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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Plastic homogeneity in nanoscale heterostructured binary and multicomponent metallic eutectics: An overview

Heterostructured materials comprised of relatively soft/hard disparate phases typically exhibit composite strengthening but lack plastic deformability at ambient temperatures. However, heterostructured systems comprised of nanoscale phases can simultaneously enhance yield strength and strain hardening, thereby promoting uniform distribution of plastic flow. In this review, the atomic-scale deformation mechanisms in model systems of eutectic alloys, Al-Al 2 Cu and Al-Si, refined to nanoscales via laser rapid solidification are discussed, and compared with literature on multi-component (high entropy) eutectics such as Ni-Al-Fe-based with Cr and/or Co additions. The nano-lamellar Al-Al 2 Cu structures exhibit unit defect mechanisms not reported in monolithic Al 2 Cu intermetallic: localized shear on {0 1 1} and shear-induced faults on {1 2 1} planes, constrained by closely-spaced dislocation arrays in Al confined by Al/Al 2 Cu interfaces. The unexpected plasticity mechanisms are enabled by slip continuity in nanoscale Al-Al 2 Cu eutectics associated with the orientation relationship and interface habit planes. In nano-fibrous Al-Si eutectic, tensile ductility at strength approaching 600 MPa is observed resulting from dislocation plasticity in the nano-Al channels and cracking in Si nanofibers. Molecular dynamics simulations show that Al dislocations easily cross-slip (screw) or climb (edge) along Al-Si interfaces, making slip transmission difficult. The propagation of nano-cracks is suppressed by surrounding strain hardening Al, retaining good ductility of the sample, in spite of lack of direct slip transmission. Finally, the critical unit mechanisms of slip transmission and interface-enabled plasticity observed in nanoscale eutectic binary systems can also explain the strength-ductility relationship in multi-component eutectics and homogeneously distributed plastic flow with increasing microstructural heterogeneity.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Influence of copper content on the high temperature tensile and low cycle fatigue behavior of cast Al-Cu-Mn-Zr alloys

The relationship between low cycle fatigue (LCF) and monotonic tensile fracture strain was investigated for cast Al-Cu-Mn-Zr alloys containing 6 and 9 wt% Cu at 250 °C. The 9% Cu alloy consisted of larger size and volume fraction of brittle intermetallic grain boundary particles that fractured during tensile tests to reduce the fracture strain by 50% compared to the 6% Cu alloy. We found that LCF life was similar between the two alloys and weakly influenced by the particles. LCF life and fracture strain were poorly correlated due to the dominant influence of particles on monotonic tensile crack but not on fatigue crack.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Modification of magnetic ground state in Tb 2 Ni 0.90 Si 2.94 by thermal annealing

In this work, we have investigated the thermal annealing effect on the physical properties of an AlB 2 -type ternary intermetallic compound, Tb 2 Ni 0.90 Si 2.94 , that undergoes spin freezing behaviour coexisting with spatially limited antiferromagnetic phase below 9.9 K in as-cast form. Thermal annealing effect is found to result in considerable changes in the magnetic ground state properties of the system. Though only one magnetic transition around 9.9 K is observed for as-cast compound, the annealed sample exhibits two distinct magnetic transitions; one around T N ~ 13.5 K and another around T f ~ 4 K. The magnetization measurements and zero field neutron diffraction study reveal that the high temperature transition is antiferromagnetic type, though of limited correlation length, while the low temperature transition corresponds to spin freezing behaviour. The ac susceptibility and heat capacity studies also confirm the existence of frustrated cluster glass state at lower temperature than the antiferromagnetic ordering temperature. Additionally, ac susceptibility data exhibits signature of an additional peak in the even lower temperature region (at 2.2 K for zero frequency) that tends to shift in opposite direction with frequency in contrast to that observed for conventional glassy transitions. The change in intrinsic local structural disorder of Ni and Si ions associated with annealing has been argued to be responsible for the different magnetic behaviour in as-cast and annealed samples.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Synthesis of bulk reactive Ni–Al composites using high pressure torsion

