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Refractory high entropy alloys (RHEAs) are candidates for replacing conventional refractory alloys. In this work, twelve new RHEAs were selected and produced. The phases present in the as-cast and heat-treated conditions were characterized and compared with CALPHAD simulations and empirical parameters. Here we propose a new interpretation for the two widely used δ and Ω empirical parameters. In this work, they are shown to be inaccurate when applied to a large group of RHEAs, but can be a powerful alloy design tool if applied on specific subsystems of alloys. Experimentally, chromium-containing alloys are shown to form Laves phases, especially when the lattice distortion (δ) is high, while aluminum-containing alloys are shown to form the A15 phase upon heat-treatment, due to their highly negative enthalpy of mixing (ΔHmix). In addition to microstructural characterization, mechanical properties of these alloys via hardness testing were assessed. A poor correlation was observed between the hardness and the atomic size and elastic modulus mismatch in these single-phase BCC RHEAs, suggesting that core structure of the screw dislocations is a crucial parameter in understanding the strength of these alloys. Keywords: Refractory high entropy alloys, Phase equilibria, CALPHAD, Materials characterization, Mechanical properties, Solid solution strengthening

Made available in DSpace on 2020-12-10T17:03:15Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-12-01 Boeing Research & Technology Brazil (BRT-Brazil) Center for Advanced Non-Ferrous Structural Alloys (CANFSA), a National Science Foundation Industry/University Cooperative Research Center (I/UCRC) at the Colorado School of Mines A design of experiments (DoE) approach was employed to systematically characterize the effects of both oxygen concentration and cooling rate on the amorphous and crystalline phase fractions in the Zr48Cu46.5Al4Nb1.5 bulk metallic glass composite (BMGC) alloy. Specifically, four distinct oxygen concentrations and five distinct cooling rates were used. Phase quantification was achieved using Rietveld refinement of synchrotron X-ray diffraction (XRD) data. Electron probe microanalysis (EPMA) using wavelength dispersive spectroscopy (WDS) revealed that the big-cube crystalline phase may scavenge oxygen from the amorphous matrix of a 2864 +/- 47 wppm oxygen sample. Transmission electron microscopy (TEM) of the alloy containing 340 +/- 25 wppm oxygen revealed the presence of an amorphous matrix with partially-transformed colonies of B2 (Pm-3m) where the transformation product was a B19' superstructure (Cm space group) instead of the more common B19' superstructure (P2(1)/m space group); this was in agreement with the Rietveld refinement results. An orientation relationship between the B2 and B19' phases was observed as expected. Finally, it was possible to produce thicker sections (5 mm vs. 3.5 mm) with the desired composite structure ( similar to 80% amorphous fraction with similar to 20% B2/ B19') compared with previous BMGC studies on similar compositions. Univ Sao Paulo, Dept Mat Engn, Sao Carlos Sch Engn, Joao Dagnone Ave 1100, BR-13563120 Sao Carlos, SP, Brazil Colorado Sch Mines, George S Ansell Dept Met & Mat Engn, 1500 Illinois St, Golden, CO 80401 USA Paulista State Univ Julio de Mesquita Filho, Chem Inst Araraquara, Francisco Degni St 55, BR-14800900 Araraquara, SP, Brazil Paulista State Univ Julio de Mesquita Filho, Chem Inst Araraquara, Francisco Degni St 55, BR-14800900 Araraquara, SP, Brazil Center for Advanced Non-Ferrous Structural Alloys (CANFSA), a National Science Foundation Industry/University Cooperative Research Center (I/UCRC) at the Colorado School of Mines: 1624836

The influence of Cr and Ni additions and quench and partition (Q&P) processing parameters on the microstructural development, including carbide formation and austenite retention during Q&P, was studied in two steels with a base composition of 0.2C-1.5Mn-1.3Si wt.% and additions of 1.5 wt.% Cr (1.5Cr) or Ni (1.5Ni). Additions of 1.5 wt.% Cr significantly slowed the kinetics of austenite decomposition relative to the 1.5Ni alloy at all partitioning temperatures, promoting greater austenite retention, lower retained austenite carbon (C) contents, and reduced sensitivity of the retained austenite amounts to processing variables. In the 1.5Cr alloy after partitioning at 400 °C for 300 s, η-carbides were identified by transmission electron microscopy (TEM) and atom probe tomography (APT) revealed no significant enrichment of substitutional elements in the carbides. In the 1.5Ni alloy after partitioning at 450 °C for 300 s, both plate-like and globular carbides were observed by TEM. APT analysis of the globular carbides clearly revealed significant Si rejection and Mn enrichment. Mossbauer effect spectroscopy was used to quantify the amount of carbides after Q&P. In general, carbide amounts below ∼0.3% of Fe were measured in both alloys after partitioning for short times (10 s), irrespective of quench or partitioning temperature, which corresponds to a relatively small portion of the bulk C. With increasing partitioning time, carbide amounts remained approximately constant or increased, depending on the alloy, quench temperature, and/or partitioning temperature.

