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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 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Grain Refinement, Microstructure and Mechanical Properties Homogeneity of Mg-Gd-Y-Nd-Zr Alloy During Multidirectional Forging

Multidirectional forging (MDF) of Mg-Gd-Y-Nd-Zr alloy was performed under decreasing temperature condition. The grain refinement, microstructure and mechanical properties homogeneity of Mg-Gd-Y-Nd-Zr alloy during MDF process were investigated. The results show that the grain refinement mechanism of Mg-Gd-Y-Nd-Zr alloy during MDF process was dynamic recrystallization. The dynamic precipitations were observed after MDF process, which were mainly located at the grain boundaries and had a significant influence on the grain refinement, and the amount of dynamic precipitations increased as the MDF passes increase. The microstructure of the billet was not homogeneous due to the deformation and temperature inhomogeneity during MDF process. Optimization MDF process has been proposed, which benefited the microstructure homogeneity and reduced the amount of the dynamic precipitations. Based on the optimization MDF process, the elongation to fracture significantly increased.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Friction Stir Welded Q235 Low-Carbon Steel

Three-millimeter-thick Q235 steel plates were friction stir welded using a tool made of tungsten-rhenium alloy in the present study. The microstructure and mechanical properties of welded joints at various traverse speeds from 50 to 150 mm/min were analyzed. It was demonstrated that traverse speed had great impacts on microstructure and mechanical properties of the joints. Phase transformation accompanied with recrystallization occurred during FSW process which resulted in different microstructures in the WNZ, and the grain size in other zones was significantly affected by traverse speed. The highest hardness was located in the nugget zone, while the lowest hardness was found in the heat-affected zone. The transverse tensile test suggested that all the joints showed slightly lower tensile strength than that of the base material, and the tensile strength increased with increasing traverse speed. All the joints fractured in the heat-affected zone with ductile fracture mode. The impact test showed that the impact toughness decreased with the increasing traverse speed and the impact fracture surface could be divided into shear lip zone and fibrous zone which showed a typical ductile fracture.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure Evolution and Stress Corrosion Cracking Susceptibility of 12Cr Martensitic Steel Upon Long-Term Service in Power Plants

After 230,000-h long-term service at 550 °C/13.7 MPa, the microstructure of a kind of 12Cr martensitic steel was investigated using scanning electron microscopy, backscattering electron microscope and x-ray diffraction analysis. The results show that the precipitation and coarsening of carbides at grain/lath boundaries are the main cause of microstructure degradation. The static immersion tests and the slow strain rate test coupled with electrochemical impedance spectroscopy were conducted on the served steel in 1.0% NaCl solution, and it turns out that the pitting corrosion resistance and repassivation ability of the steel are significantly reduced as a result of microstructure degradation. The stress corrosion cracking susceptibility of the steel was also studied. Fracture morphology analysis shows that the secondary crack in conjunction with slip lines is a result of the coalescence of micro-cracks nucleated from the pits.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Understanding Microstructural Evolution During Rapid Heat Treatment of Microalloyed Steels Through Computational Modeling, Advanced Physical Simulation, and Multiscale Characterization Techniques

An AISI 1045 steel modified with vanadium (V) and niobium (Nb) was studied to evaluate microstructural conditioning prior to and throughout a rapid heat treat process. In order to accomplish this, both computational and physical simulation techniques have been employed with the goal of assessing the microstructural evolution in a medium-carbon bar steel during the rapid austenitization and quenching procedures involved in an induction hardening process. The appropriate thermal profiles for induction hardening were obtained through finite element modeling using Flux 2D software. Physical simulations of the induction hardening process were carried out using a Gleeble{sup ®} 3500. Analysis of prior austenite grain size is complemented by observation of nanoscale carbonitride precipitation via transmission electron microscopy, scanning transmission electron microscopy, and high-energy synchrotron small-angle x-ray scattering. Through a combination of characterization techniques, this study presents a deeper understanding of nano- and microstructural changes occurring in a microalloyed steel during an induction hardening process.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Property Inhomogeneities of a Modified AZ80 Magnesium Alloy Wide Plate Under Horizontal Flat Die Extrusion

