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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 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

The Effect of Casting Speed on Microstructure, Microsegregation, and Mechanical Properties of High-Strength Mg-Nd-Zn-Zr Alloy

High-quality Mg-Nd-Zn-Zr magnesium alloy billets were successfully prepared by direct chill casting. The effect of casting speed on microstructure, microsegregation, and mechanical properties of the Mg-Nd-Zn-Zr alloy was systematically investigated. The results show that the as-cast microstructure is composed of equiaxed α-Mg and Mg{sub 12}Nd. Optimum values of mechanical properties were obtained corresponding to the casting speed of 90 mm/min. An age hardening at 200 °C took a remarkable effect on the mechanical properties of billets. The maximum ultimate tensile strength ~ 323.4 MPa was obtained in the billet undergoing T6 heat treatment. The shapes of concentration curves of various alloying elements (e.g., Nd and Zn) in Mg-Nd-Zn-Zr, as predicted by the Gulliver–Scheil model, resemble reasonably with the shapes measured by electron probe microanalysis.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Hardness Evolution in Haynes 282 Nickel-Based Superalloy During Multi-variant Aging Heat Treatment

In this paper, the effect of applied multi-variant heat treatment on microstructure, phase composition and mechanical response of Haynes 282 nickel-based superalloy was investigated. For this reason, temperatures of both stages of standard two-stage aging treatment (i.e., 1010 °C/2 h + 780 °C/8 h) were extended to 900-1100 °C/2 h and 680-880 °C/8 h ranges, respectively. Consequently, 30 different variants of heat treatment were applied. The microstructural features of heat-treated samples were investigated by means of light microscopy and SEM/EDS methods, while mechanical properties were examined via microhardness measurements. It was found that by using various combinations of temperatures of the first and second stage of aging, the room temperature hardness of Haynes 282 alloy can be decreased by ~ 100 HV units or increased by up to 25 HV units as compared to that of the alloy subjected to the standard heat treatment schedule. The mechanical response of the alloy is determined by a complex structural evolution involving the secondary precipitation of γ′, M{sub 23}C{sub 6} and M{sub 6}C phases, as well as their interaction with the fcc γ matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2024

High-Throughput Microstructural Characterization and Process Correlation Using Automated Electron Backscatter Diffraction

The need to optimize the processing conditions of additively manufactured (AM) metals and alloys has driven advances in throughput capabilities for material property measurements such as tensile strength or hardness. High-throughput (HT) characterization of AM metal microstructure has fallen significantly behind the pace of property measurements due to intrinsic bottlenecks associated with the artisan and labor-intensive preparation methods required to produce highly polished surfaces. This inequality in data throughput has led to a reliance on heuristics to connect process to structure or structure to properties for AM structural materials. In this study, we show a transformative approach to achieve laser powder bed fusion (LPBF) printing, HT preparation using dry electropolishing and HT electron backscatter diffraction (EBSD). This approach was used to construct a library of > 600 experimental EBSD sample sets spanning a diverse range of LPBF process conditions for AM Kovar. This vast library is far more expansive in parameter space than most state-of-the-art studies, yet it required only approximately 10 labor hours to acquire. Build geometries, surface preparation methods, and microscopy details, as well as the entire library of >600 EBSD data sets over the two sample design versions, have been shared with intent for the materials community to leverage the data and further advance the approach. Using this library, we investigated process–structure relationships and uncovered an unexpected, strong dependence of microstructure on location within the build, when varied, using otherwise identical laser parameters.

Characterization and Analytical Technique↗
DOE OSTI2018

Achieving ultra-high strength and ductility in equiatomic CrCoNi with partially recrystallized microstructures

Despite having otherwise outstanding mechanical properties, many single-phase medium and high entropy alloys are limited by modest yield strengths. Although grain refinement offers one opportunity for additional strengthening, it requires significant and undesirable compromises to ductility. This work therefore explores an alternative, simple processing route to achieve strength by cold-rolling and annealing an equiatomic CrCoNi alloy to produce heterogeneous, partially recrystallized microstructures. Tensile tests reveal that our approach dramatically increases the yield strength (to ~1100 MPa) while retaining good ductility (total elongation ~23%) in the single-phase CrCoNi alloy. Scanning and transmission electron microscopy indicate that the strengthening is due to the non-recrystallized grains retaining their deformation-induced twins and very high dislocation densities. Furthermore, load-unload-reload tests and grain-scale digital image correlation are also used to study the accumulation of plastic deformation in our highly heterogeneous microstructures.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Role of microstructure on the actuation fatigue performance of Ni-Rich NiTiHf high temperature shape memory alloys

