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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Microstructure and mechanical behavior of hot-work tool steels processed by Selective Laser Melting

Highlights: • H11 and a modified H11 hot work tool steels have been produced by SLM. • As-built alloys are characterized by α-Fe cells surrounded by C-rich γ-Fe regions. • Rapidly solidified structures revealed able to respond to precipitation hardening. • Mechanical behavior of AM H11 is similar to that obtained by conventional processes. - Abstract: The present study is aimed at identifying and testing high-strength alloys for tooling applications featuring suitable processability for laser-based additive manufacturing technologies. The microstructure and mechanical properties of the H11 hot-work tool steel and a leaner version of the same alloy (L-H11) processed by Selective Laser Melting were assessed as a function of specific microstructural conditions obtained by performing different heat treatments. Tempering was performed on quenched alloys or simply from as built material. The rapidly solidified microstructures revealed able to respond directly to precipitation hardening treatment without performing any prior solution annealing. The microstructure of the as-built alloys revealed characterized by α-Fe dendritic cells decorated at boundaries by C-rich γ-Fe regions. Air quenching was responsible for the transformation of the solidification cells into lath martensitic structures and for the formation of the M{sub 3}C phase, which transformed into more complex carbide species on tempering. The hardness of quenched and tempered H11 steel is similar to that obtained by processing the alloy with conventional routes, and the final hardness gap between the two SLM processed H11 and L-H11 alloys treated according to optimal tempered condition was limited to 62 HV.

36 MATERIALS SCIENCE↗
DOE OSTI2018

EBSD characterization of the eutectic microstructure in hypoeutectic Fe-C and Fe-C-Si alloys

Highlights: • EBSD investigation of eutectic cementite and austenite in as-solidified low-alloy cast iron • Crystallographically independent growth of rapidly solidified cementite/austenite eutectic • Epitaxial growth of eutectic austenite from primary dendrites • Eutectic cementite and eutectoid ferrite frequently obey Bagaryatsky or Pitsch-Petch orientation relationship. - Abstract: Hypoeutectic Fe-C and Fe-C-Si model alloys were produced at different solidification conditions. Copper mold casting yields low cooling rates promoting the formation of a eutectic microstructure, which is characterized by two morphologies: elongated cementite plates and a rod structure growing perpendicular to the plates, i.e. austenite rods in a cementite matrix. Electron beam surface remelting generates a mainly plate-like eutectic due to rapid solidification. The microstructures were characterized by light-optical microscopy and electron backscatter diffraction (EBSD). The latter allows for a spatially resolved investigation of the growth crystallography of the eutectic phases. Thereby, a possible existence of crystallographic orientations relationships between cementite and austenite within the plate-like eutectic was assessed experimentally. The eutectic phases were found to grow largely crystallographically independently. Moreover, ferrite and eutectic cementite within the decomposed eutectic microstructure frequently comply with the Bagaryatsky or the Pitsch-Petch orientation relationship. Complementary X-ray diffraction (XRD) analysis reveals a pronounced cementite {002} texture in the microstructure produced by mold casting. Characteristic changes in the lattice parameters indicate that as-cast cementite is non-stoichiometric.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure evolution in the conventional single side and bobbin tool friction stir welding of thick rolled 7085-T7452 aluminum alloy

