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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 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Effects of initial microstructures on serrated flow features and fracture mechanisms of a nickel-based superalloy

Highlights: • Uniaxial tensile tests are performed on a nickel-based superalloy at 473 K–973 K and low strain rates. • Effects of initial microstructures on serrated flow features, and fracture characteristics are analyzed. • The dislocation across slip in the matrix is responsible for the serrated flow. • Nucleation and growth of micro-voids is the main fracture mechanism of ST and HS superalloys. • The better tensile ductility is obtained with raising the deformation temperatures for the SAT superalloy. - Abstract: The uniaxial tensile experiments are performed on a nickel-based superalloy with different initial microstructures at intermediate temperatures (473–973) K and low strain rates (0.0001–0.001) s{sup −1}. Effects of initial microstructures on plastic deformation mechanisms, serrated flow features, and fracture characteristics are analyzed. It is clearly demonstrated that the plastic deformation mechanism of serrated flow is irrelevant to the initial microstructures, and the dislocation across slip in the matrix is responsible for the serrated flow. However, the effects of initial microstructures on fracture mechanisms are obvious, i.e., the main fracture mechanism of the ST (solution treated) and HS (solution plus γ′/γ″ phases aging precipitation) superalloys are the nucleation and growth of micro-voids, which accelerates the break of carbides and interfacial failure of carbides/matrix. While for the SAT (solution plus δ phases aging precipitation) superalloy, δ phases and carbides play a significant role in the nucleation of micro-voids. Furthermore, the increase of the deformation temperature reduces the elongation to fracture of the ST and HS superalloys. However, the better tensile ductility is obtained with raising the deformation temperatures for the SAT superalloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

High elongation achieved by band-like distribution of reinforcements in aluminum matrix composites

Highlights: • The high elongation of composites with high reinforcement content has been achieved by band-like distribution of ABO. • The band-like distribution of ABO could lead to more homogenous strain distribution by scattering the damages. • The cracks played a key role in the strain non-localization process, but have negligible contribution to the tensile strain. • A model of the relationship between the ductility enhancement and the band-like distribution of ABO was established. • The residual stresses induced by band-like distribution of ABO were theoretically calculated and experimentally measured. - Abstract: Ductile aluminum matrix composites reinforced by Al{sub 18}B{sub 4}O{sub 33} whisker (ABO) with band-like distribution (Band-like Aluminum Matrix Composites, BAMCs) were fabricated by powder metallurgy and extrusion followed by annealing. The cracks tend to nucleate in the ABO-rich bands, but the band-like structure induces compressive residual stress then suppresses the crack propagation and improves the strain transferring ability in the BAMCs. The denser and smaller cracks formed in the BAMCs with higher ABO volume fraction during tensile test. These cracks play an important role in the strain non-localization process, but have negligible contribution to the tensile strain.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Face-centered titanium induced by ultrasonic surface rolling process in Ti-6Al-4V alloy and its tensile behavior

Highlights: • A gradient nanostructure of fcc-Ti is formed from a depth of 440 μm to top surface. • The influence factor of hcp to fcc phase transformation is demonstrated. • The fcc phase induced by USRP decreases the yield strength of Ti-6Al-4V alloy. • The fcc phase improves combining good strength and ductility. - Abstract: The phase transformation of face-centered cubic (fcc) titanium (Ti), transformed from hexagonal close-packed (hcp) Ti, in Ti-6Al-4V alloy subjected to ultrasonic surface rolling process (USRP) and the tensile behavior were investigated. A gradient nanostructure of fcc-Ti was formed from the depth of 440 μm to the top surface. The stress concentration at the phase interface of α phase and β phase promoted hcp to fcc phase transformation in low strain and strain rate region (adjacent to the substrate) while the extremely high strain and strain rate induced this phase transformation in the surface to sub-surface regions. USRP increased the tensile strength of Ti-6Al-4V alloy slightly but decreased the yield strength dramatically, and kept a good combination strength and ductility of Ti-6Al-4V alloy mainly due to the formation of the gradient nanostructure surface layer and hcp to fcc phase transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Changing the solidification sequence and the morphology of iron-containing intermetallic phases in AA6061 aluminum alloy processed by spray forming