In the present paper, inhomogeneities in the microstructure and mechanical properties of a modified AZ80 plate under industrial production conditions were investigated. The results indicated that the microstructure of the edge and middle region shows more homogeneous and fine recrystallized grains. Additionally, more coarse recrystallized grains existed between the edge and the middle region. The ultimate tensile strength and yield strength of the edge region of the plate are maximal (287 and 162 MPa), and those near the 1/8 W region are minimal (259 and 152 MPa). The grain size increased from 19.5 to 27.2 μm along the center layer to the surface layer. The ultimate tensile stress (yield stress) increases from 258 (121 MPa) to 284 MPa (176 MPa) in the direction from the surface layer to the middle layer. The microhardness decreased from the surface layer (72HV) to the center layer (64HV). The metal strain state and temperature distribution lead to different degrees of dynamic recrystallization, which causes the microstructure and mechanical properties of the plate.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Effects of Sc Addition on the Microstructure and Mechanical Properties of Be-Al Alloy Fabricated by Induction Melting

This study investigated the alloying effects of 0.4 wt.% Sc on the microstructure, mechanical properties, and fracture behavior of a cast Be-Al alloy prepared by vacuum induction melting. The micromechanics of Sc-containing secondary-phase particles (SPs) were characterized by nanoindentation, and the fracture behavior of the alloy was investigated via tensile fractography and optical metallography. The addition of 0.4 wt.% Sc significantly modified the alloy microstructure by decreasing the secondary dendritic arm spacing (SDAS), weakening the dendritic structure of Be, and producing an equiaxed/cellular alloy microstructure with refined Be grains. Sc alloying increased the Vickers hardness by 36%, elastic modulus by 28%, yield strength by 35%, ultimate strength by 52%, but decreased the elongation from 3.6 to 2.9%. Sc-containing SPs had a higher elastic modulus, yield strength, microhardness, and a lower plasticity index than the Be and Al matrices, resulting in interface separation during the fracture process. SPs located at the Be/Al interfaces did not act as microcrack sources for the matrices, whereas highly faceted SPs within the Be grains induced microcrack formation and decreased the elongation. The failure mode of the cast Be-Al-0.4Sc alloy still involved the brittle cleavage failure of Be regions and ductile dimple failure of Al regions, which was not the case with the cast Be-Al alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure, Texture, and Mechanical Properties of AM60 Magnesium Alloy Processed by Extrusion and Multidirectional Forging

In the present paper, multidirectional forging (MDF), as a severe plastic deformation process, was carried out on an extruded AM60 magnesium alloy. The microstructure, texture, and mechanical properties of the MDFed samples were examined. Microstructural findings indicated that the grain size of the alloy decreased from 10.2 to 3.7 µm after six MDF passes, carried out at a constant temperature of 220 °C. Textural studies revealed that after six MDF passes, the fiber texture of the extruded material was transformed to another texture, in which the basal planes tend to rotate toward 45{sup o} to the transverse direction. Shear punch testing (SPT) was employed to assess the shear yield stress (SYS) and ultimate shear strength (USS) at room temperature. Both SYS and USS were enhanced after two MDF passes, but decreased in the next pressing stages, despite developing finer grains at higher levels of imposed strain. The observed drop in strength is ascribed to texture softening that counterbalances the grain refinement strengthening effects. The variation in average microhardness values after different passes of MDF showed the trend that was in concurrence with the rise and fall of the SPT results. Finally, the changes in mechanical properties were discussed, based on the microstructural and textural findings.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure Evolution and Mechanical Properties of Large-Scale AZ80 Magnesium Alloy Billets Produced by Multitemperature Multidirectional Forging

