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

Structure of the Fe-Mn-Si alloys submitted to γ ↔ ε thermocycling

Highlights: • The effect of thermal cycling Fe–Mn–Si alloys is studied. • In situ neutron diffraction, internal friction, DSC and TEM results are analysed. • Lattice defects accumulation both in martensitic and austenitic phases. • Microstrains in the γ and ε phases are estimated. - Abstract: The accumulation of defects and internal stresses and related changes of the direct and reverse martensitic transition (MT) on thermocycling over the phase transition in different Fe–Mn–Si alloys are investigated by in situ neutron diffraction, TEM, SEM-EBSD, DSC and mechanical spectroscopy. The temperature of direct MT in Fe-22Mn-3Si, Fe-26Mn-4Si and Fe-26Mn-7Si alloys decreases during thermal cycling, while that of the inverse MT increases, which causes a widening of the transformation hysteresis. Transmission electron microscopy shows that a large number of dislocations and stacking faults are formed in both martensitic and austenitic phases in the Fe-22Mn-3Si annealed alloy submitted to γ ↔ ε thermocycling. From in situ neutron diffraction tests on Fe-22Mn-3Si annealed alloy and classical Williamson-Hall analysis, we found that the microstrains in the ε-martensite after 1 and 12 thermocycles are roughly twice higher as compared with the austenite phase.

36 MATERIALS SCIENCE↗
NASA NTRS2001

Low Temperature Growth of Nanostructured Diamond Films on Metals

The field of nanocrystalline diamond and tetrahedral amorphous carbon films has been the focus of intense experimental activity in the last few years for applications in field emission display devices, optical windows, and tribological coatings, The choice of substrate used in most studies has typically been silicon. For metals, however, the thermal expansion mismatch between the diamond film and substrate gives rise to thermal stress that often results in delamination of the film. To avoid this problem in conventional CVD deposition low substrate temperatures (less than 700 C) have been used, often with the incorporation of oxygen or carbon monoxide to the feedgas mixture. Conventionally grown CVD diamond films are also rough and would require post-deposition polishing for most applications. Therefore, there is an obvious need to develop techniques for deposition of well-adhered, smooth nano-structured diamond films on metals for various tribological applications. In our work, nanostructured diamond films are grown on a titanium alloy substrate using a two-step deposition process. The first step is performed at elevated temperature (820 C) for 30 minutes using a H2/CH4/N2 gas mixture in order to grow a thin (approx. 600 nm) nanostructured diamond layer and improve film adhesion. The remainder of the deposition involves growth at low temperature (less than 600 C) in a H2/CH4/O2 gas mixture. Laser reflectance Interferometry (LRI) pattern during growth of a nanostructured diamond film on Ti-6Al-4V alloy. The first 30 minutes are at a high temperature of 820 C and the rest of the film is grown at a low temperature of 580 T. The fringe pattern is observed till the very end due to extremely low surface roughness of 40 nm. The continuation of the smooth nanostructured diamond film growth during low temperature deposition is confirmed by in-situ laser reflectance interferometry and by post-deposition micro-Raman spectroscopy and surface profilometry. Similar experiments performed without the starting nanostructured diamond layer resulted in poorly adhered films with a more crystalline appearance and a higher surface roughness. This low temperature deposition of nanostructured diamond films on metals offers advantages in cases where high residual thermal stress leads to delamination at high temperatures.

Baker, Paul A.↗
DOE OSTI2016

Accretion and orbital inspiral in gas-assisted supermassive black hole binary mergers

Many galaxies are expected to harbor binary supermassive black holes (SMBHs) in their centers. Their interaction with the surrounding gas results in the accretion and exchange of angular momentum via tidal torques, facilitating binary inspiral. Here, we explore the non-trivial coupling between these two processes and analyze how the global properties of externally supplied circumbinary disks depend on the binary accretion rate. By formulating our results in terms of the angular momentum flux driven by internal stresses, we come up with a very simple classification of the possible global disk structures, which differ from the standard constant M-dot accretion disk solution. The suppression of accretion by the binary tides, leading to a significant mass accumulation in the inner disk, accelerates binary inspiral. We show that once the disk region strongly perturbed by the viscously transmitted tidal torque exceeds the binary semimajor axis, the binary can merge in less than its mass-doubling time due to accretion. Thus, unlike the inspirals driven by stellar scattering, the gas-assisted merger can occur even if the binary is embedded in a relatively low-mass disk (lower than its own mass). This is important for resolving the “last parsec” problem for SMBH binaries and understanding powerful gravitational wave sources in the universe. We argue that the enhancement of accretion by the binary found in some recent simulations cannot persist for a long time and should not affect the long-term orbital inspiral. We also review existing simulations of SMBH binary–disk coupling and propose a numerical setup which is particularly well suited to verifying our theoretical predictions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Characterising the polymorphic phase transformation at a localised point on a Cu6Sn5 grain

