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At least 1,225 records · Page 68
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Selective laser melting of 304L stainless steel: Role of volumetric energy density on the microstructure, texture and mechanical properties

The role of volumetric energy density on the microstructural evolution, texture and mechanical properties of 304L stainless steel parts additively manufactured via selective laser melting process is investigated. 304L is chosen because it is a potential candidate to be used as a matrix in a metal matrix composite with nanoparticles dispersion for energy and high temperature applications. The highest relative density of 99 %±0.5 was achieved using a volumetric energy density of 1400 J/mm 3 . Both XRD analysis and Scheil simulation revealed the presence of a small trace of the delta ferrite phase, due to rapid solidification within the austenitic matrix of 304L. Here, a fine cellular substructure ranged between 0.4–1.8 μm, was detected across different energy density values. At the highest energy density value, a strong texture in the direction of [100] was identified. At lower energy density values, multicomponent texture was found due to high nucleation rate and the existing defects. Yield strength, ultimate tensile strength, and microhardness of samples with a relative density of 99 % were measured to be 540 ± 15 MPa, 660 ± 20 MPa and 254 ± 7 HV, respectively and higher than mechanical properties of conventionally manufactured 304L stainless steel. Heat treatment of the laser melted 304L at 1200 °C for 2 h, resulted in the nucleation of recrystallized equiaxed grains followed by a decrease in microhardness value from 233 ± 3 HV to 208 ± 8 HV due to disappearance of cellular substructure.

304L stainless steel↗
DOE OSTI2015

Crystallographic orientation variation of isothermal pearlite under high magnetic field

Crystallographic orientation (CO) variation of magnetic-induced pearlite (MIP) during its microstructure evolution in 19.8 T was investigated by electron back-scatter diffraction (EBSD). It is closely related to the isothermal temperatures (ITs) and the applied magnetic time (MT) during the process of MIP formation. The <100> easy magnetization direction in MIP colonies is strengthened with the MT within the certain transformed fraction of MIP (f{sub MIP}) at the relatively lower IT (983 K) above the eutectoid temperature but below the magnetically shifted upward eutectoid temperature, while this special CO tends to be weakened at a relatively higher IT (995 K). For the same MT, the higher the IT, the relatively larger is the proportion in <100> orientation for MIP colonies at the early growth stage. These results have demonstrated that the change of <100> orientation of MIP is closely related to the growth rate of pearlite ferrite (PF), and strengthened mainly at early transformation stage. When f{sub MIP} reaches some value, the growth rate of MIP at other COs, such as <110>, even at the hard magnetization direction, turns to present speed-up. - Highlights: • HMF can induce pearlite with different fractions above the eutectoid temperature. • CO is closely related to isothermal temperatures and applied magnetic time. • <100> direction is related to the growth rate of PF, and strengthened at early stage. • When f{sub MIP} reaches some value, the growth rate at other COs turns to present speed-up.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Hot deformation characterization of duplex low-density steel through 3D processing map development

The high temperature deformation behavior of duplex low-density Fe–18Mn–8Al–0.8C steel was investigated at temperatures in the range of 600–1000 °C. The primary constitutive analysis indicated that the Zener–Hollomon parameter, which represents the coupled effects of temperature and strain rate, significantly varies with the amount of deformation. Accordingly, the 3D processing maps were developed considering the effect of strain and were used to determine the safe and unsafe deformation conditions in association with the microstructural evolution. The deformation at efficiency domain I (900–1100 °C\10{sup −} {sup 2}–10{sup −} {sup 3} s{sup −} {sup 1}) was found to be safe at different strains due to the occurrence of dynamic recrystallization in austenite. The safe efficiency domain II (700–900 °C\1–10{sup −} {sup 1} s{sup −} {sup 1}), which appeared at logarithmic strain of 0.4, was characterized by deformation induced ferrite formation. Scanning electron microscopy revealed that the microband formation and crack initiation at ferrite\austenite interphases were the main causes of deformation instability at 600–800 °C\10{sup −} {sup 2}–10{sup −} {sup 3} s{sup −} {sup 1}. The degree of instability was found to decrease by increasing the strain due to the uniformity of microbanded structure obtained at higher strains. The shear band formation at 900–1100 °C\1–10{sup −} {sup 1} s{sup −} {sup 1} was verified by electron backscattered diffraction. The local dynamic recrystallization of austenite and the deformation induced ferrite formation were observed within shear-banded regions as the results of flow localization. - Graphical abstract: Display Omitted - Highlights: • The 3D processing map is developed for duplex low-density Fe–Mn–Al–C steel. • The efficiency domains shrink, expand or appear with increasing strain. • The occurrence of DRX and DIFF increases the power efficiency. • Crack initiation at α/γ interphase and shear banding lead to failure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Cooling Rate on Microsegregation During Solidification of Superalloy INCONEL 718 Under Slow-Cooled Conditions

