Property models are becoming more widely adopted by commercial Calphad databases, but they are not nearly as common in non-commercial or traditional academic Calphad databases. A primary driver is that user-friendly Calphad modeling tools that support property models are not widely available. Here we present new property modeling capabilities that have been implemented in ESPEI (the Extensible, Self-optimizing Phase Equilibrium Infrastructure). These capabilities include both generating property model parameters from data and improvements to the algorithmic selection of the most appropriate model from a series of candidates. Two illustrative examples are given that use ESPEI to fit different property models. First, we generate molar volume model parameters for Group IV, V, and VI refractory BCC alloys based on the model by Lu et al. (2005). Second, we demonstrate the extensibility of ESPEI’s property modeling capabilities by implementing a custom PyCalphad model for BCC elastic stiffness parameters to generate and compare parameters to the ones assessed by Marker et al. (2018) using the same data. Property models generated by ESPEI can be used in PyCalphad or further optimized with uncertainty quantification using ESPEI.

Functionally graded materials have the potential to improve upon monolithic parts by locally tailoring compositions to surrounding environmental conditions. Difficulties arise when designing composition gradients as incompatible materials can result in detrimental phase formation and failure of the gradient joint. As many alloys are multi-component, designing a composition gradient free of detrimental phases is difficult due to the large composition space available to explore. A framework was developed that improves the path planning algorithm and surrogate models with adaptive sampling schemes specific to their problem definition. A cost function was created to minimize a property (such as cracking susceptibility) along a path. This framework was applied to the Mo-Nb-Ta-Ti system as a case study to showcase the efficiency in building the surrogate models and in iterating different optimal compositionally graded paths.

Additively manufactured (AM) alloy Ti6Al4V often exhibits a distinctive alpha/alpha' microstructure due to the high cooling rates associated with AM, which can lead to increased brittleness and necessitate costly post-processing and heat treatment. In this work, we explore the use of layer-by-layer large area surface annealing with a secondary diode laser to control the microstructure of Ti6Al4V during AM and site-specifically induce more ductile, dual-phase alpha/beta Ti6Al4V. Annealing treatments were selected to target sub- and super-beta-transus temperatures at annealed layers, leading to 13 % and 20 % reductions in hardness, respectively, in the 2 mm below the annealed surfaces. Components with rapid switching between annealed and unannealed microstructures were also produced. Temperature measurements at the built surface and base plate were combined with a thermal model to validate microstructural observations, setting the stage for development of full 3-D microstructure and property control using in-situ laser annealing.

We report on the implementation of a high frequency beam oscillation (wobbling) strategy for improving process control during laser powder bed fusion of single weld tracks on Inconel 625. Oscillation frequencies ranging from similar to 600 Hz to 7000 Hz, and different oscillation trajectories (circular, parallel or perpendicular to the direction of scanning) were explored. Highspeed imaging was used to elucidate the dynamics of the melt pool induced by the wobble beams, along with in situ absorptivity measurements to substantiate our hypothesis that the dynamic nature of wobble beams reduces absorptive losses due to laser-vapor interactions and results in improved coupling at the melt pool. Operando X-ray radiography was carried out to visualize sub-surface melt flow dynamics, correlate to spatter mechanisms and optimize the window for improving process stability. Our observations indicate that wobble beams increase the aspect ratio of the melt pool by up to 4x, depending on the oscillation frequency and energy input. Highspeed imaging and X-ray radiography reveal an optimized process parameter window for reducing spatter, improving absorptivity and creating a stable melt pool at high (several kHz) oscillation frequencies. (c) 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0)

Ti5553 (Ti-5Al-5Mo-5V-3Cr wt.%) is a titanium alloy widely used for its high strength-to-weight ratio and good formability at elevated temperatures. Unlike Ti-6Al-4V, Ti5553 does not undergo martensitic transformation, preventing cracking of brittle martensite upon rapid cooling. This makes it a strong candidate for additive manufacturing (AM), particularly laser powder bed fusion (L-PBF). L-PBF offers the unique opportunity to make fine lattice structures to reduce component weight. Despite the growing field of AM, there have been limited studies on L-PBF Ti5553 lattices and how their properties differ from the bulk. The present work addresses this knowledge gap by investigating microstructures and properties of L-PBF bulk and lattice parts and the effect of post L-PBF heat treatments. Electron microscopy and mechanical testing show that the high dislocation density formed during L-PBF increases bulk part’s yield strength by approximately 100MPa compared to the conventional alloy. Digital image correlation during compression testing of octet truss lattices reveals a layer-by-layer failure mode. Compared to the bulk, the lattice contains copious ω nanoprecipitation and weaker <001> texture, smaller average grain sizes, and larger content of high-angle grain boundaries. These features elicit differences in Taylor factor distributions for the lattice depending on load direction, underlining challenges in predicting lattice mechanical response based on bulk properties. By examining the processing-structure-property relationships in the bulk and lattice, the present results delineate their microstructural and mechanical differences and establish a benchmark for the future design applications of L-PBF Ti5553.