The evolution of microstructure and microhardness of a Co-10Al-4C (at. pct) alloy during isothermal aging at 800 and 900 °C is reported. Fine κ-Co3AlC0.5 intermetallic precipitates form in an FCC α-Co matrix after aging at both temperatures. Lamellar discontinuous precipitation also occurred at grain boundaries and the lamellar transformation product consumed the fine κ-Co3AlC0.5 precipitates in the matrix during aging. The microhardness of the alloy decreased dramatically upon formation of the lamellar product. Transmission electron microscopy revealed that the lamellar product consists of α-Co, κ-Co3AlC0.5, and B2-CoAl phases at 800 °C. The orientation relationship between α-Co and κ-Co3AlC0.5 phases, and between α-Co and B2-CoAl phases were identified as cube-on-cube and Kurdjumov–Sachs, respectively. The discontinuous product at 900 °C was composed of alternating α-Co and κ-Co3AlC0.5 lamellae, without the B2-CoAl phase that formed at 800 °C. Additional continuous coarsening of κ-Co3AlC0.5 phase was observed in the lamellar aggregate during prolonged aging at 900 °C. The main driving force for the discontinuous reaction appears to be the reduction in both interfacial energy and elastic strain energy where the latter is attributed to the relatively high lattice mismatch between the κ-Co3AlC0.5 precipitates and the α-Co matrix.

Single-track laser melting experiments were performed on bulk Ti-Nb alloys to explore process parameters and the resultant macroscopic structure and microstructure. The microstructures in Ti-20Nb and Ti-50Nb (at.%) alloys exhibited cellular growth during rapid solidification, with average cell size of approximately 0.5 µm. Solidification velocities during cellular growth were calculated from images of melt tracks. Measurements of the composition in the cellular and intercellular regions revealed nonequilibrium partitioning and its dependence on velocity during rapid solidification. Experimental results were used to benchmark a phase-field model to describe rapid solidification under conditions relevant to additive manufacturing.

Cr-Co-Ni Multi-Principal Element Alloys (MPEAs) with good combinations of strength and ductility were studied to determine their attendant corrosion performance. Three alloys – Cr45Co27.5Ni27.5, Cr33.3Co33.3Ni33.3, and Cr25Co37.5Ni37.5 – were produced in recrystallized and cold-worked states, and their electrochemical response was tested in a simulated seawater electrolyte. Increasing the Cr content improved the yield strength (σy) and ultimate tensile strength (UTS), while maintaining high (>40%) uniform elongation. Potentiodynamic polarization and electrochemical impedance spectroscopy revealed high corrosion resistance of the alloys in simulated seawater, in particular the Cr-rich alloy (Cr45Co27.5Ni27.5). Furthermore, following 40% cold work, the Cr45Co27.5Ni27.5 alloy displayed a further improvement in corrosion resistance. The Cr45Co27.5Ni27.5 alloy displays mechanical and corrosion properties that exceed those of conventional structural alloys such as Ni-superalloys, stainless steels and most 3d-transition metal MPEAs, including those without appreciable ductility. Therefore, in the present work it is shown that increasing the Cr content in Cr-Co-Ni alloys leads to a better combination of mechanical properties and corrosion resistance in saline environment, as observed especially for the Cr45Co27.5Ni27.5 alloy.

The following work focuses on characterizing nitrogen in several modified alloys of the precipitation strengthened ferritic/martensitic steel HT9 (12Cr-1MoWV (wt%)). Alloy HT9 is a candidate material for fuel rod cladding and ductwork in the core of the Versatile Test Reactor and other next-generation nuclear fast reactors. Nitrogen content has been shown to have a significant effect on irradiated properties in alloy HT9. To explore that theory, a more comprehensive assessment of the location of nitrogen must be performed; however, locating nano-precipitates, such as nitrides and light interstitial elements like nitrogen in steel, has proven difficult. To begin answering questions surrounding the location of nitrogen, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) was employed as a primary analysis technique, together with extensive transmission electron microscopy, internal friction testing, and thermodynamic simulations, providing verification and context to trends found in the ToF-SIMS data. For the first time, ToF-SIMS was demonstrated to reliably measure relative differences in the vanadium carbonitride precipitate volume fraction across several chemistries and heat treatments of ferritic/martensitic steels. Additionally, associating matrix elements (iron and chromium) to nitrogen-containing ion clusters significantly reduced the ‘matrix effect that biases nitrogen detection towards nitride precipitates, providing a potential, high-throughput method for the measurement of relative interstitial nitrogen content in steels with ToF-SIMS.

The Cr40Co40Ni20 multi-principal element alloy (MPEA) displays a single-phase face centered cubic initial structure, which partially transforms to hexagonal close packed (HCP) phase by transformation-induced plasticity (TRIP) during straining, as evidenced by nanometric HCP lamellae that provide enhanced mechanical properties. This MPEA also exhibits significant yield strength—grain size dependence, given by the high Hall–Petch coefficients (k = 667 MPa/μm−0.5 and σ0 = 299 MPa). The high activation energy for grain growth (QG = 533 kJ/mol) leads to refined grain structures after conventional heat treatments. These features, combined with the large solid solution strengthening of Cr-rich Cr-Co-Ni MPEAs, grant the Cr40Co40Ni20 alloy a great combination of strength and ductility under tension. Finally, an empirical equation is proposed to describe the stacking fault energy (SFE) of Cr-Co-Ni alloys, contributing to the prediction of the acting deformation mechanisms. Such findings highlight the potential of compositional tuning to enhance multiple strength and deformation mechanisms in the Cr-Co-Ni system.

