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DOE OSTI2021

Diffusional and Partitionless Ferrite‐to‐Austenite Phase Transformations during Intercritical Annealing of Medium‐Mn Steels

Experimental research on intercritical annealing of a cold‐rolled Fe–7Mn steel with ultralow carbon concentration is presented, providing insight into the efficacy of generating Mn‐enriched austenite during relatively short (≈1000 s) heat treatments compared with batch annealing, as well as the phase transformation mechanisms that can occur during intercritical annealing. It is shown through both bulk characterization and electron microscopy in conjunction with energy‐dispersive X‐ray spectroscopy that Mn partitioning to austenite occurs via diffusional, partitioning growth of austenite during these shorter treatments and that the intercritical temperature influences the level of Mn enrichment in austenite. Evidence of massive (partitionless) and diffusional (partitioning) transformations of ferrite to austenite occurring simultaneously in a single sample are also shown along with a thermodynamic rationale for this occurrence, which is suggested to be related to residual Mn‐banding that develops during solidification.

Mueller, Josh J.↗

DOE OSTI2025

Use of Frit‐Disc Crucible Sets to Make Solution Growth More Quantitative and Versatile

The recent availability of step‐edge, frit‐disc crucible sets (generally sold as Canfield Crucible Sets or CCS) has led to multiple innovations associated with the group's use of solution growth. The use of CCS allows for the clean separation of liquid from solid phases during the growth process. This clean separation enables the reuse of the decanted liquid, either allowing for simple, economic, savings associated with recycling expensive precursor elements or allowing for the fractionation of a growth into multiple, small steps, revealing the progression of multiple solidifications. Clean separation of liquid from solid phases also allows for the determination of the liquidus line (or surface) and the creation, or correction, of composition–temperature phase diagrams. The reuse of clean decanted liquid has also allowed to prepare liquids ideally suited for the growth of large single crystals of specific phases by tuning the composition of the melt to the optimal composition for growth of the desired phase, often with reduced nucleation sites. Finally, it is discussed how solution growth and CCS use can be harnessed to provide a plethora of composition–temperature data points defining liquidus lines or surfaces with differing degrees of precision to either test or anchor artificial intelligence and/or machine‐learning‐based attempts to augment and extend the limited experimentally determined database.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Magnesium Technology 2019

Joshi, Vineet V.↗

DOE OSTI2020

Magnesium Technology 2020

Magnesium and its alloys are being investigated widely for applications that require extraordinarily high strength-to-weight ratio, vibration damping, electromagnetic shielding, reduction in carbon footprint, sustainability improvements, and low toxicity. In the past decade or so, this drive has led to increased adoption of magnesium and its alloys in everyday products and it is no longer regarded as an esoteric material. The overarching goal of its widespread adoption and replacement of conventional materials such as steel and aluminum alloys has been restricted because of the cost of the material, limited sources of primary material, alloying constraints, high chemical reactivity, and challenges associated with its plasticity. Researchers, scientists, engineers, and economists from industry, government agencies and laboratories, and academic institutions alike are actively developing roadmaps for next-generation products and addressing these challenges as quickly as possible through unique and innovative methods. The TMS Magnesium Committee has been actively involved in providing a platform to these entities for disseminating the latest information, developments, and cutting-edge research and development, and showcasing the latest research and development trends related to magnesium and its alloys through the Magnesium Technology Symposium, which takes place every year at the TMS Annual Meeting. This proceedings volume retains the essence of this longstanding tradition. The twenty-first volume in the series, Magnesium Technology 2020, is the proceedings of the Magnesium Technology Symposium held during the 149th TMS Annual Meeting & Exhibition in San Diego, CA, February 23–27, 2020. The volume captures commentaries and papers from 15 different countries. The papers have been categorized systematically based on topics pertaining to magnesium production, casting and solidification, thermomechanical processing, deformation mechanisms, modeling, corrosion, and applications. The symposium began with keynote sessions that featured several distinguished invited speakers from government organizations and academia, who provided their perspectives on the state of the art, goals, and opportunities in magnesium alloy research and development. Dr. Carlos Tomé from Los Alamos National Laboratory, U.S. Department of Energy, discussed 3D characterization of mechanical twins across grain boundaries. This was followed by a talk on hierarchically structured ultrafine-grained magnesium alloys by Dr. Rajiv Mishra from the University of North Texas, USA, who presented an example of using friction stir additive manufacturing for implementing such microstructures at a component level. Dr. Mark Horstemeyer from Liberty University, USA, presented an overview of fatigue modeling to capture the corrosion-fatigue behavior of magnesium alloys. The final talk in this session was presented by Dr. Bin Jiang from Chongqing University, China, who presented a novel extrusion approach for achieving high-strength magnesium alloy plates.