Highlights: • Comparison of microstructure evolution via EBSD of SS-FSW and BB-FSW was investigated. • Friction heat, strain rate and degree of recrystallization affected grain size. • Discontinuously dynamic recrystallization occurred in the weld nugget zone. • The degree of recrystallization was mainly controlled by the strain rate. - Abstract: The microstructure evolution during the conventional single side friction stir welding (SS-FSW) and the bobbin tool FSW (BB-FSW) of 7085-T7452 alloy thick plate was extensively studied by using the state-of-the-art materials characterization techniques. The Electron backscattering diffraction results reveal that a significant grain refinement and a larger percentage of high angle grain boundaries (HAGBs) exist in the weld nugget zone (WNZ) and the thermal-mechanical affected zone (TMAZ) compared with the base material. Meanwhile, more obviously inhomogeneous refined grains and HAGBs in WNZ along the thickness direction and a larger percentage of low angle grain boundaries (LAGBs) were observed in SS-FSW joint than that in BB-FSW joint. The change of the average grain size is the result of friction heat and strain rate of plastic deformation combined with a certain degree of recrystallization. Moreover, the recrystallization is mainly controlled by the strain rate during the FSW process. The presence of Cube and Cube ND after welding shows that the discontinuously dynamic recrystallization (DDRX) is a major factor in the final microstructural response. The research and findings help the understanding of the mechanism of microstructure homogenization and further optimizing the welding process for application.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Comparison of microstructural and mechanical properties of friction stir spot welded ultra-high strength dual phase and complex phase steels

Highlights: • The microstructure and mechanical properties of FSSW of ultra high strength DP and CP steels are compared. • The stir zone (SZ) of CP steel mainly consists of bainite and martensite; no bainite is identified in the SZ of DP steel. • The cooling was not fast enough for the CP steel to suppress the effect of chemical composition on phase transformation. - Abstract: The microstructure and mechanical properties of friction stir spot welding of ultrahigh strength dual phase (DP) and complex phase (DP) steels are compared based on the experimental results. For both DP and CP steels, FSSW joints are successfully fabricated using the same process parameter set. While the microstructure of joints are characterized in detail using scanning electron microscopy, electron back scattered diffraction, and transmission electron microscopy, the mechanical property of joints are evaluated by shear tensile tests. The result of the microstructural analysis shows that the stir zone (SZ) of CP steel mainly consists of bainite and martensite, while no bainite is identified in the SZ of DP steel. The existence of bainite in the SZ of CP steel suggests that the cooling rate, which is nearly identical for both CP and DP steels, was not fast enough to suppress the effect of the alloying composition, particularly that of Mn, on the phase transformation. The result of the present study contributes to the development of solid state joining process of the ultrahigh strength steels.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of laser rescanning on the grain microstructure of a selective laser melted Al-Mg-Zr alloy

Highlights: • Laser rescanning of a new Al-Mg-Zr alloy was investigated. • SLM of the Al-Mg-Zr alloy results in a duplex grain structure. • Rescanning of the alloy results in a refinement of the microstructure. - Abstract: The microstructures of alloys created via Additive Manufacturing (AM) can vary substantially from those present in cast or wrought products, due to the very rapid solidification associated with AM. While numerous studies have investigated the process-microstructure relationship of alloys created by Selective Laser Melting (SLM), few have investigated the effects of laser rescanning to alter the microstructure or take advantage of the rapid solidification conditions the process provides. This study investigates the effect of single- or multiple pass laser scanning upon the grain structure of Addalloy™, a new Al-Mg-Zr alloy strengthened via L1{sub 2} Al{sub 3}Zr precipitates. The bottom of the melt pools consisted of fine equiaxed grains (1.3 μm) that nucleated from primary Al{sub 3}Zr (100–400 nm) precipitates. The top of the melt pool consists of columnar grains (up to 40 μm long), consistent with lack of Al{sub 3}Zr nucleants due to Zr solute trapping from increased solidification velocities. Additional laser scanning (a second or third scan) reduces the amounts of columnar grains and increased the number equiaxed grains. The change is attributed to a shallower melt pool remelting the columnar grain region upon rescanning, due to reduced laser energy absorption and increased heat losses in the solid.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Temperature-dependent cyclic response and microstructure of AlSi10Mg(Cu) alloy

