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At least 1,477 records · Page 82
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

First-Principles Investigation of Equiatomic Quaternary Heusler Alloys NbVMnAl and NbFeCrAl and a Discussion of the Generalized Electron-Filling Rule

Plane-wave pseudo-potential methods based on density functional theory are used to study the electronic structures, magnetic, and half-metallic properties of the equiatomic quaternary Heusler alloys NbVMnAl and NbFeCrAl. Calculated results reveal that NbVMnAl and NbFeCrAl alloys are newly designed half-metallic ferrimagnets with a total magnetic moment (M{sub t}) of 2 μ{sub B}. We found that the Slater-Pauling rule, i.e., the relationship between the M{sub t} and the total number of valence electrons (Z{sub t}) of these two alloys was different. For NbVMnAl alloy, it obeys the M{sub t} = |Z{sub t} − 18| rule. For NbFeCrAl, it satisfies the M{sub t} = |Z{sub t} − 24| rule. Also, we observed that the half-metallic band gap (HM-BG) of these two alloys was different. Namely, for one material (NbFeCrAl), it is located in the spin-up channel, and in the other (NbVMnAl), it is located in the spin-down channel. To explain these phenomena, in this work, the generalized electron-filling rule was investigated, and the origin of the HM-BG of NbVMnAl and NbFeCrAl was studied. We hope that our work may provide theoretical guidance in searching for Nb-based equiatomic quaternary Heusler half-metals in the future.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Thermodynamic Properties of Magnesium-Boron Binary Alloys Determined Using Solid-State Electrochemical Measurements

This study provides the thermodynamic properties of the Mg-B binary alloys. Experiments were performed to measure the electromotive force (emf) as a function of temperature (773 K to 873 K) using a solid-state electrochemical cells that can be represented as (−)Pt,Ar/{Mg+CaF_2⋅MgF_2}∥CaF{sub 2}∥{Mg(alloy)+CaF_2⋅MgF_2}/Ar,Pt(+)The activities of Mg in Mg-B alloys were calculated from the experimental emf data. The activity coefficients of Mg in Mg-B alloys were also determined. The integral Gibbs energies of formation (∆G{sub f}{sup °}) of alloys were calculated from the activities of the Mg using tangent rule. The integral Gibbs energies of formation (∆G{sub f}{sup °}) of MgB{sub 2}, MgB{sub 4}, and MgB{sub 7} are − 15.48, − 22.03, and − 15.89 kJ/mol-atoms at 873 K.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Portevin-Le Chatelier mechanism in face-centered-cubic metallic alloys from low to high entropy

Serration phenomena during tensile testing on certain alloys with diffusing solute atoms (i.e., Portevin–Le Chatelier effect) have been observed for a long time, but detailed mechanisms are not fully clear yet. This study is intended to find the mechanism from different approaches verified by tensile testing on a series of single-phase face-centered-cubic (FCC) pure metal and alloys: Ni, CoNi, CoFeNi, CoCrFeNi, and CoCrFeMnNi, which range from low to high configurational entropy. The results of tensile tests, at strain rates from 1 × 10 -5 to 1 × 10 -2 /s and temperature from room temperature to 700 °C, show that serrations occur on stress-strain curves of CoFeNi, CoCrFeNi, and CoCrFeMnNi alloys in their specific temperature and strain-rate regime. A mechanism for dislocation pinning is proposed and verified with theoretical calculation for the present substitutional alloys. The proposed mechanism involves the in-situ rearrangement of substitutional solute atoms by “local dislocation-core diffusion” and might also be applied to similar substitutional alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2016

The microstructure of the surface layer of magnesium laser alloyed with aluminum and silicon