Jordon, J. Brian↗

DOE OSTI2022

Hybrid Additive Manufacturing of Island Grain Bicrystals

We demonstrate cylindrical island grain bicrystals grown via a hybrid additive manufacturing/directional solidification technique. The island grain bicrystal is formed using a specialized mold in which one grain (the island) is enveloped by another larger grain (the surrounding matrix). The grain boundary plane orientation varies continuously around the circumference of the island grain, making these specimens ideal for probing grain boundary structure–property relations. We report thermal groove angle measurements along the grain boundary in a tin island grain bicrystal which show the grain boundary energy as a function of a single degree of freedom in the five-dimensional grain boundary parameter space.

Ware, Logan↗

DOE OSTI2024

Design and Characterization of Hierarchically-Strengthened, Cast Al-Ce-Ni-Mn-Sc-Zr Alloys for High-Temperature Applications

We characterize the microstructure and mechanical properties of cast Al-Ce-Ni-Mn-Sc-Zr alloys designed for structural use above 300 °C. We first report on the effect of Ni on the as-cast phase formation, where all alloys consist of fine Al11Ce3 and Ni-rich eutectic phases, but the identity of the Ni-rich phase varies with Ni content. Furthermore, these alloys contain four coarsening-resistant strengthening phases as determined from SEM and APT investigations: (i) Al11Ce3 and (ii) Ni-rich micron-scale platelets formed during eutectic solidification, (iii) L12-Al3(Sc,Zr) nanoprecipitates formed during secondary aging, and (iv) Mn solute in the α-Al matrix. The Al-Ce-Ni-Mn-Sc-Zr alloy possesses higher microhardness and creep resistance than compositionally simpler alloys containing fewer of the strengthening phases, indicating that these strengthening mechanisms synergize to provide increased ambient- and elevated-temperature strength.

Ekaputra, Clement↗

DOE OSTI2021

Crystal plasticity model of residual stress in additive manufacturing using the element elimination and reactivation method

Selective laser melting is receiving increasing interest as an additive manufacturing technique. Residual stresses induced by the large temperature gradients and inhomogeneous cooling process can favour the generation of cracks. In this work, a crystal plasticity finite element model is developed to simulate the formation of residual stresses and to understand the correlation between plastic deformation, grain orientation and residual stresses in the additive manufacturing process. The temperature profile and grain structure from thermal-fluid flow and grain growth simulations are implemented into the crystal plasticity model. An element elimination and reactivation method is proposed to model the melting and solidification and to reinitialize state variables, such as the plastic deformation, in the reactivated elements. The accuracy of this method is judged against previous method based on the stiffness degradation of liquid regions by comparing the plastic deformation as a function of time induced by thermal stresses. The method is used to investigate residual stresses parallel and perpendicular to the laser scan direction, and the correlation with the maximum Schmid factor of the grains along those directions. The magnitude of the residual stress can be predicted as a function of the depth, grain orientation and position with respect to the molten pool. The simulation results are directly comparable to X-ray diffraction experiments and stress–strain curves.

