Search NASA⌕ Search

SEARCH · Search NASA

Results for “microstructure evolution”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Effects of Multiple Laser Peening impacts on Mechanical Properties and Microstructure Evolution of 40CrNiMo Steel

The effects of multiple laser peening impacts on mechanical properties and microstructure of 40CrNiMo steel were investigated. The surface residual stress and microhardness of the samples with different treatments were measured. The surface morphologies, tensile behavior and microstructure of the samples before and after laser peening were compared and analyzed. The experimental results showed that laser peening, especially multiple laser peening impacts has significant effects on the microhardness, residual stress and microstructure of 40CrNiMo steel. The surface roughness and the depth of the pit increased with increasing the impact times. After 3 laser peening impacts, the surface microhardness, compressive residual stress, the strength and ductility were significantly improved. Meanwhile, some microstructures, such as sub-grains, the precipitation of carbide and dislocations were also induced in the surface layer. Additionally, the microscopic strengthening mechanism of multiple laser peening impacts on the mechanical properties of 40CrNiMo steel was analyzed in detail.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructural evolution and thermal stability after aging of a cobalt-containing martensitic bearing steel

The microstructural changes and thermal stability of a cobalt-containing martensitic heat resistance bearing steel have been investigated in this paper. The hardness variation showed a progressive hardness decrease associated with coarsening of fine carbides at elevated temperatures. The precipitation of secondary phases during tempering at 500 °C for 10 h and 100 h has been characterized and identified in detail using transmission electron microscopy. The results revealed that the aging treatment induced very fine secondary M{sub 6}C precipitates which were responsible for the secondary hardening peak when tempered at 500 °C for 30 h. But the hardness gradually decreased due to the coarsening of M{sub 6}C carbide and other secondary phases (such as μ phase, σ phase, and χ phase) precipitation when the samples were tempered over 30 h at 500 °C. - Highlights: •Retained austenite fraction was reduced after cryogenic treatment. •Secondary hardening was responsible for M{sub 6}C precipitates. •TEM study to investigate different phases characteristics •Coarsening of carbides during aging has a significant effect on mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Probing Novel Microstructural Evolution Mechanisms in Aluminum Alloys Using 4D Nanoscale Characterization

Dispersions of nanoscale precipitates in metallic alloys have been known to play a key role in strengthening, by increasing their strain hardenability and providing resistance to deformation. Although these phenomena have been extensively investigated in the last century, the traditional approaches employed in the past have not rendered an authoritative microstructural understanding in such materials. The effect of the precipitates’ inherent complex morphology and their 3D spatial distribution on evolution and deformation behavior have often been precluded. This study reports, for the first time, implementation of synchrotron-based hard X-ray nanotomography in Al–Cu alloys to measure kinetics of different nanoscale phases in 3D, and reveals insights behind some of the observed novel phase transformation reactions. The experimental results of the present study reconcile with coarsening models from the Lifshitz–Slyozov–Wagner theory to an unprecedented extent, thereby establishing a new paradigm for thermodynamic analysis of precipitate assemblies. Lastly, this study sheds light on the possibilities for establishing new theories for dislocation–particle interactions, based on the limitations of using the Orowan equation in estimating precipitation strengthening.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Shape memory properties and microstructural evolution of rapidly solidified CuAlBe alloys