The surface layer under analysis was formed as a result of diffusion bonding of a thin AlSi20 plate to a magnesium substrate followed by laser melting. Depending on the process parameters, the laser beam melted the AlSi20 plate only or the AlSi20 plate and a layer of the magnesium surface adjacent to it. Two types of microstructure of the remelted layer were thus analyzed. If the melting zone was limited to the AlSi20 plate, the microstructure of the surface layer was typical of a rapidly solidified hypereutectic Al–Si alloy. Since, however, the liquid AlSi20 reacted with the magnesium substrate, the following intermetallic phases formed: Al{sub 3}Mg{sub 2}, Mg{sub 17}Al{sub 12} and Mg{sub 2}Si. The microstructure of the modified surface layer of magnesium was examined using optical, scanning electron and transmission electron microscopy. The analysis of the surface properties of the laser modified magnesium revealed that the thin layer has a microstructure of a rapidly solidified Al–Si alloy offering good protection against corrosion. By contrast, the surface layer containing particles of intermetallic phases was more resistant to abrasion but had lower corrosion resistance than the silumin type layer. - Highlights: •A CO{sub 2} laser was used for surface alloying of Mg with AlSi20. •Before alloying, an AlSi20 plate was diffusion bonded with the Mg substrate. •The process parameters affected the alloyed layer microstructure and properties. •With melting limited to AlSi20, the layer had a structure of rapidly solidified AlSi20. •Mg–Al and Mg–Si phases were present when both the substrate and the plate were melted.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Multi-principal element alloys for fast reactor cladding applications

Given the extensive list of multi-principal element alloys (MPEAs) within literature and the overwhelming number of alloys that can be made from only a handful of elements, this article proposes a methodology for prioritizing alloys for use as cladding within advanced reactors. This paper applies neutronic, mechanical, and chemical assessments to a collection of MPEAs available within literature for use as advanced reactor cladding. The results are compared to a reference design of HT9, a ferritic/martensitic steel, for employed in sodium-cooled fast reactor. Mechanical assessments determined pressure limits for a thin-walled tube pressure boundary, thus relating the material's mechanical properties to a minimum acceptable wall thickness. Neutronic analyses reveal a maximum allowable wall thickness that a given material must meet to provide a level of neutronic economy that is equivalent to than that of HT9. Lastly, each alloying element is compared to typical fission products found in a fast reactor fuels to mitigate deleterious phenomena such as fuel-cladding chemical interactions (FCCI). These analyses indicate that Mo-Nb-Ti-V-based alloys are likely to be advantageous and that the inclusion of elements with a high neutronic penalty (e.g., Hf) could be considered with minimal consequences.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2014

Size and alloying induced shift in core and valence bands of Pd-Ag and Pd-Cu nanoparticles

In this report, X-ray photoelectron spectroscopy studies have been carried out on Pd, Ag, Cu, Pd-Ag, and Pd-Cu nanoparticles having identical sizes corresponding to mobility equivalent diameters of 60, 40, and 20 nm. The nanoparticles were prepared by the gas phase synthesis method. The effect of size on valence and core levels in metal and alloy nanoparticles has been studied by comparing the values to those with the 60 nm nanoparticles. The effect of alloying has been investigated by comparing the valence and core level binding energies of Pd-Cu and Pd-Ag alloy nanoparticles with the corresponding values for Pd, Ag, and Cu nanoparticles of identical sizes. These effects have been explained in terms of size induced lattice contractions, alloying induced charge transfer, and hybridization effects. The observation of alloying and size induced binding energy shifts in bimetallic nanoparticles is important from the point of view of hydrogen reactivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Electronic properties of liquid Hg-In alloys : Ab-initio molecular dynamics study

Ab-initio molecular dynamics simulations are performed to study the structural properties of liquid Hg-In alloys. The interatomic interactions are described by ab-initio pseudopotentials given by Troullier and Martins. Three liquid Hg-In alloys (Hg{sub 10}In{sub 90}, Hg{sub 30}In{sub 70,.} Hg{sub 50}In{sub 50}, Hg{sub 70}In{sub 30}, and Hg{sub 90}Pb{sub 10}) at 299 K are considered. The calculated results for liquid Hg (l-Hg) and lead (l-In) are also drawn. Along with the calculated results of considered five liquid alloys of Hg-In alloy. The results obtained from electronic properties namely total density of state and partial density of states help to find the local arrangement of Hg and In atoms and the presence of liquid state in the considered five alloys.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Optical Properties of Tellurium-Based Chalcogenide Alloys in the Far Infrared Region (λ > 30 μm)