Metal additive manufacturing technologies keep receiving a great deal of interest as well as strong requests to develop methods to link the process science to printed parts performance and understand how to overcome inherent limitations. A high-fidelity model based on the multiphysics ALE3D code was developed to reproduce the directed energy deposition process down to the powder scale. This includes resolving the laser-powder-melt pool interactions (powder impingement and incorporation into melt pool, hydrodynamics flow condition and laser absorption inefficiencies) as well as the resulting solidification microstructure. This micrometer scale digital twin captured the effect of powder incorporation process and powder flow rate on porosity. Furthermore, it was used to explore how a ring laser beam profile instead of the standard Gaussian laser profile could decrease the thermal gradient along the solidification front in the melt pool, which in turn can increase propensity for more desirable equiaxed grains.

The aerogels industry has been growing with a market capacity of 909 billion USD in 2019. Silica aerogels are the most extensively used due to their ease of synthesis. It can possess many useful properties such as high optical transparency, high-surface area, low thermal conductivity, and a wide accessible density range. However, silica aerogels are brittle and friable, making them unsuitable for many applications. Polymeric aerogels are typically much more robust but lack the uniform fine nanostructure which gives silica its exceptional properties. Herein, mechanically robust polyimide (PI) aerogels with uniform <20 nm porosity and exceptional transparency are demonstrated. Optimization was achieved by minimizing the phase separation during gelation, minimizing the light scattering, and yielding the exceptionally transparent aerogels. The aromatic PI backbone results in a high modulus while retaining a low thermal conductivities and high thermal stability. We demonstrate methods for inducing phase separation to increase the pore size and the effects on bulk properties. This study presents a better understanding of the route to producing transparent polymer aerogels.

Effective optimization of the production of Ti-6Al-4 V using AM requires a fundamental understanding of the relative importance of different microstructural features to the deformation and failure mechanisms, particularly features that vary between production methods. In this study, the tensile response and deformation mechanisms of electron beam melted (EBM) AM Ti-6Al-4 V material loaded in different orientations and produced using various powder sizes were compared to those of selective laser melted (SLM) AM Ti-6Al-4 V material. The density and morphology of pores, phase fractions, prior-β grains, and defect microstructures were evaluated using scanning electron microscopy, X-ray computed tomography, electron backscatter diffraction, and transmission electron microscopy before and after deformation. The results were used to evaluate the relative importance of each feature on strengthening, deformation, and failure initiation mechanisms. Results focused primarily on coarse-powder EBM materials indicated that phase distribution and defect density were most influential for determining material yield strength as well as maximum possible strain to failure. Porosity was lower overall in EBM Ti-6Al-4 V than in SLM, allowing for occasional increases in part strain to failure, but remained a limiting factor determining overall part ductility.

This paper reports on spall damage mechanisms in laser powder bed fusion (LPBF) fabricated stainless steel 316L (SS316L) subjected to uniaxial, strain plate-impact loading and explores failure evolution with increasing impact velocity and peak pressure. Analysis of velocimetry profiles reveals a heterogeneous failure response with several different failure mechanisms activated during impact and resulting spall. The analysis is supported by optical microscopy and electron backscattered diffraction (EBSD) observation of soft recovered impacted samples, which show evidence of localized nano-twinning, twin growth, severe grain rotation, and grain refinement along the spall plane, in addition to pore nucleation, coalescence, and growth. The observed failure mechanisms, including crack formation and propagation along high angle grain boundaries (HAGBs), are not consistently indicative of ductile fracture typical of wrought stainless steel.

The fundamental mechanism of hydrogen embrittlement was investigated in the high-entropy alloy FeNiCoCrMn using slow strain rate tensile tests with and without internal hydrogen. Hydrogen induced intergranular failure and reduced the average grain elongation parallel to the tensile axis, but also increased the local plasticity within grains. The influence of hydrogen on plasticity establishes a compatibility constraint across grain boundaries, which results in failure along the hydrogen-weakened grain boundaries. This study is the first to directly confirm the presence of the hydrogen-enhanced compatibility constraint in a high-entropy alloy and highlights the importance of developing a physical understanding of grain-scale interactions.