Grain refinement is an effective method to improve the mechanical properties of magnesium (Mg) alloys. In the present research, the combination of multidirectional forging (MDF) and forward extrusion for sizing with decreasing temperature pass by pass was employed to deform as-cast AZ80 magnesium alloy billet with large size to obtain fine grains and homogeneous microstructure. It was found that with the increasing number of MDF passes, the average grain size decreases; the homogeneity of microstructures and the percentages of dynamic recrystallised grains increase simultaneously. After six-pass MDF, fine and homogeneous microstructure with an average grain size of 15 μm was achieved; however, grain refinement became very difficult with a further increase in the MDF pass number. After eight-pass MDF plus forward extrusion for sizing, excellent mechanical properties of large-scale AZ80 magnesium alloy billet were obtained. The tensile strength, the yield strength and the elongation of billet are 410 MPa, 292 MPa and 14%, respectively. Furthermore, during multitemperature MDF, the inverse of the square root of grain size and tensile yield strength fit well with the Hall–Petch relationship.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure, Texture, and Mechanical Properties of Continuously Extruded and Rolled AZ31 Magnesium Alloy Sheets

It is difficult to fabricate magnesium alloy sheets at room temperature due to their hexagonal close-packed structure. In this paper, we proposed a new process that can be used to fabricate AZ31 magnesium alloy sheets with fine-grained microstructure and good mechanical properties. AZ31 magnesium alloy sheets with dimensions of 160 mm × 8 mm were obtained successfully by a continuous extrusion technique. The extruded sheets were then rolled by two different rolling routes, Route 1: the extruded sheet was heated at 350 °C and then rolled without heating of rolls and Route 2: the sheet was rolled without any previous heating by heating rolls at 280 °C. The microstructure and texture evolution as well as mechanical properties of the continuously extruded and rolled sheets were examined experimentally. The magnesium alloy sheets with relatively fine grains of 4.5-8.3 µm were obtained by continuous extrusion followed by rolling processes. The tensile strength of continuously extruded AZ31 Mg alloy sheets can be effectively increased by rolling using heating rolls (Route 2), while the ductility was enhanced by rolling heated sheets (Route 1). The mixed microstructure, consisting of refined DRX grains and deformed grains, contributes to enhancement in tensile strength of the sheet samples rolled by Route 2 due to grain-boundary strengthening and work hardening. Lower temperature and larger rolling reduction are two main factors for improving the tensile strength of AZ31magnesium alloy sheets.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructural Evolution and Toughness of the Various HAZs in 1300-MPa-Grade Ultrahigh-Strength Structural Steel

The effect of heat input on the microstructure and toughness of simulated subregions was investigated by welding thermal simulations and Charpy impact tests. The results indicate that the microstructures of the simulated coarse and fine grain heat affected zone (CGHAZ and FGHAZ) gradually changed from lath martensite to a mixture of lath martensite/lath bainite and finally to granular bainite with the increase in heat input. The microstructure of the simulated intercritical heat affected zone (ICHAZ) was mainly composed of granular bainite and blocky martensite regardless of heat input. When the heat input increased, the toughness of the simulated CGHAZ and ICHAZ continuously decreased. Nevertheless, the simulated FGHAZ still displayed good toughness (53.16 J) due to its fine structure. The occurrence of martensite-austenite (M-A) constituents was the main reason for the decrease in crack initiation energy of the simulated CGHAZ and ICHAZ at high values of heat input, and the toughness deteriorated as the size of M-A constituents increased. It should be noted that high-misorientation packet and block boundaries can effectively deviate or arrest the propagation of microcracks. When the heat input was in the approximate range of 8.63-14.95 kJ cm{sup −1}, all of the simulated subregions exhibited good toughness.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Composition-Dependent Microstructure-Property Relationships of Fe and Al Modified Ti-12Cr (wt.%)

b-Titanium (Ti) alloys have applications in several industries (e.g. aerospace, automotive, and biomedical) where material performance requirements vary widely. To tailor the microstructure and mechanical properties of b-Ti alloys for various applications, it is critical to understand the influence of individual alloying elements. Toward this goal, we investigated the effect of individual alloying additions on the microstructure and resultant mechanical properties of four model b-Ti alloys: Ti-12Cr, Ti-12Cr-3Al, Ti-12Cr-1Fe, and Ti-12Cr-1Fe3Al (wt.%). The microstructures of these alloys were studied using x-ray diffraction, electron microscopy, and atom probe tomography. The mechanical properties were analyzed via Vickers and Rockwell hardness measurements and tensile testing. The addition of 1 wt.% Fe resulted in an approximate 5% increase in elongation-to-failure (ef), while the addition of 3 wt.% Al did not appear to significantly affect ef. The addition of Fe and Al decreased the yield and ultimate tensile strengths.