Here, the focus of the present study was to systematically investigate the influence of microstructure on the actuation fatigue performance of a Ni-rich NiTiHf high temperature shape memory alloy (HTSMA). Different aging heat treatments led to the formation of H-phase nano-precipitates with different sizes and morphology within the matrix, enhancing thermo-mechanical stability and enabling control of transformation temperatures. The actuation fatigue testing of specimens was performed until failure through thermally-induced reversible martensitic transformation under a constant stress (300 MPa) with two distinct upper cycle temperatures (UCT) of 300 °C and 350 °C. Consequently, the specimens heated to 700 °C and furnace cooled to 100 °C in 48 h, with relatively large precipitates, failed at average fatigue life of 15,500 cycles and exhibited 1.0% average actuation strain in the 300 °C UCT experiments, while those aged at 550 °C for 3 h, with the precipitate sizes less than 20 nm, attained an average fatigue life of 10,800 cycles and an actuation strain of 2.5%. Samples with intermediate precipitate sizes after aging at 600 °C for 10 h failed at the shortest average fatigue life of 8,200 cycles with an intermediate average actuation strain of 2.0%. Furthermore, increase in UCT decreased the fatigue life and resulted in larger average actuation strains for all samples. Overall, the current findings constitute the first systematic results demonstrating the microstructure dependence of the evolution of actuation strain, irrecoverable strain, and transformation temperatures during actuation fatigue, and actuation fatigue life of the Ni-rich Ni 50.3 Ti 29.7 Hf 20 HTSMA, demonstrating the importance of controlling the H-phase precipitate size.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Modeling the role of local crystallographic correlations in microstructures of Ti-6Al-4V using a correlated structure visco-plastic self-consistent polycrystal plasticity formulation

This paper presents a multi-level crystal plasticity-based simulation framework for modeling mechanical response and microstructure evolution of Ti-6Al-4V with α-lath/lamellar microstructures. Here, the model is a correlated structure visco-plastic self-consistent (CS-VPSC) formulation linking three scales: a single crystals micro-scale, a lath/lamellar colony meso-scale, and a lath/lamellar aggregate macro-scale. A selected hardening law for the evolution of critical resolved shear stress per slip system used in CS-VPSC is phenomenological. However, it adjusts the resistances of basal and prismatic slip systems based on the geometry of slip transfer between adjacent lamellae. Consistent with experimental evidences, the resolved shear stress on the pyramidal slip planes is dependent not only on the stress in the direction of slip but also on the two orthogonal shear stress components and the three normal stress components (non-Schmid effects). Electron backscatter diffraction (EBSD) data in conjunction with a procedure relying on α→β phase transformation is used to construct paired variants of α-laths/lamellae satisfying their local crystallographic correlations. The procedure fits volume fractions of individual laths/lamellae with the experimental EBSD data and selects a distribution of habit planes between adjacent variants with respect to the loading direction. The simulation framework is applied to interpret the deformation behavior in tension and compression along two sample directions of Ti-6Al-4V fabricated via laser powder bed fusion. Moreover, the model is used to simulate texture evolution during rolling of the material to large strains. It is demonstrated that the model is capable of predicting plastic anisotropy/asymmetry and the concomitant texture evolution. While the model reveals a significant effect of habit plane inclination with respect to the loading direction on yield stress, the comparison of the data and model predictions shows that a random distribution of habit planes fits the flow response. It is further inferred that the tension-compression asymmetry arises from the non-Schmid effects.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Experiments and simulations on solidification microstructure for Inconel 718 in powder bed fusion electron beam additive manufacturing