Self-propagating exothermic reactions, for instance in the nickel-aluminum (Ni-Al) system, have been widely studied to create high performance intermetallic compounds or for in-situ welding. Their easy ignition once the phase spacing is reduced below the micron scale, makes top-down methods like high-energy ball milling, ideal to fabricate such reactive nanostructures. A major drawback of ball milling is the need of a sintering step to form bulk pieces of the reactive material. However, this is not possible, as the targeted reactions would already proceed. Therefore, we investigate the ability of high pressure torsion as an alternative process, capable to produce bulk nanocomposites from powder mixtures. Severe straining of powder mixtures with a composition of 50 wt% Ni and 50 wt% Al enables fabrication of self-reactive bulk samples with microstructures similar to those obtained from ball milling or magnetron sputtering. Additionally, samples deformed at ambient temperature are highly reactive and can be ignited significantly below the Al melting point, finally predominantly consisting of Al 3 Ni 2 and Al 3 Ni, independent of the applied strain. Although the reaction proceeds first at the edge of the disk, the strain gradient present in the disks does not prevent reaction of the whole sample.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Indium segregation in Gd 5 (Si, Ge) 4 magnetocaloric materials

Chemical substitution is one of the most efficient tools to tune and optimize magnetic and magnetocaloric properties of the giant magnetocaloric materials. In particular, Indium substitutions could be useful both for tuning properties of these interesting intermetallic materials and to unveil their local-scale behavior across the magnetostructural transition via hyperfine techniques. Hence, in order to investigate the effect of Indium additions on the crystal structure, micro-structure, magnetic and magnetocaloric properties, a series of In-containing samples derived from the base Gd 5 Si 1.2 Ge 2.8 stoichiometry were prepared. Here, the major findings are that while In is insoluble in the 5:4 phase, it will instead promote the emergence of the impurity 5:3 phase and segregates into this phase. Hence, In leads to major crystallographic changes, which enhance atomic disorder and disrupt the Si to Ge ratio in the 5:4 phase. Subsequently, a higher 5:4 unit cell volume and a lower magnetic ordering temperature are found in the In-substituted samples. Finally, the magnetocaloric properties of the In-substituted samples reveal a detrimental effect on the maximum magnetic entropy change.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Neutron vibrational spectroscopic evidence for short H∙∙∙H contacts in the RNiInH 1.4; 1.6 (R = Ce, La) metal hydride

Intermetallic metal hydrides are critical materials for hydrogen storage applications, however, metal hydrides with greater storage capacities are still needed. Within metal hydrides, the volumetric storage capacities are limited by the number of hydrogen-accommodating interstitial sites which can be simultaneously occupied given a minimum hydride nearest-neighbor distance of ~2.1 A, according to the Switendick-Westlake criterion. To date, violations of this criterion are rare. Perhaps the most well studied compounds violating this criterion are the RNiInHx compounds (R = Ce, La, Nd). Previous neutron diffraction studies on the deuterated species revealed the presence of Ni-D∙∙∙D-Ni-D∙∙∙D-Ni chains with anomalously close D∙∙∙D contacts of ~1.6 A. Yet there are no neutron vibrational spectroscopic investigations reported for these atypical hydrides. Here we use neutron vibrational spectroscopy (NVS) measurements to probe the hydrogen dynamics in LaNiInHx (x = 0.67, 1.6) and CeNiInH 1.4 . For x > 0.67, the presence of close H...H contacts yields two related features in the vibrational spectrum centered near ~90 meV corresponding to the oscillations of paired H atoms simultaneously occupying neighboring R 3 Ni tetrahedra. Notably, these features are energetically distinct from comparable vibrational motions for "unpaired" H atoms when x = 0.67. To compare, we also present powder neutron diffraction and NVS measurements for the newly characterized, chemically similar Sn compounds CeNiSnH, CeNiSnH 2 , and CeNiSnD 2 . These compounds also contain R 3 Ni tetrahedra, however, the H-occupied tetrahedra are well separated from each other with the closest H∙∙∙H distances exceeding 2.1 A, and the Switendick-Westlake criterion is not violated. Consequently, the spectral signature of the close H∙∙∙H contacts is absent in these hydrides.

