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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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High temperature polymer creep simulation of aluminum alloy stress-strain distribution using photoelastoplastic stress measurement
Cast Monel alloys are used in many industrial applications that require a combination of good mechanical properties and excellent resistance to corrosion. Despite relative widespread use, there has been limited prior research investigating the fundamental composition–structure–property relationships. Here in this work, microstructural characterization, thermal analysis, electron probe microanalysis, tensile testing, and Varestraint testing were used to assess the effects of variations in nominal composition on the solidification path, microstructure, mechanical properties, and solidification cracking susceptibility of cast Monel alloys. It was found that Si segregation caused the formation of silicides at the end of solidification in grades containing at least 3 wt pct Si. While increases to Si content led to significant improvements in strengthening due to the precipitation of β 1 -Ni 3 Si, the silicide eutectics acted as crack nucleation sites during tensile loading which severely reduced ductility. The solidification cracking susceptibility of low-Si Monel alloys was found to be relatively low. However, increases to Si concentration and the onset of associated eutectic reactions increased the solidification temperature range and drastically reduced cracking resistance. Increases in the Cu and Mn concentrations were found to reduce the solubility limit of Si in austenite which promoted additional eutectic formation and exacerbated the reductions in ductility and/or weldability.
Mullite is promising as a protective coating for silicon-based ceramics in aggressive high-temperature environments. Conventionally plasma-sprayed mullite on SiC tends to crack and debond on thermal cycling. It is shown that this behavior is due to the presence of amorphous mullite in the conventionally sprayed mullite. Heating the SiC substrate during the plasma spraying eliminated the amorphous phase and produced coatings with dramatically improved properties. The new coating exhibits excellent adherence and crack resistance under thermal cycling between room temperature and 1000 to 1400 C. Preliminary tests showed good resistance to Na2CO3-induced hot corrosion.
A wedge-loaded tapered DCB specimen is proposed for fracture toughness and stable crack growth measurements of ceramic materials at elevated temperatures. The specimen is wedge-loaded by a compression pin seated in the starter notch. By appropriate selection of the specimen's taper and starter notch angle, the stress intensity factor is maintained relatively constant through 50 percent of the test section. Room temperature fracture toughness testings of commercial grade polycrystalline alumina specimens indicate that the crack will run straight for approximately 50 percent of the length of the test section without side grooving. A moderate 10 percent side grooving was sufficient to maintain straight crack extension throughout the entire specimen length.
Abstract Polyolefin waste is the largest polymer waste stream that could potentially serve as an advantageous hydrocarbon feedstock. Upcycling polyolefins poses significant challenges due to their inherent kinetic and thermodynamic stability. Traditional methods, such as thermal and catalytic cracking, are straightforward but require temperatures exceeding 400 °C for complete conversion because of thermodynamic constraints. We summarize and critically compare recent advances in upgrading spent polyolefins and model reactants via kinetic (and thermodynamic) coupling of the endothermic C─C bond cleavage of polyolefins with exothermic reactions including hydrogenation, hydrogenolysis, metathesis, cyclization, oxidation, and alkylation. These approaches enable complete conversion to desired products at low temperatures (<300 °C). The goal is to identify challenges and possible pathways for catalytic conversions that minimize energy and carbon footprints.
Abstract Polyolefin waste is the largest polymer waste stream that could potentially serve as an advantageous hydrocarbon feedstock. Upcycling polyolefins poses significant challenges due to their inherent kinetic and thermodynamic stability. Traditional methods, such as thermal and catalytic cracking, are straightforward but require temperatures exceeding 400 °C for complete conversion because of thermodynamic constraints. We summarize and critically compare recent advances in upgrading spent polyolefins and model reactants via kinetic (and thermodynamic) coupling of the endothermic C─C bond cleavage of polyolefins with exothermic reactions including hydrogenation, hydrogenolysis, metathesis, cyclization, oxidation, and alkylation. These approaches enable complete conversion to desired products at low temperatures (<300 °C). The goal is to identify challenges and possible pathways for catalytic conversions that minimize energy and carbon footprints.