In this work, the effects of Be addition on the microstructure and phase transformation temperatures of Cu–12Al–xBe (x = 0.4, 0.5 and 0.6 wt.%) shape memory alloys fabricated by using the arc-melting and melt-spinning techniques have been investigated. X-ray diffraction analysis revealed that the arc-melted alloys consisted of austenitic β{sub 1}, martensitic β{sub 1}′ and γ{sub 2} precipitate phases, whereas melt-spun ribbons were composed of a fully martensitic phase. The average grain size of martensitic phases in melt-spun ribbons was determined by electron microscopy images, showing a decrease with increasing Beryllium (Be) amount. Moreover, it was found that the Be addition in the arc-melted alloys had a distinct effect on the morphology of the γ{sub 2} precipitate phase. Transmission electron microscopy analysis showed that the thickness of martensitic plates in the melt-spun ribbons reduced with increasing Be addition. In a differential scanning calorimeter analysis, no martensitic transformation (M{sub s}) peak was observed in arc-melted alloys, but it was clearly detected in melt-spun ribbons, in which M{sub s} decreased dramatically with increasing Be addition. The improvement in the shape memory ability of melt-spun ribbons was explained in terms of the refinement in grain size and martensitic plates. - Highlights: • The CuAlBe SMAs were produced by means of arc-melter and melt-spinner techniques. • MT was directly obtained in melt-spuns without any intermediate process. • The transformation temperatures decreased with increasing Be amount. • The thickness of martensitic plates in the ribbons reduced with increasing Be. • SMP of CuAl was improved by the addition of Be together with rapid solidification.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Interfacial Microstructural Evolution and Metallurgical Bonding Mechanisms for IN718 Superalloy Joint Produced by Hot Compressive Bonding

The novel metallurgical joining process for bonding IN718 superalloy was investigated by hot compressive bonding (HCB) process under the deformation temperature range of 1000 °C to 1150 °C and true strains ranging from 0 to 0.5 at a strain rate of 0.001 s{sup −1}. The effect of HCB process parameters on the tensile strength was analyzed. Both the as-deformed and the interfacial microstructures were characterized using the optical microscope, electron backscattered diffraction and transmission electron microscope (TEM) analysis. The results of tensile property revealed that the degree of metallurgical bonding is promoted by increasing deformation temperature and strain. The evolution of the interfacial microstructure showed that the migration of interfacial grain boundary (IGB), which is characterized by discontinuous dynamic recrystallization, is the dominant metallurgical bonding mechanism in the early stages of bonding. TEM analysis indicated that the dislocation density is distributed heterogeneously over both sides of IGB, which is the significant reason for the migration of IGB, during the initial stage of HCB process.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Phase Field Simulations of Microstructure Evolution in IN718 using a Surrogate Ni–Fe–Nb Alloy during Laser Powder Bed Fusion

The solidification microstructure in IN718 during additive manufacturing was modeled using phase field simulations. The novelty of the research includes the use of a surrogate Ni–Fe–Nb alloy that has the same equilibrium solidification range as IN718 as the model system for phase field simulations, the integration of the model alloy thermodynamics with the phase field simulations, and the use of high-performance computing tools to perform the simulations with a high enough spatial resolution for realistically capturing the dendrite morphology and the level of microsegregation seen under additive manufacturing conditions. Heat transfer and fluid flow models were used to compute the steady state temperature gradient and an average value of the solid-liquid (s-l) interface velocity that were used as input for the phase field simulations. The simulations show that the solidification morphology is sensitive to the spacing between the columnar structures. Spacing narrower than a critical value results in continued growth of a columnar microstructure, while above a critical value the columnar structure evolves into a columnar dendritic structure through the formation of secondary arms. These results are discussed in terms of the existing columnar to dendritic transition (CDT) theories. The measured interdendritic Nb concentration, the primary and secondary arm spacing is in reasonable agreement with experimental measurements performed on the nickel-base superalloy IN718.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Heterogeneous microstructural evolution during hydrodynamic penetration of a high-velocity copper microparticle impacting copper

Microparticle hydrodynamic penetration (HDP) may be associated with the erosion regime in cold spray processing and other high-velocity impact events. Here, in an experimental approach where we can individually launch particles and study the impact sites, we explore copper microparticles impacted on copper substrates at velocities above 900 m/s where HDP begins. We lift cross-sectional lamellae from the impact sites with a focused-ion beam for further microstructural characterization using electron backscatter diffraction and scanning transmission electron microscopy. Due to the gradients of strain, strain rate, and temperature associated with HDP, heterogeneous microstructures result. The structural evolution processes observed include deformation twinning and multiple dislocation-mediated grain recrystallization mechanisms—geometric dynamic recrystallization (gDRX), discontinuous DRX (dDRX), and meta DRX (mDRX). The higher strains at the interface lead to the most significant structural changes and complex mechanisms. In contrast, there is a gradient to more conventional dislocation plasticity away from the interface (on either the particle or substrate side). Here, these microstructural observations are consistent with the deformation map for copper and extend the observations of impact-induced recrystallization across new regimes of behavior.