08 HYDROGEN↗
DOE OSTI2021

On the interactions between molten aluminum and high entropy alloy particles during aluminum matrix composite processing

The feasibility of adding AlCrTiV high entropy alloy (HEA) particles to molten aluminum to make metal matrix composites was studied. Equiatomic AlCrTiV HEA particles, produced by a solid state reduction process, were dispersed into pure aluminum and 7075 alloy (Al-5.8Zn-2.3Mg-1.4Cu, all weight %) melt and by a ultrasonic dispenser. It was found that the AlCrTiV particles reacted with molten aluminum, leading to the formation of Al 45 (Cr,V) 7 and Al 3 Ti intermetallic phases in the aluminum matrix composites. Although the AlCrTiV HEA particles did not survive in molten aluminum during composite processing, the final composite materials showed significantly improved mechanical properties, due to grain refinement and dispersion strengthening. Finally, the research suggests that the interactions between matrix aluminum alloys and the particles are key in controlling the final microstructure and properties of the composites.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Structural and magnetic properties of magnetostrictive Fe-Ga-Zr nanocrystalline alloy

(Fe 1–x Ga x ) 92 Zr 8 amorphous and nanocrystalline alloys with x = 0.15 to x = 0.36 were investigated to improve the magnetic softness of Galfenol-type alloys and to evaluate their magnetostrictive properties. The samples were prepared by melt spinning of arc melted ingots. The rapidly solidified ribbons were annealed at 823 K for 1 h to produce a nanocrystalline structure. X-ray Diffraction data showed that after annealing, body-centered cubic (BCC) Fe-Ga phase crystallized for x = 0.15 to x = 0.26. When x exceeded 0.26, the ribbons crystallized into BCC and a ternary intermetallic phase (i.e., ZrFe 6 Ga 6 ) which has deteriorating effect on saturation magnetization, magnetic softness and magnetostriction coefficient. The annealed ribbons’ saturation magnetization value decreases from 126 Am 2 /kg to 54 Am 2 /kg as the Ga content increased from x = 0.15 to x = 0.36. The alloy with x = 0.26 annealed at 823 K for 1 h shows a peak magnetostriction of 10 ppm with saturation magnetization of 110 Am 2 /kg and coercivity of 260 A/m.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Large adiabatic temperature change and magnetic frustration in triangular lattice antiferromagnet Dy 2 IrSi 3

While previous studies reveal that the formation of polycrystalline R 2 TX 3 (R = rare-earth, T = transition metal, X = p-block element) type compounds with T = Ir and Co is not achievable without deliberate introduction of atomic vacancies of Ir/Co and Si, here we report successful synthesis of Dy 2 IrSi 3 with fully stoichiometric ratio of constituent elements. An antiferromagnetic transition is manifested in the compound at 6.6 K where only about 50 % of Dy spins take part, while the rest remain in highly frustrated state due to edge-sharing triangular lattice geometry. The magnetic frustration is conclusively probed through magnetization, heat capacity and magnetocaloric study and the frustration parameter (f = |θ p |/T N , θ p , T N being Curie-Weiss and Néel temperatures, respectively) in this case is as large as ~ 3. An additional cluster-glass type transition at ~2.8 K observed in this system has likely been originated due to this magnetic frustration. The magnetocaloric effect reveals an adiabatic temperature change of ~7.5 K for 70 kOe magnetic field, which is one of the largest values reported in intermetallic compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Transition from an incommensurate spin density wave to a commensurate magnetic order in a triangular lattice compound Ho 2 PdAl 6 Ge 4

