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Accumulative roll bonding (ARB) of aluminum alloy strips is often limited by severe edge cracking due to large, single-pass rolling reductions. Edge cracks are often caused by either poor alignment of the roll-bonded strips or lateral spreading. This work shows that lateral spreading can be reduced significantly by roll-bonding strips of interest within a frame of sacrificial material. For the conditions tested, it has been found that frames with 6.3 mm of constraint width are optimal in reducing edge cracking and maintaining thickness homogeneity during ARB. This methodology was found to greatly improve sample quality and yield when applied to multiple ARB cycles.

The Co55Cr40Ni5 multicomponent alloy was designed to be single-phase face centered cubic (FCC), display high strength, and have the ability to undergo transformation-induced plasticity (TRIP) during plastic deformation. The alloy was produced, and phase stability was monitored in-situ by synchrotron X-ray diffraction during thermomechanical deformation to observe the formation and evolution of the hexagonal close packed (HCP) martensite phase as a function of applied stress. The alloy displays a good combination of strength and ductility, owing largely to the TRIP effect, which increases the strain hardening rate during deformation.

Compression stress-strain curves of a number of refractory high entropy alloys (RHEAs) were generated at temperatures ranging from room temperature to 1000 °C. It is shown that solid-solution strengthening in these alloys has both an athermal and a thermal component. Results from mechanical testing are combined with literature data to develop solid-solution strengthening models for both components that incorporate the particularities of single-phase body centered cubic (BCC) materials. The athermal component is affected by a combination of atomic size mismatch and elastic modulus mismatch, which depend upon average values from each alloy, thereby allowing this component to be estimated in a high-throughput fashion. On the other hand, the thermally-activated yield stress component does not correlate with any parameter that can be calculated by averaging pure elemental atomic properties and it is observed to be larger than the values found for pure BCC refractory metals and their dilute alloys. Overall, RHEAs are found to have larger thermal and athermal yield stress components compared to pure or conventional refractory alloys, which explains their relatively high strengths at room temperature.

Two low-carbon 9-Cr ferritic-martensitic steels were designed with the aim of decreasing M23C6 and maintaining or increasing MX phase fraction. A low-carbon (LC) alloy and a low-carbon, zero-niobium (0Nb) alloy were fabricated, their designs based upon the P92 alloy system. Solutionizing temperatures to maximize V and Nb in solution while avoiding δ-ferrite were determined to be 1170 °C for the P92 alloy and 1050 °C for LC and 0Nb, significantly lower than predicted using ThermoCalc® modeling. As was intended, the M23C6 phase fraction was reduced for LC and 0Nb alloys after both heat treating and aging relative to the base P92 alloy, as determined by wide angle x-ray scattering (WAXS) analysis. Dislocation density measurements from x-ray line broadening in P92 and LC suggest these alloys had more stable dislocation substructures than 0Nb at lower temperature and shorter time aging conditions. While LC exhibited lower microhardness than P92 at room temperature, the tensile properties were comparable at 650 °C, suggesting that elevated temperature strength can be achieved with lower carbon contents. Aging studies showed that P92 had a more stable microstructure for higher temperature and longer time aging conditions. The P92 alloy also had a longer stress rupture life, implying that the M23C6 precipitate contribution to thermal stability is important. Evidence of Z-phase was discovered for the LC alloy aged 10,000 h at 650 °C, corresponding to decreased strength and increased ductility. Overall, the stress rupture lives of the modified heat-treatment variations of P92 and LC compare favorably to literature values for 9% Cr steels with conventional heat treatments.

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 screening methods is paramount to alloy design. Here, an 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 for a total of 8 compositions. Both are compared to a solid solution strengthening model, again yielding a good correlation. The experimental and simulation results indicate that, in this system, the strength is maximized when the atomic size mismatch is maximized. Furthermore, it is necessary to consider the strain hardening of these alloys to accurately estimate their strength by nanoindentation. A pathway is presented here. This work shows that high-throughput methodologies for predicting and measuring properties are promising for designing new HEAs with desirable combinations of properties.

Variant selection of Ni2(Mo,Cr) precipitates (γ′) in a low coefficient of thermal expansion Ni-Mo-Cr-W alloy has been investigated as a function of precipitate size and applied stress. The lenticular-shaped, coherent γ′ precipitates form with a {110}M/(010)p habit plane. During conventional aging, all 6 variants of the Ni2(Mo,Cr) precipitates form and coarsen with aging time. With the application of external loading below the yield stress during aging, variants with {110}M habit planes that experience the largest dilations resulting from the externally applied stress are favored. Phase field modeling, coupled with elasticity theory, has been employed to simulate the development of variant selection of γ′ precipitates with and without applied external loading. Similar to the experimental results, the γ′ precipitates tend to form parallel to each other during coarsening and display variant selection when external stress is applied during aging. The mechanism is believed to be the reduction of elastic strain energy caused by the precipitate/matrix lattice mismatch. Thus, the nature of the selection indicates that the sign of the misfit is positive along [010]p. Because the misfit strain energy increases with increasing precipitate size, variant selection occurs during coarsening.