12Cr-1MoWV (wt.%) ferritic/martensitic (F/M) steel is a candidate material for fuel cladding in advanced nuclear reactors. As such, understanding the relationship between microstructure and mechanical properties in the context of irradiation environments for these steels is critical. Here we reveal the presence of ultrafine scale (2-5 nm), intralath V(C,N) precipitates in conventionally heat treated 12Cr-1MoWV steel for the first time. Lower N content results in finer intralath precipitates, whereas higher N content results in larger, elongated disks or needles. N content significantly alters the strength, but not the strain hardening behavior, by its impact on precipitate characteristics. Finer precipitates could have an impact on irradiated behavior, specifically their capacity as defect sinks. The presence of ultrafine scale V(C,N) precipitates in conventionally heat treated 12Cr-1MoWV steel, controlled by N variations, provides a new means for tailoring the strength and irradiation response of F/M steels for nuclear applications.

We follow an Al-12 at. pct Cu alloy sample from the liquid state to mechanical failure, using in situ X-ray radiography during directional solidification and tensile testing, as well as three-dimensional computed tomography of the microstructure before and after mechanical testing. The solidification processing stage is simulated with a multi-scale dendritic needle network model, and the micromechanical behavior of the solidified microstructure is simulated using voxelized tomography data and an elasto-viscoplastic fast Fourier transform model. This study demonstrates the feasibility of direct in situ monitoring of a metal alloy microstructure from the liquid processing stage up to its mechanical failure, supported by quantitative simulations of microstructure formation and its mechanical behavior.

• This article focuses on reviewing modeling and simulation effort in metal additive manufacturing taking places at U.S. Department of Energy national laboratories.

In-situ neutron diffraction measurements were performed during heat-treating and uniaxial loading of additively manufactured (AM) GP1 material. Although the measured chemical composition of the GP1 powder falls within the composition specifications of 17-4 PH steel, a fully martensitic alloy in the wrought condition, the crystal structure of the as-built GP1 material is fully austenitic. Chemical analysis of the as-built material shows high oxygen and nitrogen content, which then significantly decreased after heat-treating in a vacuum furnace at 650 °C for one hour. Significant austenite-to-martensite phase transformation is observed during compressive and tensile loading of the as-built and heat-treated material with accompanied strengthening as martensite volume fraction increases. During loading, the initial average phase stress state in the martensite is hydrostatic compression independent of the loading direction. Preferred orientation transformation in austenite and applied load accommodation by variant selection in martensite are observed via measurements of the texture development.

In situ dynamic transmission electron microscope (DTEM) imaging of Al-Si thin-film alloys was performed to investigate rapid solidification behavior. Solidification of alloys with compositions from 1 to 15 atomic percent Si was imaged during pulsed laser melting and subsequent solidification. Solely α-Al solidification was observed in Al-1Si and Al-3Si alloys, and solely kinetically modified eutectic growth was observed in Al-6Si and Al-9Si alloys. A transition in the solidification mode in eutectic and hypereutectic alloys (Al-12Si and Al-15Si) from nucleated α-Al dendrites at lower solidification velocities to planar eutectic growth at higher solidification velocities was observed, departing from trends previously seen in laser-track melting experiments. Comparisons of the growth modes and corresponding velocities are compared with previous solidification models, and implications regarding the models are discussed.

We study microstructure selection during directional solidification of a thin metallic sample. We combine in situ X-ray radiography of a dilute Al-Cu alloy solidification experiments with three-dimensional phase-field simulations. We explore a range of temperature gradient G and growth velocity V and build a microstructure selection map for this alloy. We investigate the selection of the primary dendritic spacing Λ and tip radius ρ. While ρ shows a good agreement between experimental measurements and dendrite growth theory, with ρ ∼ V − 1 / 2 , Λ is observed to increase with V ( ∂ Λ / ∂ V > 0 ), in apparent disagreement with classical scaling laws for primary dendritic spacing, which predict that ∂ Λ / ∂ V 0 . We show through simulations that this trend inversion for Λ ( V ) is due to liquid convection in our experiments, despite the thin sample configuration. We use a classical diffusion boundary-layer approximation to semi-quantitatively incorporate the effect of liquid convection into phase-field simulations. This approximation is implemented by assuming complete solute mixing outside a purely diffusive zone of constant thickness that surrounds the solid-liquid interface. This simple method enables us to quantitatively match experimental measurements of the planar morphological instability threshold and primary spacings over an order of magnitude in V. We explain the observed inversion of ∂ Λ / ∂ V by a combination of slow transient dynamics of microstructural homogenization and the influence of the sample thickness.

