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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Microstructural evolution of an aluminide coating on alloy 625 during wet air exposure at 900 °C and 1000 °C

The microstructural changes of the aluminized alloy 625 during cyclic oxidation in air + 6% H 2 O at 900 °C and 1000 °C were analyzed using optical metallography (OM), scanning electron microscopy (SEM) with energy and wave length dispersive X-ray analysis (EDX/WDX) as well as electron backscatter diffraction (EBSD). An in-house developed thermodynamic-kinetic procedure was employed to predict the microstructural evolution of aluminized alloy 625 during high temperature exposure by considering simultaneously occurring surface oxidation and interdiffusion processes. Because of the lack of mobility data for the relevant alloying elements in the σ-phase, assumptions for the mobilities were made based on the value of the mobilities in α-Cr. Despite these assumptions, the calculated results were found to be in good agreement with experimental observations. The complete depletion of β-NiAl in the coating observed during exposure at 1000 °C was correctly predicted by the model. The model was also able to infer dissolution of the precipitate phases α-Cr and σ in the interdiffusion zone during exposures at 900 °C and 1000 °C. The model was yet unable to predict the formation of the μ-phase in the alloy after 1000 h of exposure at 1000 °C. The developed modelling method offers the potential to predict microstructural changes of aluminized nickel base alloys thus reducing cost and time consuming experimental efforts.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Relationship between microstructure evolution and properties enhancement of carbon nanotubes‐filled polybutylene terephthalate/polypropylene blends induced by thermal annealing

Abstract Thermal annealing induced microstructure evolution and properties enhancement of polybutylene terephthalate/polypropylene/carbon nanotubes (PBT/PP/CNTs) nanocomposites was methodically investigated. The electrical conductive PBT/PP/CNTs nanocomposites with a low percolation threshold were obtained by simple melt blending due to the selective location of the CNTs in PBT phase and the formation of the double percolation structure. Thermal annealing further remarkably decreased the volume resistivity of the nanocomposites, with the maximum reduction of seven orders of magnitude. Scanning electron microscope observation and rheology tests demonstrated that the relatively large CNTs agglomerates were redistributed to build more homogeneous CNTs networks after annealing. The phase coalescence during annealing promoted the formation of the more perfect co‐continuous structure. These phenomena helped to enhance the electrical conductivity of the nanocomposites. Flexural properties of the nanocomposites were also enhanced after annealing, which might be due to the more effective stress transfer between the polymer and CNTs induced by the reconstructed CNTs networks. The nanocomposites obtained in this study possess excellent electrical and mechanical properties, which can be applied to the fields of sensors, electromagnetic interference shielding, anti‐static materials, and so forth.

Shang, Mengyao↗
DOE OSTI2019

Influence of the Composition on the Solidification Path, Microstructure Evolution and Mechanical Properties of Al-Cu-Mg Alloys

The influence of the initial composition on the solidification path, microstructure evolution, thermophysical properties and mechanical properties of Al-Cu-Mg alloys was investigated. The solidification paths of the investigated alloys were determined by analyzing the solidification structure, and then the experimentally determined paths were compared with those from the calculated results. Three of the investigated alloys experienced typical ternary eutectic solidification, and the other four alloys experienced quasiperitectic reactions. Due to differences in the initial compositions, different eutectic morphologies were formed during the solidification process. The binary eutectic phases, (α-Al + Al{sub 2}Cu) and (α-Al + Al{sub 6}CuMg{sub 4}), and the (α-Al + Al{sub 6}CuMg{sub 4} + Al{sub 8}Mg{sub 5}) ternary eutectic phase tended to exhibit divorced growth, and the (α-Al + Al{sub 2}CuMg), (α-Al + Al{sub 6}CuMg{sub 4}) and (α-Al + Al{sub 2}Cu + Al{sub 2}CuMg) eutectic phases tended to exhibit coupled growth. The results indicated that the initial composition, solidification path, microstructure and morphology could greatly affect the melting enthalpy, microhardness and compressive strength of the investigated Al-Cu-Mg alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructural evolution of ion-irradiated sol–gel-derived thin films