316 stainless steel↗

DOE OSTI2021

Thermodynamics and kinetics of precipitation and austenite reversion during aging of Ti-free grade 300 maraging steel manufactured by laser powder bed fusion (LPBF)

Austenite reversion and intermetallic precipitation are two key phase transformations in maraging steels which determine the strength and ductility. Usually, an optimum aging temperature/time is determined using trial and error approach with inputs from thermodynamic databases and a-posteriori microscopy/hardness evaluations. In this study, a thermo-kinetic model is developed to predict the precipitation and austenite reversion kinetics during various post-fabrication heat treatments in a Ti-free variant of grade 300 maraging steel manufactured using laser powder bed fusion. The thermo-kinetic model was based on the principles of Scheil solidification and the classical nucleation theory. The predictions from the thermo-kinetic model were verified using suitable microscopy techniques, and the results from the thermo-kinetic model agree well with the experimental measurements. Overall, results highlight the fact that an Integrated Computational Materials Engineering approach can be used to predict the precipitation and austenite reversion kinetics, and be used as an input for an efficient design of heat treatments for achieving optimum mechanical properties.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Time-resolved 3D characterisation of early-age microstructural development of Portland cement

Abstract Time-resolved in-situ synchrotron X-ray microtomography reveals new levels of detail about the chemical and physical processes that take place as Portland cement hardens. The conversion of a fluid paste into a hardened product can be monitored on a sub-minute time-scale, and with sample movement/settlement corrections applied to enable individual particles to be tracked as they react, hydrate, and become interconnected into a single strong monolith. The growth of the strength-giving hydrate phases surrounding cement grains, and of the fluid-filled pore network that surrounds them, is able to be directly viewed at the level of individual cement particles through the application of this tracking protocol. When cement is brought into contact with water, a layer which differs in density from the bulk of the cement grains becomes observable on the grain surfaces during the induction period (during which time the heat evolution from the paste is relatively low). As hydration continues, reaction products grow both from particle surfaces into the initially fluid-filled region, and also into the space originally occupied by the cement particles, forming a density gradient within the microstructure. As the reaction accelerates and larger volumes of solid phases precipitate, the newly-formed solid structure percolates via interconnection of agglomerated low-density outer hydrates, which then densify as hydration continues. This eventually leads to solidification of the structure into a hardened porous matrix. Graphical abstract

36 MATERIALS SCIENCE↗

DOE OSTI2023

Microstructural refinement in ultrasonically modified A356 aluminum castings

Two A356 aluminum alloys (Al-Si-Mg), one with 0.09 wt.% Fe and one with 0.91 wt.% Fe, were cast in a graphite mold with the simultaneous application of local ultrasonic intensification to refine the as-cast microstructure. Ultrasonication during casting transformed the morphology of primary Al grains from dendritic (~140-290 microns in size) to globular (~33-36 microns in size). The alloy with high Fe exhibited globular grains at distances up to 45 mm away from the ultrasound probe, while the alloy with low Fe exhibited globular grains at distances only up to 6 mm away from the ultrasound probe. Near the location of the ultrasound probe (< 2 mm away), a second non-dendritic microstructural morphology was observed with fine aluminum grains (~9-25 microns in size). This unique fine-grained morphology has not been previously reported, contains a greater concentration of Si relative to the globular microstructure, and may be a large, fully eutectic region. Ultrasonication during casting also transformed the morphology of the ß-Al 5 FeSi phase particles (which are deleterious to the strength and ductility of the alloy) in the high Fe alloy from needle-like to rectangular, which could enable the greater use of secondary Al alloys. Thermodynamic simulations conducted to calculate the solidification paths of the two alloys studied predict that the ß-Al 5 FeSi phase begins to form earlier in the alloy with high Fe. Finally, data suggest that the ß-Al 5 FeSi phase (which is more abundant in alloys with high Fe content) may enhance ultrasonically-induced grain refinement.

36 MATERIALS SCIENCE↗

DOE OSTI2024

A Gaussian process autoregressive model capturing microstructure evolution paths in a Ni–Mo–Nb alloy