Highlights: • The cyclic behavior of AlSi10Mg(Cu) alloy in LCF regime is temperature-dependent. • The transition temperature of the cyclic behavior is about 180 °C. • Microstructure transitions were observed with increasing temperature. - Abstract: In the present work, the cyclic stress response and microstructure evolution of AlSi10Mg(Cu) alloy at high temperature were investigated. Samples were tested at high temperature, under total strain control and tensile-compression loading (R = −1) until failure. The cyclic stress response was analyzed, as well as the microstructures were examined by TEM. The results demonstrated that the cycle stress response of the studied alloy is experimental temperature dependence. Microstructure analysis indicated the precipitated phase was disruptive changed by high temperature compared to the as-received alloy. A lot of coherent β″-Mg{sub 2}Si precipitates and GP zones were observed in the as-received alloy, resulting in sustaining cyclic hardening by pinning effect. At high temperature, the vast majority of the tiny coherency precipitates and clusters transformed to the large semi-coherency Q′ and β′-Mg{sub 2}Si precipitates, leading to the sharp decrease of pinning number and the change of dislocation interaction mechanism, which is believed to be one of the reasons for softening. Another reason is that high temperatures can promote dislocations movement.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Quantification of complex-phase steel microstructure by using combined EBSD and EPMA measurements

Highlights: • In CP800 steel, bainite is the dominant phase with a typical lath (width 300 nm) structure and cementite precipitates. • Martensite is dispersed as small islands and only a few narrowed martensite bands are observed which has been induced by annealing process of cold rolled sheet. • The precise carbon content is measured in bainite and martensite utilizing EPMA line scans, which is around 0.2 wt% and 0.4 wt%, respectively. • Digitalization of the microstructure for CP800 is achieved by combining EBSD and EPMA techniques. • The final constituent map consists of plenty of microstructural features including topography, crystal structures, orientations and elemental distribution. - Abstract: We combined electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) techniques to characterize the microstructure of industrial produced complex-phase CP800 steel. First, three major constituents (ferrite, martensite and bainite) are characterized. Then, transformation-induced dislocation zones in ferrite and bainitic regions consisting of different sub-structures are quantified and segmented. A final digitalized component map including plenty of microstructural features including crystal structures, orientations and elemental distribution are successfully achieved. The precise carbon content is measured in bainite and martensite utilizing EPMA line scans, which is around 0.2 wt% and 0.4 wt%, respectively. In the bainite phase of CP800, a typical Bagaryatski orientation relationship between cementite and lath-like bainitic ferrite is revealed based on the select diffraction patterns of transmission electron microscopy measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of niobium and phase transformation temperature on the microstructure and texture of a novel 0.40% C thermomechanically processed steel

Field emission scanning electron microscopy, electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) have been employed to investigate the effect of niobium and phase transformation temperature on the evolution of microstructure and texture in a novel thermomechanically processed, medium‑carbon, low-alloy steel intended for slurry pipeline applications. Thermomechanical processing consisted of hot-rolling in the austenitic region with deformation both above the recrystallization limit temperature and below the recrystallization stop temperature. Immediately after rolling, specimens were directly quenched in water to two different temperatures of 560 °C and 420 °C and subsequently furnace cooled from those temperatures to simulate the cooling of coiled strip on a hot strip mill. The microstructure of samples quenched to 560 °C mostly comprised of upper bainite, whereas the samples quenched to 420 °C mainly consisted of lath-type lower bainite. The transformation texture of all samples at the mid-thickness position consisted of α, γ and ε-fibers with high intensities close to the transformed copper, transformed brass and rotated cube components. The addition of 0.013 wt% Nb refined the microstructure and sharpened the texture. The texture of the small fraction of retained austenite present in the final microstructures indicated that the main bcc texture components result from the brass and copper components in the parent austenite.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Selective electron beam melting of TiAl alloy: Microstructure evolution, phase transformation and microhardness