Ternary telluride alloys of Ge–Se(Sb)–Te and Si–Ge(Ga)–Te systems are synthesized in glassy and crystalline states for use in the terahertz frequency range. The transmission spectra of the obtained alloys are measured and studied in a wide wavelength range from 0.75 to 300 μm. The possible mechanisms of their formation are discussed. A comparative analysis of the results shows that the Ge{sub 14}Sb{sub 28}Te{sub 56} alloy of the GST system is most promising. Its phonon spectrum is in the range of 40–280 cm{sup –1}, limiting the long-wavelength transmission window of this alloy by 35 μm. Optimization of the Ge{sub 14}Sb{sub 28}Te{sub 56} composition, the removal of impurities, and heat treatment will promote a further decrease in the absorbance in the far-infrared spectrum of this alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Fundamental Effects of Al and Ta on Microstructure and Phase Transformations in the Al–Cr–Mo–Ta–Ti Refractory Complex Concentrated Alloy System

The effect of aluminum and tantalum concentrations on a refractory metal complex concentrated alloy is reported, particularly with respect to their effect on microstructure and phase composition of the alloy in cast and annealed form. Alloys with an equiatomic composition, (AlCrMoTaTi), an aluminum-lean composition (Al 0.75 CrMoTaTi), and a tantalum-lean composition (AlCrMoTa 0.75 Ti) are produced via arc melting. The alloys exhibit multiphase structures, confirmed by X-ray diffraction, microstructural characterization, and thermal analysis. The minor off-equiatomic adjustments of aluminum and tantalum in this alloy system did not drastically alter the prevalence of the Cr–Ta-based Laves phase. Correlations between thermodynamic predictions and observed phase transformations via thermal analysis are improved upon refinement of calculations removing impractical intermediate phases. As a result, experimental findings provide information for the refinement of thermodynamic modeling and deliver additional insight into the optimization of alloy compositions within this five-component system.

36 MATERIALS SCIENCE↗
DOE OSTI2018

High‐Strength Nanotwinned Al Alloys with 9R Phase

Abstract Light‐weight aluminum (Al) alloys have widespread applications. However, most Al alloys have inherently low mechanical strength. Nanotwins can induce high strength and ductility in metallic materials. Yet, introducing high‐density growth twins into Al remains difficult due to its ultrahigh stacking‐fault energy. In this study, it is shown that incorporating merely several atomic percent of Fe solutes into Al enables the formation of nanotwinned (nt) columnar grains with high‐density 9R phase in Al(Fe) solid solutions. The nt Al–Fe alloy coatings reach a maximum hardness of ≈5.5 GPa, one of the strongest binary Al alloys ever created. In situ uniaxial compressions show that the nt Al–Fe alloys populated with 9R phase have flow stress exceeding 1.5 GPa, comparable to high‐strength steels. Molecular dynamics simulations reveal that high strength and hardening ability of Al–Fe alloys arise mainly from the high‐density 9R phase and nanoscale grain sizes.

Li, Qiang↗
DOE OSTI2020

The Impact of Impurities on Alloy Behavior in Supercritical CO 2 at 700 °C

As part of round robin testing, specimens of commercial alloys 316, 120, 625, and 740 were exposed to 20 MPa research grade CO 2 at 700 °C for up to 1500 h. The first set of specimens had higher mass gain likely due to impurities not flushed from the autoclave at startup. After this issue was corrected, an identical set of specimens exhibited lower mass gains for both the Fe- and Ni-based alloys. The differences in reaction products were characterized to understand the effect of impurities under these conditions. As suggested by the mass change, thicker oxides were formed in each case, primarily for the Fe-based alloys. Furthermore, for the Ni-based alloys, the difference was primarily in the transient stage of oxidation with no change in rate constant. Alloy 120 exhibited increased internal attack, and differences in the scale phases formed were characterized to better understand the effect of impurities. Alloy 740 primarily exhibited a greater depth of internal attack in the first run.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Microstructure and Mechanical Properties of Laves Phase-strengthened Fe-Cr-Zr Alloys