Previous research on the powder bed fusion electron beam additive manufacturing of Inconel 718 has established a definite correlation between the processing conditions and the solidification microstructure of components. However, the direct role of physical phenomena such as fluid flow and vaporization on determining the solidification morphology have not been investigated quantitatively. In this work, we investigate the transient and spatial evolution of the fusion zone geometry, temperature gradients, and solidification growth rates during pulsed electron beam melting of the powder bed with a focus on the role of key physical phenomena. The effect of spot density during pulsing, which relates to the amount of heating of the build area during processing, on the columnar-to-equiaxed transition of the solidification structure was studied both experimentally and theoretically. Predictions and the evaluation of the role of heat transfer and fluid flow were established using existing solidification theories combined with transient, three-dimensional numerical heat transfer and fluid flow modeling. Metallurgical characteristics of the alloy’s solidification are extracted from the transient temperature fields, and microstructure is predicted and validated using optical images and electron backscattered diffraction data from the experimental results. Simulations show that the pure liquid region solidified quickly, creating a large two-phase, mushy region that exists during the majority of solidification. While conductive heat transfer dominates in the mushy region, both the pool geometry and the solidification parameters are affected by convective heat transfer. Lastly, increased spot density during processing is shown to increase the time of solidification, lowering temperature gradients and increasing the probability of equiaxed grain formation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of scan pattern and geometry on the microstructure and soft-magnetic performance of additively manufactured Fe-Si

The influence of geometry and scan pattern on the microstructure evolution and magnetic performance of additively manufactured Fe-3Si components was investigated. To reduce eddy current losses, novel geometries were designed and built and the microstructure and properties of these samples were characterized. The laser scan pattern was shown to strongly influence both the as-built grain structure and strength of the crystallographic texture, resulting in measurable changes in the as-built magnetic performance. In thin wall samples, heat treatment resulted in an increase in the maximum relative magnetic permeability and decrease in power losses in most samples, consistent with grain growth. However, decreases in the spacing between thin walls to increase the stacking factor of the cross-section was shown to result in unwanted electrical shorting between walls and an increase in eddy current losses. Compared to simple parallel plate construction and a mesh structure, a novel cross-section design based on the Hilbert space filling curve was found to produce the lowest power losses. The mechanisms behind these results were explored using a combination of heat conduction and electromagnetic simulations, providing a route for future component and process optimization.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Tuning the fluid wetting dynamics on gold microstructures using photoactive compounds

This work demonstrates how photoactive compounds can be used to tune the surface chemistry, surface free energy, and the wetting velocity of fluids on Spiropyran functionalized surfaces with different surface microstructures. Evidence of this photowetting effect is based on data showing: (1) the cyclic changes in the static, advancing, and receding contact angles $\theta$ CA for multiple UV$\rightleftarrows$vis photoswitching cycles with both smooth and microstructured surfaces and (2) the changes in the fluid wicking velocity (via UV$\rightleftarrows$vis photoswitching) on different Spiropyran functionalized hemiwicking surfaces. X-ray photoelectron spectroscopy is used to determine the efficiency of photoswitching caused by the quality of the Spiropyran functionalization. Reversible photoisomerization of Spiropyran generates a gradient in surface free energy, resulting in a similar change in contact angle ($\theta$ CA ) in the photoswitchable surfaces. In conclusion, by incorporating these changes into the Owens-Wendt and Van Oss surface energy models, the energy of smooth Au surfaces can also be predicted for both UV and visible light irradiations, where reversible contact angle variations of $\Delta$$\theta$ CA ≈ 5–10° are achieved with both water and water-ethanol mixtures due to the conversion of SP to merocyanine, which corresponds to a total free energy change of ~13% on the photoswitchable surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Material processing, microstructure, and composite properties of low carbon Engineered Cementitious Composites (ECC)

Traditional PVA fiber-reinforced Engineered Cementitious Composites (ECC) show high tensile ductility and superior durability with tight crack width, but the high cost and embodied carbon can hinder its wider application in infrastructures. The objective of this study is to develop a better understanding of the fresh and hardened properties of an ECC that employs a lower embodied-carbon binder, Limestone Calcined Clay Cement (LC3), and lower-cost PP fiber that is widely available. Specifically, the interrelations between material processing, microstructure, and composite properties were studied experimentally. The results showed that ECC with high tensile ductility up to 9% tensile strain and tight crack width with 50 μm at 2% tensile strain can be achieved. It was found that a matrix paste with higher viscosity generally enhanced fiber dispersion uniformity and robustness in tensile strain-hardening. The paste viscosity is increased when OPC is replaced by LC3 and can be tuned with superplasticizer content. Larger maximum flaw size leads to lower first crack strength, beneficial for microcrack initiation and multiple cracking. This study generates fundamental knowledge linking processing-microstructure-performance of PP-LC3-ECC. This class of low embodied carbon ECC with tight crack width is expected to contribute to reducing the carbon footprint of the built environment.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of fly ash and metakaolin on the microstructure and compressive strength of magnesium potassium phosphate cement paste