Highlights: cThe microstructure variation with build height of TiAl alloy was investigated. • The microhardness rapidly increased with build height increasing from 19 mm to 20 mm. • The mechanism of microstructure evolution was illuminated during SEBM. - Abstract: In this research, a Ti-47Al-2Cr-2Nb alloy was fabricated by selective electron beam melting (SEBM). The microstructures at different heights of SEBM-produced TiAl alloy sample and their contribution to microhardness were investigated. The results revealed that the last deposited layers displayed near lamellar structure with fine α{sub 2} lamellae. Lamellar colonies degraded at the height of 19 mm of the as-built sample, accompanied with the α{sub 2} lamellae decomposition and B2 precipitation along primary α{sub 2} lamellae. Meanwhile, equiaxed γ and blocky B2 phase dispersed on the colony boundaries. Lamellar colonies further degraded and colony boundaries further coarsen at the heights of 15 mm and 5 mm due to the in-situ heat treatment during SEBM process. Three kinds of phase transformations could be observed during SEBM process, which were α{sub 2} → γ, α{sub 2} → B2 and α{sub 2} + γ → B2. The microhardness exhibited slight fluctuation as the building height increased from 3 mm to 19 mm, and the microhardness rapidly increased with the building height increasing from 19 mm to 20 mm. Eventually, the mechanism of microstructure evolution during SEBM process was discussed in detail.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Plastic flow and microstructural instabilities during high-pressure torsion of Cu/ZnO composites

Highlights: • Hybrid bulk metal - ceramic nanoparticle material was processed by HPT. • ZnO nanoparticles exhibited significant plasticity of up to 86%. • Hydrodynamic instability determined the flow mode of ZnO particle clusters. • Microstructural stability of Cu decreased due to the co-deformation with ZnO. - Abstract: We conducted high-pressure torsion (HPT) of hybrid materials composed of two copper disks with a layer of ZnO ceramic nanoparticles sandwiched in-between. A detailed microstructural analysis of the processed samples revealed a surprisingly high level of plastic deformation introduced in individual ZnO particles, even though they possess a much high hardness than the surrounding Cu. Moreover, we demonstrated that by tailoring the initial geometry of the system (i.e. introducing holes in the cladding Cu disks), we can effectively change the flow mode of ZnO particle clusters from laminar to turbulent one and facilitate their deagglomeration process. Finally, the presence of ZnO particles decreased the microstructural stability of the Cu matrix. We discussed the obtained results in terms of hydrodynamic and microstructural instabilities during plastic deformation. This work expands our understandings of the cooperative severe plastic deformation of two dissimilar materials and sheds new light on the design of hybrid materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Elucidation of microstructural evolution of beta-type titanium alloy joint during friction stir welding using liquid CO2 cooling

Highlights: • Microstructural evolution during FSW was elucidated along real material flow line. • Stirring and cooling stages of FSW can be investigated separately using CO{sub 2} cooling. • In stirring stage, discontinuous dynamic recrystallization predominantly occurred. • In cooling stage, static recovery and grain growth mainly occurred. - Abstract: Normal friction stir welding (FSW) and liquid CO{sub 2} cooling FSW were conducted on the β-type Ti-15V-3Cr-3Al-3Sn (Ti-15-3, mass%) alloy plates under a processing condition. Since liquid CO{sub 2} cooling with high cooling efficiency can freeze the microstructure formed during the high-temperature stirring stage of FSW, it is possible to separately elucidate the microstructural evolution of the Ti-15-3 alloy during the high-temperature stirring stage and the post-stirring cooling stage in FSW. The deformation temperature during the normal and the CO{sub 2} cooling FSW was similar in the top and the bottom of stir zone, which is considered to be owing to that the top and bottom are located adjacent to the heat sources, i.e., the shoulder and probe surfaces. In the high-temperature stirring stage of FSW, discontinuous dynamic recrystallization predominantly occurred, and a simple shear texture generated in an early stage of the material flow around the tool, then the microstructure started to evolve steadily after a steady-state dynamic recrystallization. In the post-stirring cooling stage of FSW, static recovery and grain growth were found to mainly occur within the stir zone.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Modeling deformation and failure in AlSi-polyester abradable sealcoating material using microstructure-based finite element simulation

