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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 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Black hole entropy and soft hair

A set of infinitesimal Virasoro L Ⓧ Virasoro R diffeomorphisms are presented which act non-trivially on the horizon of a generic Kerr black hole with spin J. The covariant phase space formalism provides a formula for the Virasoro charges as surface integrals on the horizon. Integrability and associativity of the charge algebra are shown to require the inclusion of ‘Wald-Zoupas’ counterterms. A counterterm satisfying the known consistency requirement is constructed and yields central charges c L = c R = 12 J . Assuming the existence of a quantum Hilbert space on which these charges generate the symmetries, as well as the applicability of the Cardy formula, the central charges reproduce the macroscopic area-entropy law for generic Kerr black holes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Approximate Recovery and Relative Entropy I: General von Neumann Subalgebras

We prove the existence of a universal recovery channel that approximately recovers states on a von Neumann subalgebra when the change in relative entropy, with respect to a fixed reference state, is small. Our result is a generalization of previous results that applied to type-I von Neumann algebras by Junge at al. [arXiv:1509.07127]. We broadly follow their proof strategy but consider here arbitrary von Neumann algebras, where qualitatively new issues arise. Our results hinge on the construction of certain analytic vectors and computations/estimations of their Araki–Masuda L p norms. We comment on applications to the quantum null energy condition.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Research on Bulk-metallic Glasses and High-entropy Alloys in Peter K. Liaw’s Group and with His Colleagues

Bulk-metallic glasses (BMGs) and high-entropy alloys (HEAs) have attracted extensive attention in the field of metallic materials research for several decades due to their extraordinary properties. Many scientists and researchers have significantly contributed to developing new classes of metallic alloys, such as BMGs and HEAs, for various applications. Liaw’s group and his colleagues have focused on the fundamental understanding of unique features, structures, and properties in BMGs and HEAs as well as the development of new types of metallic materials. Here, we summarized the research work of Liaw’s group and his colleagues by reviewing relevant papers. The goal is to provide an understanding of the current research progression in BMGs and HEAs while further encouraging young and junior researchers to be involved in the field of structural materials research pertaining to these classes of exotic alloy systems.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Homogenization of Dendritic Structures and the High-Temperature Strength of the Refractory High-Entropy Alloy MoNbTaVW

MoNbTaVW, a pioneering refractory high-entropy alloy (RHEA), is renowned for its exceptional strength at elevated temperatures. Like most RHEAs, it solidifies into a dendritic microstructure with steep concentration gradients, necessitating heat treatment for equilibration. Steep concentration gradients typically complicate mechanical property analysis and modeling, key challenges for high-throughput alloy discovery. This study examines the effects of high-temperature homogenization (1800 °C for 8 hours) on the microstructure and mechanical properties of MoNbTaVW across temperatures up to 1200 °C. Postmortem microstructural analysis, coupled with chemical, crystallographic, and mechanical assessments, revealed that yield strength was surprisingly insensitive to homogenization, with less than a 3 pct difference between as-cast and heat-treated materials. Nanoindentation confirmed minimal nanohardness changes across the dendritic structure at room temperature. However, homogenization significantly enhanced high-temperature work hardening, producing higher peak compressive strength. Both as-cast and homogenized MoNbTaVW exhibited room-temperature strengths of 1400 MPa, exceeding previously reported values. These findings demonstrate that high-temperature treatment enables microstructural homogenization without compromising strength, a unique behavior among RHEAs. In conclusion, while not universally applicable, this insight highlights the importance of understanding microstructural development in experimental alloys and offers a pathway to design alloys that achieve homogeneous-like properties without heat treatment.