In this paper, the effects of ion irradiation on the microstructural evolution of sol–gel-derived silica-based thin films were examined by combining the results from Fourier transform infrared, Raman, and X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and elastic recoil detection. Variations in the chemical composition, density, and structure of the constituent phases and interfaces were studied, and the results were used to propose a microstructural model for the irradiated films. It was discovered that the microstructure of the films after ion irradiation and decomposition of the starting organic materials consisted of isolated hydrogenated amorphous carbon clusters within an amorphous and carbon-incorporated silica network. A decrease in the bond angle of Si–O–Si bonds in amorphous silica network along with an increase in the concentration of carbon-rich SiO x C y tetrahedra were the major structural changes caused by ion irradiation. Finally, in addition, hydrogen release from free carbon clusters was observed with increasing ion energy and fluence.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructure evolution in the fusion zone of laser-welded Mg–Gd–Y–Zr alloy during solution and aging treatment

The microstructure evolution in the fusion zone of laser-welded Mg-Gd-Y-Zr alloy during solution and aging treatment is investigated. The morphology of the Mg{sub 24}(Gd,Y){sub 5} in the divorced eutectic at the grain boundary transforms from a continuous network to disconnected and fragmentized islands and then to spheroidal particles before complete dissolution during the solution treatment at 430 °C. During the subsequent aging treatment at 225 °C, the precipitation sequence in the fusion zone follows the order of supersaturated solid solution (SSSS) → βʺ(D0{sub 19}) → βʹ(cbco) → β{sub 1}(fcc) → β(fcc). High-density precipitates are present at the original grain boundaries of the fusion zone from the welded structure but there are less precipitates in the interior of the original grains. The grain growth during the solution treatment at 430 °C comprises the slowly increasing stage, rapidly increasing stage, and stable stage. The network-distributed Mg{sub 24}(Gd,Y){sub 5} impedes migration of the grain boundaries, restricts grain growth in the first slowly increasing stage, and segregation of zirconium near the grain boundaries also affects migration of the grain boundaries. - Highlights: •Different quantities of precipitates are present at different location of grain. •The network-distributed Mg{sub 24}(Gd,Y){sub 5} restricts grain growth. •Segregation of Zr affects migration of grain boundaries.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of annealing parameters on microstructural evolution of a typical nickel-based superalloy during annealing treatment

Highlights: • Adopting an annealing treatment after deformation can improve the homogeneity of microstructure. • Heterogeneous grains become homogeneous firstly, and then heterogeneous with increasing annealing time. • The optimum annealing parameters are the temperature of 980 °C and the time of 10 min. - Abstract: It is usually one of important goals to obtain the homogeneous microstructure for forgings by hot working, such as hot die forging. However, there is always a large inhomogeneous deformation in die forgings during hot deformation. It often results in an inhomogeneous microstructure of forging due to the different dynamic recrystallization (DRX) fractions and grain sizes in different strain regions. In this study, a method was proposed to improve the homogeneity of deformed microstructure by annealing treatment. The microstructural evolution of a typical nickel-based superalloy during annealing treatment has been investigated. The Optical Microscope (OM), Electron Backscatter Diffraction (EBSD) and Scanning Electron Microscope (SEM) technologies were applied to observe microstructures. It is found that adopting an annealing treatment after deformation can effectively improve the homogeneity of microstructure due to the static recrystallization. There is an incubation period which is not less than 5 min for the occurrence of static recrystallization. The initial heterogeneous grains become homogeneous firstly, and then heterogeneous again when the annealing time is increased from 5 to 30 min. The deformation accelerates the dissolution of delta phase in annealing treatment. The fraction of delta phase decreases at the early stage of annealing whether the temperature exceeds the solution temperature. Based on a comprehensive consideration of all factors, the optimum annealing parameters, which are the temperature of 980 °C and the time of 10 min, have been obtained.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Enhanced Surface Properties and Microstructure Evolution of Cr12MoV Using Ultrasonic Surface Rolling Process Combined with Deep Cryogenic Treatment