Highlights: • A single-targeted Cu{sub 6}Sn{sub 5} grain adjacent to Sn-rich sites and Cu{sub 3}Sn was investigated using HVTEM. • Phase transformation from hexagonal to monoclinic in a single-targeted Cu{sub 6}Sn{sub 5} grains. • A time-temperature transformation (TTT) diagram was constructed using the HVTEM results. - Abstract: Cu{sub 6}Sn{sub 5} is an important intermetallic for electric interconnects and anode material of Li-ion batteries, with a polymorphic transformation from hexagonal eta-Cu{sub 6}Sn{sub 5} at temperatures higher than 186 °C to monoclinic eta′-Cu{sub 6}Sn{sub 5} at lower temperatures. The polymorphic transformation of Cu{sub 6}Sn{sub 5} has the potential to generate internal stresses in soldered joints during both the soldering process and subsequent device operation involving thermal cycling. Here we developed a systematic method to characterise the polymorphic transformations at localised points in a single grain of Cu{sub 6}Sn{sub 5} that is in intimate contact with adjacent Sn rich and Cu{sub 3}Sn phases to establish a time–temperature transformation (TTT) diagram of the eta to eta′-Cu{sub 6}Sn{sub 5} transformation. The method involves in situ temperature-controlled high-voltage transmission electron microscopy (HVTEM) of a single-targeted Cu{sub 6}Sn{sub 5} grain with observation of diffraction patterns during multiple heating and cooling experiments on a 0.5 μm thick sample made by focused ion beam milling (FIB). The results are compared to previously reported powder X-ray diffraction data. It is concluded that there is good agreement between both techniques and the transformations observed in the powder are representative of those in individual grains present in polycrystalline structures. Based on the newly developed TTT, it was established that the high-temperature eta-Cu{sub 6}Sn{sub 5} could be stablised down to room temperature if the cooling rate exceeded 20 °C min{sup −1}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Generalized similarity for accretion/decretion disks

Decretion (or external) disks are gas disks freely expanding to large radii due to their internal stresses. They are expected to naturally arise in tidal disruption events, around Be stars, in mass-losing post-main-sequence binaries, as a result of supernova fallback, etc. Their evolution is theoretically understood in two regimes: when the central object does not exert torque on the disk (a standard assumption for conventional accretion disks) or when no mass inflow (or outflow) occurs at the disk center. However, many astrophysical objects—circumbinary disks, Be stars, neutron stars accreting in a propeller regime, etc.—feature non-zero torque simultaneously with the non-zero accretion (or ejection of mass) at the disk center. We provide a general description for the evolution of such disks (both linear and nonlinear) in the self-similar regime, to which the disk should asymptotically converge with time. We identify a similarity parameter λ, which is uniquely related to the degree, to which the central mass accretion is suppressed by the non-zero central torque. The known decretion disk solutions correspond to the two discrete values of λ, while our new solutions cover a continuum of its physically allowed values, corresponding to either accretion or mass ejection by the central object. A direct relationship between λ and central M-dot and torque is also established. We describe the time evolution of the various disk characteristics for different λ, and show that the observable properties (spectrum and luminosity evolution) of the decretion disks, in general, are different from the standard accretion disks with no central torque.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Emergent SO( 3 ) Symmetry of the Frictionless Shear Jamming Transition

Here, we study the shear jamming of athermal frictionless soft spheres, and find that in the thermodynamic limit, a shear-jammed state exists with different elastic properties from the isotropically-jammed state. For example, shear-jammed states can have a non-zero residual shear stress in the thermodynamic limit that arises from long-range stress-stress correlations. As a result, the ratio of the shear and bulk moduli, which in isotropically-jammed systems vanishes as the jamming transition is approached from above, instead approaches a constant. In contrast to these striking differences, we argue that in a deeper sense, the shear jamming and isotropic jamming transitions actually have the same symmetry, and that the differences can be fully understood by rotating the six-dimensional basis of the elastic modulus tensor.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Modeling of Powder Bed Manufacturing Defects

