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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 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Microstructural control of FeCrAl alloys using Mo and Nb additions

The effects of Mo and Nb additions on the microstructure and mechanical properties of two FeCrAl alloys were studied in this paper. Fine and uniform recrystallized grain structures (~ 20–30 μm) were achieved in both alloys through suitable annealing after warm-rolling. The formation of Fe 2 Nb-type Laves phase precipitates in the Nb-containing FeCrAl alloy effectively stabilized the deformed and recrystallized microstructures. The Mo-containing FeCrAl alloy exhibited strong γ texture fiber after annealing at 650–900 °C, whereas the annealed Nb-containing FeCrAl alloy had much weaker texture. Finally, both strength and ductility decreased as the grain size increased in both alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and mechanical properties of extruded Mg-Gd-Y-Zn-Zr alloys filled with intragranular LPSO phases

Highlights: • Intragranular thin-platelet LPSO phases strongly restrict the DRX. • The low extrusion ratio fails to widely break the intragranular LPSO. • The large extrusion ratio results in the full DRX via the CDRX mechanism. • Finer DRX-ed grains and LPSO induce the higher yield strength and elongation. • The larger grain size causes an increase in the strain hardening rate. - Abstract: A comparison has been made on the dynamic recrystallization (DRX), texture, and mechanical properties of the Mg-5.5Gd-4.4Y-1.1Zn-0.5Zr (wt%) alloys extruded with different ratios (ER, 36 and 7.1). Intragranular long-period stacking ordered (LPSO) phases strongly restrict the DRX by restraining the lattice rotation of α-Mg grains and hindering the migration of the boundaries of DRX-ed grains. Extrusion with ER of 36 can widely break the intragranular LPSO phases and create a fully DRX-ed microstructure with a relatively random texture, while the lower ER of 7.1 fails to break the LPSO and leads to a bimodal microstructure with a strong basal fiber texture. Finer DRX-ed grains and smaller LPSO phases account for the superior tensile yield strength and elongation of the ER36 alloy. However, the ultimate tensile strength of the ER7.1 alloy catches up because its larger grain size causes an increase in the strain hardening rate.

36 MATERIALS SCIENCE↗
DOE OSTI2018

EBSD characterization of boundary types after discontinuous precipitation in a grit-blasted nickel-based superalloy

Highlights: • During recrystallization, high-angle grain boundaries increased dramatically. • The reaction fronts are high-angle GBs, but are not always correspond to CSL type. • Σ3 twin boundaries have been confirmed to be positive on reducing residual stress. • No advancing reaction front corresponds to Σ3 twin boundary type. - Abstract: In the present work, direct and detailed insights with respect to the statistical information on boundary type of discontinuous precipitates and their advancing reaction fronts in a Re-containing single crystal superalloy DD6 have been obtained by EBSD. With the annealing time prolonging, the reaction fronts proceeded into the original matrix for gradual release of grit-blast induced stress. The average recrystallized grain diameter and the average depth of recrystallization were respective 0.83 μm and 3.98 μm for the specimen annealed at 1050 °C for 10 min, which, after 16 h, had grown to the average values of 3.22 μm and 31.17 μm. The reaction fronts of discontinuous precipitation have been confirmed to be high-angle grain boundaries, but do not always correspond to CSL type. Besides the role of strain energy dissipation for recrystallization, the formation of twin structure has no direct relationship with the proceeding of reaction fronts. The information of boundary type of discontinuous precipitation and its advancing reaction fronts would provide new insight into the cellular recrystallization behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Twining and dynamic recrystallization in austenitic Alloy 718 during friction welding

Highlights: • DDRX was the primary grain refinement mechanism in friction welding of Alloy 718. • Two models were developed to explain the twin formation mechanism. • Intragranular twins were caused by grain boundary growth incidents. • Intergranular twin boundaries were ascribed to boundary growth stagnation. - Abstract: Quantitative boundary misorientation analysis verified that discontinuous dynamic recrystallization (DDRX) was the primary grain refinement mechanism in friction welding of Alloy 718. Two models were developed to explain the formation mechanism of intragranular twins and intergranular twins, respectively. Intragranular twins were caused by grain boundary growth incidents. After twin initiation, the twin boundary migrated toward the plastic-deformed grain interior to reduce the stored energy. Additionally, intergranular twin boundaries were ascribed to boundary growth stagnation. When boundary growth stagnation occurred, twin boundaries could form and altered the orientation of the boundary migration front, so that boundary migration could progress, leaving a single twin boundary behind.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of velocity of pressing in RCS processed AA8090 Al-Li alloy