Cold spray process↗
DOE OSTI2019

Electron Beam Welding of Nimonic 80A Superalloy: Microstructure Evolution and EBSD Study After Aging Heat Treatment

The purpose of this research was to evaluate integrity and microstructure changes of the electron beam-welded Nimonic 80A and the base metal after aging heat treatment process, i.e., 700 °C/72 h/air cooling. Here, microstructure, microhardness and tensile were investigated by various tools, i.e., scanning electron microscope with energy-dispersive spectroscopy and electron backscatter diffraction, x-ray diffraction analysis and Vickers hardness. Evaluation of microstructure shows that the weld zone mainly consists of columnar dendrites. The effect of aging heat treatment on the microstructure morphology and phase structure is very limited. Also, it was observed that there are obvious segregation and precipitates in intracrystalline and along the grain boundaries after aging heat treatment. Electron backscatter diffraction results show that the weld zone and base metal have a clear difference in the grain boundary characterization and kernel average misorientation at room temperature and after aging heat treatment, which leads to a difference in microhardness of the weld zone. In addition, the relationship between microhardness and tensile strength is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Microstructural evolution of rapidly solidified hypoeutectic Al 10Cu alloy during non-isothermal annealing transients induced by nano-second laser pulses

The evolution of characteristic nonequilibrium features presenting in morphologically distinct regions of rapid solidification (RS) microstructures in a hypoeutectic Al—10Cu (atomic %) in response to non-isothermal annealing transients has been studied by transmission electron microscopy (TEM). The capabilities of the Movie-Mode Dynamic TEM (MM-DTEM) instrument were used to expose select regions of the RS microstructure to sequences of rapid heating and cooling transients induced by nanosecond laser pulses while permitting in-situ observation. Partial melting, microstructural scale coarsening, morphological changes of the nonequilibrium features in the multi-phase RS microstructure, and solid-state phase transformation were observed. Heterogeneous nucleation of nanoscale θ-Al 2 Cu phase involved metastable supersaturated α-Al and the θ'-Al 2 Cu phases, establishing different sets of orientation relationships for the stable θ-Al 2 Cu and α-Al phases. Replacement of banded morphology grains that formed under conditions driven farthest from equilibrium by an equiaxed nanocrystalline structure comprised of α-Al phase, the primary solidification product, and an intergranular network of Al 2 Cu crystals has been attributed to local remelting. Here the experimental approach explored, permitted discovery of mechanistic details of location-specific transformation pathways activated in the multi-phase RS microstructure of hypoeutectic Al—Cu during subsequent nonisothermal transients.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Microstructural evolution of thin film vanadium oxide prepared by pulsed-direct current magnetron sputtering

Vanadium oxide (VO{sub x}) thin films have been deposited by pulsed-DC magnetron sputtering using a metallic vanadium target in a reactive argon and oxygen environment. While the process parameters (power, total pressure, oxygen-to-argon ratio) remained constant, the deposition time was varied to produce films between 75 {+-} 6 and 2901 {+-} 30 A thick, which were then optically and electrically characterized. The complex dielectric function spectra ({epsilon} = {epsilon}{sub 1} + i{epsilon}{sub 2}) of the films from 0.75 to 5.15 eV were extracted by ex situ, multiple-angle spectroscopic ellipsometry (SE) measurements for the series of varied thickness VO{sub x} samples. Significant changes in {epsilon} and resistivity occur as a function of thickness, indicating the correlations exist between the electrical and the optical properties over this spectral range. In addition, in situ measurements via real time SE (RTSE) were made on the film grown to the largest thickness to track optical property and structural variations during growth. RTSE was also used to characterize changes in the film occurring after growth was completed, namely during post sputtering in the presence of argon and oxygen while the sample is shielded, and atmospheric exposure. RTSE indicates that the exposure of the film to the argon and oxygen environment, regardless of the shutter isolating the target, causes up to 200 A of the top surface of the deposited film to become more electrically resistive as evidenced by variations in {epsilon}. Exposure of the VO{sub x} thin film to atmospheric conditions introduces a similar change in {epsilon}, but this change occurs throughout the bulk of the film. A combination of these observations with RTSE results indicates that thinner, less ordered VO{sub x} films are more susceptible to drastic changes due to atmospheric exposure and that microstructural variations in this material ultimately control its environmental stability.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Identification of crystal plasticity model parameters by multi-objective optimization integrating microstructural evolution and mechanical data