We present the results of a comprehensive phase-field study of columnar grain growth competition in bi-crystalline samples in two dimensions (2D) and in three dimensions (3D) for small sample thicknesses allowing a single row of dendrites to form. We focus on the selection of grain boundary (GB) orientation during directional solidification in the steady-state dendritic regime, and study its dependence upon the orientation of two competing grains. In 2D, we map the entire orientation range for both grains, performing several simulations for each configuration to account for the stochasticity of GB orientation selection and to assess the average GB behavior. We find that GB orientation selection depends strongly on whether the primary dendrite growth directions have lateral components (i.e. components perpendicular to the axis of the temperature gradient) that point in the same or opposite directions in the two grains. We identify a range of grain orientations in which grain selection follows the classical description of Walton and Chalmers. We also identify conditions that favor unusual overgrowth of favorably-oriented dendrites at a converging GB. We propose a simple analytical description that reproduces the average GB orientation selection from 2D simulations within statistical fluctuations of a few degrees. In 3D, we find a similar GB orientation selection as in 2D when secondary branches grow in planes parallel and perpendicular to the sample walls. Remarkably, quasi-2D behavior is also observed even when those perpendicular sidebranching planes are rotated by a finite azimuthal angle about the primary dendrite growth axis as long as the absolute values of those azimuthal angles are equal in both grains. In contrast, when the absolute values of those azimuthal angles differ markedly, we find that unusual overgrowth events at a converging GB are promoted by a high azimuthal angle in the least-favorably-oriented grain. We also find that diverging GBs can be strongly affected by those azimuthal angles, while converging GBs exhibit a weak dependence on those angles. For diverging GBs, GB orientation is also strongly affected by the relative signs of the lateral components of the primary dendrite growth directions in both grains.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The mechanical properties of uranium-niobium alloys evolve with aging at relatively low temperatures due to subtle microstructural changes. In-situ neutron diffraction measurements during aging of a monoclinic U-6Nb alloy at temperatures to 573 K were performed to monitor these changes. Further, in-situ neutron diffraction studies during deformation of U-6Nb in the as-quenched state and after aging for two and eight hours at 473 K were completed to assess the influence of microstructural evolution on mechanical properties. With heating, large anisotropic changes in lattice parameter were observed followed by relaxation with time at the aging temperature. The lattice parameters return to nearly their initial values with cooling. The active plastic deformation mechanisms including, in order of occurrence, shape-memory de-twinning, mechanical twinning, and slip-mediated deformation do not change with prior aging. However, the resistance to motion of the as-quenched martensitic twin boundaries increases following aging, resulting in the observed increase in initial yield strength.

Complicating factors that have stymied understanding of uranium-niobium’s aging response are briefly reviewed, including (1) niobium inhomogeneity, (2) machining damage effects on tensile properties, (3) early-time transients of ductility increase, and (4) the variety of phase transformations. A simple Logistic-Arrhenius model was applied to predict yield and ultimate tensile strengths and tensile elongation of U-4Nb as a function of thermal age. Fits to each model yielded an apparent activation energy that was compared with phase transformation mechanisms.

Conventional alloys have undesirably coarse-grained microstructures when used in 3D printing. A designer alloy overcomes this problem, potentially opening the way to the widespread adoption of 3D metal printing. 3D-printed metals that have similar properties to structural alloys.

We present a mathematical formulation of a multiscale model for solidification with convective flow in the liquid phase. The model is an extension of the dendritic needle network approach for crystal growth in a binary alloy. We propose a simple numerical implementation based on finite differences and step-wise approximations of parabolic dendritic branches of arbitrary orientation. Results of the two-dimensional model are verified against reference benchmark solutions for steady, unsteady, and buoyant flow, as well as steady-state dendritic growth in the diffusive regime. Simulations of equiaxed growth under forced flow yield dendrite tip velocities within 10% of quantitative phase-field results from the literature. Finally, we perform illustrative simulations of polycrystalline solidification using physical parameters for an aluminum-10wt% copper alloy. Resulting microstructures show notable differences when taking into account natural buoyancy in comparison to a purely diffusive transport regime. The resulting model opens new avenues for computationally and quantitatively investigating the influence of fluid flow and gravity-induced buoyancy upon the selection of dendritic microstructures. Further ongoing developments include an equivalent formulation for directional solidification conditions and the implementation of the model in three dimensions, which is critical for quantitative comparison to experimental measurements.

Al-Fe-V-Si and Al-Fe-Cr-Si quaternary alloys were produced by arc melting and chill casting, and the crystal structures of the \( \upalpha \)-phase intermetallics (Pm-3 or Im-3, a = 1.25–1.27 nm) that formed are compared with those in the well-characterized Al-Mn-Si and Al-Fe-Mn-Si systems produced by arc melting. The crystal structures were refined using synchrotron x-ray powder diffraction, and compositions were determined using electron probe microanalysis (EPMA). It was shown that the Al-Fe-Cr-Si, Al-Fe-V-Si, and Al-Fe-Mn-Si systems appear to have the same \( \upalpha \)-phase structures after heat treatment at 500 °C. Additionally, a new \( \upalpha \)-phase with low Si content was observed in the Al-Fe-V-Si alloys in the as-cast condition.

The fatigue performance of nitrided medium carbon steels, alloyed with V and Si, and heat treated to form martensite and bainite microstructures, was evaluated using cantilever bending fatigue testing. Hardness testing and X-ray diffraction residual stress measurements were used in conjunction with an analysis of the applied stress distributions to determine vulnerable regions of crack initiation. The analysis was correlated with experimental scanning electron microscopy observations of crack initiation location. Increases in V and Si content lead to higher core hardness and higher core fatigue strength which increase the applied stress needed to initiate fatigue failure originating subsurface below the case. Increases in V and Si content also increase the magnitude of compressive residual stress, which increases the applied stress needed for surface initiated failure. The combination of increased core fatigue strength and magnitude of compressive residual stress improve the fatigue performance after nitriding compared to other nitrided medium carbon steels reported in literature.