Highlights: • Spray forming alters the solidification path of the Fe contaminated AA 6061 alloy. • Thus forming primary α-Al{sub 15}(Fe, Mn){sub 3}Si{sub 2} with polyhedral morphology • Thus preventing the formation of AlFeSi with deleterious morphology • Despite Fe contamination final microstructure presented important ductility. - Abstract: Typically, aluminum alloys are contaminated by iron during recycling cycles. Such a procedure leads to the formation of intermetallic phases with detrimental morphology, which acts as stress risers, impairing mechanical properties and consequently hindering further mechanical processing and restricting recycling. This paper aimed at studying the solidification of an AA6061 alloy highly contaminated with iron (1.4 wt%). Spray forming processing allowed altering the solidification path of the Fe-modified AA 6061 alloy, resulting in the formation of primary α-Al{sub 15}(Fe, Mn){sub 3}Si{sub 2} phase with polyhedral morphology, which traps Fe and Si at elevated temperature. Furthermore, spray forming prevented the formation of intermetallic AlFeSi phases with platelet-like morphology, such as β-AlFeSi as well as the typical Chinese script morphology, resulting in a final ductile microstructure, allowing further processing by hot extrusion.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of niobium interlayer in dissimilar friction stir welding of aluminum to titanium

Highlights: • In the present study, FSW of Al to Ti with Nb interlayer has been characterized. • Fragmentation and distribution of Nb and Ti have been correlated with properties. • Ternary mixing and chemical reaction reduced the brittleness of the joint. • Al in the weld nugget showed a gradient in microstructural features. • The weld with Nb interlayer exhibited higher ductility. - Abstract: The most important issue associated with dissimilar welding is the formation of detrimental intermetallics and consequent reduction in mechanical properties of the weld. It was expected that addition of an interlayer would modify the pattern of the intermetallic formation. The present investigation deals with Friction Stir Welding of aluminum (Al) to titanium (Ti) in presence of niobium (Nb) interlayer. X-ray Computed Tomography results highlighted that fine particles of both Ti and Nb are homogeneously distributed. Large Ti particles are substantially fragmented whereas large Nb particles remain as flakes. Microstructural features of the nugget zone indicated a complex mixing of materials. Jointing had been perceived through elemental diffusion at the interfaces of Al/Ti and Ti/Nb. However, Nb restricted the reaction between Al and Ti, and acted as an efficient interlayer to retard the formation of brittle Al{sub 3}Ti intermetallic phase. Mechanisms of microstructural evolution in Al side of the weld have been identified as Dynamic Recovery, Dynamic Recrystallization and particle stimulated nucleation. Significant improvement in the tensile ductility had been observed due to the presence of finer particles. The presence of Nb in the weld nugget influences the microstructural evolution and tensile properties of the weld by reducing the formation of intermetallics.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of temperature on microstructure and mechanical properties of ECAPed (TiB + La2O3)/Ti-6Al-4V composites

Highlights: • Dynamic recrystallization resulted in ultrafine grains near TiB interface region. • Tensile strength of ECAPed composites increased by up to 18%. • The average aspect ratio of TiB reduced with increasing ECAP temperature. • Interface debonding of TiB led to crack propagation and low ductility. TiB short fibers and La{sub 2}O{sub 3} particles reinforced titanium matrix composites (TMCs) were successfully equal-channel angular pressed (ECAPed) at 600 °C–900 °C. The effect of ECAP temperature on the microstructure and mechanical properties of the TMCs was investigated. The results indicated that dislocation tangling formed cell structures in matrix at lower ECAP temperature. In contrast, continuous dynamic recrystallization occurred at higher ECAP temperature, and promoted the formation of numerous new ultrafine grains. The average aspect ratio of TiB short fibers decreased with increasing ECAP temperature, which induced interfacial debonding. The tensile strength of TMCs ECAPed at 800 °C was up to 1128 MPa, which was 18% higher than the as-received composites. Interfacial debonding led TiB short fibers in low aspect ratios disabled to bear load, while the formation of ultrafine grains increased the strength of TMCs. The coupling of these two factors resulted in little effect of ECAP temperature on the tensile strength. However, the elongation of ECAPed TMCs generally decreased due to the loss of strain hardening capacity. Stress concentration tended to occur around the tips of TiB short fibers due to the interfacial debonding and cause crack propagation. It led to the premature fracture and low ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Fracture toughness and corrosion resistance of semisolid AlSi5 alloy