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, including a fourfold increase in yield strength, without commensurate losses in ductility, are exhibited, overcoming the strength/ductility tradeoff. This is achieved by nanoscale ω phase precipitates produced by natural and artificial aging, in concert with TRansformation Induced Plasticity (TRIP), in alloy Ti-10V-2Fe-3Al (wt.%) for the first time. These deformation mechanisms can be tailored to design microstructures, opening up the mechanical property and performance landscape for lightweight, Ti structural applications, analogous to the design of advanced high strength steels with unique properties for automotive applications that result in energy savings via lightweighting and crashworthiness

The microstructures and hardnesses of Co-10Al-9W-1C, Co-7Al-5W-1C and Co-7Al-5W (at %) alloys are reported. Homogenization of the Co-10Al-9W-1C alloy was unsuccessful at 1300 °C and both B2-CoAl and η carbide phases remained in the interdendritic regions. However, the lower-solute Co-7Al-5W-1C and Co-7Al-5W alloys did homogenize at 1300 °C. Upon aging the Co-10Al-9W-1C alloy at 800 °C, the undissolved η carbide transformed into D019-Co3W phase and subsequently into γ'-Co3(Al,W) phase, consistent with the higher stability of the γ' phase in the C-doped alloy. Also, high γ' volume fractions and low γ' coarsening rates were revealed by transmission electron microscopy in the various Co-Al-W-C alloys. These observations are explained by the effect of C on the phase equilibria and γ/γ' interfacial energy in the Co-Al-W system. Finally, the microhardness is increased while the density is decreased by alloying with C, which could result in higher specific strengths in the C-doped alloys.

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.

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.

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.

• 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.

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.

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.

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.

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

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.

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 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.

The quenching and partitioning (Q&P) process is a new heat treatment for the creation of advanced high-strength steels. This treatment consists of an initial partial or full austenitization, followed by a quench to form a controlled amount of martensite and an annealing step to partition carbon atoms from the martensite to the austenite. In this work, the microstructural evolution during annealing of martensite–austenite grain assemblies has been analyzed by means of a modeling approach that considers the influence of martensite–austenite interface migration on the kinetics of carbon partitioning. Carbide precipitation is precluded in the model, and three different assumptions about interface mobility are considered, ranging from a completely immobile interface to the relatively high mobility of an incoherent ferrite–austenite interface. Simulations indicate that different interface mobilities lead to profound differences in the evolution of microstructure that is predicted during annealing.

Low temperature aging (

The quenching and partitioning (QP extension of this methodology to include carbon partitioning kinetics is developed here. The final austenite fraction is less sensitive to quench temperature than previously predicted, in agreement with experimental results.

Detailed examinations of shape memory effect (SME) deformation structures in martensite of U–14 at.% Nb were performed with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). An accommodation strain analysis, which has been previously used to predict SME deformation structures and texture evolution in polycrystalline material, was also performed. Martensite variants and twin relationships observed with EBSD after compressive or tensile deformation were determined to be consistent with those expected from calculated accommodation strains. Focused ion beam (FIB) was used to select twinned regions identified with EBSD for more detailed TEM analysis to verify the presence of these specific twins. The observed SME twinning systems in the martensite agree with previous TEM observations and the predicted { 1 ¯ 76 } twinning system was observed experimentally for the first time in U–14 at.% Nb using these complementary techniques.

The goal of the quench and partition (Q&P) process for steel heat treatment is to enrich austenite with carbon during a partitioning treatment after initial quenching below the martensite start temperature ( M s ). Two proposed mechanisms for austenite carbon enrichment during partitioning include carbon transport from martensite and/or the formation of carbide-free bainite. Theoretical calculations show experimentally measured austenite fractions are difficult to explain based upon a mechanism involving solely bainite formation. Carbon partitioning from martensite provides a more satisfactory explanation, although the formation of bainite during partitioning cannot be completely excluded.

Microcompression specimens, 10–15 µm in diameter by 20–30 µm in height, were produced from individual parent grains in a polycrystalline U–13 at.%Nb shape-memory alloy using the focused ion beam technique. The specimens were tested in a nanoindentation instrument with a flat diamond tip to investigate stress–strain behavior as a function of crystallographic orientation. The results are in qualitative agreement with a single-crystal accommodation strain (Bain strain) model of the shape-memory effect for this alloy.