Titanium (Ti) alloys are ubiquitous in the aerospace, defense, and biomedical industries for their high strength to density ratios and good corrosion resistance. Ti alloys are typically optimized for strength, but improved work hardening characteristics and ductility would broaden their use in engineering applications. Here we show desirable combinations of strength and ductility in Ti-10V-2Fe-3Al (wt.%), including a fourfold increase in yield strength without commensurate losses in ductility, overcoming the classic strength/ductility tradeoff. This is achieved by nanoscale ω phase precipitates produced by natural and artificial aging, in concert with TRansformation Induced Plasticity (TRIP). Opportunity exists to tailor these strengthening mechanisms to design microstructural and mechanical response, opening up the performance landscape for lightweight Ti alloys for structural applications, analogous to the design of advanced high strength steels that result in lightweighting and improved crashworthiness for transportation applications.

Whether it be the mechanical response of biomaterials or the crack propagation pathways within metal alloys, observing how damage occurs (both spatially and temporally) is critical to understanding materials behavior. Here, nanoscale transmission X-ray microscopy (TXRM) is used to follow the initiation and propagation of damage during quasi-static mechanical testing of natural, crystalline, and metallic materials. The coupling of a novel load stage and TXRM for in situ mechanical testing enables both radiographic (2D) and tomographic (3D) characterization. With an imaging resolution down to 50 nm during uniaxial nanoindentation, compression, or tension, TXRM is ideally suited for the characterization of materials degradation. Several applications are demonstrated including nanoindentation of dentin, compression of a single crystal of high explosive, and tensile testing of both beetle cuticle and Al-Cu alloy. These experiments highlight the capability of the new experimental fixture to provide enhanced insight on material performance through four dimensional (3D + time) observation and analysis.

The influence of partitioning temperature on microstructural evolution during quenching and partitioning was investigated in a 0.38C 1.54Mn 1.48Si wt.% steel using Mossbauer spectroscopy and transmission electron microscopy. η-carbide formation occurs in the martensite during the quenching, holding, and partitioning steps. More effective carbon partitioning from martensite to austenite was observed at 450 than 400 °C, resulting in lower martensite carbon contents, less carbide formation, and greater retained austenite amounts for short partitioning times. Conversely, greater austenite decomposition occurs at 450 °C for longer partitioning times. Cementite forms during austenite decomposition and in the martensite for longer partitioning times at 450 °C.

Additive manufacturing (AM) of metals and alloys is becoming a pervasive technology in both research and industrial environments, though significant challenges remain before widespread implementation of AM can be realized. In situ investigations of rapid alloy solidification with high spatial and temporal resolutions can provide unique experimental insight into microstructure evolution and kinetics that are relevant for AM processing. Hypoeutectic thin-film Al–Cu and Al–Si alloys were investigated using dynamic transmission electron microscopy to monitor pulsed-laser-induced rapid solidification across microsecond timescales. Solid–liquid interface velocities measured from time-resolved images revealed accelerating solidification fronts in both alloys. The observed microstructure evolution, solidification product, and presence of a morphological instability at the solid–liquid interface in the Al–4 at.%Cu alloy are related to the measured interface velocities and small differences in composition that affect the thermophysical properties of the alloys. These time-resolved in situ measurements can inform and validate predictive modeling efforts for AM.

An application of nuclear physics, a facility for using protons for flash radiography, has been developed at the Los Alamos Neutron Science Center (LANSCE). Protons have proven far superior to high energy x-rays for flash radiography because of their long mean free path, good position resolution, and low scatter background. Although this facility is primarily used for studying very fast phenomena such as high explosive driven experiments, it is finding increasing application to other fields, such as tomography of static objects, phase changes in materials and the dynamics of chemical reactions. The advantages of protons are discussed, data from some recent experiments will be reviewed and concepts for new techniques are introduced.

Strengthening mechanisms influenced by silicon (Si) content during tempering of martensite and bainite between 500 and 650 °C were investigated. Microstructural features such as dislocation density, subgrain size, cementite size, and microalloy precipitate volume fraction and size were evaluated in low and high Si alloys heat treated to form bainite or martensite and tempered at 500 or 650 °C for 1 h. Higher Si contents increase the hardness of both bainite and martensite more after tempering at 500 °C, as compared to 650 °C. The increase in Si content leads to a greater increase in hardness in bainite compared to martensite after tempering at 500 °C. A strength model was used to calculate the increase in hardness from the quantitatively measured microstructural features affected by Si content. The strength model suggests that after tempering at 650 °C, increased Si content primarily increases the hardness by solid solution strengthening. However, after tempering at 500 °C, increased Si contents result in higher dislocation densities in martensite and bainite. A recovery model based on solute drag was evaluated and the results indicate that increases in Si content can delay recovery and result in higher dislocation densities after tempering at 500 °C. Increases in Si content also lead to finer cementite sizes in bainite after tempering at 500 °C. A cementite coarsening model was evaluated, and the results indicate that cementite refinement in bainite tempered at 500 °C cannot be accounted for by Si diffusion controlled coarsening alone. The lower transformation temperature employed to form high Si bainite, indirectly influenced by Si decreasing the martensite start temperature, is suggested to result in refinement of both cementite and subgrain size after tempering at 500 °C. Microalloy precipitation is not significantly affected by Si content in the present study.