The aim of this work was to investigate fracture toughness and corrosion resistance of semisolid AlSi5 castings, compared to samples obtained from conventional casting operations. In order to have a semisolid microstructure, the melt alloy was treated by means of ultrasound during solidification and then poured into permanent moulds. Mechanical properties of semisolid and conventional castings were compared by means of ultimate tensile strength (R{sub m}), yield stress (Rp{sub 02}) and hardness (HV) measurements. Fracture mechanics tests were carried out on Single Edge Notched Bend (SENB) specimens, machined from castings, and pre-cracked by fatigue. These tests were performed to determine the effect of the microstructure on the J-Integral resistance (J-R) behavior and to deeply understand the ductile fracture behaviour of semisolid parts. The J-Integral versus spaced crack extension (J-{Delta}a) curves showed an improved resistance of the semisolid microstructure, due to the higher ductility. Finally, the corrosion behaviour of semisolid samples was compared to that of castings coming from solidification of fully liquid alloy by means of electrochemical potentiodynamic polarization tests. It was observed that the globular microstructure offers better quality, in terms of higher mechanical properties, as a consequence of a more uniform distribution of the solute.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Process design of press hardening with gradient material property influence

Press hardening is currently used in the production of automotive structures that require very high strength and controlled deformation during crash tests. Press hardening can achieve significant reductions of sheet thickness at constant strength and is therefore a promising technology for the production of lightweight and energy-efficient automobiles. The manganese-boron steel 22MnB5 have been implemented in sheet press hardening owing to their excellent hot formability, high hardenability, and good temperability even at low cooling rates. However, press-hardened components have shown poor ductility and cracking at relatively small strains. A possible solution to this problem is a selective increase of steel sheet ductility by press hardening process design in areas where the component is required to deform plastically during crash tests. To this end, process designers require information about microstructure and mechanical properties as a function of the wide spectrum of cooling rates and sequences and austenitizing treatment conditions that can be encountered in production environments. In the present work, a Continuous Cooling Transformation (CCT) diagram with corresponding material properties of sheet steel 22MnB5 was determined for a wide spectrum of cooling rates. Heating and cooling programs were conducted in a quenching dilatometer. Motivated by the importance of residual elasticity in crash test performance, this property was measured using a micro-bending test and the results were integrated into the CCT diagrams to complement the hardness testing results. This information is essential for the process design of press hardening of sheet components with gradient material properties.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Effects of irradiation on the mechanical behavior of twined SiC nanowires

Irradiation is known to bring new features in one-dimensional nano materials. In this study, we used molecular dynamics simulations to investigate the irradiation effects on twined SiC nanowires. Defects tend to accumulate from outside toward inside of the twined SiC nanowires with increasing irradiation dose, leading to a transition from brittle to ductile failure under tensile load. Atomic chains are formed in the ductile failure process. The first-principles calculations show that most of the atomic chains are metallic.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Deformation and fracture behavior of composite structured Ti-Nb-Al-Co(-Ni) alloys

Tensile ductility of the Ti-based composites, which consist of a β-Ti phase surrounded by ultrafine structured intermetallics, is tunable through the control of intermetallics. The two Ti-based alloys studied exhibit similar compressive yield strength (about 1000 MPa) and strain (about 35%–40%) but show a distinct difference in their tensile plasticity. The alloy Ti{sub 71.8}Nb{sub 14.1}Ni{sub 7.4}Al{sub 6.7} fractures at the yield stress while the alloy Ti{sub 71.8}Nb{sub 14.1}Co{sub 7.4}Al{sub 6.7} exhibits about 4.5% of tensile plastic deformation. To clarify the effect of microstructure on the deformation behavior of these alloys, tensile tests were carried out in the scanning electron microscope. It is shown that the distribution as well as the type of intermetallics affects the tensile ductility of the alloys.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