Ballor, Joann P.↗
DOE OSTI2020

Evolution of Microstructure, Residual Stress, and Tensile Properties of Additively Manufactured Stainless Steel Under Heat Treatments

This study investigated the microstructure, residual stress and tensile properties of the directed energy deposited stainless steel 316L (SS316L) under thermal annealing. Microstructure characterization shows the as-printed sample has laser-generated patterns where dendritic structure is observed at the edge of the patterns and cellular structure dominates the interior region. Here, the thermal annealing at 983 and 1093°C effectively removes the dendritic/cellular structures. Synchrotron X-ray diffraction reveals that the as-fabricated SS316L exhibits compressive residual stress of -197.4 MPa. The stress is greatly relieved to -53.8 MPa after annealing at 1093°C. The room temperature tensile testing indicates that the yield strength and ultimate tensile strength drop from 378 and 502 MPa in the as-printed sample to 258 and 446 MPa in the annealed samples (1093°C), respectively. Our study provides insights into the relationship between microstructure, residual stress, and tensile properties of laser additive manufactured SS316L.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Microstructure and Residual Stress in Functionally Graded 316L Stainless Steel/Inconel 625 Alloys Fabricated by Direct Energy Deposition

Functionally graded materials (FGMs) provide a unique solution to combine distinct properties within a single component to enhance its overall performance. Understanding the microstructure and residual stress distribution is particularly important as the material dissimilarity in FGMs can result in high residual stress that affects the mechanical integrity of structural components. Here, this work aims to study the microstructure and residual stress of additively manufactured FGMs and the effects of isothermal annealing. In this study, 316L stainless steel/Inconel 625 FGMs were built by direct energy deposition, and characterized by optical and electron microscopy as well as synchrotron-based X-ray diffraction. Our study reveals that thermal annealing at 500°C for 3 h has minimal effects on the microstructure and chemistry of the graded materials, but effectively relieves the residual stress and leads to a uniform distribution of residual stress.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Relating microstructure to defect behavior in AA6061 using a combined computational and multiscale electron microscopy approach

In this paper, a multiscale electron microscopy-based approach is applied to understanding how different aspects of the microstructure in a notched AA6061-T6, including grain boundaries, triple junctions, and intermetallic particles, promote localized dislocation accumulation as a function of applied tensile strain and depth from the sample surface. Experimental measurements and crystal plasticity simulations of dislocation distributions as a function of distance from specified microstructural features both showed preferential dislocation accumulation near intermetallic particles relative to grain boundaries and triple junctions. High resolution electron backscatter diffraction and site-specific transmission electron microscopy characterization showed that high levels of dislocation accumulation near intermetallic particles led to the development of an ultrafine sub-grain microstructure, indicative of a much higher level of local plasticity than predicted from the coarser measurements and simulations. In addition, high resolution measurements in front of a crack tip suggested a compounding influence of intermetallic particles and grain boundaries in dictating crack propagation pathways.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Microstructure and properties of a high temperature Al–Ce–Mn alloy produced by additive manufacturing

An Al–10Ce-8Mn (wt%) alloy was designed and fabricated by laser powder bed fusion additive manufacturing (AM). The rapid cooling rates of the AM process produced a refined microstructure with a large fraction of reinforcing intermetallic phases. The tensile properties of the alloy were characterized in the as-fabricated state and following thermal exposure. The properties of the as-fabricated microstructure showed exceptional high-temperature performance and strength retention at elevated temperatures up to 400 °C relative to benchmark wrought Al and AM Al alloy properties. Characterization of the microstructure and thermodynamic modeling of the ternary Al–Ce–Mn system rationalized the solidification and solid-state phase transformations. Finally, analysis of the relevant strengthening mechanisms for both the as-fabricated and thermally exposed conditions was performed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Uniaxial compression of [001]-oriented CaFe 2 As 2 single crystals:the effects of microstructure and temperature on superelasticity Part I: Experimental observations