Powder bed additive manufacturing offers unmatched capabilities. The deposition resolution achieved is extremely high enabling the production of innovative functional products and materials. Achieving the desired final quality is, however, hampered by many potential defects that have to be managed in due course of the manufacturing process. Defects observed in products manufactured via powder bed fusion have been studied experimentally. In this effort we have relied on experiments reported in the literature and—when experimental data were not sufficient—we have performed additional experiments providing an extended foundation for defect analysis. There is large interest in reducing the effort and cost of additive manufacturing process qualification and certification using integrated computational material engineering. A prerequisite is, however, that numerical methods can indeed capture defects. A multiscale multiphysics platform is developed and applied to predict and explain the origin of several defects that have been observed experimentally during laser-based powder bed fusion processes. The models utilized are briefly introduced. The ability of the models to capture the observed defects is verified. The root cause of the defects is explained by analyzing the numerical results thus confirming the ability of numerical methods to provide a foundation for rapid process qualification.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A Study on Structural, Corrosion, and Sensitization Behavior of Ultrafine and Coarse Grain 316 Stainless Steel Processed by Multiaxial Forging and Heat Treatment

Cubic specimens from AISI 316 stainless steel were multiaxially forged to 15 passes and annealed at 1200 °C for 1, 2, and 3 h and finally sensitized at 700 °C for 24 h. Examination of samples indicated that the hardness of the annealed samples was reduced from 153 to 110, 81, and 74 HV for as-received sample and under 1, 2, and 3 h of annealing, and increased from 245 to 288 HV for samples forged at 3 and 7 passes. However, no significant changes were observed in a large number of passes and at about 300 HV. Degree of sensitization of samples was increased to approximately 27.3% at 3-h annealing but reduced to 1.23% by 15 passes of MF. The potentiodynamic polarization test shows that the breakdown potentials decreased with annealing time from 0.6 to − 102 (mV/SCE) for as-received and 3-h annealed specimen. These potentials increased to approximately − 16.5 mV with the increase in MF passes to 15. These observations indicated that the chromium carbide deposition affects Cr-depleted zone, which can subsequently affect the degree of sensitization and pitting corrosion resistance of AISI 316 austenitic stainless steel.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Enhancement of Hot Corrosion Resistance of Modified 9Cr-1Mo Steel Through Surface Nanostructuring and Pre-oxidation

Nanosize grains of ~ 65 nm were developed with large misorientation angles of grain boundaries in surface region of normalized and tempered modified 9Cr-1Mo steel by ultrasonic shot peening (USSP) with hard steel balls of 3 mm diameter. The surface roughness and the depth of nanostructured surface region were found to increase with the duration of USSP from 0.5 to 5 min. Hot corrosion tests were performed for the Un-USSP and the samples USSPed for the durations of 0.25 and 1.5 min and also for the samples subjected to USSP and subsequently pre-oxidation at 600 °C for 4 h. The hot corrosion resistance at 600 °C under 100% NaCl coating was significantly improved by USSP for 1.5 min and subsequent pre-oxidation at 600 °C for 4 h. The results are discussed in terms of the formation of more uniform and effective protective iron–chromium oxide layer on the USSPed surface from the subsequent pre-oxidation treatment, prior to hot corrosion of the specimens under 100% NaCl coating at 600 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Joining of C{sub f}/SiBCN Composite with a Ni-Cr-Pd(Si,B) Filler Alloy

Joining of C{sub f}/SiBCN composite was conducted by a newly designed Ni-Cr-Pd(Si,B) system filler alloy. The dynamic wettability on the C{sub f}/SiBCN composite was measured by the sessile drop method. The joint microstructure and the fracture surface were analyzed by scanning electron microscope and x-ray diffraction spectrometer. The results showed that the Ni-Cr-Pd(Si,B) filler alloy exhibited a low contact angle of 11.6° on the composite after holding at 1170 °C for 30 min. Element Cr played an active role in the interfacial reaction, and a Cr{sub 7}C{sub 3} reaction layer with a thickness of 8 µm was formed at the surface of the C{sub f}/SiBCN composite. Under the brazing condition of 1170 °C for 10 min, the joint strength at the room temperature reached 66.8 MPa. When the test temperature increased to 600 and 800 °C, the joint strength was even elevated to 90.3 and 81.2 MPa, respectively. The joint strength at high temperature was higher than that at room temperature, and this should be attributed to the improvement of the joint plasticity and the release of the residual thermal stresses within the brazed joint.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Dilation Behavior of Thermal Spray Coatings