Laves phase-reinforced alloys have shown some preliminary promising performance at room temperatures. This paper aims at evaluating mechanical properties of Laves phase-strengthened alloys at elevated temperatures. Three Fe-Cr-Zr alloys were designed to favor the formation of eutectic microstructures containing Laves and body-centered cubic phases with the aid of thermodynamic calculations. Microstructural characterization was carried out on the alloys in as-processed and aged states using optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The effect of thermal aging and alloy composition on microstructure has been discussed based on microstructural characterization results. Mechanical properties have been evaluated by means of Vickers microhardness measurements, tensile testing at temperatures up to 973.15 K (700.15 °C), and creep testing at 873.15 K (600.15 °C) and 260 MPa. Alloys close to the eutectic composition show significantly superior strength and creep resistance compared to P92. Finally, however, their low tensile ductility may limit their applications at relatively low temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Comparative evaluation of cast aluminum alloys for automotive cylinder heads: Part II: Mechanical and thermal properties

The first part of this study documented the as-aged microstructure of five cast aluminum alloys namely, 206, 319, 356, A356, and A356+0.5Cu, that are used for manufacturing automotive cylinder heads (Roy et al. in Metall Mater Trans A, 2016). In the present part, we report the mechanical response of these alloys after they have been subjected to various levels of thermal exposure. In addition, the thermophysical properties of these alloys are also reported over a wide temperature range. The hardness variation due to extended thermal exposure is related to the evolution of the nano-scale strengthening precipitates for different alloy systems (Al-Cu, Al-Si-Cu, and Al-Si). The effect of strengthening precipitates (size and number density) on the mechanical response is most obvious in the as-aged condition, which is quantitatively demonstrated by implementing a strength model. Significant coarsening of precipitates from long-term heat treatment removes the strengthening efficiency of the nano-scale precipitates for all these alloys systems. Thermal conductivity of the alloys evolve in an inverse manner with precipitate coarsening compared to the strength, and the implications of the same for the durability of cylinder heads are noted.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A General Model for the Ductility of Intermetallics Applied to Fe-Co Alloys

The mechanical properties, specifically ductility, of high performing soft magnets such as Fe-Co alloys are a limiting factor to their broader use in a number of systems. The understanding of the mechanical robustness in these materials is currently insufficient to be able to support the growing interest in applications such as magnetic shielding or electric motors. Fe-Co alloys provide the highest commercially available magnetic saturation and high magnetic permeability but have very poor ductility. The addition of vanadium to these alloys has allowed for significant commercialization and some ductility improvements, but the fundamental reasons for the observed improvements are not well understood. In most published work on mechanical properties in these alloys, the precise chemistry of the alloys investigated, often a critical aspect of intermetallics, is not reported or controlled and thermal history is unclear. This work creates a ductility model that is sensitive to changes in chemistry and can predict relative strain to failure as well as brittle fracture mode for intermetallics and is applied to Fe-Co alloys. Through the application of density functional theory (DFT), this model identifies defect stabilities, anti-phase boundary (APB) energies, the energy necessary to cross-slip, and cleavage energies and combines them through energetic competition to determine a relative failure strain. The model correctly predicts ductility improvements with the addition of vanadium as well as the transition from intergranular to transgranular cleavage, though more precise experiments are necessary to appropriately validate the various improvements observed.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Full-Strength Additive Manufacturing of Pure Aluminum Alloy 7075 Using Liquid Metal Jet Printing

Additive manufacturing (AM) of high-strength aluminum alloys can produce high-performance aerospace and biomedical parts with unique geometries and functions. However, many high-performance alloy systems such as AA 7075 pose challenges during AM due to thermal cycles that cause hot cracking, limiting AM to casting-friendly alloys. Stock AA 7075 alloy poses additional challenges due to the excessive growth of insoluble precipitates during casting and needs to be thermomechanically processed to obtain acceptable mechanical properties. Here, this work uses liquid metal jet printing to produce high-resolution pure AA 7075 parts at high strength (593 MPa) and ductility (10 pct), surpassing the specific strength of any type of steel. The parts are printed by depositing fine droplets on a high-temperature substrate that reduces cooling rates, preventing hot cracking. The results show that fully dense, crack-free parts can only be printed at substrate temperatures of 500 °C with a cooling rate threshold of < 2000 K/s to prevent cracking and < 500 K/s to eliminate porosity. The tensile strength, microhardness, and ductility of the parts matched or were beyond the wrought alloy specifications after heat treatment, possibly due to high solidification rates that produce finer insoluble precipitates. Our results indicate that many challenging high-performance alloy systems can now be 3D printed based on the critical thresholds for cooling rates using any AM method that can satisfy these conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A Low-Cost Lightweight Entropic Alloy with High Strength