Rietema, C. J. [Lawrence Livermore National Labora↗
DOE OSTI2019

Improvement of Microstructure and Mechanical Properties of CoCrCuFeNi High-Entropy Alloys By V Addition

V element had positive effect in improving the strength of many alloys, so it was possible that V had potential to strengthen CoCrCuFeNi high-entropy alloys (HEAs) with face-centered cubic (FCC) crystal structure, which was relatively weak in strength and had outstanding ductility. In this paper, we studied the alloying effect of V on the phase evolution, microstructure and the mechanical properties of the (CoCrCuFeNi){sub 100−x}V{sub x} (x = 0-16, atomic ratio, hereafter in at.%) HEAs systematically. The results showed that V element had capacity to induce sigma phase precipitation. The volume fraction of sigma phase increased from 0 to 12%; the compressive yield stress of (CoCrCuFeNi){sub 100−x}V{sub x} HEAs increased from 300 to 613 MPa with V content increasing from 0 to 16% (atomic ratio, hereafter in at.%). However, the compression fracture strain decreased from 50 to 28%. V addition was beneficial in improving the strength of CoCrCuFeNi HEA, and the increase in sigma phase volume fraction was the key factor for the improvement of the (CoCrCuFeNi){sub 100−x}V{sub x} HEAs in yield stress.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Properties of AlCoCrFeNiTi High-Entropy Alloy Coating on AISI1045 Steel Fabricated by Laser Cladding

AISI1045 steel is widely used in mechanical engineering. In spite of the favorable toughness and strength, the surface properties of AISI1045 steel, such as hardness, wear resistance and corrosion resistance, are not ideal. Therefore, surface modification of AISI1045 is necessary, especially for the parts suffering severe work condition. Laser cladding is a promising surface modification technology. In this work, the AlCoCrFeNiTi high-entropy alloy (HEA) coatings were prepared on the AISI1045 steel to improve its surface properties. Metallurgical bonding is obtained between the coating and the substrate. The microstructure and surface properties of the coating were characterized by scanning electron microscope, energy-dispersive spectrometry, x-ray diffraction, electrochemical workstation, microhardness tester and pin-on-ring wear tester. The microstructure of the coating produced by laser cladding is dendritic. The AlCoCrFeNiTi HEA coating is mainly composed of disordered body-centered cubic phase (Fe-Cr), ordered B2 phase (AlNi) and intermetallic phase (Ti-rich). The coating shows excellent abrasion resistance and corrosion resistance in comparison with the substrate. The maximum microhardness of the coating reaches approximately 865 HV, which is 4.5 times of AISI1045 steel.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Properties of a Multiphase FeCrCuMnNi High-Entropy Alloy

A FeCrCuMnNi high-entropy alloy was produced using vacuum induction melting, starting from high-purity raw materials. The microstructure and mechanical properties of the as-cast FeCrCuMnNi alloy were studied, considering x-ray diffraction (XRD), scanning electron microscopy, and hardness and tensile tests. XRD results revealed the existence of two FCC phases and one BCC phase. Microstructural evaluation illustrated that the as-cast alloy has a typical cast dendritic structure, where dendrite regions (BCC) were enriched in Cr and Fe. Interdendritic regions were saturated with Cu and Ni and revealed G/B(T) {110} 〈111〉 and Brass {110} 〈112〉 as the major texture components. The produced alloy revealed an excellent compromise in mechanical properties due to the mixture of solid solution phases with different structures: 300 HV hardness, 950 MPa ultimate tensile strength and 14% elongation. Microhardness test results also revealed that the BCC phase was the hardest phase. The fracture surface evidenced a typical ductile failure. Furthermore, heat treatment results revealed that phase composition remained stable after annealing up to 650 °C. Phase transformation occurred at higher temperatures in order to form more stable phases; therefore, FCC2 phase grew at the expense of the BCC phase.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nanoindentation Creep Behavior of CoCrFeNiMn High-Entropy Alloy under Different High-Pressure Torsion Deformations

CoCrFeNiMn high-entropy alloy (HEA) has gained increasing attention in recent years. In this work, CoCrFeNiMn HEA was prepared by arc melting and casting method; and high-pressure torsion (HPT) technology was used to refine the grains. A series of nanoindentation experiments was used to investigate the effect of different loading rates and deformations on the nanoindentation creep behavior of the CoCrFeNiMn HEA. The results show that the grain size of the CoCrFeNiMn HEA refines to the nanoscale after HPT. However, grain refinement is not obvious with further increasing HPT rotation. The loading rate and HPT rotations have an obvious effect on the nanoindentation creep behaviors of the CoCrFeNiMn HEA. The creep behaviors of the as-cast and HPT-treated CoCrFeNiMn HEA may be controlled by dislocations and the collective effect of GB sliding and dislocations, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructural Transition of (CuFeMnNi){sub 1−x}Cr{sub x} (x = 0-0.25) High-Entropy Alloys