The thermal expansion of a thermally sprayed coating material is an important design-relevant property from both processing and application service points of view. High thermal stresses and residual strains can arise during deposition, and thermal exposure in service. The linkage to process conditions which can impact both physical and chemical attributes of the microstructure and consequently its expansion behavior is of great importance. The observed dilation response of various coatings is discussed based on the processing effects that induce differences in microstructural features and the impact of residual strain and annealing of defects on the dilation behavior during first thermal exposure. The coefficient of thermal expansion (CTE) of coatings was found to approach the bulk when the material is not sensitive to oxidation in-flight. For those materials sensitive to oxidation, mathematical models used to describe effective properties of composite materials were adopted to describe the CTE property. A large irreversible dilation change was found in the first heating cycle due to defect annealing (referred to as thermal shakedown). Several reasons are hypothesized for the magnitude of such dilation change. Finally, together, the paper presents a comprehensive analysis of thermal expansion behavior of a diversity of thermal spray processed materials.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Phase-field modeling of thermally-grown oxide and damage evolution in environmental barrier coatings

Silicon carbide-based ceramic matrix composites protected by environmental barrier coatings (EBCs) present a promising materials solution for next-generation gas turbines. Developing more robust and efficient EBCs is therefore of significant technological importance. During the service in high-temperature oxidative environments, there is a thermally grown oxide (TGO) layer, spontaneously formed in the EBC system. TGO is recognized as a critical factor for the degradation and failure of EBCs, yet the detailed mechanisms of TGO growth and its effect on EBC failure remain unclear. In this study we develop a comprehensive chemo-mechano-phase-field model to simulate growth of the TGO in EBCs, factoring in creep and deformation, and especially the cracking behaviors. The volume expansion due to TGO growth and the resulting large inelastic deformation are addressed by using our recently developed, so-called incremental realization of inelastic deformation (IRID) algorithm, in combination with an adapted Hu-Chen spectral solver for elasticity. Simulations of TGO growth are performed considering different growth modes of TGOs determined mainly by the ratio of oxidant permeability in the topcoat to that in the TGO itself. Large-scale three-dimensional (3D) simulations are performed to model the formation of interconnecting vertical/channel cracks (often called ‘mud cracks’). The simulated crack morphology are in excellent agreement with the experimental observations from the literature. The simulations also provide insights into the cracking of EBCs and its dependence on the structure and constituent properties of the coating system. Furthermore, these results demonstrate the developed damage model can be a useful tool for design of more durable EBCs.

Cracking↗
DOE OSTI2020

Crack nucleation and growth during dynamic indentation of chemically-strengthened glass

Dynamic point impact loading is a primary cause of fracture of glass screens on mobile devices. An improved understanding of crack initiation and evolution under high-speed indentation could contribute to the development of materials with better performance, but experimental observations are challenging due to the short timescales and limited depth-of-field of optical microscopy. To address this need, we have observed fracture of a chemically-strengthened glass during dynamic indentation using in situ x-ray phase-contrast imaging (XPCI). Median cracks initiate below the surface of the glass, at a depth approximately corresponding to the depth at which the surface residual compressive stress diminishes to zero. These cracks initially propagate at ~10ms –1 , rapidly accelerating to > 100ms –1 . We also observe some evidence for rate-dependent behavior, in that indentation at the lowest rates studied here (below about ~0.15ms –1 ) fails to initiate cracks regardless of the depth of the indentation (up to 18μm), while indentation at higher rates produces median cracks that either arrest or cause complete fracture, depending on the depth of indentation.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Learning interpretable surface elasticity properties from bulk properties via neural network equation learners

Surface elasticity is central to understanding the mechanics and stability of surfaces and interfaces. It is characterized by quantities such as surface tension, residual surface stress, and surface stiffness. However their analytical expressions are typically difficult to derive from atomistic data, and depend strongly on modeling choices. This work presents a neural network-based equation learner which combines customized activation functions and connection-based pruning to discover parsimonious, closed-form equations for surface elasticity from atomistic simulations. Applying the method to seven face-centered cubic (FCC) metals, our equation learner uncovers interpretable equations that describe both low-Miller index and high-Miller index surface properties, capturing long-tail property distributions accurately. The discovered expressions are decoupled into two components: a universal, geometry-driven orientation function, and material-specific baseline coefficients. We find that lower-order properties such as surface tension are fundamentally geometry dependent, while higher-order properties such as surface stress and elasticity show more complex geometry and material dependence. We also relate material dependent coefficients to bulk properties, forming a clear map from bulk material properties to surface elasticity. Overall, this approach demonstrates that interpretable neurosymbolic machine learning can bridge the gap between atomistic simulations and physical laws, enabling the discovery of generalizable structure–property relationships for materials science phenomena such as surface elasticity.