Rare-earth (RE) intermetallics on a triangular lattice are promising candidates for generating interesting magnetic phases due to the complex interplay between Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction and geometrical frustration. Here, in this work, we report the exotic magnetic structure of a layered compound Ho 2 PdAl 6 Ge 4 with triangular lanthanide nets. Magnetization and heat capacity measurements in zero magnetic field reveal two magnetic phase transitions at T N1 = 10.8 K and T N2 = 6.0 K. Neutron powder diffraction demonstrates a commensurate antiferromagnetic phase with k 1 = (0, 0, 1.5) below T N2 . With increasing temperature, another incommensurate vector appears and therefore, the magnetic structure of the intermediate state is identified as an unusual incommensurate spin density wave with two propagation vectors k 1 = (0, 0, 1.5) and k 2 = (0.0492, 0.0492, 1.5). The magnetic moments in the intermediate state rotate continuously and form an unusual S-shaped wave arrangement in the ab plane, sharing similarities with typical cycloid and helix magnetic orders. These results identify Ho 2 PdAl 6 Ge 4 as a candidate for exploring field-induced topological magnetic phases such as skyrmions, opening the way for further investigations on the family of RE 2 PdAl 6 Ge 4 materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Microstructural features underpinning the mechanical behavior of powder metallurgy Cr-based alloys

Refractory materials such as Cr-based alloys offer the potential of enhanced elevated temperature performance but have been limited by their poor formability. However, powder metallurgy has been shown to be a viable pathway to fabricate these alloys. Nanophase separation sintering (NPSS) in particular has been used in the literature to accelerate the densification of powder metallurgy Cr- and W-based alloys. Here, we explore microstructure evolution during NPSS in a binary Cr 85 Ni 15 alloy consolidated via (i) cold pressing & pressureless sintering and (ii) hot isostatic pressing followed by hot extrusion & hot upsetting, and the role of these different processing routes on resulting material properties. The alloy lacked room temperature tensile ductility regardless of consolidation process, with multi-length scale characterization, including scanning electron microscopy, transmission electron microscopy, atom probe tomography, X-ray diffraction, and uniaxial tensile testing, revealing that brittleness was due to intrinsically poor Cr grain boundary cohesion. Tensile testing conducted at 760 °C showed marked strength reduction for the extruded & upset (94 %) condition compared to the pressed & sintered (18 %). The formation of orthorhombic CrNi 2 intermetallics functioned as precipitate strengtheners and prevented elevated temperature softening in the pressed and sintered condition. The findings offer foundational insights into aiding the future development of Cr-based alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Scalable fabrication of high activity nanoporous copper powders for electrochemical CO 2 reduction via ball milling and dealloying

Electrochemical CO 2 reduction (ECR) is a promising technology to close the anthropic CO 2 circle using renewable energy to achieve carbon neutrality. Future commercialization of ECR will require the development of new catalyst synthesis routes that will allow significant upscaling of catalyst production from current research-level milligram quantities to the kilogram scale and beyond while maintaining the activity and selectivity demonstrated at the research level. Here, we report on generating and testing submicron-sized nanoporous copper (npCu) particles by using a scalable approach consisting of ball milling brittle Cu-based intermetallics followed by dealloying to add nanoporosity for high surface area. The resulting npCu particles have been tested in an industry-relevant large area (25 cm 2 ) electrolyzer platform and showed Faraday efficiencies (FE) for ethylene up to 34 % at current densities of 75–100 mA/cm 2 while keeping FE for hydrogen less than 30 %. Our results demonstrate that a combination of ball milling and dealloying is a promising approach to generate large quantities of high activity and high surface area npCu particles for ECR at an industry-relevant scale.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2025

Spatial mapping of the localized corrosion behavior of a magnesium alloy AZ31B tungsten inert gas weld

Sections of a magnesium alloy, AZ31B, joined with tungsten inert gas (TIG) welding, were examined with scanning electrochemical microscopy (SECM) and scanning Kelvin probe force microscopy (SKPFM) to investigate corrosion mechanisms by correlating observed corrosion behavior with weld-affected microstructural variations. Insight into the changing nature of the galvanic couples between weld zones and at localized microgalvanic sites were investigated using SECM and SKPFM to map both electrochemically active regions and Volta potential differences across the weld-affected zones. The formation of an Al-Zn solidification network in the fusion zone (FZ) at and near the TIG weld epicenter differs from the outer heat-affected zone (HAZ), where intermetallic particles (IMPs) are the notable secondary phase from the magnesium matrix. These microstructures were mapped with SKPFM before and after brief exposure to a salt solution, revealing micro-galvanic couples as the main driving force to corrosion initiation and propagation within each zone. The IMPs and Al-Zn solidification network act as strong cathodes and govern the corrosion processes. The galvanic coupling and evolution of the intrinsic corrosion behavior between the weld zones is explained by monitoring the hydrogen evolution reaction (HER) with SECM over time. Anodically induced cathodic activation is confirmed for this welded material, as micro-galvanic couples between microstructural features are found to transition over time to broad electrochemically active areas within the weld-affected zones, resulting in polarity reversal as time of exposure proceeds.