The influences of fly ash and metakaolin added as substitutions (by up to 50 wt%) of magnesium potassium phosphate cement (MKPC) on the microstructures and compressive strengths of the MKPC pastes were investigated. The results indicate that the aluminosilicate fractions of both fly ash and metakaolin are involved in the acid-base reaction of MKPC system, leading to a preferential formation of irregular crystalline struvite-K incorporated with Al and Si elements and/or amorphous aluminosilicate phosphate products. Metakaolin is more reactive than fly ash in the MKPC system. For the same addition dosage, the MKPC pastes containing metakaolin exhibit higher compressive strengths than the pastes containing fly ash. This is attributed to the formation of more highly reinforced microstructures and denser interfaces between the metakaolin particle and hydration products (e.g. struvite-K) in the MKPC paste containing metakaolin. Addition of 30 wt% metakaolin increases the compressive strengths of MKPC pastes at all test ages.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Mineralogical and microstructural alterations in a portland cement paste after an accelerated decalcification process

Cement paste leaching and concomitant decalcification is a common cause of concrete structure deterioration. Such alterations entail microstructural changes that condition concrete durability. This study analysed those changes in cement pastes exposed to accelerated decalcification, layer-by-layer, from the surface in contact with the leaching agent, 6 M NH{sub 4}NO{sub 3,} to the unaltered area. The microstructural changes were analysed with BSEM, MIP and BET whilst the mineralogical and nanostructural alterations in each layer were assessed with XRD, FTIR, {sup 29}Si and {sup 27}Al MAS NMR and BSD. Decalcification affected not only portlandite and CSH gel, but also the anhydrous clinker and ettringite, inducing major micro- and nano-structural changes, particularly in the two layers closest to the leaching agent. CSH gel decalcification was more intense in the outer layers where long chain gels co-existed with silicon rich gels. BDS, in turn, revealed differences between the amount of bound water in the surface interfacing with the leaching agent and in the leaching front. More specifically, larger numbers of water molecules were tightly bound to silanol groups in the outer, whereas hydroxyl group distribution was more orderly in the inner layers.

36 MATERIALS SCIENCE↗
DOE OSTI2021

GeoMicro3D: A novel numerical model for simulating the reaction process and microstructure formation of alkali-activated slag

Highlights: • The GeoMicro3D model was innovatively developed for simulating the reaction process and microstructure formation of AAS. • In GeoMicro3D, the dissolution of slag and reactions of aqueous ions are fundamentally described in the local lattice cell. • A novel approach is proposed to speed up the nucleation simulations. • GeoMicro3D was implemented and verified with the relevant experimental data and thermodynamic calculation results. For the first time, this study developed a novel model, named GeoMicro3D, to simulate the reaction process and microstructure formation of alkali-activated slag. The GeoMicro3D model consists of four modules that are designed to simulate, respectively: (i) the initial spatial distribution of real-shape slag particles in alkaline activator, (ii) the dissolution of slag and diffusion of ions via the transition state theory and lattice Boltzmann method, respectively, (iii) the spatial distribution of reaction products using a nucleation probability theory, and (iv) the chemical reactions with thermodynamic modelling. Afterwards the GeoMicro3D model was implemented and verified. The simulation results were discussed and compared with the relevant experimental data and thermodynamic calculation results using GEMS. A good agreement was found in the comparisons, showing the strong simulation capability of GeoMicro3D.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Influence of supplementary cementitious materials on microstructure and transport properties of spacer-concrete interface

Reinforcement spacers are a critical component of concrete structures. Their presence affects microstructure and transport properties of concrete cover though this is not widely appreciated. This paper presents the first study to determine whether the negative effects of spacers can be mitigated through the use of supplementary cementitious materials such as silica fume, fly ash and blast-furnace slag. Concrete samples (>200) with different spacers, binders, curing and drying regimes were prepared and tested for diffusion, permeation, absorption, electrical conductivity, carbonation and microstructure. It was found that spacers increase all transport properties, the extent depending on type of spacer, drying regime and transport mechanism. The spacer-concrete interface is weak, porous and micro-cracked, and this lowers the resistance of concrete to ingress of aggressive agents. The beneficial effects of SCMs (strength enhancement and densification) and prolonged curing (120-day) are insufficient to overcome the negative effects of spacers. Implications for durability are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Microstructure modeling of nuclear structural materials: Recent progress and future directions