Additive manufacturing is increasingly being employed to produce components of complex geometries in structural alloys because of the expected energy savings associated with the near-net-shape capability and the ability to build in novel internal features that are not possible with many conventional manufacturing approaches. However, because of the extreme thermal conditions encountered, the non-equilibrium microstructures produced during powder bed-based additive manufacturing processes must be subjected to custom post-heat treatment processes to recover the target mechanical properties. Phase-field models and simulation techniques have matured to a state where the microstructure evolution paths, and the morphologies of the resulting precipitate phases can be predicted reasonably accurately, considering alloy-specific thermodynamic and kinetic aspects of the nucleation and growth processes. However, phase-field simulations are computationally intensive, which precludes the ability to apply the simulations directly to the length scale of the entire component. Therefore, it is highly desirable to develop low-computational-cost surrogate models that effectively capture the physics at the microstructural length scale, while facilitating the design of optimized processing conditions resulting in location-specific targeted microstructures at the component scale. The work presented here demonstrates the application of the materials knowledge system framework to develop a surrogate model that effectively captures the microstructural path during annealing of a Ni–Mo–Nb alloy containing different Mo and Nb compositions known to segregate during solidification under additive manufacturing conditions. Specifically, the surrogate model built in this work is based on a Gaussian process autoregressive model informed by statistical representation of simulated microstructures using two-point correlations and dimensionality reduction through principal component analysis. In conclusion, this surrogate model is shown to capture the bifurcation of the microstructural path during precipitation, which yields a microstructure dominated by the $\gamma^{\prime\prime}$ phase at high Nb concentrations and the $\delta$ phase at low Nb concentrations.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Design of Novel Precipitate-Strengthened Al-Co-Cr-Fe-Nb-Ni High-Entropy Superalloys

Here, a series of non-equiatomic Al-Co-Cr-Fe-Nb-Ni high-entropy alloys, with varying levels of Co, Nb and Fe, were investigated in an effort to obtain microstructures similar to conventional Ni-based superalloys. Elevated levels of Co were observed to significantly decrease the solvus temperature of the γ' precipitates. Both Nb and Co in excessive concentrations promoted the formation of Laves and NiAl phases that formed either during solidification and remained undissolved during homogenization or upon high-temperature aging. Lowering the content of Nb, Co, or Fe prevented the formation of the eutectic type Laves. In addition, lowering the Co content resulted in a higher number density and volume fraction of the γ' precipitates, while increasing the Fe content led to the destabilization of the γ' precipitates. Various aging treatments were performed which led to different size distributions of the strengthening phase. Results from the microstructural characterization and hardness property assessments of these high-entropy alloys were compared to a commercial, high-strength Ni-based superalloy RR1000. Potentially, precipitation-strengthened high-entropy alloys could find applications replacing Ni-based superalloys as structural materials in power generation applications.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Asymmetric Cracking in Mar-M247 Alloy Builds During Electron Beam Powder Bed Fusion Additive Manufacturing

In the electron beam powder bed fusion (EB-PBF) process, a substantial number of high-gamma prime Ni-based superalloys are considered as non-printable due to a high propensity to form cracks. In this research, we focused on computational modeling framework to predict solidification-related cracking phenomena in EB-PBF processes. The cracking analysis was performed on cylindrical overhang structures where the cracks are observed only on one side of the part. Comprehensive microstructural characterization correlated the cracking tendency to low-melting point liquid-film formation along columnar grain boundaries with high misorientation angles due to partitioning of alloying elements. Uncoupled numerical thermal and mechanical models were used to rationalize the relationship between process parameters, build geometry, and cracking. The simulations showed asymmetric temperature distributions and associated asymmetric tensile thermal stresses over a cross section due to differences in section modulus and periodic changes in beam scanning directions. Here, the results provide a potential pathway based on spatially varying beam scanning strategies to reduce the cracking tendency during additive manufacturing of complex geometries on the overhang structure in high-gamma prime nickel-based superalloys.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Process-Defect-Structure-Property Correlations During Laser Powder Bed Fusion of Alloy 718: Role of In Situ and Ex Situ Characterizations

Components made by laser powder bed fusion (L-PBF) additive processes require extensive trial and error optimization to minimize defects and arrive at targeted microstructure and properties. In this paper, in situ infrared thermography and ex situ surface roughness measurements were explored as methodologies to ensure Inconel ® 718-part quality. For a given laser energy of 200 Watts, prismatic samples were produced with different exposure times (80 to 110 µs) and point spacings (80 to 110 µm). The infrared intensities from laser–material interaction zones were measured spatially and temporally. The conditions leading to higher IR intensity and lowest surface roughness values correlated well with less porosity and coarse solidification grain structure. The transition from highly columnar to misoriented growth is attributed to changes in thermal gradients and liquid–solid interface velocities. Hardness measurements and electron microscopy of the as-processed and post-processed heat-treated samples show complex transitions in microstructural states including the heavily dislocated FCC matrix, reduction of dislocation density, and copious precipitation, respectively. These results show that the geometry-process-structure-property correlations are dynamic, and they cascade depending on the transitions of phase states from powder to liquid to solid, as well as phase decompositions and deformations within the solid FCC phase. Finally, validity of using analytical weld process models to describe the above phenomena is also highlighted.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Manipulating Retained Austenite Fraction and Stability With Controlled Chemical Heterogeneities in Q&P Steels