The elevated-temperature mechanical behavior of two aligned eutectics (Ni-8.1 wt % Al-26.4 wt % Mo and Ni-6.3 wt % Al-31.2 wt % Mo) has been investigated utilizing monotonic and cyclic testing in vacuum. Tensile yield strength and fatigue resistance increased from 25 to 725 C, but then were reduced at 825 C. The fatigue lives of specimens tested at 725 C decreased sharply with decreasing frequency. A shift from surface to internal crack initiation was observed upon increasing the test temperature from 725 to 825 C. Stage II crack propagation was observed at both temperatures, in contrast to stage I cracking at 25 C. The test results are compared to those for other nickel and cobalt-base aligned eutectics to show that the frequency effect on fatigue life is not limited to the Ni-Al-Mo system.
Inconel® alloy 740H® (UNS N07740) was the first age-hardenable nickel-based alloy approved by the ASME Boiler & Pressure Vessel Code for use in pressure-boundary applications. In recent years, advanced energy systems such as supercritical CO2 power cycles have utilized alloy 740H in large demonstration projects driven by the requirement for higher fluid temperatures and pressures. Stress relaxation cracking (SRxC) following post-weld heat-treatment (PWHT), also known as strain age cracking (SAC), has been identified in a limited number of weldments during these industrial builds resulting in focused research to further clarify factors influencing this cracking tendency. This paper will summarize some of the findings from shop and field fabrication leading to successful welds and characteristics of observed SRxC. Laboratory experiments supported by microstructural characterization will be presented to highlight the importance of variables such as strain, material starting condition, and PWHT temperatures. Finally, the results will be summarized within the context of practical guidance for industry to successfully weld the material in boiler, heat exchanger, and piping applications.
Supersonic transport structural materials design, considering fatigue behavior, crack propagation and residual static strength under temperature and cyclic load effects
The measurement of compressional velocity, shear velocity, static compressibility, and thermal expansion of (1) a suite of shocked rocks fron the Ries impact in Germany, (2) a suite of samples cracked by thermal cycling to high temperatures, (3) many terrestrial igneous rocks, and (4) lunar basalts, gabbroic anorthosites, and breccias, indicate that shock metamorphism is the primary cause for values of physical properties of lunar rocks being diffferent from their intrinsic values. Large scale thermal metamorphism, thermal cycling between temperatures of lunar day and night, large thermal gradients, or thermal fatigue could possibly cause minor cracking in the top few centimeters of the lunar regolith, but are probably not important mechanism for extensively changing values of physical properties of lunar rocks.-
Abstract The combined effects of high-temperature and heavy-ion irradiation on Mrozowski cracks (MC) and nuclear graphite crystallographic dimensions have been studied using in situ heating and in situ ion-irradiation in the transmission electron microscope (TEM). Electron transparent lamella of nuclear graphite, IG-110, was irradiated using a 2.8 MeV Au beam at an ion flux of 3.991 ×10 10 ion cm −2 s −1 for 70 min at 800 °C. Upon high-temperature irradiation, Mrozowski crack closure was studied quantitatively. The analysis showed linear, positive expansion of nuclear graphite which is significantly different from the dimensional changes previously reported for low-dose neutron irradiation of nuclear graphite in which the material undergoes negative to positive expansion via a turnaround radiation dose. The trend of the thermal expansion coefficient (CTE) of pristine IG-110 in this study is consistent with previous reports in the 100 °C–800 °C temperature region in which the dimensional change ranges from negative to positive values.
The high melting temperature of tungsten (W) makes it an attractive candidate for energy generation applications; however, its use is limited by its poor ductility at low temperatures. This limitation affects even melt-based additive manufacturing (AM) processes such as laser powder bed fusion (PBF-LB), with as-fabricated pure W exhibiting both longitudinal and transverse cracks. Metallurgical and processing factors that affect these cracks are still being explored. This work utilizes powderless single-tracks on pure W plates made over a range of power and velocity combinations in three unique PBF-LB setups: one with flowing argon, one with continuous vacuum, and an in situ high-speed synchrotron X-ray radiography setup. Each had different oxygen activities in their build environments. Longitudinal cracks with oxides appearing to exude from the crack were found for tracks deposited in a build environment with argon shield gas, while no such cracks were observed for builds conducted in vacuum. A calculation showed that direct oxygen ingress into the melt pool from the build environment is negligible when argon shield gas is present. Thus, incorporation of spatter is a likely mechanism for oxygen ingress into the melt pool. Radiography showed extensive keyholing, spatter generation, and crack formation at keyhole porosity. A heat transfer calculation showed the crack formation time was consistent with cooling below its ductile-to-brittle transition (DBT) temperature. This work identifies solidification cracking as a feasible mechanism in pure W beyond the well-known DBT related cracking.