Equation learning↗
DOE OSTI2020

Spark plasma sintering (SPS) densified U 3 Si 2 pellets: Microstructure control and enhanced mechanical and oxidation properties

Dense U 3 Si 2 pellets with controlled grain structure and enhanced thermal-mechanical and oxidation properties are synthesized with spark plasma sintering (SPS). Microstructure and phase composition of the SPS densified pellets are characterized systematically using SEM, EDS, and XRD. Thermal-mechanical properties and oxidation behavior of the sintered silicide fuel pellets are analyzed by laser flash, indentation, and dynamic thermogravimetric analysis. Dense U 3 Si 2 pellets are consolidated by combining high energy ball milling and rapid sintering by SPS, and the microstructure structures are controlled from micron-sized (~5.7 μm grain size) for conventional silicide to a nanocrystalline matrix with an average grain size of ~280 nm. A dominant phase of distorted U 3 Si 2 was identified with lattice expansion due to residual thermal stress upon SPS consolidation and rapid cooling processes. Both micron-sized and nano-sized pellets show exceptional thermal transport properties, consistent with monolithic silicides reported in literature. The SPS-densified pellets possess simultaneously high hardness and fracture toughness. The SPS-densified silicide pellets also demonstrate exceptional oxidation performance with extended onset oxidation temperature above 500 °C and reduced oxidation kinetics, particularly for nano-sized pellets. A strong strain effect was proposed in which compressive stress in nano-sized pellets enhances the oxidation resistance of silicide fuels, as evidenced by the degradation of oxidation performance upon strain relaxation by isothermal annealing. The correlation among the sintering process – microstructure control – physical properties and fuel behavior is established. A new concept of strain engineering is proposed further properties optimization, enabling the development of potential oxidation and corrosion-resistant silicides with extended performance, the key technological challenge of U 3 Si 2 as the leading concept of accident tolerant fuels.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Intense cyclic heating effects on thermo-fracture and thermal shock of solid tungsten and open-cell tungsten foam

Here we investigate here the effects of transient (cyclic) arc-jet plasma and laser heating on fracture behavior of W-foam and solid tungsten. The two key parameters that control the foam thermomechanical response are its density and mean cell size expressed in Pores Per Inch (PPI). Tungsten foam samples were fabricated with Chemical Vapor Deposition (CVD) with variety of PPI and relative density. These were tested under two types of qualitatively different conditions: (1) high-enthalpy arc-jet, and (2) high-power cyclic laser heating. None of the foam samples showed macroscopic through-thickness cracks. However, distributed micro-cracks were observed on ligaments and their triple junctions. Under the same loading conditions, W-foam and solid tungsten showed similar crack network pattern and characteristic length-scale. However, Crack Opening Displacement (COD) was twice as large in solid W as compared to W-foam. Foam samples that have been previously exposed to a low-pressure helium plasma showed significant changes in their surface forming nano-texture fuzz which was removed by subsequent testing in the arc-jet. Extensive fracture and re-crystallization were observed in the thin solid W disk that was fully-constrained from expansion. Thicker and fully-constrained solid W disks did not display recrystallization, grain growth, and extensive cracking. However, thicker disks that were free to expand showed some recrystallization and extensive through-thickness cracks due to less effective cooling and thus higher temperatures. Laser beam testing showed no visible damage formation at 0.19 GW/m 2 and 0.38 GW/m 2 for both low-density (23%) and high-density (43%) foams at low pulses (100-1000). Micro-cracks were observed after 10,000 pulses at 0.19 GW/m 2 in both foams, and in low-density foam after 100,000 at 0.38 GW/m 2 . The nature of thermomechanical damage in W-foam exposed to extreme power (GW/m 2 ) short-duration laser pulses was found to be qualitatively similar to that of high power (MW/m 2 ) long-duration arc-jet.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Limited-constraint WAAM fixture for hybrid manufacturing

This paper describes a limited-constraint alternative to the traditional over-constrained build plate clamping in metal additive manufacturing. A fixture design is presented that enables build plate thermal growth within its plane while restricting deformation perpendicular to that plane. A prototype fixture is designed, fabricated, and tested for wire arc additive manufacturing (WAAM). Experimental results are presented for depositing 1.2 mm diameter 5356 aluminum wire on a 5052 aluminum build plate. The build plate deformation is measured for various clamping torques and a 77.3% reduction in distortion is achieved using the limited-constraint approach with identical WAAM parameters.

36 MATERIALS SCIENCE↗