Anodically induced cathodic activation↗
DOE OSTI2025

Evaluating the bond strength and fracture mechanisms of cold-sprayed zinc coating on AZ91 magnesium substrate via a combined experimental and computational approach

Magnesium (Mg) alloys are ideal candidates for automotive applications due to their high strength to weight ratio, castability, recyclability etc., however, they lack corrosion and oxidation resistance. Solid-state deposition techniques, such as cold spray, have been demonstrated to enhance their corrosion resistance as it relies on the severe plastic deformation of powder particles upon impact with the substrate to form a metallurgical bond with the substrate and within the coating. At cold sprayed interfaces, a heterogeneous microstructure is formed that includes some porosity, oxides and intermetallics which can significantly affect coating performance. Thus, establishing a direct correlation between the interface microstructure and its properties can aid in designing optimal cold spray parameters. In this study, we investigated the microstructure and mechanical properties of a zinc (Zn) coating deposited on a high pressure die cast (HPDC) AZ91 Mg substrate via high resolution scanning transmission electron microscopy, in situ micro-tensile testing, and finite element method (FEM) modeling. Micro-tensile pillars fabricated using the plasma focused ion beam (PFIB) successfully isolates the coating-substrate interface within the gauge length. The average bond strength of Zn-Mg interface was determined to be ∼140 MPa with failure occurring partially at the interface and mostly into the coatings. A detailed microstructural characterization revealed evidence of a strong metallurgical bonding at the Zn-Mg interface and formation of the C14 MgZn 2 laves phase interlayer resulting in a mixed mode of fracture during the micro-tensile experiments. FEM modeling reveals the stress distribution along the interfaces and suggests that a MgZn 2 layer thickness between 200–400 nm is optimum to increase the bond strength and minimize the triaxiality. Such a site-specific interfacial analysis with correlative computational modeling provides crucial insight into the overall performance of cold spray interfaces.

Bond strength↗
DOE OSTI2021

Friction Stir Dovetailing of AA7099 to Steel with AA6061 Interlayer for Reduced Zn Embrittlement at Dissimilar Interface

Joining of high strength Al alloy (AA7099) to steel (rolled homogeneous armor) with and without AA6061 interlayer was achieved using a friction stir dovetailing (FSD) method. Detailed investigations were conducted to establish correlations between mechanical performance and microstructural evolution of FSD joints. Lap shear tensile testing revealed that incorporation of the AA6061 interlayer combined with tailored processing methodology significantly improved both mechanical interlocking and load transmission across the dovetail interface. Following incorporation of the interlayer, the maximum load carrying capacity and peak extension (of the AA7099/AA6061/steel joint) are 60% and 210% higher, respectively, than results obtained from an AA7099/steel joint without an interlayer. Without an AA6061 interlayer, formation of zinc rich intermetallic compounds led to brittle interfacial failure of the AA7099-steel FSD joint. Incorporation of novel tool and joint configuration eliminated asymmetric joint performance as a function of loading direction, which is generally observed in friction stir lap welding. The efficacy of the use of an AA6061 interlayer in joining AA7099 to steel was demonstrated through mechanical testing as well as high resolution analytical electron microscopy characterization.