Extensive constant stress-rate testing for 96 wt % alumina was conducted in room-temperature distilled water using four different specimen/loading configurations: rectangular beam test specimens under four-point uniaxial flexure, square plate test specimens in ring-on-ring biaxial flexure, square plate test specimens in ball-on-ring biaxial flexure, and dog-boned tensile test specimens in pure tension. The slow crack growth (SCG) parameter n was almost independent of specimen/loading configurations, in either four-point uniaxial flexure, ring-on-ring biaxial flexure, ball-on-ring biaxial flexure, or pure tension, ranging from n = 35 to 47 with an average value of n = 41.1 +/- 4.5. The prediction of fatigue strength/reliability based on the four-point uniaxial flexure data by using the CARES/Life design code as well as a simple PIA model was in good agreement with both the ring-on-ring biaxial and the ball-on-ring biaxial flexure data. A poor prediction using the PIA model was observed for the dog-boned tensile test specimens, presumably due to different flaw population involved in the tensile test specimens.
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The crack initiation and crack propagation capability was extended to the previously developed generalized plane strain, finite element micromechanics analysis. Also, an axisymmetric analysis was developed, which contains all of the general features of the plane analysis, including elastoplastic material behavior, temperature-dependent material properties, and crack propagation. These analyses were used to generate various example problems demonstrating the inelastic response of, and crack initiation and propagation in, a boron/aluminum composite.
The crack initiation and growth fracture resistance of Al-Cu-Li and Al-Cu-Li-In alloys were characterized and optimized for cryogenic tank applications. Presently, the effects of stress state and temperature is being determined on the fracture toughness and fracture mechanisms of commercially available Vintage 3 2090-T81 and experimental 2090+In-T6. Precracked J-integral specimens of both alloys were tested at ambient and cryogenic temperatures in the plane stress and plane strain conditions. Considering ambient temperature, results showed that 2090-T81 exhibited the highest toughness in both plane strain and plane stress conditions. For the plane strain condition, reasonable crack initiation and growth toughness of 1090-T81 are associated with a significant amount of delamination and transgranular fracture. Plane stress toughnesses were higher and fracture was characterized by shear cracking with minimal delaminations. In comparisons, the fracture behavior of 2090+In-T6 is significantly degraded by subgrain boundary precipitation. Toughness is low and characterized by intersubgranular fracture with no delamination in the plane stress or plane strain conditions. Intersubgranular cracking is a low energy event which presumably occurs prior to the onset of slip band cracking. Copious grain boundary precipitation is atypical of commercially available 2090. At cryogenic temperatures, both alloys exhibit increased yield strength, toughness, and amount of delamination and shear cracking. The change in fracture mode of 2090+In-T6 from intersubgranular cracking at ambient temperature to a combination of intersubgranular cracking, shear cracking, and delamination at cryogenic temperature is the subject of further investigation.
Oxidation behaviors of bulk SiC at 1000–1400 °C in Ar-20 vol% H 2 O-10 vol% air were investigated. The thickness of the passivation oxide layer showed a parabolic increase with time. Dense and uniform layers formed below 1200 °C, whereas pores and cracks were observed at higher temperatures. An analytical model was proposed to estimate the oxide layer growth and SiC mass loss rate. The SiC layer should be effective in retaining the fission products after oxidation by excessive water and air. This work is the first detailed study of SiC layer oxidation during loss of coolant accidents (LOCA) for HTGRs.
The present work includes results on material characterization conducted under COBRA Hydrogen Cooled Nozzle Program and was funded by NASA MSFC. The nozzle requires a material that has high strength at ambient and high (up to l200 F) temperatures in air and hydrogen. Presently, a precipitation hardened steel; A-286 is used in nozzles for Space Shuttle Engines. The A-286 alloy has limited hydrogen compatibility and weldability. The present work focused on selection and characterization of JBK-75 alloy that has significantly higher capability in hydrogen and weldability in addition to other attributes. The alloy was evaluated at different temperatures and environments. Tungsten Inert Gas (TIG) and Electron Beam welding techniques were used to evaluate the weldability of material. Brazing was also conducted on the alloy and evaluated. The characterization of base JBK-75 alloy, welded and brazed alloy included tensile properties, low cycle fatigue and crack growth resistance at different temperatures in air and hydrogen environments. The results indicated that JBK-75 has excellent tensile and fatigue properties in air and hydrogen. The welded and brazed alloy also showed very good properties.