr/IMPTi67 interface was found to be at 63 MPa, while the first stages of failure occurred at a stress state of 35 MPa. The adhesion of the film with the tubular structure proved to have a significantly larger adhesion strength of 246 MPa for the complete failure and 203 MPa for incipient stages of spallation. Finally, the considered surface modification can be an effective step in determining the interface strength between biomedical implants and the nanostructured coatings.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Exceptionally high spallation strength for a high-entropy alloy demonstrated by experiments and simulations

High-entropy alloys are materials with an increasing number of technological applications. Amongst them, the Cantor alloy, FeMnCoCrNi, shows desirable mechanical properties at normal loading conditions. In this study we focus on the performance of the Cantor alloy at the ultra-high deformation rates of shock waves. We study shock-induced spallation using both experiments and atomistic simulations. Experimental loading is achieved using high power laser, with VISAR to obtain velocity profiles and spall strength, followed by transmission electron microscopy of the recovered samples. Molecular Dynamics (MD) simulations of shock-induced spallation are compared with experiments. Both experiments and simulations show a high spall strength which would be beneficial for certain applications, with experiments giving ~8 GPa at ~10 7 s -1 and MD giving almost ~30 GPa at ~10 9 s -1 . The difference between experiments and simulations can be explained by the difference in strain rate. Post-mortem analysis of the experimental samples shows nanotwins near the spall plane, while MD simulations show a highly disordered region giving rise to void nucleation and spall during loading.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Characterization of a mixed high-energy spallation neutron–proton field using monoisotopic activation detectors

In this paper, we used monoisotopic Co, Au, and Bi threshold activation detectors to characterize the high-energy ( > 5 MeV) portion of a mixed spallation neutron–proton (n/p) field. The field was produced by irradiating the natural uranium target–blanket subcritical assembly Quinta with 660-MeV protons from Phasotron accelerator at the Joint Institute for Nuclear Research (JINR) in Dubna. Experimental yields of the (n,x) and (p,x) reactions were compared to the calculated reaction rates, which represent the convolution of particle flux and reaction cross section. The flux was simulated using the radiation transport code MCNPX 2.7.0, and the cross sections were extracted from the following three sources: the intranuclear cascade model INCL4.2, the phenomenological spherical optical model from TALYS-1.8, and the evaluated nuclear data file TENDL-2015. We show that, in most cases, the evaluated data provide a closer agreement with the experimental data in comparison with INCL4.2 model, which is an integral part of MCNPX. Finally, our results bring a contribution to those fields of physics research that utilize computer prediction of neutron generation in the energy range of tens to hundreds MeV, such as the medical radioisotope production, radiation shielding of spallation neutron sources, and nuclear transmutation in accelerator-driven systems.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2012

Big-bang nucleosynthesis with a long-lived charged massive particle including {sup 4}He spallation processes in a bound state

We propose helium-4 spallation processes induced by long-lived stau in supersymmetric standard models, and investigate an impact of the processes on light elements abundances. We show that, as long as the phase space of helium-4 spallation processes is open, they are more important than stau-catalyzed fusion and hence constrain the stau property.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Generation of negative pressures and spallation phenomena in diamond exposed to a picosecond laser pulse

The spallation phenomena in poly- and single-crystal synthetic diamonds have been experimentally investigated. A shockwave impact on a target was implemented using a 70-ps laser pulse in the Kamerton-T facility. The ablation pressure of 0.66 TPa on the front target surface was formed by pulsed radiation of a neodymium phosphate glass laser (second harmonic λ = 0.527 mm, pulse energy 2.5 J) with an intensity as high as 2 × 10{sup 13} W cm{sup -2}. The maximum diamond spall strength σ* ≈ 16.5 GPa is found to be 24% of the theoretical ultimate strength. Raman scattering data indicate that a small amount of crystalline diamond in the spallation region on the rear side of the target is graphitised. (extreme light fields and their applications)

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

Jet formation in spallation of metal film from substrate under action of femtosecond laser pulse

It is well known that during ablation by an ultrashort laser pulse, the main contribution to ablation of the substance is determined not by evaporation, but by the thermomechanical spallation of the substance. For identical metals and pulse parameters, the type of spallation is determined by film thickness d{sub f}. An important gauge is metal heating depth d{sub T} at the two-temperature stage, at which electron temperature is higher than ion temperature. We compare cases with d{sub f} < d{sub T} (thin film) and d{sub f} ≫ d{sub T} (bulk target). Radius R{sub L} of the spot of heating by an optical laser is the next (after d{sub f}) important geometrical parameter. The morphology of film bulging in cases where d{sub f} < d{sub T} on the substrate (blistering) changes upon a change in radius R{sub L} in the range from diffraction limit R{sub L} ∼ λ to high values of R{sub L} ≫ λ, where λ ∼ 1 μm is the wavelength of optical laser radiation. When d{sub f} < d{sub T}, R{sub L} ∼ λ, and F{sub abs} > F{sub m}, gold film deposited on the glass target acquires a cupola-shaped blister with a miniature frozen nanojet in the form of a tip on the circular top of the cupola (F{sub abs} and F{sub m} are the absorbed energy and the melting threshold of the film per unit surface area of the film). A new physical mechanism leading to the formation of the nanojet is proposed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Spallation reactions in shock waves at supernova explosions and related problems