In this study, the enhanced surface properties and microstructure evolution of quenched and tempered cold work die steel Cr12MoV, which were induced by ultrasonic surface rolling process combined with deep cryogenic treatment (UDCT), were systematically investigated. The results indicated that UDCT had an advantage over conventional ultrasonic surface rolling process (USRP) in improving surface mechanical properties, including smaller surface roughness and smoother machined surface with less cracks and defects; higher surface microindentation hardness with the value of ~ 890 HV (increased by ~ 6.2% compared with USRP); and lower friction coefficient. Such enhancements in surface properties are mainly attributed to the martensitic transformation of retained austenite, the dispersion strengthening of small secondary carbides and the homogenized carbides distribution during UDCT.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Microstructural Evolution of Steel During Magnetic Field-Assisted Processing

Advancing magnetic field-assisted processing, as an energy-efficient method for tailoring steel microstructures, requires a thorough understanding of how the high magnetic field impacts microstructural evolution, particularly its effect on prior austenite grain structures. The current investigation of a near-eutectoid composition, Fe-C alloy, uses electron backscatter diffraction to examine the morphology and orientation of martensite and pearlite microstructures, and to reconstruct the parent austenite microstructures present during equivalent heating under varied magnetic field strengths (0-T, 2-T, 5-T, and 9-T). It was observed that the magnetic field has a negligible effect on martensite lath/block width, slightly decreases prior austenite grain size, and increases the fraction of austenite grains with annealing twins. Additionally, the magnetic field increases the phase fraction of proeutectoid ferrite but has a negligible effect on pearlite block size and the distribution of boundary misorientation angles. No preferred texture was induced by the magnetic field, regardless of the applied field direction, in the proeutectoid ferrite phase or the martensite and prior austenite microstructures. Furthermore, the observed results contradict previous literature, and the differences are discussed.

Magnetic Materials↗
DOE OSTI2020

Transient Microstructure Evolutions and Local Properties of Dual-Phase 980 MPa Grade Steel Via Friction Stir Spot Processing

Friction stir processing is a novel solid-state process to modify microstructures and their properties by intense, localized plastic deformation. However, little research has been reported for microstructure evolutions of advanced high-strength steels during the process. The present work focuses on the study of transient microstructure changes and local mechanical properties for friction stir spot processed dual-phase (DP) 980 MPa grade steel (DP980) under different peak temperatures. A pinless silicon nitride ceramic tool was used to produce relatively simple material deformation and flow near the tool. Friction stir spot processed steel samples were characterized by optical and electron microscopies. Furthermore, Vickers microhardness and nano-indentation measurements were used to study local mechanical properties for correlation with microstructures. A swallow layer of refined grains (<0.6 µm) was obtained with a low peak temperature (under 400 °C), whereas higher peak temperatures (>Ac 1 ) led to a change in grain size with different microstructures (fine-grained DP or martensite). Electron back-scattered diffraction characterizations revealed a large deformation in the as-received microstructures (mixture of ferrite and tempered martensite) induced by friction stir spot processing, leading to recrystallization and grain refinement around the stirred zone. Also, nano-indentation measurements showed a higher hardness than the hardness of the as-received DP980. Friction stir processing with different process conditions effectively changed microstructures and local mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Microstructural evolution of compositionally complex solid-solution alloys under in-situ dual-beam irradiation