Dissimilar metal joining, Friction stir dovetailin↗
DOE OSTI2021

Evidence of lithium mobility under neutron irradiation

Understanding the evolution of intermetallic materials in high-radiation environments is of great importance for fusion science and national security. Tritium ( 3 H) and lithium ( 6 Li, 7 Li) transport within neutron irradiated claddings coated with aluminide (FeAl3) has been investigated using state-of-the-art multimodal imaging. Specifically, scanning electron microscopy – focused ion beam (SEM-FIB) was used to prepare irradiated coating lift-out samples for follow-on microanalysis. Here, scanning transmission electron microscopy (STEM) was used to acquire atomic-scale information on the carbonaceous structure and elemental mapping. Atomic force microscopy (AFM) was used to determine lift-out dimensions nondestructively. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) spectral and depth profiling show unexpected lithium isotopic distributions in the irradiated cladding, raising an important evidence of possible light isotope mobility within the cladding under elevated temperature and irradiation. ToF-SIMS three-dimensional chemical mappings of mid- and bottom-cladding coatings show light isotopic (e.g., 3 H, 6 Li, 7 Li) distributions in the irradiated coating and give new insights into the fundamental mechanism related to transport mechanisms within the cladding. Multimodal imaging is power to link microstructure, chemistry, and nanoscale defects that impact reliability of these materials. Furthermore, chemical mapping offers observations of the microstructural evolution due to irradiation and provides insights into unexpected material transport under extreme conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Cantor-derived medium-entropy alloys: bridging the gap between traditional metallic and high-entropy alloys

The year 2004 marked the beginning of a new era in the design of metallic materials, as the concept of multiple principal component alloys, commonly known as High-Entropy Alloys (HEAs), was proposed by Cantor and Yeh. The unexpected single-phase microstructure, instead of the expected brittle intermetallic compounds, was attributed to the large entropy of mixing and immediately caught the attention of the scientific community. Today, HEAs are considered important advanced materials and a broad range of alloys using nominally the same design principle have been investigated. Despite that, the CrMnFeCoNi (Cantor) alloy stands out as the most successful HEA due to its outstanding mechanical properties and microstructure. In this scenario, variants of the Cantor alloy, named medium-entropy alloys (MEAs), are gaining significant interest as they display a better industrial potential than both HEAs and traditional alloys. These variants of the Cantor alloy with only three or four main elements result in 15 possible combinations. The microstructure of these alloys is discussed in terms of advanced characterization as well as thermodynamic parameters and computational simulation. Their phase stability is addressed over a wide range of temperatures and strain rates. The mechanical properties, especially the fracture toughness, of the CrFeCoNi and CrCoNi alloys have been reported to be even superior to those of the Cantor alloy and most modern engineering alloys. This is associated with the formation of a continuous sequence of strengthening mechanisms, including hierarchical twin networks, which serve to prolong the strain hardening. The present article reviews and critically assesses, for the first time, recent advances in these Cantor-derived MEAs.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Collaborative or competitive phase transformation processes in an additively manufactured-maraging steel M300

Although retained austenite is present in minor amounts in as-built additively manufactured-maraging steels, its evolution during ageing is key to controlling the final microstructure. This study investigates the relationship between austenite reversion and precipitation evolution during ageing, particularly whether these processes compete or cooperate. Using Scanning/Transmission Electron Microscopy, Atom Probe Tomography and Thermo-Calc® PRISMA simulations, the presence and evolution of retained austenite, Ni 3 Ti and Fe 7 Mo 6 intermetallic phases was characterised across different ageing conditions. Experimental results revealed the presence of non-enriched austenite in the as-built condition. Nevertheless, with increasing ageing temperature, Ni and Mo segregation became evident. High-Resolution Transmission Electron Microscopy revealed that Ni 3 Ti precipitation primarily occurred within the martensitic matrix, while Fe 7 Mo 6 nucleated preferentially at the austenite-martensite interface. PRISMA simulations indicated early and rapid precipitation in solute-enriched areas (i.e., intercellular regions), with Ni 3 Ti forming prior to Fe 7 Mo 6 . Both experimental and simulation results suggested Ni diffusion controls retained austenite growth, while Ti and Mo drive Ni 3 Ti and Fe 7 Mo 6 precipitation, respectively. The evidence reported in this work supports a both collaborative and independent phase transformation mechanism, where austenite reversion and precipitation co-evolve.

42 ENGINEERING↗