The solidification sequence, microstructural evolution, solid-liquid interface variation, interdendritic segregation, and elemental distribution of as-cast IN718 alloy at three slow-cooling rates (5, 10, and 20 °C/min) were investigated by differential scanning calorimetry (DSC), confocal laser scanning microscopy (CLSM), optical microscopy (OM), field-emission scanning electron microscopy (FESEM), and electron-probe microanalysis (EPMA) techniques. The results indicate that as the cooling rate decreases, the constitutional supercooling at the solidification front affects the solid-liquid interface more significantly, and the size and quantity of the Laves phase increase. However, the composition of the Laves phase is insensitive to the cooling rate in the range of conditions studied here. In dendrite core, the contents of Ni, Cr, Fe, and Al follow a slight downward trend with an increasing cooling rate, whereas the Nb, Mo, and Ti contents show an upward trend. Additionally, Mo shows a stronger propensity to segregate under slow-cooling conditions because its effective partition coefficient almost linearly decreased with decreasing cooling rate, which is same as Nb. Using the parameters experimentally determined in this study and the Clyne–Kurz equation, we achieved reasonable agreement between the calculated and measured liquid composition change during the solidification process at different cooling rates. All experimental and theoretical programs in this research were undertaken with the aim of gaining further understanding of the microsegregation behaviors in large-scale IN718 ingots, whose cooling rates are within a lower range.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Chromium Tolerant, Highly Active and Stable Electrocatalytic Internal Surface Coating for Cathode of Commercial SOFCs (Final Report)

This project is aimed to develop a chromium (Cr) tolerant, highly active, and stable coating layer on the internal surfaces of the porous composite cathode from commercially available SOFCs. Such coating layer was developed using the additive manufacturing process of Atomic Layer Deposition (ALD) and has been applied on the cathode consisting of either an electronic conductor of LaxSr 1-x MnyO 3-δ (LSM) or mixed ionic and electronic conducting La x Sr 1-x Co y Fe 1-y O 3-δ (LSCF). PI's work has demonstrated that the internal surface of cathode from the commercial cells, can be further tailored using ALD coating to dramatically enhance the cell performance. For instance, ALD layer consisting heterostructured nano composite of nano-Pt and nano-(Mn 0.8 Co 0.2 ) 3 O 4 oxide on the internal surface of porous LSM/YSZ cathode from SOFCs, has resulted in the large reduction of the cell polarizations resistance by up to 53%, and enormous increase of power density over 370%. For the cells with LSCF/Sm 2 O 3 doped CeO 2 (SDC) cathode, the conformal layer of nano-composite consisting of superjacent CoOx and subjacent minimum amount of Pt nano-grains has resulted in the power density enhancement by 126% for the large scale industry tubular cells at 750°C, and both the performance enhancement and nanostructure of the ALD layer are stable over ~ 2000 h continuous operation performed at industry test station. In the meanwhile, those ALD coating layer developed by PI's work is also inherently Cr-tolerant, and could act as physical barrier for preventing Cr diffusion into the cathode backbone, so as to mitigate the Cr poisoning effect on the cathode. In this project, the impact of Cr on the performance of those ALD coated commercial cells has been evaluated. Based on evolution of the cell performance, the ALD coating layer chemistry and ALD coating layer thickness has been optimized to maximize the overall Cr tolerance, cell power density and cell longevity. Different ALD coating has been applied onto the internal surface of LSM/YSZ and LSCF/SDC backbone respectively. The architecture/scaffold structures on the internal surface of different cathode, designed by this project, was catalogued and analyzed using High Resolution Transmission Electron Microscopy (HRTEM), and cell power/durability performance are assured via comprehensive electrochemical performance testing in the industry operation relevant conditions. The impact of the electrochemical operation current density, the water humidity, the cell operation temperature, and cell operation duration on the Cr tolerance of ALD coated cells has been systematically investigated. There is completely different nanostructure degradation mechanisms between LSM and LSCF cells induced by Cr contamination. For the LSCF/SDC baseline cell, With the Cr source, there is no apparent Sr surface segregation phase even for the baseline cell operated for 3000 h at 750 °C. With the Cr source, there is significant amorphous (SrCr)Ox phase accumulated in the original pore region. For the commercial baseline cells, Cr contaminants on the LSM electrode severely impacted the entire cell's electrochemical performance and nanostructure degradation. Those degradations include (1). Peak power density loss of 64 % after 109 h of operation. The dramatic increase in Rp (2). They are cracking at LSM/SSZ interface, LSM grains. SSZ remains intact but with (CrMn)Ox. By contrast, ALD coating (MnCo)Ox/Pt dramatically improves the Cr resistance, as follows (1). ALD-coated cell with a power density is 280-380 % of the baseline cell, depending on the ALD layer thickness. (2). For a cell with a 20 nm thick ALD layer, there is a large performance enhancement (> 200 % power density) induced by ALD coating of Cr-tolerant Mn 0.8 Co 0.2 Ox. (3). For a cell with a 20 nm thick ALD layer, after 168 h at 750 °C power density of the ALD-coated cell is ~ 600% of that baseline cell upon operation with Cr contamination for 109 h. The ALD coating on the internal surface of cathode developed by this project integrated multi-functions. Those multi-functions include (1). Dramatically improving the cell power density for the commercial cells; (2). Dramatically improving contamination resistance of the cathode, for being an excellent protection coating layer sealing off Cr contamination. (3). Dramatically increasing the cell longevity by potentially preventing the microstructure evolution and grain coarsening of the cathode. Overall, this project will provide a simple solution to simultaneously enhance power density and increase the reliability, robustness, and endurance of commercial SOFCs, over the entire operating temperature range of 650-800 °C. For the inherently functional SOFC, the ALD coating of LSM based cathode mitigate the Cr-contamination. Power density of ALD-coated cell is ~ 600% of that baseline cell upon operation with Cr contamination. In addition to SOFCs, the novel on-demand design approach and creation of multifunctional heterogeneous architecture on the electrode surface presented in this work opens further research for their application in other types of fuel cells, batteries, and sensors for which electrochemical reactions on the surface are similarly critical.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Microstructural features underpinning the mechanical behavior of powder metallurgy Cr-based alloys