Micropillar compression experiments on [001]-oriented CaFe 2 As 2 single crystals have recently revealed the existence of superelasticity with a remarkably high elastic limit of over 10%. The collapsed tetragonal phase transition, which is a uni-axial contraction process in which As-As bonds are formed across an intervening Ca-plane, is the main mechanism of superelasticity. Usually, superelasticity and the related structural transitions are affected strongly by both the microstructure and the temperature. As such, in this study, we investigated how the microstructure and temperature affect the superelasticity of [001]-oriented CaFe 2 As 2 micropillars cut from solution-grown single crystals, by performing a combination of in-situ cryogenic micromechanical testing and transmission electron microscopy studies. Our results show that the microstructure of CaFe 2 As 2 is influenced strongly by the crystal growth conditions and by subsequent heat treatment. The presence of Ca and As vacancies and FeAs nanoprecipitates affect the mechanical behavior significantly. In addition, the onset stress for the collapsed tetragonal transition decreases gradually as the temperature decreases. These experimental results are discussed primarily in terms of the formation of As-As bonds, which is the essential feature of this mechanism for superelasticity. Our research outcomes provide a more fundamental understanding of the superelasticity exhibited by CaFe 2 As 2 under uni-axial compression.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Designing shape-memory-like microstructures in intercalation materials

During the reversible insertion of ions, lattices in intercalation materials undergo structural transformations. These lattice transformations generate misfit strains and volume changes that, in turn, contribute to the structural decay of intercalation materials and limit their reversible cycling. In this paper, we draw on insights from shape-memory alloys, another class of phase transformation materials, that also undergo large lattice transformations but do so with negligible macroscopic volume changes and internal stresses. We develop a theoretical framework to predict structural transformations in intercalation compounds and identify crystallographic design rules necessary for forming shape-memory-like microstructures in intercalation materials. We use our approach to systematically screen open-source structural databases comprising $n$ > 5000 pairs of intercalation compounds. We identify candidate compounds, such as Li x Mn 2 O 4 (Spinel), Li x Ti 2 (PO 4 ) 3 (NASICON), that approximately satisfy the crystallographic design rules and can be precisely doped to form shape-memory-like microstructures. Throughout, we compare our analytical results with experimental measurements of intercalation compounds. We find a direct correlation between structural transformations, microstructures, and increased capacity retention in these materials. These results, more generally, show that crystallographic designing of intercalation materials could be a novel route to discovering compounds that do not decay with continuous usage.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Understanding coarsening of a post-corrosion microstructure in a molten salt by combining phase-field modeling and in situ tomography

Alloys corroding in molten salt have been observed to form bicontinuous, nanoporous microstructures via dealloying, which subsequently undergo coarsening due to facile transport in high-temperature conditions. In this work, we describe a methodology to elucidate the underlying transport mechanisms during coarsening of a bicontinuous microstructure via quantitative comparisons between phase-field simulations and four-dimensional in situ experiments, in this case X-ray nanotomography of the coarsening of a dealloyed 80 wt% Ni-20 wt% Cr microwire in molten KCl-MgCl 2 at 800°C. We conduct phase-field simulations initialized from experimental data to model coarsening via three different transport mechanisms: surface diffusion, solid bulk diffusion, and liquid bulk diffusion. These simulations reproduce key features of the experiment, such as the densification of the outer layer of the dealloyed wire and the reduction in radius over time. We quantitatively compare different microstructural characteristics between the simulations and experiment and extract temporal scaling factors that optimally match the time scales of the simulations to that of the experiment. This allows us to evaluate morphological similarity between the simulations and experiment and relate the experimental coarsening kinetics to fundamental material properties. We find that surface diffusion is most likely to be the dominant coarsening mechanism, and its kinetics imply a surface diffusivity of D S = 8.9 x 10 -20 m 3 /s, which is within the range of reported values for Ni-vacuum interfaces at 800°C. However, the difference between the experiment and the surface diffusion simulation increases substantially at late times, suggesting that other mechanisms, such as the dissolution of residual Cr, may be at play.

36 MATERIALS SCIENCE↗