Highlights: • For the first time the Influence of RCS on the properties of AA 8090 has been studied by processing velocity. • It is found that the properties vary according to pressing velocity and number of passes. • Reported significant property improvement achieved through the pressing velocity of 1.5mm/s. - Abstract: The effect of the velocity of pressing on grain refinement in AA8090 Al-Li alloy sheets during repetitive corrugation and straightening (RCS) at 300 °C is studied. V-groove corrugation dies and flattening dies were used at three different pressing velocities of 1.5 mm/s, 2 mm/s and 2.5 mm/s. The specimen temperature was maintained at 300 °C. Grain refinement was significantly influenced by the pressing velocity. The samples processed for 8 passes at a pressing velocity of 1.5 mm/s had an improved microhardness value of 104 HV and an increased tensile strength of 405 MPa which is respectively 39% and 113% more than the corresponding values in the parent material. TEM analysis showed that samples processed at a pressing velocity of 1.5 mm/s had in places a grain size down to 50 nm and the EBSD results revealed that the average grain size was 2 μm, with 86% of the total number of grain boundaries of the high angle type.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A comparative research on bobbin tool and conventional friction stir welding of Al-Mg-Si alloy plates

Highlights: • 6061-T4 Al plates were successfully welded by both BT-FSW and C-FSW processes. • A wide welding parameter window was achieved with bobbin tool. • BT-FSW joints showed good strength similar to that of C-FSW joints. • Proving that an Isothermal Dissolution Layer Theory also apply to BT-FSW - Abstract: In our work, Al-Mg-Si (6061Al -T4) plates of 6 mm thickness were subjected to bobbin tool friction stir welding (BT-FSW) and conventional friction stir welding (C-FSW) for comparison. How the welding process affects the microstructure and mechanical properties was investigated at various rotation rates and welding speeds. The results showed that butt FSW joints with high quality could be produced at the selected parameters. The joint strength rose with enhanced welding speed and was nearly independent of the rotation rate in both the welding processes. The strength of the joints produced with BT-FSW reached the same level as that of the C-FSW. The maximum joint strength of 229 MPa was 93% of the base material (BM), which is superior to the reported results of 60–80% for the T4 condition. The fracture position of most of the joints was in the heat-affected zone (HAZ), which is the lowest hardness zone.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Formation mechanism of an α2 phase-rich layer on the surface of Ti-22Al-25Nb alloy

Highlights: • This paper reports the formation mechanism of a α{sub 2}-rich layer on the surface of the Ti-22Al-25Nb alloy. • The α{sub 2}-rich layer is significantly harder than the alloy interior. • The α{sub 2}-rich layer is formed owing to the Ti{sub 2}O{sub 3} particles produced by oxygen infiltration at the surface. • The relative orientation of the α{sub 2} and Ti{sub 2}O{sub 3} phases is (1–100)α{sub 2}//(1–100)Ti{sub 2}O{sub 3}. - Abstract: In order to investigate the formation mechanism of the α{sub 2}-rich layer in the surface of the Ti-22Al-25Nb alloy, systematic analyses are carried out on the surface of the alloy by using SEM, XRD and EBSD. The results reveal that this α{sub 2}-rich layer is formed by oxidation of the surface layer. In this surface oxidation layer, there are abundant Ti{sub 2}O{sub 3} particles similar with the α{sub 2} phase in structure. These Ti{sub 2}O{sub 3} particles serve as nucleation points and lower the nucleation energy of the α{sub 2} phase, promoting the precipitation and growth of α{sub 2} phase on the surface layer of the alloy. This surface layer has a hardness of 684 HV, significantly exceeding the value of 312 HV for the interior of the alloy and thus considerably improves the surface properties of the alloy, serving as a high-performance, self-generated surface strengthening layer.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Impact of forging direction on the recrystallization behaviour of nickel base superalloy AD730 billet material at subsolvus temperatures