Refractory materials such as Cr-based alloys offer the potential of enhanced elevated temperature performance but have been limited by their poor formability. However, powder metallurgy has been shown to be a viable pathway to fabricate these alloys. Nanophase separation sintering (NPSS) in particular has been used in the literature to accelerate the densification of powder metallurgy Cr- and W-based alloys. Here, we explore microstructure evolution during NPSS in a binary Cr 85 Ni 15 alloy consolidated via (i) cold pressing & pressureless sintering and (ii) hot isostatic pressing followed by hot extrusion & hot upsetting, and the role of these different processing routes on resulting material properties. The alloy lacked room temperature tensile ductility regardless of consolidation process, with multi-length scale characterization, including scanning electron microscopy, transmission electron microscopy, atom probe tomography, X-ray diffraction, and uniaxial tensile testing, revealing that brittleness was due to intrinsically poor Cr grain boundary cohesion. Tensile testing conducted at 760 °C showed marked strength reduction for the extruded & upset (94 %) condition compared to the pressed & sintered (18 %). The formation of orthorhombic CrNi 2 intermetallics functioned as precipitate strengtheners and prevented elevated temperature softening in the pressed and sintered condition. The findings offer foundational insights into aiding the future development of Cr-based alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Non-isothermal creep aging behaviors of an Al-Zn-Mg-Cu alloy