The isotopic anomalies of some extinct radionuclides testify to the outburst of a nearby supernova just before the collapse of the protosolar nebula, and to the fact that the supernova was Sn Ia, i.e. the carbon-detonation supernova. A key role of spallation reactions in the formation of isotopic anomalies in the primordial matter of the Solar System is revealed. It is conditioned by the diffusive acceleration of particles in the explosive shock waves, which leads to the amplification of rigidity of the energy spectrum of particles and its enrichment with heavier ions. The quantitative calculations of such isotopic anomalies of many elements are presented. It is well-grounded that the anomalous Xe-HL in meteoritic nanodiamonds was formed simultaneously with nanodiamonds themselves during the shock wave propagation at the Sn Ia explosion. The possible effects of shock wave fractionation of noble gases in the atmosphere of planets are considered. The origin of light elements Li, Be and B in spallation reactions, predicted by Fowler in the middle of the last century, is argued. All the investigated isotopic anomalies give the evidence for the extremely high magnetohydrodynamics (MHD) conditions at the initial stage of free expansion of the explosive shock wave from Sn Ia, which can be essential in solution of the problem of origin of cosmic rays. The specific iron-enriched matter of Sn Ia and its MHD-separation in turbulent processes must be taking into account in the models of origin of the Solar System.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Management of tritium European Spallation Source

The European Spallation Source (ESS) will produce tritium via spallation and activation processes during operational activities. Within the location of ESS facility in Lund, Sweden site it is mandatory to demonstrate that the management strategy of the produced tritium ensures the compliance with the country regulation criteria. The aim of this paper is to give an overview of the different aspects of the tritium management in ESS facility. Besides the design parameter study of the helium coolant purification system of the target the consequences of the tritium releasing into the environment were also analyzed. Calculations show that the annual release of tritium during the normal operations represents a small fraction from the estimated total dose. However, more refined calculations of migration of activated-groundwater should be performed for higher hydraulic conductivities, with the availability of the results on soil examinations. With the assumption of 100% release of tritium to the atmosphere during the occurring of the extreme accidents, it was found as well that the total dose complies with the constraint. (authors)

43 PARTICLE ACCELERATORS↗
DOE OSTI2019

The Development of the Generalized Spallation Model [Thesis]

The generalized spallation model (GSM) is a general spallation physics event generator with a physics-rich history dating back to the early 1970s during the development of the CEM. GSM estimates the outcome of high-energy hadron-nucleus and nucleus-nucleus collisions, where collision energies often exceed hundreds of MeV. Such a collision may include a neutron incident on carbon, for example. The thesis presented here outlines the continued development and modernization of GSM, whereby it was migrated from a legacy Fortran code to utilize an advanced and modern Fortran architecture. Verification and validation of GSM also took place throughout the continued development of GSM. The results of this work have transformed GSM into a highly modern and powerful event generator, where numerous benefits have been fostered and precipitated, including but not limited to portability, modularization, flexibility, robustness, and scalability. GSM is a robust and well developed software library that clients and end-users may confidently use, within the scope outlined here, as a result of this work.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Target Operational Experience at the Spallation Neutron Source

The Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) has operated at unprecedented power levels for a short-pulse spallation source. Target operations have been successful but not without difficulties. Three targets out of the eight used to date have ended life unexpectedly causing interruptions to the neutron science users. The first of a kind mercury target design experiences beam-pulse induced cavitation damage that is suspected in one of the target leaks. The two other targets suffered early failures due to defective welds. Diagnosing the causes of target leaks and understanding of the progression of cavitation erosion and radiation damage effects has made use of post-irradiation examination (PIE) capabilities. As a result of PIE, review of quality assurance practices and related investigations, design changes are being implemented and manufacturing oversight improved. This paper describes SNS target operating experience, including the more important observations and lessons learned.

Acclerator↗
DOE OSTI2020

Coherent elastic neutrino-nucleus scattering at the European Spallation Source

The European Spallation Source (ESS), presently well on its way to completion, will soon provide the most intense neutron beams for multi-disciplinary science. Fortuitously, it will also generate the largest pulsed neutrino flux suitable for the detection of Coherent Elastic Neutrino-Nucleus Scattering (CEνNS), a process recently measured for the first time at ORNL’s Spallation Neutron Source. We describe innovative detector technologies maximally able to profit from the order-of-magnitude increase in neutrino flux provided by the ESS, along with their sensitivity to a rich particle physics phenomenology accessible through high-statistics, precision CEνNS measurements.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2022

Quantifying the reduction in cavitation-induced erosion damage in the Spallation Neutron Source mercury target by means of small-bubble gas injection

Here, a model developed to represent the progress of erosion damage in liquid-metal spallation target vessels was modified to incorporate the effect of gas injection on the erosion rate. The liquid mercury target system for the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory now operates with helium gas injection to reduce target vessel fatigue stress and cavitation-induced erosion damage. Erosion damage is a primary degradation phenomenon affecting the service life of SNS target vessels, and cavitation mitigation techniques, such as small-bubble gas injection, have been implemented to reduce damage and extend target lifetimes. Erosion depths in samples removed from SNS targets after operation were measured using laser line scanning. These measurements confirmed that gas injection reduced erosion damage. However, quantifying the damage reduction due to gas injection was complicated by variations in lifetime, power, and gas injection rates between different targets. In this study, the operating power and gas injection rate of targets were incorporated into an erosion damage prediction model to quantify their effects on erosion damage reduction. Values of a power scaling factor, β, were calculated by comparing modeled with measured erosion damage. These values indicate that the use of gas injection at the SNS reduced damage to a level equivalent to operating targets without gas injection at 35–47% of the actual beam power. To account for the gas injection effect on the cavitation damage, a simple exponential form based on analysis of the scaling factor β was developed to incorporate the gas rate history with a scaling factor γ in the erosion damage modeling.

47 OTHER INSTRUMENTATION↗