Crystal plasticity models evolve a polycrystalline yield surface using meso-scale descriptions of deformation mechanisms. The activation of deformation mechanisms is governed by crystallography and a set of model parameters, which are typically calibrated through the fitting of mechanical data such as stress–strain curves and elastic lattice strains. Microstructural data such as phase fractions and texture evolution are used for verifying crystal plasticity parameters. In this study, we use a multi-objective genetic algorithm to identify hardening parameters from flow stress curves with an option to incorporate texture into the optimization approach. Robust, generalized objective functions are developed and used to identify sets of parameters pertaining to dislocation density-based hardening laws in visco-plastic and elasto-plastic self-consistent (VPSC and EPSC) homogenization models. First, the parameters are identified for pure Nb directly from texture using an objective function based on generalized spherical harmonics. Since texture evolution is driven by the relative contribution of active slip systems, the parameters governing the evolution of slip resistance ratios can be recovered from fitting discrete textures at a series of strains. Next, a comprehensive set of load reversal data for dual phase (DP) 780 steel is used to fit a hardening law and a back-stress law in EPSC. Finally, parameters pertaining to a complex hardening law for the evolution of slip and twinning in pure α-Ti are identified. Remarkably, using texture as an objective in combination with stress–strain objectives constrains the model of Ti to fully reproduce not only stress–strain and texture evolution but also hierarchical twinning measurements as a function of initial grain size and texture. Furthermore, given an appropriate model fit to representative experimental texture evolution, underlying twin volume fractions contributing to texture evolution can be predicted.

42 ENGINEERING↗
DOE OSTI2021

Physical-chemical-mechanical quantitative assessment of the microstructural evolution in Portland-limestone cement pastes exposed to magnesium sulfate attack at low temperature

The changes in structural integrity and microstructure of Portland-limestone cement pastes were investigated in the course of magnesium sulfate attack at low temperature. A deterioration front, consisting of three distinct layers (brucite, gypsum, leached cement matrix), swelled in time due to the expansive nature of the deterioration products, generating cracks and subsequently detaching from the sound cement matrix, continuously promoting the process. Gypsum and thaumasite characterized the leached matrix, which experienced extensive cross-linking of the aluminosilicate structures, as a result of decalcification and dealumination of the calcium silicate hydrates (C(A)SH), impairing the overall mechanical performance. CSH of low packing density was most severely affected by the process, as confirmed by the significant drop in nano-mechanical properties. The increased rate of deterioration with limestone content was tentatively attributed to the prevalent morphology of the CSH phase. Results were validated by thermodynamic simulations, indicating that the real systems did not reach equilibrium.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Ghost microstructure evolution and identification in the coarse grain heat affected zone of 2.25Cr-1Mo-V-Ti steel using tint etching