Highlights: • A novel non-isothermal creep aging (NICA) process is proposed to improve sheet metal forming efficiency. • Appropriate heating-cooling rate is conducive to obtain desired creep deformation and material properties. • The time consumption in optimized NICA process is about half of that in conventional creep aging process. • In NICA, the precipitates formed in heating will be dissolved nearby the peak temperature and re-precipitated in cooling. - Abstract: To shorten production period and reduce manufacturing cost of high strength aluminum alloy integral panel components, a novel non-isothermal creep age forming (CAF) technology was developed. Taking an Al-Zn-Mg-Cu alloy as the case material, the non-isothermal creep aging (NICA) behavior and the conventional isothermal creep aging (ICA) behavior were comparatively investigated. The corresponding creep deformations, material performance variations and precipitate microstructure evolutions were studied by creep aging tests, mechanical property and corrosion resistance tests, and transmission electron microscope (TEM) and scanning electron microscope (SEM) characterizations. Unlike the ICA which contains short heating and cooling stages and long holding stage, the NICA only has heating and cooling stages, viz., the alloy is heated slowly and the cooling stage begins immediately when reaching the peak temperature. In the NICA process, the precipitates nucleate and grow up in the first half of heating stage, resulting in the reduction of creep rate and enhancement of mechanical strength. The coarsening of precipitates occurs in the second half of heating stage, elevating the creep rate and significantly improving the corrosion resistance. Nearby the peak temperature, the primary precipitates partially dissolve, and the creep rate, mechanical strength and corrosion resistance decrease. Distinctive from the ICA process, a secondary precipitation phenomenon takes place in the cooling stage of NICA, leading to the recovery of high mechanical strength. The higher heating-cooling rate (1 °C/min) is not beneficial to accumulating creep deformation in the shorter creep aging period, while the lower one (0.25 °C/min) causes the loss of mechanical strength due to the excessive dissolution of precipitates. The moderate heating-cooling rate (0.5 °C/min) can lead to larger creep deformation, better mechanical properties and more satisfactory corrosion resistances in the NICA treated alloy, reaching 87.0% of creep strain, 105.7% of yield strength and 97.9% of electric conductivity in the ICA treated alloy. It is worth noting that the time consumption of the optimal NICA process is only 52.9% of the ICA process.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Laser Welding-Brazing of Immiscible AZ31B Mg and Ti-6Al-4V Alloys Using an Electrodeposited Cu Interlayer

Metallurgical bonding between immiscible system AZ31B magnesium (Mg) and Ti-6Al-4V titanium (Ti) was achieved by adding Cu interlayer using laser welding-brazing process. Effect of the laser power on microstructure evolution and mechanical properties of Mg/Cu-coated Ti joints was studied. Visually acceptable joints were obtained at the range of 1300 to 1500 W. The brazed interface was divided into three parts due to temperature gradient: direct irradiation zone, intermediate zone and seam head zone. Ti{sub 3}Al phase was produced along the interface at the direct irradiation zone. Ti-Al reaction layer grew slightly with the increase in laser power. A small amount of Ti{sub 2}(Cu,Al) interfacial compounds formed at the intermediate zone and the (α-Mg + Mg{sub 2}Cu) eutectic structure dispersed in the fusion zone instead of gathering when increasing the laser power at this zone. At the seam head zone, Mg-Cu eutectic structure was produced in large quantities under all cases. Joint strength first increased and then decreased with the variation of the laser power. The maximum fracture load of Mg/Cu-coated Ti joint reached 2314 N at the laser power of 1300 W, representing 85.7% joint efficiency when compared with Mg base metal. All specimens fractured at the interface. The feature of fracture surface at the laser power of 1100 W was characterized by overall smooth surface. Obvious tear ridge and Ti{sub 3}Al particles were observed at the fracture surface with increase in laser power. It suggested atomic diffusion was accelerated with more heat input giving rise to the enhanced interfacial reaction and metallurgical bonding in direct irradiation zone, which determined the mechanical properties of the joint.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Imaging of dynamic processes in materials with a laser-wakefield accelerator

Betatron x rays generated from laser-wakefield accelerators are a promising source for imaging dynamic processes in materials. Here, we present proof-of-concept imaging of microstructural evolution in a hypermonotectic Al-Bi alloy, which consists of liquid Bi particles in a solid Al matrix. The images capture the elongation and fragmentation of Bi particles upon isothermal annealing. Because of the femtosecond time scale of the betatron source, the images are not subject to motion blur, whereas the accessibility of the source allows for studies of long-term processes such as annealing. The high-resolution data reveal that the evolution of Bi particles is mediated by an interplay of grain-boundary wetting and morphological instability, in stark contrast to an earlier proposal for (inverse) coarsening.

Alloys↗
DOE OSTI2018

Achieving Ultralow Wear with Stable Nanocrystalline Metals

Abstract Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near‐melting temperatures. Pt–Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt–Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all‐metallic material exhibiting a specific wear rate on the order of 10 −9 mm 3 N −1 m −1 , comparable to diamond‐like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt–Au alloy, despite the substantially higher hardness of the ceramic probe materials. High‐resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue‐driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear‐resistant materials such as DLC.