Relating a crystal's microscopic structure—such as orientation and size—to a material's macroscopic properties is of great importance in materials science. Although most crystal orientation microscopy is performed in the scanning electron microscope (SEM), transmission electron microscopy (TEM)-based methods have a number of benefits, including higher spatial resolution. Current TEM orientation methods have either specific hardware requirements or use software that has limited scope, utility, or availability. In this article, a technique is described for orientation mapping using Kikuchi diffraction patterns generated from a focused STEM probe. One key advantage is that indexing and analysis of the patterns and maps occurs in the robust OIM Analysis software, currently widely used for electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD) analysis. It was found that with minimal to no image processing and by changing only a few software parameters, reliable indexing of Kikuchi patterns is possible. Three samples, a deformed β-Titanium (Ti), a medium carbon heat-treated steel, and BaCe0.8Y0.2O3-δ were tested to determine the effectiveness of the approach. In all three measurements the algorithms effectively and reliably determined the phases and the crystal orientations of the features measured. For the two orientation maps produced, less than 5% of the patterns were misindexed including boundary areas where overlapping patterns existed. An angular resolution of 0.15° was achieved while features

Additive manufacturing has been around for many years, yet the underlying physics of thermal gradients, local pressure environment, and other non-steady state manufacturing conditions are not fully understood. A Multi-University Research Initiative (MURI) is currently ongoing to measure liquid/solid and solid/solid interface stabilities in AM Ti-6Al-4V. Samples were produced with different beamscanning strategies in order to study the role of thermal gradients on the resulting microstructure. The motivation is to determine which beam-scanning strategy leads to desired grain size and texture. Orientation at different length scales (from mm to nm) can be quantified and compared with a combination of techniques including Precession Electron Diffraction (PED), Electron Backscatter Diffraction (EBSD) and Neutron diffraction. This new information will help predict properties of additively manufactured parts.

High Entropy Alloys (HEAs) are new classes of structural metallic materials that show remarkable property combinations. Yet, often times interesting compositions are still found by trial and error. Here we show an “Effective Atomic Radii for Strength” (EARS) methodology, together with different semi-empirical and first-principle models, can be used to predict the extent of solid solution strengthening to discover and design new HEAs with unprecedented properties. We have designed a Cr45Ni27.5Co27.5 alloy with a yield strength over 50% greater with equivalent ductility than the strongest HEA (Cr33.3Ni33.3Co33.3) from the CrMnFeNiCo family reported to date. We show that values determined by the EARS methodology are more physically representative of multicomponent concentrated solid solutions. Our methodology permits high throughput, property-driven discovery and design of HEAs, enabling the development of future high-performance advanced materials for extreme environments.

While some high entropy alloys (HEAs) have been shown to display remarkable combinations of properties, exploration of the extensive multicomponent space by conventional methods is experimentally intractable. Thus, identifying and developing high-throughput methods is paramount to alloy design. Here, a high-throughput experimental methodology is developed for rapid yield strength estimations of single-phase HEAs, which involves the production and testing of a compositionally-graded sample made by a diffusion-multiple approach. The sample is analyzed by a combination of nanoindentation and microstructural characterization, where the nanohardness results are analyzed by different conversion equations to determine yield strength. The values estimated by nanohardness agree with bulk tensile properties. Both are compared to a high-throughput solid solution strengthening model, again yielding a good correlation. This work shows that high-throughput methodologies for predicting and measuring properties are promising for designing new HEAs with desirable combinations of properties.

Low-enriched uranium-10 wt.% molybdenum (LEU-10wt.%Mo) is of interest for the fabrication of monolithic fuels to replace highly-enriched uranium (HEU) dispersion fuels in high performance research and test reactors around the world. In this work, depleted uranium-10 wt.%Mo (DU-10wt.%Mo) is used to simulate the solidification and microstructural evolution of LEU-10wt.%Mo. Electron backscatter diffraction (EBSD) and complementary electron probe microanalysis (EPMA) reveal significant microsegregation present in the metastable γ-phase after solidification. Homogenization is performed at 800 and 1000 °C for times ranging from 1 to 32 h to explore the time–temperature combinations that will reduce the extent of microsegregation, as regions of higher and lower Mo content may influence local mechanical properties and provide preferred regions for γ-phase decomposition. We show for the first time that EBSD can be used to qualitatively assess microstructural evolution in DU-10wt.%Mo after homogenization treatments. Complementary EPMA is used to quantitatively confirm this finding. Homogenization at 1000 °C for 2–4 h may the regions that contain 8 wt.% Mo or lower, whereas homogenization at 1000 °C for longer than 8 h effectively saturates Mo chemical homogeneity, but results in substantial grain growth. The appropriate homogenization time will depend upon additional microstructural considerations, such as grain growth and intended subsequent processing. Higher carbon LEU-10wt.%Mo generally contains more inclusions within the grains and at grain boundaries after solidification. The effect of these inclusions on microstructural evolution (e.g. grain growth) during homogenization and as potential γ-phase decomposition nucleation sites is unclear, but likely requires additional study.