Microstructural characteristics of the CGHAZ (coarse grained heat affected zone) made of the 2.25Cr-1Mo-V-Ti material for the thermal power plant boiler tube were discussed using the technique of tint etching. To conduct the micro structural characterization, the sample on which CGHAZ was produced by using a high temperature thermal cycle simulator, Gleeble 3500 equipment was used for comparative analyses using the existing Nital etching (ASTM E407-74) and the alkaline etching (ASTM E40785). The latter was used to observe a specific phase. For the microstructure on which the alkaline etching was experimented, the shape of a black strip (Ghost microstructure) in a few microns was observed, which was not observed from the Nital etching. It was found from the phase identifications based EPMA, EBSD and TEM experiments that the image of the black strip in a few microns represented the alpha phase in which C, Cr and Mo became segregated. In addition, it was verified that austenite and M{sub 23}C{sub 6} phase were present around the segregated zone. Based on such results, the mechanism by which the image of the black strip in a few microns was formed at the CGHAZ. In this study, we have investigated the mechanism of the appeared black strip in the CGHAZ. - Highlights: •Ghost microstructure was observed which was not observed from the nital etching. •Ghost microstructure has high concentrations of carbon and molybdenum than matrix. •Schematic illustration proposed of why Ghost microstructure was generated. •Ghost microstructure caused by partial dissolution of M{sub 23}C{sub 6} precipitation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Microstructure evolution of gadolinium doped cerium oxide under large thermal gradients

In this report the effects of large thermal gradient annealing on the microstructure of 10 mol% gadolinium doped ceria (GDC) were investigated. GDC powder was prepared by solvent deficient method and sintered at 1650 °C for 10 h to achieve dense ceramics with ~8 μm grain size. The densified GDC samples were subsequently annealed using a 60 W infrared laser at over 2100 °C for 1 h under a thermal gradient equivalent to ~0.3–0.5 °C/μm. The post-annealed samples at 2150 °C for 1 h exhibit grains with average length and width of 37 and 28 μm, respectively. Electron backscattered diffraction (EBSD) analysis revealed that the post-annealed sample at 2150 °C consists of grains oriented close to five principal directions (<4 3 10>, <0 0 1> and <13 1 14> on [0 0 1], and <7 6 20> and <7 2 7> on [0 1 0]) within a tolerance angle of ±10°, whereas the grains of the pre-annealed sample are randomly oriented. Gadolinium diffuses 20–30 μm away from the irradiated surface, with the measured composition of regions deeper than 30 μm, Ce 0.86 Gd 0.14 O 1.93 , is close to that of the pre-annealed sample, Ce 0.87 Gd 0.13 O 1.94 . Enhancement of total conductivity of the post-annealed GDC (1.1 × 10 -3 S cm -1 at 500 °C, and 2.1 × 10 -2 S cm -1 at 700 °C) is observed when compared to the pre-annealed GDC (3.1 × 10 -5 S cm -1 at 500 °C, and 1.7 × 10 -3 S cm -1 at 700 °C), and points to the decrease in the grain boundary (GB) resistivity. This could be attributed to both the change in GB area and grain alignment.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Understanding the solidification and microstructure evolution during CSC-MIG welding of Fe–Cr–B-based alloy

The present is a study of the solidification and microstructure of Fe–28.2%Cr–3.8%B–1.5%Si–1.5%Mn (wt.%) alloy deposited onto a 1020 plain carbon steel substrate using the controlled short-circuit metal inert gas welding process. The as-solidified alloy was a metal matrix composite with a hypereutectic microstructure. Thermodynamic calculation based on the Scheil–Gulliver model showed that a primary (Cr,Fe){sub 2}B phase formed first during solidification, followed by an eutectic formation of the (Cr,Fe){sub 2}B phase and a body-centered cubic Fe-based solid solution matrix, which contained Cr, Mn and Si. Microstructure analysis confirmed the formation of these phases and showed that the shape of the (Cr,Fe){sub 2}B phase was irregular plate. As the welding heat input increased, the weld dilution increased and thus the volume fraction of the (Cr,Fe){sub 2}B plates decreased while other microstructural characteristics were similar. - Highlights: • We deposit Fe–Cr–B-based alloy onto plain carbon steel using the CSC-MIG process. • We model the solidification behavior using thermodynamic calculation. • As deposited alloy consists of (Cr,Fe){sub 2}B plates embedded in Fe-based matrix. • We study the effect of the welding heat input on the microstructure.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Crystallization Mechanisms of Poly(vinylidene Fluoride- co -chlorotrifluoroethylene): Nucleation Transitions, Growth Kinetics, and Microstructure Evolution