The influence of Cr on the microstructure and hardness of the cast and annealed (CuFeMnNi){sub 1−x}Cr{sub x} (x = 0, 0.05, 0.1, 0.15, 0.2 and 0.25) high-entropy alloys were investigated systematically. The results revealed that the microstructures of (CuFeMnNi){sub 1−x}Cr{sub x} (x = 0, 0.05 and 0.1) alloys were composed of (Fe, Ni and Cr)-rich FCC1 and (Cu, Mn)-rich FCC2 phases. No obvious change in the dendritic size was detected between cast and related annealed alloys, indicating the excellent thermal structure stability of the studied alloys. The microstructures of the cast (CuFeMnNi){sub 1−x}Cr{sub x} (x = 0.15, 0.2 and 0.25) alloys contained FCC1, FCC2 and (Cr, Fe)-rich BCC phases, while those of the annealed alloys were composed of FCC1, FCC2 and ρ phases. The significant transition from BCC phase to ρ phase was detected. Moreover, the hardness of both cast and annealed (CuFeMnNi){sub 1−x}Cr{sub x} alloys increased with the increasing Cr content. The hardness of annealed (CuFeMnNi){sub 1−x}Cr{sub x} (x = 0, 0.05 and 0.10) alloys reduced slightly as compared to corresponding cast alloys, whereas the hardness of the annealed (CuFeMnNi){sub 1−x}Cr{sub x} (x = 0.15, 0.2 and 0.25) alloys increased.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Oxidation Resistance of Ti-Al-Cr-Nb-Based High-Entropy Alloys in Air at 1073 K

The effects of prolonged exposure to air at 1073 K on Al20Cr25Nb20Ti20Zr15 and Al20Cr25Nb19Ti20Zr15Y1 (at.%) high-entropy alloys (HEAs) were investigated in this work. Combined scanning electron microscopy and energy-dispersive x-ray spectroscopy (SEM–EDS) analysis revealed that scales containing all major elements in the systems are formed during the oxidation process. Thermogravimetric analysis carried out under the above-mentioned corrosive conditions indicates an initial parabolic kinetics course in the case of the sample with 1 at.% Y addition. However, after a certain period of time, the correlation between mass gain per unit surface area and time becomes linear. In the other case, oxidation proceeds according to the parabolic rate law for the entire process duration. The addition of 1 at.% Y decreases the parabolic rate constants by 1 order of magnitude, thereby improving the chemical stability of the studied Al-Cr-Nb-Ti-Zr system. This is confirmed by visual evaluation, as well as SEM–EDS cross-sectional analysis. Additionally, x-ray diffraction studies indicate that a multiphase oxide scale is formed on the metallic core of both samples. This means that selective oxidation does not occur and all constituent elements took part in the reaction. Taking all of the above into account, it can be concluded that more research is required to fully understand and improve the corrosion resistance of Ti-Al-Cr-Nb-based HEAs.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Long Term Creep Behavior of a CoCrFeNiMn High Entropy Alloy

The potential of high entropy alloys (HEAs) to meet or exceed austenitic stainless-steel performance with the additional benefit of improved hot corrosion/oxidation resistance makes FCC HEAs attractive for use in energy applications. While shorter term creep tests have been reported in the literature on HEAs, not all methodologies utilize repeatable techniques. This manuscript reports on over 23,500 accumulated hours of tensile creep testing with adherence to ASTM standards on a melt solidified ingot of CoCrFeNiMn HEA converted to wrought plate using conventional thermo-mechanical processing techniques. The typical standard creep analyses are reported, i.e., Larson-Miller parameter, Monkman-Grant relationship, activation energy for creep, and creep stress exponents were calculated and compared to previously reported short-term creep tests. Additionally, characteristics of creep fracture and microstructural evolution are reported with cursory dislocation mechanisms investigated.