Curry, John F.↗
DOE OSTI2018

Achieving Ultralow Wear with Stable Nanocrystalline Metals

Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near-melting temperatures. In this paper, we show that PtAu, a proposed stable alloy consisting of two noble metals, exhibits extraordinary resistance to wear. Ultra-low wear rates, less than a monolayer of material removed per sliding pass, were measured for PtAu thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all-metallic material exhibiting a specific wear rate on the order of 10 -9 mm 3 /N-m, comparable to diamond-like carbon and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments were either higher or comparable to that of the PtAu alloy, despite the substantially higher hardness of the ceramic probe materials. High-resolution microscopy showed negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Finally, mitigation of fatigue-driven delamination enabled a transition to wear by atomic attrition, a regime previously limited to highly wear resistant materials such as diamond-like carbon.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Mapping of Texture and Phase Fractions in Heterogeneous Stress States during Multiaxial Loading of Biomedical Superelastic NiTi

Thermoelastic deformation mechanisms in polycrystalline biomedical-grade superelastic NiTi are spatially mapped using in situ neutron diffraction during multiaxial loading and heating. The trigonal R-phase is formed from the cubic phase during cooling to room temperature and subsequently deforms in compression, tension, and torsion. The resulting R-phase variant microstructure from the variant reorientation and detwinning processes are equivalent for the corresponding strain in tension and compression, and the variant microstructure is reversible by isothermal loading. The R-phase variant microstructure is consistent between uniaxial and torsional loading when the principal stress directions of the stress state are considered (for the crystallographic directions observed here). The variant microstructure evolution is tracked and the similarity in general behavior between uniaxial and torsional loading, in spite of the implicit heterogeneous stress state associated with torsional loading, pointed to the ability of the reversible thermoelastic transformation in NiTi to accommodate stress and strain mismatch with deformation. This ability of the R-phase, despite its limited variants, to accommodate stress and strain and satisfy strain incompatibility in addition to the existing internal stresses has significance for reducing irrecoverable deformation mechanisms during loading and cycling through the phase transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Unraveling Thermodynamic and Kinetic Contributions to the Stability of Doped Nanocrystalline Alloys using Nanometallic Multilayers

Abstract Targeted doping of grain boundaries is widely pursued as a pathway for combating thermal instabilities in nanocrystalline metals. However, certain dopants predicted to produce grain‐boundary‐segregated nanocrystalline configurations instead form small nanoprecipitates at elevated temperatures that act to kinetically inhibit grain growth. Here, thermodynamic modeling is implemented to select the Mo–Au system for exploring the interplay between thermodynamic and kinetic contributions to nanostructure stability. Using nanoscale multilayers and in situ transmission electron microscopy thermal aging, evolving segregation states and the corresponding phase transitions are mapped with temperature. The microstructure is shown to evolve through a transformation at lower homologous temperatures (<600 °C) where solute atoms cluster and segregate to the grain boundaries, consistent with predictions from thermodynamic models. An increase in temperature to 800 °C is accompanied by coarsening of the grain structure via grain boundary migration but with multiple pinning events uncovered between migrating segments of the grain boundary and local solute clustering. Direct comparison between the thermodynamic predictions and experimental observations of microstructure evolution thus demonstrates a transition from thermodynamically preferred to kinetically inhibited nanocrystalline stability and provides a general framework for decoupling contributions to complex stability transitions while simultaneously targeting a dominant thermal stability regime.

25 ENERGY STORAGE↗
DOE OSTI2025

Deformation Behaviors in Single BCC‐Phase Refractory Multi‐Principal Element Alloys under Dynamic Conditions

The mechanical behavior and microstructural evolution of a BCC-phase NbTaTiV refractory multi-principal element alloy (RMPEA) is studied over a wide range of strain rates (10 −3 to 10 3 s −1 ) and temperatures (room temperature to 850 °C). The mechanical property of present RMPEA shows less strain-rate dependence and strong resistance to softening at high temperatures. Under high strain-rate loading, the formation of thin type-I twins is observed, which could lead to an increase in strain-hardening rates. However, this hardening mechanism competes with adiabatic heating effects, resulting in the deterrence of strain-hardening behaviors. In contrast, substantial strain-hardening occurs at cryogenic temperatures due to the formation of twins, which act as stronger barriers to dislocation motion and interact with each other. To further understand the different strain-hardening behaviors, density functional theory (DFT) calculations predict relatively low stacking fault energies and high twinning stress for the NbTaTiV RMPEA.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Unraveling the Voltage Decay Phenomenon in Li-Rich Layered Oxide Cathode of No Oxygen Activity