Additive manufacturing (AM) is a promising means of production of austenitic stainless steel (SS) parts for hydrogen service. The hydrogen embrittlement resistance of SS 316 L parts manufactured by powder-bed-fed selective laser melting (SLM) and directed energy deposition (DED) was examined using slow strain rate tensile testing. The influence of the hierarchical AM microstructures on mechanical response, microstructural evolution, and void formation were analyzed using multiscale electron microscopy. The presence of hydrogen reduced ductility in as-built DED materials, but did not significantly influence the response in as-built SLM material or heat-treated materials. Microstructural features driving these different responses are discussed.

While metal additive manufacturing has seen significant growth in recent years, the surfaces produced often need post-processing to improve surface finish, mitigate residual stresses, and remove surface-connected porosity. Laser polishing, by means of remelting a thin layer of the surface, is one post-processing method being investigated for surface finish improvements and other surface enhancements. In this work, the surface morphology and microstructure of laser powder bed fused (L-PBF) stainless-steel 316 L (316 L) before and after laser polishing are characterized by optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), and transmission electron microscopy (TEM). In addition, the cross-sectional microhardness of the samples is measured and reported. Additionally, the as-built and laser-polished sample's tensile properties are characterized using uniaxial tension tests. The results indicate that the surface roughness of as-built 316 L (Sa = 4.84 mu m) can be substantially reduced through laser polishing (Sa = 0.65 mu m). After laser polishing, the average grain diameter is reduced and the proportion of low angle grain boundaries (2 degrees similar to 5 degrees) is increased in the L-PBF 316 L. The maximum sub-surface hardness reaches 262 HV, and both the tensile strength and ductility of 316 L are increased after laser polishing. This enhancement is attributed to thermal cycling stresses, grain refinement, the elimination of surface defects, and dislocation strengthening after laser polishing.

High-entropy alloys (HEAs) are a class of alloys that can exhibit promising properties including enhanced irradiation resistance, high-temperature strength, and corrosion resistance. However, they exist in a relatively unexplored region of quasi-limitless composition space. Thus, to enable the development of promising compositionally complex alloys, such as HEAs, high-throughput methods are needed. Such high-throughput capabilities are developed and presented in this work. In situ alloying through additive manufacturing was employed to produce arrays of different HEA compositions. Sample arrays were then characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD), all while remaining on the build plate. The top surface of each sample was compositionally homogeneous, as determined by EDS, and each sample exhibited a single-phase, disordered crystal structure, as determined by XRD. CALculation of PHAse Diagrams (CALPHAD) modeling was used to determine the equilibrium phases of each HEA composition at lower temperatures, the results of which were compared with XRD results. Implications of high-throughput synthesis techniques and the coupling of high-throughput characterization and modeling techniques are discussed in the context of alloy development.