Highlights: • Large unrecrystallized grains present a fine distribution of intragranular γ′ precipitates; • Intragranular precipitates play a key role on strain distribution and subsequent recrystallization behaviour for AD730; • Continuous dynamic recrystallization (CDRX) promotes the gradual consumption of large unrecrystallized grains. • Undissolved secondary precipitates play a key role on the microstructural evolution of AD730 • No significant effect associated to the forging direction on the recrystallization behaviour of AD730 was found. AD730™ is a newly developed Ni-based superalloy for turbine disk applications produced by cast and wrought processes. Conventional ingot-to-billet conversion is an expensive and very complex operation. Because of the difficulties to achieve a uniform strain for recrystallization, large unrecrystallized grains are retained along this process. Heterogeneities of grain size have a negative impact on subsequent forging processes as well as ultrasonic inspectability of the destination part. The main aim of this work is to analyse the impact of both forging direction (billet, cogging) and strain (ε = 0.3–2) on the microstructural evolution of AD730 at subsolvus temperatures from a semi-finished product (billet) under conditions representative of both cogging and hot forging operations. Special attention to the presence of large unrecrystallized grains was paid. Double truncated cones (DTCs) were hot forged at subsolvus temperatures followed by air cooling. SEM and EBSD analysis were conducted in the as-received (billet) and the as-forged conditions. AD730 alloy presents a complex microstructure characterized by the presence of large unrecrystallized grains, aligned in the billet direction, with remarkable differences in γ′ precipitates distribution as compared to recrystallized structures. The fine distribution of primary γ′ precipitates (pinning effect) plays a key role on grain size control but also on the recrystallization behaviour. Continuous dynamic recrystallization (CDRX) mechanism was found to be operating in the large unrecrystallized grains, promoting the formation intragranular DRX grains and the gradual recrystallization of these grains. Strain presents a strong effect on the microstructural evolution of AD730, increasing the recrystallization fraction and refining the structure. By contrast, no significant effect associated to the forging direction was found. Conditions representative of cogging operations (ε ≤ 0.6) at subsolvus temperatures were translated into large fractions of unrecrystallized structures (strain accumulation).

36 MATERIALS SCIENCE↗
DOE OSTI2023

Microstructure and texture evolution of friction stir lap welded dissimilar multi-aluminum stack

Here this work investigates the microstructural evolution of a dissimilar multiple-aluminum alloy stack (AA7055, AA7055, AA6022) produced by friction stir lap welding (FSLW). A through-thickness analyses of grain orientation map, grain size and distribution, strain distribution, and texture evolution by electron backscattered diffraction-based characterization technique were performed. Two different welding parameters (combination of tool rotation and traverse speed) were used to study the effects of the welding parameters on these microstructural characteristics. The study shows that higher welding speed and higher rotation speed of the FSLW tool yield finer grains. FSLW resulted in shear texture formation in the weld regions due to high welding speeds. This texture development in the nugget zone is consistent with shear deformation, and (111) pole figures show ideal shear texture components. The Stir Zone (SZ) exhibits shear texture (A/A ̅ and B/B ̅ components), consistent with other shear-assisted processes. Recrystallization mechanisms observed are CDRX in SZ and DDRX in TMAZ (Thermo-Mechanically Affected Zone). Interface analysis reveals the presence of oxide layers between different aluminum sheets, affecting joint mechanical performance. The mechanical property of the lap welded joints was presented in terms of microhardness distribution across the weld cross section. Slightly higher hardness in higher speed welding setup was observed throughout the SZ and AA6022 layer, indicating greater grain refinement and uniform grain structure distribution.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of preheat temperature and post-process treatment on the microstructure and mechanical properties of stainless steel 410 made via ultrasonic additive manufacturing

Ultrasonic additive manufacturing (UAM) is a solid-state manufacturing technology for producing near-net shape metallic parts combining additive ultrasonic metal welding and subtractive machining. Even though UAM has been demonstrated to produce robust metal builds in Al–Al, Al–Ti, Al-steel, Cu–Cu, Al–Cu, and other material systems, UAM welding of high strength steels has proven challenging. This study investigates process and post-processing methods to improve UAM steel weld quality and demonstrates the UAM fabrication of stainless steel 410 (SS 410) builds which possess, after post-processing, mechanical properties comparable with bulk material. Unlike UAM fabrication of softer metals, this study shows that increasing the baseplate temperature from 38°C (100°F) to 204°C (400°F) improves interfacial strength and structural homogeneity of the UAM steel samples. Further improvement in strength is achieved through post-processing. The hot isostatic pressing (HIP) post treatment improves the shear strength of UAM samples to 344 MPa from 154 MPa for as-welded samples. Microstructural analyses with SEM and EBSD show no evidence of body centered cubic (BCC) ferrite to face centered cubic (FCC) austenite transformation taking place during UAM welding of SS 410. The weld quality improvement of UAM steel at higher baseplate temperatures is believed to be caused by the reduction of the yield strength of SS 410 at elevated temperature. Finaly, HIP treatment is shown to increase the overall hardness of UAM SS 410 from 204 ± 7 HV to 240 ± 16 HV due to the formation of local pockets of martensite. Nanohardness tests show that the top of layer n is harder than the bottom of layer n+1 due to grain boundary strengthening.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Dynamic properties of FeCrMnNi, a high entropy alloy