The crystallization kinetics of FK-800, a commercial semicrystalline copolymer of chlorotrifluoroethylene and vinylidene fluoride (poly(CTFE-co-VDF)), dictate its microstructure and functional performance. Crystallization occurs between the glass transition (T g ≈ 31 °C) and melting point (T m ≈ 110 °C), with grain boundaries playing a key role in applications such as memristors, where they regulate filament growth. This study integrates in situ atomic force microscopy (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and differential scanning calorimetry (DSC) to characterize nucleation, growth, and morphology across the full crystallization window. Hot-stage AFM reveals a transition from homogeneous to heterogeneous nucleation near 45 °C and a shift from reaction-limited to diffusion-limited growth above 60 °C. Kinetics are analyzed using Lauritzen–Hoffman, Turnbull–Fisher, and Avrami models to extract surface free energies and activation barriers. A multiscale modeling framework is developed in which nucleation densities and growth rates measured by AFM are used to reconstruct Avrami kinetics, which are then validated against coverage data, and extrapolated to predict bulk crystallization behavior observed by DSC. GIWAXS confirms a primarily flat-on chain orientation at all temperatures, linking orientation to domain morphology. This integrative approach quantitatively connects nanoscale crystallization dynamics with macroscopic phase evolution, establishing a quantitative framework for modeling crystallization across time, temperature, and length scales.

Crystallization↗
DOE OSTI2014

Growth kinetics and microstructural evolution during hot isostatic pressing of U-10 wt.% Mo monolithic fuel plate in AA6061 cladding with Zr diffusion barrier

Phase constituents and microstructure changes in RERTR fuel plate assemblies as functions of temperature and duration of hot-isostatic pressing (HIP) during fabrication were examined. The HIP process was carried out as functions of temperature (520, 540, 560 and 580 °C for 90 min) and time (45–345 min at 560 °C) to bond 6061 Al-alloy to the Zr diffusion barrier that had been co-rolled with U-10 wt.% Mo (U10Mo) fuel monolith prior to the HIP process. Scanning and transmission electron microscopies were employed to examine the phase constituents, microstructure and layer thickness of interaction products from interdiffusion. At the interface between the U10Mo and Zr, following the co-rolling, the UZr2 phase was observed to develop adjacent to Zr, and the a-U phase was found between the UZr2 and U10Mo, while the Mo2Zr was found as precipitates mostly within the a-U phase. The phase constituents and thickness of the interaction layer at the U10Mo-Zr interface remained unchanged regardless of HIP processing variation. Observable growth due to HIP was only observed for the (Al,Si)3Zr phase found at the Zr/AA6061 interface, however, with a large activation energy of 457 ± 28 kJ/mole. Thus, HIP can be carried to improve the adhesion quality of fuel plate without concern for the excessive growth of the interaction layer, particularly at the U10Mo-Zr interface with the a-U, Mo2Zr, and UZr2 phases.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2018

Microstructural evolution of NF709 austenitic stainless steel under in-situ ion irradiations at room temperature, 300, 400, 500 and 600 °C

Irradiation induced microstructural changes in the NF709 austenitic stainless steel were investigated under 1 MeV Kr ion irradiations to a dose of 3 dpa at room temperature (RT), 300, 400, 500 and 600 °C, and a dose of 20 dpa at RT. The irradiation-induced defects and the stability of precipitates were characterized with transmission electron microscopy (TEM). Frank dislocation loops were observed in all the irradiated samples, and the loop sizes were much larger at 600 °C than those at lower temperatures. “Raft” defect structures, formed through self-alignment of small dislocation loops, were also observed in all irradiated samples. M23C6 precipitates were amorphorized under irradiations at RT and 300 °C, but remained to be crystalline at 400 °C and above. MX precipitates were stable under irradiations at RT up to 20 dpa, and at temperatures below 600 °C to 3 dpa. At 600 °C, some MX precipitates were observed to dissolve during in-situ irradiation, suggesting possible precipitate instability at this irradiation temperature.

36 MATERIALS SCIENCE↗