Rozman, Kyle A.↗
DOE OSTI2021

Thermodynamics and Kinetics of High-Entropy Alloys

Here, this editorial introduces a special issue of the Journal of Phase Equilibria and Diffusion that is comprised of 19 high-quality papers that report the state-of-the-art frontier research in various high entropy alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Insights into Defect-Mediated Nucleation of the Equilibrium B2 Phase in Face-Centered Cubic High-Entropy Alloys

A fine scale second phase distribution is important for strengthening metallic materials. High precipitate density is often difficult to achieve due to sluggish precipitation kinetics and limited nucleation sites. Based on the CALPHAD predictions, transition element-based complex concentrated alloys (CCAs)/ High Entropy Alloys (HEAs), have a highly stable face-centered cubic (FCC) phase and intermetallic B2 phase. The precipitation of the strengthening B2 phase is often limited to low fractions in these alloys due to a high nucleation barrier in the FCC matrix. Here, by illustrating in three different alloys, we present a defect mediated nucleation of a high fraction equilibrium B2 phase. The degree of cold work is used to manipulate the phase fraction, morphology, and distribution of B2 precipitates. Our approach synergistically improves the strength and ductility of these alloys and is widely applicable to many other complex alloys.

Sharma, Abhishek↗
DOE OSTI2024

Data-Driven Insights into the Structural Essence of Plasticity in High-Entropy Alloys

The heterogeneous mechanical response of a crystalline alloy with multiple principal elements was investigated using molecular dynamics simulations. The local configuration of the alloy in its quiescent state was characterized by the variables derived from the gyration tensor and the atomic electronegativity. A multivariate analysis identified the geometric and chemical factors that influenced the atomic packing variations. Further, upon straining, the non-affine displacement exhibited spatial heterogeneity. A statistical correlation was established between the local yield events and the specific features of the local configuration. Our findings, validated by the performance metrics analysis, provided a structural criterion for the instability mechanisms in high-entropy alloys (HEAs) and enhanced the understanding of their plasticity.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Adiabatic shear localization in the CrMnFeCoNi high-entropy alloy

The mechanical behavior of the single phase (fcc) CrMnFeCoNi high-entropy alloy (HEA) is examined in the dynamic regime. Here a series of experiments by dynamic-loading hat-shaped specimens using stopper rings to control the displacement are performed, and the alloy resists adiabatic shear-band formation up to a very large imposed shear strain of ~7. It is proposed that the combination of the excellent strain-hardening ability and moderate thermal-softening effect retard shear localization. Recrystallized ultrafine-grained grains (diameters of 100–300 nm) with twins are revealed inside the shear band. Their formation is explained by the rotational dynamic recrystallization mechanism. The stability of the structure at high strain rates strongly suggests a high ballistic resistance for this class of alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Deformation mechanisms and ductile fracture characteristics of a friction stir processed transformative high entropy alloy

Deformation mechanisms of a friction stir processed Fe 40 Mn 20 Co 20 Cr 15 Si 5 transformative high entropy alloy using three different process parameters were studied to explain the microstructural dependence of tensile response in these specimens. The relative strain hardening contribution due to transformation and twinning effects was different for the three different microstructural conditions and thus resulted in different magnitude of work hardening. Crystal plasticity simulations confirmed that synergistic activity of face centered cubic and hexagonal close packed slip and twin mechanisms resulted in sustained work hardening and enhanced uniform elongation. The non-uniform ductility and extent of void nucleation and growth in these specimens were limited. Nevertheless, fractography and X-ray microscopy of fractured tensile specimens verified that microstructural flexibility induced different propensities for void growth and ductile fracture mode. Thus, alloy design induced phase stability, adaptive phase evolution due to friction stir processing, extent and kinetics of martensitic transformation and related work hardening capability of the individual phases combined together to determine the overall tensile response. The crystallographic orientation dependence of deformation induced phase transformation was also thoroughly studied. Finally, this study included quantitative determination of the crystallographic orientation where resolved shear stress on leading and trailing partial dislocations favored separation of the partials to cause transformation at a critical value of applied stress.

36 MATERIALS SCIENCE↗