Dendrite fragmentation is an important phenomenon in microstructural development during solidification. For instance, it plays a key role in initiating the columnar-to-equiaxed transition (CET). Here, we use x-ray radiography to study dendrite fragmentation rate in a Sn-39.5 wt.% Bi alloy during directional solidification. Experiments were performed in which solidification was parallel and anti-parallel to gravity, leading to significantly different fragmentation rates. We quantify the distribution of fragmentation rate as a function of distance from the solidification front, time in the mushy zone, and volume fraction of solid. While the observed fragmentation rate can be high, there is no evidence of a CET, illustrating that it requires more than just fragmentation to occur.

The effect of partitioning parameters on mechanical properties and carbon partitioning in quenching and partitioning (Q&P)-treated C-1.5Mn-1.5Si steels was studied using 0.3 wt.% and 0.4 wt.% carbon compositions. Fully austenitized specimens were quenched to a fixed quenching temperature followed by partitioning at 400°C and 450°C for varying times. In most cases, increasing the partitioning temperature decreased UTS and increased TE, and both UTS and TE decreased with increasing partitioning times. Similar ultimate tensile strength levels were obtained for the 0.3C alloy partitioned at 400°C and the 0.4C alloy partitioned at 450°C. Increasing alloy carbon content increased retained austenite fractions. Mossbauer effect spectroscopy results were used to investigate carbon redistribution after Q&P processing in the 0.4C alloy.

We present a three-dimensional extension of the multiscale dendritic needle network (DNN) model. This approach enables quantitative simulations of the unsteady dynamics of complex hierarchical networks in spatially extended dendritic arrays. We apply the model to directional solidification of Al-9.8 wt.%Si alloy and directly compare the model predictions with measurements from experiments with in situ x-ray imaging. We focus on the dynamical selection of primary spacings over a range of growth velocities, and the influence of sample geometry on the selection of spacings. Simulation results show good agreement with experiments. The computationally efficient DNN model opens new avenues for investigating the dynamics of large dendritic arrays at scales relevant to solidification experiments and processes.

Quenching and partitioning (Q&P) produces steel microstructures with martensite and austenite that exhibit promising property combinations for third generation advanced high strength steels. Understanding the kinetics of reactions that compete for available carbon, such as carbide formation, is critical for alloying and processing design and achieving austenite enrichment and retention during Q&P. Mossbauer effect spectroscopy (MES) was used to characterize Q&P microstructures in a 0.38C–1.54Mn–1.48Si wt.% steel after quenching to 225 °C and partitioning at 400 °C for 10 or 300 s, with an emphasis on transition carbides. The recoilless fraction for η-carbide was calculated and a correction for saturation of the MES absorption spectrum was applied, making quantitative measurements of small amounts of η-carbide, including non-stoichiometric η-carbide, possible in Q&P microstructures. Complementary transmission electron microscopy confirmed the presence of η-carbides, and MES and X-ray diffraction were used to characterize the austenite. The amount of η-carbide formed during Q&P ranged from 1.4 to 2.4 at.%, accounting for a substantial portion (∼24% to 41%) of the bulk carbon content of the steel. The amount (5.0 at.%) of η-carbide that formed after quenching and tempering (Q&T) at 400 °C for 300 s was significantly greater than after partitioning at 400 °C for 300 s (2.4 at.%), suggesting that carbon partitioning from martensite to austenite occurs in conjunction with η-carbide formation during Q&P in these specimens.

Solidification from a multiphase fluid involves many unknown quantities due to the difficulty of predicting the impact of fluid flow on chemical partitioning. Real-time x-ray radiography has been used to observe liquid–liquid phase separation in Al90In10 prior to solidification. Quantitative image analysis has been used to measure the motion and population characteristics of the dispersed indium-rich liquid phase during cooling. Here we determine that the droplet growth characteristics resemble well known steady-state coarsening laws with likely enhancement by concurrent growth due to supersaturation. Simplistic views of droplet motion are found to be insufficient until late in the reaction due to a hydrodynamic instability caused by the large density difference between the dispersed and matrix liquid phases.

X-ray imaging, which permits the microscopic visualization of metal alloy solidification dynamics, can be coupled with controlled solidification to create microstructures by design. This x-ray image shows a process-derived composite microstructure being made from a eutectic Al-17.1 at.%Cu alloy by successive solidification and remelting steps.

The development of oxide dispersion strengthened ferrous alloys has shown that microstructures designed for excellent irradiation resistance and thermal stability ideally contain stable nanoscale precipitates and dislocation sinks. Based upon this understanding, the microstructures of conventionally manufactured ferritic and ferritic-martensitic steels can be designed to include controlled volume fractions of fine, stable precipitates and dislocation sinks via specific alloying and processing paths. The concepts proposed here are categorized as advanced high-Cr ferritic-martensitic (AHCr-FM) and novel tailored precipitate ferritic (TPF) steels, which have the potential to improve the in-reactor performance of conventionally manufactured alloys. AHCr-FM steels have modified alloy content relative to current reactor materials (such as alloy NF616/P92) to maximize desirable precipitates and control phase stability. TPF steels are designed to incorporate nickel aluminides, in addition to microalloy carbides, in a ferritic matrix to produce fine precipitate arrays with good thermal stability. Both alloying concepts may also benefit from thermomechanical processing to establish dislocation sinks and modify phase transformation behaviors. Alloying and processing paths toward designed microstructures are discussed for both AHCr-FM and TPF material classes.