High resolution transmission electron microscopic in-situ observations of plastic deformation of compressed nanocrystalline gold

Nanocrystalline (nc) metals possess extremely high strength, while their capability to deform plastically has been debated for decades. Low ductility has hitherto been considered an intrinsic behavior for most nc metals, due to the lack of five independent slip systems actively operating during deformation in each nanograin. Here we report in situ high resolution transmission electron microscopic (HRTEM) observations of deformation process of nc gold under compression, showing the excellent ductility of individual and aggregate nanograins. Compression causes permanent change in the profile of individual nanograins, which is mediated by dislocation slip and grain rotation. The high rate of grain boundary sliding and large extent of widely exited grain rotation may meet the boundary compatibility requirements during plastic deformation. The in situ HRTEM observations suggest that nc gold is not intrinsically brittle under compressive loading.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Elastic properties and atomic bonding character in metallic glasses

The elastic properties of glasses from different metallic systems were studied in the light of the atomic packing density and bonding character. We found that the electronegativity mismatch (Δe{sup −}) between the host- and the major solute-elements provides a plausible explanation to the large variation observed for Poisson's ratio (ν) among metallic glasses (MGs) (from 0.28 for Fe-based to 0.43 for Pd-based MGs), notwithstanding a similar atomic packing efficiency (C{sub g}). Besides, it is found that ductile MGs correspond to Δe{sup −} smaller than 0.5 and to a relatively steep atomic potential well. Ductility is, thus, favored in MGs exhibiting a weak bond directionality on average and opposing a strong resistance to volume change.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

High-strength bioresorbable Fe–Ag nanocomposite scaffolds: Processing and properties

High strength ductile iron-silver nanocomposite scaffolds were fabricated employing high energy attrition milling of micron-submicron powders, followed by cold sintering/high pressure consolidation. Particulate leaching method with soluble Na{sub 2}SO{sub 4} and K{sub 2}CO{sub 3} salts as porogens was used to create scaffolds with 50, 55, 60 and 73% volume fraction of pores. Part of specimens was annealed at 600, 800 and 900°C. Specimens were characterized employing X-ray diffraction, scanning electron microscopy (SEM) with electron probe microanalysis (EDS) and high resolution SEM. Mechanical properties were measured in compression and permeability was measured in permeameter based on Darcy’s law. Scaffolds with 50% and 55% porosity exhibited high compressive strength (18–22 MPa), compressive strength of 8–12 MPa was observed for scaffolds with 73% porosity. Treatments at 800 and 900°C result in increase of strength and ductility with some coarsening of microstructure. Best combination of compressive strength (15 MPa) and permeability (0.6{sup −6} cm{sup 2}) is close to the range of trabecular bone.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Ab-initio study of B{sub 2}-type technetium AB (A=Tc, B=Nb and Ta) intermetallic compounds

The structural, electronic and elastic properties of AB type (A = Tc, B = Nb and Ta) technetium intermetallic compounds are studied using full potential linearized plane wave (FP-LAPW) method within generalized gradient approximation (GGA). The calculated lattice parameters agree well with the experimental results. The elastic constants obey the stability criteria for cubic system. Ductility for these compounds has been analyzed using the Pugh’s rule and Cauchy’s pressure and found that all the compounds are ductile in nature. Bonding nature is discussed in terms of Fermi surface and band structures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electronic structure, elasticity, bonding features and mechanical behaviour of zinc intermetallics: A DFT study