In this study, a series of lightweight Al–Mg system entropic alloys containing Zn, Cu, and Si were designed based on the order/disorder or entropy, and eutectic concepts. The alloys of Al{sub 58.5}Mg{sub 31.5}Zn{sub 4.5}Cu{sub 4.5}Si{sub 1}; Al{sub 63}Mg{sub 27}Zn{sub 4.5}Cu{sub 4.5}Si{sub 1}; Al{sub 66.7}Mg{sub 23.3}Zn{sub 4.5}Cu{sub 4.5}Si{sub 1}; Al{sub 80}Mg{sub 14}Zn{sub 2.7}Cu{sub 2.7}Si{sub 0.6}; Al{sub 85}Mg{sub 10.5}Zn{sub 2.025}Cu{sub 2.025}Si{sub 0.45}; and Al{sub 90}Mg{sub 7}Zn{sub 1.35}Cu{sub 1.35}Si{sub 0.3} were prepared by induction melting under a high-purity argon atmosphere and then casted into stainless steel molds. The microstructures which were tested in the as-cast state exhibited multiphases and contained apparent volume fractions of intermetallic compounds and solid solutions. Then, the compressive mechanical properties of the alloys were measured, and high fracture strengths of 577, 677, 590, 498, 814, and 794 MPa, respectively, were determined. Strong hardening phenomena were observed in the Al{sub 80}Mg{sub 14}Zn{sub 2.7}Cu{sub 2.7}Si{sub 0.6}, Al{sub 85}Mg{sub 10.5}Zn{sub 2.025}Cu{sub 2.025}Si{sub 0.45}, and Al{sub 90}Mg{sub 7}Zn{sub 1.35}Cu{sub 1.35}Si{sub 0.3} alloys at room temperature, with amazing plasticity percentages of 13.8, 24.8, and 32.7%, respectively. The property differences in the lightweight alloys were analyzed using the following parameters: the critical values of the enthalpy of mixing; atomic size differences; ratio of entropy to enthalpy; valence electron concentration (VEC); and Pauling electronegativity difference. Finally, three of the aforementioned parameters (atomic size difference, enthalpy of mixing, and Pauling electronegativity difference) were regarded in this study as the crucial rules for the lightweight multicomponent alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructural Evolution and Properties of 24CrNiMoY Alloy Steel Fabricated by Selective Laser Melting

24CrNiMoY alloy steel samples were fabricated by selective laser melting (SLM), and a phase transformation model was established to study the alloy steel microstructural evolution. Meanwhile, microhardness and tensile properties of 24CrNiMoY alloy steel prepared by different laser energy densities (E{sub v}) were investigated. Results indicate that the microstructural evolution of 24CrNiMoY alloy steel is consistent with the phase transformation model. The main microstructure changed from martensite to bainite with the increase in thermal cycle numbers. In addition, a suitable E{sub v} plays an important role in refining the bainite structure and improving the alloy steel properties. When the E{sub v} decreases from 210 to 140 J/mm{sup 3}, the bainite lath width reduces from 1.7 to 0.6 μm. Simultaneously, the relative density, tensile strength and microhardness of the fabricated samples increase first and decrease later. 24CrNiMoY alloy steel sample prepared by 160 J/mm{sup 3} has fine mechanical properties: The tensile strength is 850 MPa and microhardness is 360 HV{sub 0.2}.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Aqueous Corrosion Behavior of Cast CoCrFeMnNi Alloy

CoCrFeMnNi high-entropy alloy demonstrates good mechanical properties and can be further strengthened by thermomechanical processes. A sample alloy prepared by vacuum casting in a graphite mold was further investigated for its corrosion resistance. One-millimeter thin strips of cast alloy were used to investigate its corrosion behavior at room temperature. Linear potentiodynamic tests of the alloy in sodium chloride solution confirmed uniform corrosion property comparable to stainless steel AISI 304. However, a positive cyclic hysteresis loop indicated susceptibility to pitting corrosion. In sodium hydroxide solution, metastable pit formation was observed, while sulfuric acid solution passivated the alloy. In both cases, the alloy did not show any spontaneous pitting tendency.

36 MATERIALS SCIENCE↗