The goal of this paper was to assess the dynamic properties of a new class of materials, High entropy alloys (HEAs). Specifically, gas gun experiments coupled with recovery were performed on a specific HEA, FeCrMnNi, to measure its equation of state (EOS) and its spall strength, which is a measure of the stress required to nucleate voids under shock loading conditions. While there has been a plethora of work performed to assess the mechanical properties of these HEAs under uniaxial stress conditions as a function of strain rate, investigation of its properties in dynamic extremes remains rare. The current work fills this gap in knowledge for this novel class of materials. Our results show that the measured EOS for this material at one velocity was in reasonable agreement with an estimated Hugoniot. Furthermore, while the spall strength for this material was found to be ~1.9 GPa, with some variation based on sample location, the main failure mode was brittle. This brittle failure leading to formation of cracks in the material is different than the ductile failure observed in pure iron and its alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Spherical nanoindentation stress–strain curves

Although indentation experiments have long been used to measure the hardness and Young's modulus, the utility of this technique in analyzing the complete elastic–plastic response of materials under contact loading has only been realized in the past few years – mostly due to recent advances in testing equipment and analysis protocols. This paper provides a timely review of the recent progress made in this respect in extracting meaningful indentation stress–strain curves from the raw datasets measured in instrumented spherical nanoindentation experiments. These indentation stress–strain curves have produced highly reliable estimates of the indentation modulus and the indentation yield strength in the sample, as well as certain aspects of their post-yield behavior, and have been critically validated through numerical simulations using finite element models as well as direct in situ scanning electron microscopy (SEM) measurements on micro-pillars. Much of this recent progress was made possible through the introduction of a new measure of indentation strain and the development of new protocols to locate the effective zero-point of initial contact between the indenter and the sample in the measured datasets. As a result, this has led to an important key advance in this field where it is now possible to reliably identify and analyze the initial loading segment in the indentation experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Irradiated recycled plastic as a concrete additive for improved chemo-mechanical properties and lower carbon footprint

Concrete production contributes heavily to greenhouse gas emissions, thus a need exists for the development of durable and sustainable concrete with a lower carbon footprint. This can be achieved when cement is partially replaced with another material, such as waste plastic, though normally with a tradeoff in compressive strength. This study discusses progress toward a high/medium strength concrete with a dense, cementitious matrix that contains an irradiated plastic additive, recovering the compressive strength while displacing concrete with waste materials to reduce greenhouse gas generation. Compressive strength tests showed that the addition of high dose (100 kGy) irradiated plastic in multiple concretes resulted in increased compressive strength as compared to samples containing regular, nonirradiated plastic. This suggests that irradiating plastic at a high dose is a viable potential solution for regaining some of the strength that is lost when plastic is added to cement paste. X-ray Diffraction (XRD), Backscattered Electron Microscopy (BSE), and X-ray microtomography explain the mechanisms for strength retention when using irradiated plastic as a filler for cement paste. By partially replacing Portland cement with a recycled waste plastic, this design may have a potential to contribute to reduced carbon emissions when scaled to the level of mass concrete production.

42 ENGINEERING↗
DOE OSTI2014

Structure of the near-surface layer of NiTi on the meso- and microscale levels after ion-beam surface treatment

Using the EBSD, SEM and TEM methods, the structure of surface layer of polycrystalline NiTi alloy samples was examined after the modification of material surface by the pulsed action of mean-energy silicon ion beam. It was found that the ion beam treatment would cause grain fragmentation of the near-surface layer to a depth 5÷50 μm; a higher extent of fragmentation was observed in grains whose close-packed planes were oriented approximately in the same direction as the ion beam was. The effect of high-intensity ion beam treatment on the anisotropic behavior of polycrystalline NiTi alloy and the mechanisms involved were also examined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Synchrotron-based investigation of transition-metal getterability in n-type multicrystalline silicon

Solar cells based on n-type multicrystalline silicon (mc-Si) wafers are a promising path to reduce the cost per kWh of photovoltaics; however, the full potential of the material and how to optimally process it are still unknown. Process optimization requires knowledge of the response of the metal-silicide precipitate distribution to processing, which has yet to be directly measured and quantified. To supply this missing piece, we use synchrotron-based micro-X-ray fluorescence (μ-XRF) to quantitatively map >250 metal-rich particles in n-type mc-Si wafers before and after phosphorus diffusion gettering (PDG). We find that 820°C PDG is sufficient to remove precipitates of fast-diffusing impurities and that 920°C PDG can eliminate precipitated Fe to below the detection limit of μ-XRF. Thus, the evolution of precipitated metal impurities during PDG is observed to be similar for n- and p-type mc-Si, an observation consistent with calculations of the driving forces for precipitate dissolution and segregation gettering. Measurements show that minority-carrier lifetime increases with increasing precipitate dissolution from 820°C to 880°C PDG, and that the lifetime after PDG at 920°C is between the lifetimes achieved after 820°C and 880°C PDG.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