Extensive efforts have been devoted to unraveling the true cause of voltage decay in Li, Mn-rich layered oxides. An initial consensus was reached on structural rearrangement, then leaned towards the newly discovered lattice oxygen activity. It is challenging to differentiate their explicit roles because these events typically co-exist during the electrochemical reaction of most Li-rich layered oxides. Here, we probe the voltage decay behavior in Li 1.2 Ni 0.2 Ru 0.6 O 2 , a structurally and electrochemically relevant compound to Li, Mn-rich layered oxide, but of no oxygen activity. Such intriguing characteristics allow us to explicitly decouple the contribution of transition metal migration and lattice oxygen activity to voltage decay in Li-rich layered oxides. Our results demonstrate that the microstructural evolution, mainly originating from transition metal migration, is a direct cause of voltage decay, and lattice oxygen activity likely accelerates the decay.

25 ENERGY STORAGE↗
DOE OSTI2023

Convoluted filtering for process cycle modeling

Principles of materials science and engineering, physics, mathematics, and information science are used to extract knowledge and insights from the process-structure–property-performance relationships hidden in materials data. The process-structure modeling can be accelerated without loss of interpretability, with artificial intelligence tools that mimic the salient features of the process and process-structure relations. In this work, a novel convoluted model-filtering technique was exploited to build and successfully train the Convoluted Filter (CoFi) artifacts for Fe-based alloy heat treatment cycles. The artifacts were pre-trained to filter out deep models that change the surrogate microstructure state after the heat treatment at ambient conditions. Direct representation of the thermal cycle features within knowledge Graph facilitated development of meaningful data models for microstructure evolution, which reduce overfitting to limited datasets.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of annealing and supercritical CO 2 exposure at 750°C on the tensile properties of stainless steel and Ni-based structural alloys

Subsize dogbone tensile specimens of alloys 304H, 310HCbN, and 740H were exposed to 30 MPa supercritical CO 2 (sCO 2 ) for up to 4000 h at 750°C. Similar exposures in laboratory air were used to assess the specific role of sCO 2 exposure on the alloy tensile properties. Type 304H specimens were only embrittled by exposure in sCO 2 due to the formation of a thick Fe-rich oxide and C ingress in the bulk of the alloy. For 310HCbN and 740H, no effect of the sCO 2 environment was observed and a decrease of the alloy's ductility was associated with previously observed microstructure evolutions, for example, grain boundary carbide formation. The 740H was tested in the solution annealed condition and self-aging took place during 750°C exposures in sCO 2 and air with the formation of fine gamma prime precipitates leading to significant tensile strength increase. It was found that both 740H and 310HCbN have adequate sCO 2 compatibility for high-temperature commercial applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Materials data analytics for 9% Cr family steel

A materials data analytics (MDA) methodology was developed in this study to evaluate publicly available information on 9% Cr family steel and to handle nonlinear relationships and the sparsity in materials data for this alloy class. The overarching goal is to accelerate the design process as well as to reduce the time and expense associated with qualification testing of new alloys for fossil energy applications. Data entries in the analyzed data set for 82 iron–base alloy compositions, several processing parameters, and results of tensile mechanical tests selected for this study were arranged in 34 columns by 915 rows. While detailed microstructural information was not available, it is assumed that the compositional space for the 9 to 12% Cr steels is limited such that all data entries have a tempered martensitic microstructure during service. Establishing a hierarchy of first–order trends in the publicly available data requires the MDA to filter out the biases. Complexity of the phase transformations and microstructure evolution in the multicomponent alloys (using 21 chemical elements) with major influence on mechanical properties, leads to inefficiency in direct application of unbiased linear regression across the entire data space. To address the nonlinearity, analyses of tensile data were performed in composition–based clusters. Clusters corresponding to moderately frequent patterns and maximized information gain were further refined by using p–norm distance measures, matching the alloy classification groups adopted by industry. Lastly, the evolutionary method of propagating an ensemble of competing cluster–based models proved to be a viable option in dealing with scarce, multidimensional data.

36 MATERIALS SCIENCE↗