Here, this work attempts to link the microstructural evolution of single-phase compositionally complex alloy (CCA) compositions under dual-beam irradiation to their Mn-content via the stacking-fault energy (SFE) and vacancy migration energies. Two alloys, Cr 18 Fe 27 Mn 27 Ni 28 and Cr 15 Fe 35 Mn 15 Ni 35 , along with less compositionally complex pure Ni and Fe 56 Ni 44 binary were irradiated at 500 and 600 °C under dual-beam 1 MeV Kr 2+ and 16 keV He + heavy-ions up to 7 displacements per atom (dpa) with a He/dpa ratio of 0.75 %/dpa using in situ transmission electron microscopy (TEM). Due to the bubble-stabilizing effect of implanted He, bubbles were observed in all irradiations, and populations of faulted interstitial loops were characterized in Cr 18 Fe 27 Mn 27 Ni 28 and Cr 15 Fe 35 Mn 15 Ni 35 . A reduction in swelling was observed in the two CCAs compared to pure Ni and Fe 56 Ni 44 . Although swelling increased from 500 to 600 °C in Fe 56 Ni 44 , Cr 15 Fe 35 Mn 15 Ni 35 and Cr 18 Fe 27 Mn 27 Ni 28 both swelled slightly more at 500 °C. This was attributed to the difference in vacancy mobility, stronger pinning effect of vacancies on He, and the sink strength of faulted dislocation loops. Faulted interstitial loops nucleated with a higher number density and dislocation line density in Cr 15 Fe 35 Mn 15 Ni 35 at both temperatures, and at 600 °C in both materials. The differences in faulted loop population and temperature effect on swelling are correlated to the Mn-content and the measured SFE (20.2 ± 6.7 mJ/m 2 for Cr 18 Fe 27 Mn 27 Ni 28 and 9.2 ± 3.4 mJ/m 2 for Cr 15 Fe 35 Mn 15 Ni 35 ).

36 MATERIALS SCIENCE↗
DOE OSTI2019

Multiscale Modeling of Microstructural Evolution in Fused-Coating Additive Manufacturing

A quantitative phase field model with adaptive-computational-domain-expansion (ACDE) method was developed to simulate the microstructural evolution in fused-coating additive manufacturing process. A heat transfer and fluid flow model was established to investigate the solidification behavior of the molten pool in a single deposited layer. The solidification parameters such as temperature gradient and solidification rate were calculated from the macroscopic model and then fed into the phase field model to get the microstructure at different positions within a single deposited layer. Columnar dendrites were formed at the bottom of the deposited layer, and equiaxed dendrites appeared in the upper part of the layer. The ACDE method is used to improve the simulation efficiency. The metallographic experiments and energy-dispersive spectrometer analysis for composition were conducted on the fused-coating specimen, and the results were in good accordance with the simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructural evolution and mechanical properties of maraging steel produced by wire + arc additive manufacture process

Highlights: • Wire + arc additive manufacture (WAAM) was applied to produce maraging steel structures. • Microstructure evolution of WAAM alloy was characterized and correlated with the layer-by-layer building manner. • Mechanical properties of WAAM alloy were tested and correlated with the aging response of WAAM microstructure. • Retained austenite content was measured and correlated with the tensile strength loss. - Abstract: Wire + arc additive manufacture is developed for producing large-scale metallic components. In this paper, maraging steel parts were produced, and the microstructure and mechanical properties were investigated. The microhardness and tensile strength of the as deposited alloy reduced from the bottom to the top due to the transient thermal cycling, which resulted in partial aging and non-uniform formation of intermetallic compounds along the building direction. Solutionizing, followed by 3 h aging, significantly reduced the microstructural heterogeneity and increased the mechanical properties by 24.7% through the formation of large amounts of finely distributed precipitates. The as deposited alloy possessed superior strength to the wrought alloy in solutionized condition but inferior to the later in aged condition, which was attributed to the less pronounced aging response of the low-angle columnar grains characterized microstructure and the presence of retained and reverted austenite.