Advanced high-strength steels go through complex heat treatment processes with the goal of retaining austenite to enhance the mechanical properties. Martensite start temperature predictions are generally based on bulk composition when designing heat treatments, so compositional heterogeneities are not considered. Here, the effects of compositional heterogeneities on retained austenite fractions in a quenching and partitioning steel are investigated. In conclusion, the results show that Mn banding from solidification can be used to change retained austenite fractions.

36 MATERIALS SCIENCE↗

DOE OSTI2026

Full-Strength Additive Manufacturing of Pure Aluminum Alloy 7075 Using Liquid Metal Jet Printing

Additive manufacturing (AM) of high-strength aluminum alloys can produce high-performance aerospace and biomedical parts with unique geometries and functions. However, many high-performance alloy systems such as AA 7075 pose challenges during AM due to thermal cycles that cause hot cracking, limiting AM to casting-friendly alloys. Stock AA 7075 alloy poses additional challenges due to the excessive growth of insoluble precipitates during casting and needs to be thermomechanically processed to obtain acceptable mechanical properties. Here, this work uses liquid metal jet printing to produce high-resolution pure AA 7075 parts at high strength (593 MPa) and ductility (10 pct), surpassing the specific strength of any type of steel. The parts are printed by depositing fine droplets on a high-temperature substrate that reduces cooling rates, preventing hot cracking. The results show that fully dense, crack-free parts can only be printed at substrate temperatures of 500 °C with a cooling rate threshold of < 2000 K/s to prevent cracking and < 500 K/s to eliminate porosity. The tensile strength, microhardness, and ductility of the parts matched or were beyond the wrought alloy specifications after heat treatment, possibly due to high solidification rates that produce finer insoluble precipitates. Our results indicate that many challenging high-performance alloy systems can now be 3D printed based on the critical thresholds for cooling rates using any AM method that can satisfy these conditions.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Duplex Heterogeneous Nucleation Behavior of Precipitates in C-Mn Steel Containing Sn

The two successive heterogeneous nucleation behaviors of FeSn{sub 2}-MnS-Al{sub 2}O{sub 3} complex precipitates in ultrahigh Sn-bearing steel were investigated. First, Al{sub 2}O{sub 3} was the nucleation site of the MnS at the end of solidification. Then, FeSn{sub 2} nucleated heterogeneously on the MnS particles that nucleated on the Al{sub 2}O{sub 3} particles. The formation sequence of the precipitated phase caused the duplex heterogeneous nucleation to occur consecutively at most twice.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Welding Heat Input Influence on UNS S82441 Lean Duplex Stainless Steel Corrosion Resistance Assessed by Scanning Vibrating Electrode Technique (SVET)

The precipitation of the intermetallic phases can impair mechanical and corrosion properties. In the case of lean duplex stainless steels, they present solidification mode as primary ferrite, with austenite precipitation in the solid state, during cooling. This means the heat-affected zone balanced microstructure can be changed during the welding by the cooling rate, which is associated to the welding heat input, the thickness of the plate and the physical properties of the material. This work intends to present the influence of welding heat input on the corrosion resistance of UNS S82441 lean duplex stainless steel. The gas metal arc welding process was used to make the weld beads, using ER2209 as filler metal, which characterizes a dissimilar welded joint. The corrosion resistance of these specimens was analyzed through scanning vibrating electrode technique. It was observed that welding heat input variation distinctly influences corrosion in different zones of the weld beads over the specimen exposure time. The weld bead under intermediate welding heat input showed the highest corrosion rate and the one under the highest welding heat input, the lowest corrosion rate intensity.

36 MATERIALS SCIENCE↗