A plasma-sprayed aluminum-silicon (AlSi)/polyester coating is applied in modern gas turbine engines as an abradable sealcoating to maintain tight clearances between the rotating blades and the static casing. While running the engine, the rotating blades “rub” with the abradable coating, which results in extreme strain rate (up to 10 6 ) dynamics and a high-temperature environment. Due to the difficulty of collecting direct measurements, predictive computational models are important for analyzing the deformation and failure of the abradable material, to help meet the design target of avoiding damage to the blade tip and maintaining high fuel efficiency. In this research, a microstructure-based finite element (FE) computational model was developed to capture the complex mechanical behavior of the AlSi/polyester microstructure. The model is based on a virtual representative-volume-element (RVE) of a metal-polymer microstructure, reconstructed from x-ray computed tomography. It models the plastic deformation of, and damage to, each AlSi and polyester constituents, as well as the failure at their interface. The model was calibrated and validated with uniaxial tension and compression experiments, conducted at two temperatures (298 K and 533 K) at an applied strain rate of 10 3 - 10 4 s -1 . The material exhibited strongly asymmetric tension-compression behavior and a sensitivity to temperature, which was well captured by the model. The model was further applied to investigate changes in mechanical behavior due to variations in constituents’ volume fractions, which provides guidance to the microstructural design of AlSi/polyester abradable materials. The model is expected to facilitate the development of improved abradable materials by bypassing the conventional trial-and-error approach and extensive testing requirements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Studies on phase formation, microstructure development and elastic properties of reduced NiO-8YSZ anode supported bi-layer half-cell structures of solid oxide fuel cells

Highlights: Black-Right-Pointing-Pointer Detailed study on the development of microstructure and phase in NiO-8YSZ anodes. Black-Right-Pointing-Pointer Detailed study on elastic properties at high temperatures in air/reducing atmosphere. Black-Right-Pointing-Pointer Effects of initial porosity, composition and other issues are evaluated in detail. -- Abstract: Half-cell structures of solid oxide fuel cells (SOFCs) with a thin and dense electrolyte layer of 8YSZ supported by a thick and porous NiO-8YSZ anode precursor structure were reduced in a gas mixture of 5% H{sub 2}-95% Ar at 800 Degree-Sign C for selected time periods in order to fabricate cermets with desired microstructure and composition, and to study their effects on the elastic properties at ambient and reactive atmospheres. It appears that 2 h of exposure to the reducing conditions is enough to reduce {approx}80% of NiO with an enhanced porosity value of 35%. The Ni-8YSZ cermet phase formation in the anode was analyzed with X-ray diffraction (XRD) in correlation with its microstructure. The elastic properties were determined after the reduction, at room and elevated temperatures using the impulse excitation technique. At room temperature the decrease in the Young's modulus was about 44% (after 8 h of reduction) and can be attributed mainly to the changes in the microstructure, particularly the increase in porosity from {approx}12% to 37%. Young's moduli of the as-received precursor and reduced anodes were evaluated as a function of temperature in air and reducing atmosphere. The results were explained in correlation to the initial porosity, composition and oxidation of Ni at the elevated temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Li-ion battery design through microstructural optimization using generative AI

Lithium-ion batteries are used across various applications, necessitating tailored cell designs to enhance performance. Optimizing electrode manufacturing parameters is a key route to achieving this, as these parameters directly influence the microstructure and performance of the cells. However, linking process parameters to performance is complex, and experimental or modeling campaigns are often slow and expensive. This study introduces a fast computational optimization framework for electrode manufacturing parameters. A generative model, trained on a small dataset of microstructural images associated with different manufacturing parameters, efficiently generates representative microstructures for new parameters. This model is integrated into a Bayesian optimization loop that includes microstructure generation, characterization, and simulation, aiming to find optimal manufacturing parameters for a particular application. Significant improvement in the energy density of a 4680 cell is achieved through bespoke cell design, highlighting the importance of cell-scale normalization. The framework’s modularity allows its application to various advanced materials manufacturing scenarios.

batteries↗
DOE OSTI2021

Effect of heterogeneous microstructure on the tensile and creep performances of cast Haynes 282 alloy