36 MATERIALS SCIENCE↗
DOE OSTI2020

In situ microstructural evolution in face-centered and body-centered cubic complex concentrated solid-solution alloys under heavy ion irradiation

This study characterizes the microstructural evolution of single-phase complex concentrated solid-solution alloy (CSA) compositions under heavy ion irradiation with the goal of evaluating mechanisms for CSA radiation tolerance in advanced fission systems. Three such alloys, Cr 18 Fe 27 Mn 27 Ni 28 , Cr 15 Fe 35 Mn 15 Ni 35 , and equimolar NbTaTiV, along with reference materials (pure Ni and E90 for the Cr-FeMnNi family and pure V for NbTaTiV) were irradiated at 50 K and 773 K with 1 MeV Kr ++ ions to various levels of displacements per atom (dpa) using in-situ transmission electron microscopy. Cryogenic irradiation resulted in small defect clusters and faulted dislocation loops as large as 12 nm in face-centered cubic (FCC) CSAs. With thermal diffusion suppressed at cryogenic temperatures, defect densities were lower in all CSAs than in their less compositionally complex reference materials indicating that point defect production is reduced during the displacement cascade stage. High temperature irradiation of the two FCC CSA resulted in the formation of interstitial dislocation loops which by 2 dpa grew to an average size of 27 nm in Cr 18 Fe 27 Mn 27 Ni 28 and 10 nm in Cr 15 Fe 35 Mn 15 Ni 35 . This difference in loop growth kinetics was attributed to the difference in Mn-content due to its effect on the nucleation rate by increasing vacancy mobility or reducing the stacking-fault energy.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Through-thickness microstructural evolution during grain boundary engineering type thermomechanical processing and its implication on sensitization behavior in austenitic stainless steel

Highlights: • GBE-treated 304L steel exhibits thickness-dependent microstructural evolution. • Degree of sensitization increases from surface to interior in GBE specimen. • Low fractal dimension and large TRD size lead to lesser sensitization at surface. • As-received specimen shows high albeit thickness-independent sensitization behavior. • Higher residual strain leads to higher sensitization at surface of deformed sample. - Abstract: Through-thickness microstructural variations and their synergistic influence on degree of sensitization in grain boundary engineering (GBE) treated 304L stainless steel was compared with solution-annealed and marginally cold-worked (~ 3% thickness reduction) specimens. The solution-annealed specimen has shown higher degree of sensitization due to the presence of lower fraction of Σ3 (as well as other low Σ CSL) boundaries and dominant random high-angle boundaries (RHAGBs) connectivity. However, it has shown thickness-independent sensitization behavior as grain boundary character distribution, residual strain and grain boundary network topology towards thickness direction are similar. Owing to higher residual strain, as revealed by local misorientation analysis, the cold-worked specimen showed maximum degree of sensitization (higher than solution-annealed condition) at surface. The GBE specimen has shown much lower degree of sensitization at surface due to higher-fraction of Σ3 boundaries and discontinuous RHAGBs connectivity. However, increasing fraction of RHAGBs and their connectivity, as confirmed by twinning-related domain and fractal analysis, cause more sensitization at interior of this specimen. In spite of this, the degree of sensitization at the interior of GBE specimen is lower than the solution-annealed or cold-worked specimen. Even though the residual strain in GBE specimen is higher than the solution-annealed condition, the lower degree of sensitization indicates that the grain boundary character distribution and grain boundary network topology have higher implication than residual strain in controlling sensitization.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Accelerate microstructure evolution simulation using graph neural networks with adaptive spatiotemporal resolution

Abstract Surrogate models driven by sizeable datasets and scientific machine-learning methods have emerged as an attractive microstructure simulation tool with the potential to deliver predictive microstructure evolution dynamics with huge savings in computational costs. Taking 2D and 3D grain growth simulations as an example, we present a completely overhauled computational framework based on graph neural networks with not only excellent agreement to both the ground truth phase-field methods and theoretical predictions, but enhanced accuracy and efficiency compared to previous works based on convolutional neural networks. These improvements can be attributed to the graph representation, both improved predictive power and a more flexible data structure amenable to adaptive mesh refinement. As the simulated microstructures coarsen, our method can adaptively adopt remeshed grids and larger timesteps to achieve further speedup. The data-to-model pipeline with training procedures together with the source codes are provided.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Electron Backscattered Diffraction Study of Microstructural Evolution During Isothermal Deformation of High-N Mn18Cr18 Alloy