The structural stability, electronic structure, elastic and mechanical properties of TiZn and ZrZn intermetallics have been studied using ab-initio full potential linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation for exchange and correlation potentials. The various structural parameters, such as lattice constant (a{sub 0}), bulk modulus (B), and its pressure derivative (B’) are analysed and compared. The investigation of elastic constants affirm that both TiZn and ZrZn are elastically stable in CsCl (B{sub 2} phase) structure. The electronic structures have been analysed quantitatively from the band structure which reveals the metallic nature of these compounds. To better illustrate the nature of bonding and charge transfer, we have also studied the Fermi surfaces. The three well known criterion of ductility namely Pugh’s rule, Cauchy’s pressure and Frantsevich rule elucidate the ductile nature of these compounds.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

High-throughput exploration of thermoelectric and mechanical properties of amorphous NbO{sub 2} with transition metal additions

To increase the thermoelectric efficiency and reduce the thermal fatigue upon cyclic heat loading, alloying of amorphous NbO{sub 2} with all 3d and 5d transition metals has systematically been investigated using density functional theory. It was found that Ta fulfills the key design criteria, namely, enhancement of the Seebeck coefficient and positive Cauchy pressure (ductility gauge). These quantum mechanical predictions were validated by assessing the thermoelectric and elastic properties on combinatorial thin films, which is a high-throughput approach. The maximum power factor is 2813 μW m{sup −1} K{sup −2} for the Ta/Nb ratio of 0.25, which is a hundredfold increment compared to pure NbO{sub 2} and exceeds many oxide thermoelectrics. Based on the elasticity measurements, the consistency between theory and experiment for the Cauchy pressure was attained within 2%. On the basis of the electronic structure analysis, these configurations can be perceived as metallic, which is consistent with low electrical resistivity and ductile behavior. Furthermore, a pronounced quantum confinement effect occurs, which is identified as the physical origin for the Seebeck coefficient enhancement.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Multiplicity of morphologies in poly (L-lactide) bioresorbable vascular scaffolds

Poly(L-lactide) (PLLA) is the structural material of the first clinically approved bioresorbable vascular scaffold (BVS), a promising alternative to permanent metal stents for treatment of coronary heart disease. BVSs are transient implants that support the occluded artery for 6 mo and are completely resorbed in 2 y. Clinical trials of BVSs report restoration of arterial vasomotion and elimination of serious complications such as late stent thrombosis. It is remarkable that a scaffold made from PLLA, known as a brittle polymer, does not fracture when crimped onto a balloon catheter or during deployment in the artery. We used X-ray microdiffraction to discover how PLLA acquired ductile character and found that the crimping process creates localized regions of extreme anisotropy; PLLA chains in the scaffold change orientation from the hoop direction to the radial direction on micrometer-scale distances. This multiplicity of morphologies in the crimped scaffold works in tandem to enable a low-stress response during deployment, which avoids fracture of the PLLA hoops and leaves them with the strength needed to support the artery. Furthermore, the transformations of the semicrystalline PLLA microstructure during crimping explain the unexpected strength and ductility of the current BVS and point the way to thinner resorbable scaffolds in the future.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2026

Uniaxial tensile mechanical properties from a scratch on ultra-fine grained tungsten

We demonstrate that surface-based mechanical testing can be used to quantify a wide range of mechanical properties determined using traditional uniaxial tension experiments. These include mapping of dynamic stress–strain behaviour, which can be used as a screening tool for uniaxial tensile ductility (uniform elongation), fracture toughness (K C ), which is shown to be in good agreement with reported plane-strain K IC , as well work-hardening exponents (n), activation volumes (V) energies (Q) and strain-rate sensitivity (m) exponents as a function of temperature. Advantages of the surface-based approach are the ability to work with significantly smaller volumes and accommodate processing defects like pores, voids and cracks. These methods are applied to ultra-fine grained tungsten (average grain size of 650 nm), as it is an ideal candidate for alloy development efforts in advanced energy systems. Tungsten is a useful baseline as it exhibits a near-ambient temperature (approx. 200°C) ductile-to-brittle transition temperature (DBTT), enabling validation efforts for the proposed methods, and extraordinary material properties, including the highest melting temperature of any metal and complex temperature and strain-rate dependencies linked to transitions in dislocation mechanisms.

36 MATERIALS SCIENCE↗