Precipitation-strengthened Ni-based superalloys are leading candidate materials for advanced ultra-supercritical (A-USC) plants with steam conditions up to 760 °C (1400 °F) and 35 MPa (5 ksi). This study evaluates representative specimens from a large casting of Haynes 282 to study the effect of microstructural heterogeneity on the mechanical behavior of this alloy. The tensile test results of cast Haynes 282 over the temperature range 20–816 °C exhibited lower tensile strength and ductility in comparison with the reference wrought Haynes 282. However, the creep rupture tests of the cast alloy below 704–788 °C and 190–431 MPa presented a similar stress-Larson-Miller parameter to that of the wrought material. Microstructural and dislocation characterizations using scanning electron microscopy, conventional transmission electron microscopy, and scanning transmission electron microscopy in conjunction with energy dispersive X-ray spectroscopy were performed to understand the microstructural evolution before and after the mechanical tests. The heterogeneous microstructures of the cast Haynes 282 material, including the coarse grains, potential casting defects, and a bimodal size distribution of γ' precipitates, were detrimental to the tensile behavior, whereas the coarse-scale grains had a positive effect on the creep performance because diffusional creep was the dominant creep mechanism.

tensile testing↗
DOE OSTI2024

Alloying effects on the microstructure and properties of laser additively manufactured tungsten materials

A large body of literature within the additive manufacturing (AM) community has focused on successfully creating stable tungsten (W) microstructures due to significant interest in their application for extreme environments. However, cracking and additional embrittling features at grain boundaries have resulted in poorly performing materials, stymying the application of AM as a manufacturing technique for W. Several alloying strategies, such as ceramic particles and ductile elements, have emerged with the promise to eliminate cracking while simultaneously enhancing stability against recrystallization. Here, in this work, we provide new insights regarding the defects and microstructural features that result from the introduction of ZrC for grain refinement and NiFe as a ductile reinforcement phase – in addition to the resulting thermophysical and mechanical properties. ZrC is shown to promote microstructural stability with increased hardness due to the formation of ZrO 2 dispersoids. Conversely, NiFe forms into micron-scale FCC phase regions within a BCC W matrix, producing enhanced toughness relative to pure AM W. A combination of these effects is realized in the WNiFe + ZrC system and demonstrates that complex chemical environments coupled with the tuning of AM microstructures provides an effective pathway for enabling laser AM W materials with enhanced stability and performance.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Digital polycrystalline microstructure generation using diffusion probabilistic models

Accurate micromechanical simulation of polycrystalline materials requires a realistic digital representation of the grain scale microstructure. Here, this work demonstrates the use of a generative diffusion probabilistic model for synthesizing single phase polycrystalline realizations. The model performs well and is capable of producing realistic microstructures consisting of not just simple equiaxed structures but also structures exhibiting more complex spatial arrangements. Masked microstructure generation reveals that the model is context aware of morphological descriptors which may be encoded in the latent space. Training on more diverse data sets, with scaled up architectures, may enable development of future models capable of synthesizing even more complex microstructural features.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Mesoscale Modeling Approach for Quantifying Microstructure-Aware Micromechanical Responses in Metal Hydrides

Metal hydrides can undergo significant volume changes upon hydrogen uptake and release, which induce a mechanical response that depends not only on the evolving hydrogen composition but also on the microstructure. We present a comprehensive mesoscale modeling framework based on microelasticity theory to quantify the micromechanical responses of metal hydrides, specifically focusing on a hydrogenating polycrystalline MgH 2x particle within a host material as a model micromechanical system. Utilizing digitally generated realistic microstructures and density-functional-theory-derived parameters, we analyzed highly nonuniform local stress profiles in the polycrystalline hydrides under the clamping force exerted by the host during hydrogenation. Our framework also allows us to predict the corresponding strain energy accumulation and mechanical hot spots formation in the hydrides, highlighting their roles in thermodynamic destabilization and mechanical failure, respectively. Through extensive parametric simulations, we further quantified the influence of interface type, crystallinity, grain size, loading ratio, and host stiffness, providing practical guidance for optimizing microstructural design and host material selection. This proposed approach is broadly applicable to micromechanical systems with complex microstructural features involving chemical reaction- and/or phase-transformation-induced deformation.

36 MATERIALS SCIENCE↗