The relative chemistry from grain interiors to grain boundaries help explain why grain boundaries may be beneficial, detrimental or benign towards device performance. 3D Nanoscale chemical analysis extracted from atom probe tomography (APT) (10’s of parts-per-million chemical sensitivity and sub-nanometer spatial resolution) of twenty grain boundaries in a high-efficiency Cu(In, Ga)Se2 solar cell shows the matrix and alkali concentrations are wide-ranging. The concentration profiles are then related to band structure which provide a unique insight into grain boundary electrical performance. Fluctuating Cu, In and Ga concentrations result in a wide distribution of potential barriers at the valence band maximum (VBM) (−10 to −160 meV) and the conduction band minimum (CBM) (−20 to −70 meV). Furthermore, Na and K segregation is not correlated to hampering donors, (In, Ga)Cu and VSe, contrary to what has been previously reported. In addition, Na and K are predicted to be n-type dopants at grain boundaries. An overall band structure at grain boundaries is presented.

X-ray computed tomography across multiple length scales provides an opportunity to non-destructively visualize and quantify the micro- to nano-scale microstructural features of solidification structures in three dimensions. Aluminum–7 at.%copper samples were directionally solidified at three cooling rates (0.44, 0.67, and 1.33 °C/s), resulting in systematic changes in the as-solidified microstructure, which are difficult to quantify using traditional microscopic techniques. The cooling rate of a material affects its ultimate microstructure, and characterizing that microstructure is key to predicting and understanding its bulk properties. Here, two different laboratory X-ray computed tomography instruments were used to characterize as-solidified microstructures, including micro-scale computed tomography with approximately 1 mm field-of-view, ~ 1.7 μm resolution, and nano-scale X-ray computed tomography ~ 65 μm FOV, 150 nm resolution. Micro-scale X-ray radiography and computed tomography enabled a quantitative investigation of changes in the primary dendritic solidification structure with increasing cooling rate. Nano-scale absorption contrast X-ray computed tomography resolved the distinct phases of the lamellar eutectic structure and three dimensional measurements of the ~ 1 μm interlamellar spacing. It is found that the lamella eutectic structure thickness is inversely proportional to the cooling rate. Nano-scale Zernike phase contrast was also used to image voids at eutectic colony boundaries. The application and resolution of these two instruments are discussed with respect to the resolvable features of the solidification structures.

Atom probe tomography and transmission electron microscopy (TEM) have been used to determine the location and distribution of carbon and alloying elements associated with the complex structural changes that occur at the atomic and nanoscale in 4340 steel after quenching to martensite and tempering at 325, 450 or 575 °C. Tempering at 325 °C resulted in carbide formation without partitioning of chromium, manganese, molybdenum, aluminum, nickel or phosphorus, but with early-stage silicon rejection from the carbide. TEM verified the presence of cementite and the Bagaryatsky orientation relationship with the tempered martensite matrix and detected complex precipitate structures. Tempering at 450 or 575 °C developed concentrations of all alloying elements at ferrite–cementite interfaces: chromium, manganese and molybdenum partitioned into the cementite, and silicon, aluminum, nickel and phosphorus were clearly rejected from the cementite. These results provide direct evidence for staged cementite growth, where early-stage growth likely occurs under paraequilibrium conditions, followed by initial silicon redistribution and subsequent alloying element redistribution during late-stage growth. Tempering at 575 °C induced spheroidization of the cementite, loss of the Bagaryatsky orientation relationship, and phosphorus concentrations at Cottrell atmospheres within the cementite and at ferrite–cementite interfaces, correlating with early observations of the retardation of spheroidization by phosphorus.

Quenching and partitioning is a relatively new heat treatment concept to generate microstructures containing retained austenite stabilised by carbon partitioning from martensite. Research on quench and partitioning has been conducted by numerous groups, and this critical assessment provides some of the authors’ perspectives on progress and understanding in the field, with particular focus on the physical metallurgy and transformation mechanisms, process variations, mechanical behaviour, and industrial implementation. While much progress has been made, the field provides rich opportunity for further understanding and development.

In situ characterization techniques are now affording direct interrogation of opaque materials during synthesis and processing. In this work, synchrotron X-ray radiography and tomography were performed at Argonne National Laboratory’s Advanced Photon Source to monitor metallic alloys during melting and solidification. X-ray radiographs of microstructure evolution in Al-7at.%Cu during continuous heating and cooling were obtained; the influence of cooling rate on microstructure evolution was also explored. X-ray tomography results of solidification progression in the mushy zone are also presented. These results demonstrate that synchrotron X-ray radiography and tomography can nondestructively and sequentially reveal metallic alloy melting and solidification over the micron length scale in 2D and 3D. In situ characterization will permit advances in solidification theory and allow for the development of predictive solidification and microstructure evolution models. Feedback from real-time imaging will ultimately enable in-process parameter adjustments to control microstructure evolution.