In order to understand the dynamic recrystallization (DRX) mechanism of Mn18Cr18N austenitic stainless steel during hot deformation, the investigation of microstructure evolution is significant. Isothermal compression tests were conducted on a Gleeble-3500 thermomechanical simulator in the temperature range of 950 °C to 1150 °C and strain rate range between 0.001 and 1 s{sup −1}. Optical microscopy (OM) and electron backscattered diffraction (EBSD) were employed to study the resultant microstructures. More grain boundary information of hot-deformed specimens was provided by EBSD. It was found that the fraction of low-angle grain boundaries (LAGBs) increases with the decrease of deformation temperature or the increase of strain rate and can be expressed as a function of the Zener–Hollomon parameter. DRX easily occurs at the high deformation temperature and low strain rate. From the flow stress curves, the occurrence of single peak behavior and multiple transient steady-state (MTSS) behavior illustrated the existence of different DRX mechanisms. The EBSD analysis indicated that both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) existed during hot deformation. For the low fraction of 10 to 15 deg misorientation, CDRX is the minor DRX mechanism while DDRX is the major DRX mechanism for the studied steel.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanical Properties, Microstructural Evolution, and the Effect of Friction on the Plastic Flow of the AISI 321 Austenitic Stainless Steel Tube During Cold Pilgering: An Experimental and Simulation Analysis

The mechanical properties, microstructural evolution, and the effect of friction on the plastic flow of the AISI 321 austenitic stainless steel (ASS) tube were investigated during the cold pilger process. The elastic–plastic behavior of as-received tube was simulated by the Johnson-cook model. The model parameters were obtained by the compression and tensile tests. The mechanical properties of the material were examined by the tensile and microhardness tests. Based on the ring compression test results, three different situations for friction conditions were considered. The stress and strain states at these conditions were examined for the outer and inner surfaces of the tube. Numerical evaluation of plastic shearing due to the cold pilgering of the AISI321 tube was performed. The X-ray diffraction, ferrite scope tests, and optical microscope were also used for the microstructure evaluation and verification of the simulation results, respectively. The Latham-Cockcraft damage was calculated for different conditions by introducing the new subroutine, showing that it was strongly dependent on the friction conditions and the shear strain, ε{sub zr}, respectively. It was shown that the friction coefficient of 0.3 resulted in the minimum damage of the tube and the different friction conditions between the two surfaces of the tube enhanced the damage function. Also, the strain induced ά-martensite increased the work-hardening capacity and affected the ductility of the tube.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Microstructural evolution and transmutation in tungsten under ion and neutron irradiation

This study aims to compare the effects of neutron and self-ion irradiation on the mechanical properties and microstructural evolution in W. Neutron irradiation at the HFR reactor to 1.67 dpa at 800 °C resulted in the formation of large Re and Os rich clusters and voids. The post-irradiation composition was measured using APT and verfified against FISPACT modelling. The measured Re and Os concentration was used to create alloys with equivalent concentrations of Re and Os. These alloys were exposed to self-ion irradiation to a peak dose of 1.7 dpa at 800 °C. APT showed that self-ion irradiation leads to the formation of small Os clusters, wheras under neutron irradiation large Re/Os clusters form. Voids are formed by both ion and neutron irradiation, but the voids formed by neutron irradiation are larger. By comparing the behaviour of W-1.4Re and W-1.4Re-0.1Os, suppression of Re cluster formation was observed. Irradiation hardening was measured using nanoindentation and was found to be 2.7 GPa, after neutron irradiation and 1.6 GPa and 0.6 GPa for the self-ion irradiated W-1.4Re and W-1.4Re-0.1Os. The higher hardening is attributed to the barrier strength of large voids and Re/Os clusters that are observed after neutron irradiation.

36 MATERIALS SCIENCE↗