Modeling and simulation of microstructures are essential to understand the complex responses and behaviors of nuclear materials in extreme environments. The needs to assess the extended life operation as well as the growing interest in accelerating nuclear materials development and qualification have stimulated the use of high-fidelity multiscale models aided by empirical and ab initio data. This paper reviews the role of various models across different length and time scales in investigating irradiation effects on microstructure evolution and degradation, in particular the embrittlement caused by radiation induced or enhanced formation of nanoscale chemical heterogeneities. The strength and limitations of these models, including classical rate theories, cluster dynamics, phase-field methods, and atomistic models informed by ab initio energies, are discussed with seminal examples. Challenges regarding the lack of thermo-kinetic data and theoretical treatments considering chemical complexities and magnetic excitations, as well as the stabilizing effect by excess point defects in nuclear structural materials are presented, along with potential solutions based on ab initio informed surrogate energy models and statistical sampling by Monte Carlo simulations. Further, the review then highlights the opportunities to leverage the advantages of different methods by establishing hybrid models by shared variables or coupled codes and applications. Finally, the review concludes with forward-looking remarks on how the use of physics-based models can aid the improvement of machine-learning models of property degradation and vice versa.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Fatigue of additive manufactured Ti-6Al-4V, Part I: The effects of powder feedstock, manufacturing, and post-process conditions on the resulting microstructure and defects

Additive manufacturing provides an appealing means to process titanium alloy parts with new levels of conformability, complexity, and weight reduction. However, due to the heating/cooling rates and heat transfer associated with directed energy source material processing, the as-built AM parts contain unique material and microstructural features. In order to confidently manufacture fatigue critical additive manufactured (AM) Ti-6Al-4V parts, a better understanding of the interrelationships between powder feedstock, AM processes, structure of the processed parts, their resulting mechanical properties, and their performance under realistic loadings is necessary. Part I of this two-part collection focuses on the powder-process-structure relationships and how powder feedstock, manufacturing, and post-processing conditions can affect the microstructure and defect features that ultimately contribute to the fatigue performance of Ti-6Al-4V parts. The material and physical phenomena inherent to the AM process of Ti-6Al-4V are discussed in detail and related to the phase composition/structure, grain morphology, surface characteristics, defect size/distribution, and post-process treatments available for AM parts. This investigation is the foundation for the structure-performance relationships that will be covered in detail in Part II.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Gradient microstructure induced shear band constraint, delocalization, and delayed failure in CuZr nanoglasses

The mechanical properties of metallic glasses (MGs) can be significantly tuned by introducing a nanoscale glass-glass interface microstructure. Here, we design three types of Cu 64 Zr 36 MG seamless gradient microstructures by combining glassy grain sizes from 3 to 15 nm. We perform tensile loading molecular dynamics simulations on the named gradient nanoglasses to characterize their mechanical behavior. In all cases, results indicate the generation of gradient plasticity following the gradient in grain size, i.e. local plasticity increases from large to small grain size regions. Loading parallel to the gradient direction results in initial broad necking followed by catastrophic failure. In contrast, loading perpendicular to the gradient direction results in diffused shear band propagation from large to small grain size regions, delaying the generation of a dominant shear band and further delocalizing plastic deformation. In conclusion, the results demonstrate the synergistic effects generated by heterogeneous gradient designs in the mechanical behavior of metallic glasses.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Elevated temperature microstructure evolution of a medium-entropy CrCoNi superalloy containing Al,Ti

A new medium-entropy superalloy was produced based on the compositions of equiatomic CrCoNi and Ni-base superalloy Inconel 740H. Initial alloy design was performed using Thermo-Calc. The aging response and microstructural stability were assessed following heat treatment at temperatures between 600 and 900 °C and durations up to 100 h. Aging from a fully recrystallized state resulted in negligible grain growth and produced γ’ and σ phases. The same phases were present after aging from a cold-rolled state, but partially recrystallized microstructures resulted in multi-modal size distributions and heterogeneous spatial arrangements. Finally, room temperature hardness measurements were used to correlate aging conditions with quantitative precipitate measurements and mechanical properties.

36 MATERIALS SCIENCE↗