In this experiment, the origin of dislocation structures in AM stainless steels was systematically investigated by controlling the effect of thermal stress through geometric constraints for the first time. Stainless steel 316L parts were produced in the form of "1D" rods, "2D" walls, and "3D" rectangular prisms to evaluate the effect of constraints to thermal expansion/shrinkage on the development of defect microstructures and to elucidate the origin of additively manufactured (AM) dislocation microstructures. Dislocation density, organization, chemical micro-segregation, precipitate structures, and misorientations were analyzed as a function of increasing constraints around solidifying material in 1D, 2D, and 3D components built using both directed energy deposition (DED) and powder-bed selective laser melting (SLM). In DED parts, the dislocation density was not dependent on local misorientations or micro-segregation patterns, but evolved from approximately rho(perpendicular to)approximate to 10(12) m(-)(2) in 1D parts to rho(perpendicular to) approximate to 10(14) m(-2) in 3D parts, indicating that it is primarily thermal distortions that produce AM dislocation structures. In DED 3D parts and SLM parts, dislocation densities were highest (rho(perpendicular to) approximate to 10(14) m(-2)) and corresponded to the formation of dislocation cells approximately 300-450 nm in diameter. Dislocation cells overlapped with dendrite micro-segregation in some but not all cases. The results illustrate that dendritic micro-segregation, precipitates, or local misorientations influence how the dislocations organize during processing, but are not responsible for producing the organized cell structures. This work shows that AM dislocation structures originate due to thermal distortions during printing, which are primarily dictated by constraints surrounding the melt pool and thermal cycling. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The integration of topology optimization (TO) and additive manufacturing (AM) has the potential to revolutionize modern design and manufacturing. However, few instances of manufactured optimized designs are documented, and even fewer examples of experimentally-tested designs are available. The lack of validation combined with the influence of AM process on material properties leaves a gap in our understanding of processmicrostructure-property relationships that is essential for developing holistic design optimization frameworks. In this work, a functional design was topologically optimized and fabricated using both directed energy deposition (DED) and selective laser melting (SLM) methods. This is the first direct comparison of these AM methods in the context of TO. Mechanical properties of SS316L and the optimized components in as-fabricated and heat-treated conditions were investigated under uniaxial displacement-controlled tensile loading and compared to finite element modeling (FEM) predictions. Optimized samples provided regions of both compressive and tensile loading in the test specimen. Experimental results showed the FEM predictions to be conservative. Microstructural analysis revealed that this difference is due to refined microstructures formed during the additive manufacturing process that strengthen the material in regions with high stress levels. Moreover, SLM samples showed higher yield strength compared to DED samples due to a more refined grain size and denser dislocation structures. TO results are sensitive to the AM method, post-processing conditions, and differences in mechanical properties. Thus, a TO for AM framework can be best optimized with the incorporation of microstructure features to account for localized microstructural variations in fabricated components.

Metal additive manufacturing offers potential advantages for producing structural materials, such as austenitic stainless steels, in nuclear power systems. However, the microstructure developed during metal additive processing is notably different from the one developed in conventional processing, and the influence of the microstructural differences on performance in radiation environments has not been fully quantified. Using heavy ion irradiation and transmission electron microscopy, the radiation-induced swelling response of a laser powder-bed fusion-manufactured austenitic stainless steel was investigated at high doses. The influence of solidification-induced dislocation and precipitate structures was studied by comparing the radiation-induced swelling response of a 316 L stainless steel in three microstructural states: as-fabricated, solution annealed, and fully recrystallized. Void swelling was approximately twice as pronounced in the as-fabricated state compared to post-processed states. In the framework of the rate theory for radiation effects, the higher swelling in the as-fabricated state can be explained by the strong sink bias for interstitial point defects exerted by the intermediate density of pre-existing dislocations. Void swelling was inhibited in the vicinity of pre-existing precipitates, but the density of precipitates in the as-fabricated material was not enough to compensate for the increase in swelling caused by dislocations. (C) 2019 Elsevier B.V. All rights reserved.

A multi-scale experimental approach was used to determine the fundamental mechanisms responsible for the hydrogen-induced transition in failure mode from ductile transgranular to intergranular in polycrystalline Ni during uniaxial loading. Hydrogen accelerated the evolution of the deformation microstructure, producing smaller dislocation cells and microbands, and causing significantly different orientation deviations to develop in neighboring grains, while inducing less evolution of texture, less grain rotations, less elongation of the grains parallel to the tensile axis, and greater out-of-surface distortion of the grains. These observations are explained in terms of the hydrogen-enhanced plasticity mechanism, which results in a redistribution of hydrogen that stabilizes the deformed microstructure and increases the hydrogen coverage on the grain boundaries. The stabilization of the microstructure manifests as a reduced ability of grains to cooperatively accommodate evolving deformation structures, which introduces an additional compatibility constraint across grain boundaries. The combination of this compatibility constraint across grain boundaries, the locking of the microstructure in a specific configuration by hydrogen, and the hydrogen-weakening of the grain boundaries drives the hydrogen-induced intergranular failure.

The influence of internal hydrogen on the tensile properties of an equi-molar FeNiCoCrMn alloy results in a significant reduction of ductility, which is accompanied by a change in the fracture mode from ductile microvoid coalescence to intergranular failure. The introduction of 146.9 mass ppm of hydrogen reduced the plastic strain to failure from 0.67 in the uncharged case to 0.34 and 0.51 in hydrogen-charged specimens. The reduction in ductility and the transition in failure mode are clear indications that this alloy exhibits the classic signs of being susceptible to hydrogen embrittlement. The results are discussed in terms of the hydrogen-enhanced plasticity mechanism and its influence on hydrogen-induced intergranular failure. Furthermore, a new additional constraint that further promotes intergranular failure is introduced for the first time.