Low angle grain boundaries (LAGBs) form during directional solidification of Ni-base single crystal superalloys and extend over distances of one to several average dendrite spacings (mm scale). Their influence on the superalloys' behavior has often been overlooked. In the present work we use the high-resolution rotation vector base line electron back scatter diffraction (RVB-EBSD) technique to locate LAGBs, and focused ion beam (FIB) micromachining to prepare specimens for investigations by scanning transmission electron microscopy (STEM) and atom probe tomography (APT). STEM confirms that LAGBs consists of fine dislocation networks. APT provides experimental evidence for the segregation of Rhenium (Re) to these LAGBs in the as-cast state. RVB-EBSD results show that during a multi-step post cast heat treatment, irregular shaped LAGBs straighten out and move over distances of the order of several 10 mu m. Considering this LAGB movement, and in the light of previous reports (He et al., 2020), we propose that Re segregates to LAGBs during Bridgman processing, redissolves during the high temperature (> 1300 degrees C) solution phase, allowing LAGB migration, and then resegregates during the medium temperature (< 1000 degrees C) precipitation phase of the multi-step heat treatment, which immobilizes the LAGBs during creep loading in the 1000 degrees C temperature range. The finding of the present study shades a new light on the behavior of Re and LAGBs in Ni-base single crystal superalloys.

In laser-based powder bed fusion of metals (PBF-LB/M), variations in laser scanner movements, particularly lesser-studied parameters like scanner delays that control laser directional changes, can influence the microstructure in a part during fabrication as each of typically millions of individual laser vectors impact part thermal history and resulting microstructure. While the impact of commonly researched parameters such as laser power, scan speed, hatch spacing, and layer thickness on part microstructure have been well studied, considerably less attention has been given to scanner delays such as the polygon delay. This study uses electron backscatter diffraction to investigate the microstructural variations caused by polygon delay values ranging from 0 to 450 microseconds, beginning with individual scan tracks. The study then extends single tracks to a simple three-dimensional part to examine if microstructure differences due to polygon delays may be influenced by localized heating and cooling caused by nearby hatch vectors and successive layers. The results reveal that varying polygon delay clearly affects grain morphology during individual scan tracks, although these effects are less clear during a three-dimensional build. Future PBF-LB/M studies should focus more on understanding time-resolved laser beam processing effects to better reduce inconsistencies and improve part quality.

Advanced electron backscatter diffraction (EBSD) and electron dispersive spectroscopy (EDS) techniques were used to systematically quantify meso-scale microstructural descriptors in an advanced powder processed polycrystalline Ni-base superalloyNi-base superalloys containing elevated levels of refractory alloying additions. The microstructural changes of the alloy as a function of effective strain were tracked and related to the subsequent heat-treated microstructuresMicrostructure. This emerging field of “microstructure informaticsMicrostructure informatics” extends beyond the conventionally used metrics of grain and precipitatePrecipitates sizes and distributions. Due to the multidimensional nature of the data, manual microstructure characterizationMicrostructure characterization becomes virtually impossible, especially when a multitude of different material states must be considered. This motivated the development of an automated microstructure characterizationMicrostructure characterization procedure, which extracts useful geometric, crystallographic, and chemical microstructureMicrostructure features through a batch process. These features provide a level of microstructureMicrostructure detail that has not traditionally been demonstrated at a statistically significant scale capable of effectively capturing the level of intrinsic heterogeneity that is present in polycrystalline Ni-base superalloysNi-base superalloys. In this study, microstructural descriptors from the deformed material were evaluated and used to understand the grain growthGrain growth response during super-solvus heat treatmentHeat treatment. Compared to traditional qualitative and semi-quantitative approaches for characterizing microstructuresMicrostructure, the innovative methodology used in this investigation provide insightful, quantitative microstructureMicrostructure metrics that lead to the generation of new knowledge and scientific understanding.

Materials characterization using electron backscatter diffraction (EBSD) requires indexing the orientation of the measured region from Kikuchi patterns. The quality of Kikuchi patterns can degrade due to pattern overlaps arising from two or more orientations, in the presence of defects or grain boundaries. In this work, we employ constrained nonnegative matrix factorization to segment a microstructure with small grain misorientations, (<1∘), and predict the amount of pattern overlap. First, we implement the method on mixed simulated patterns—that replicates a pattern overlap scenario, and demonstrate the resolution limit of pattern mixing or factorization resolution using a weight metric. Subsequently, we segment a single-crystal dendritic microstructure and compare the results with high-resolution EBSD. By utilizing weight metrics across a low-angle grain boundary, we demonstrate how very small misorientations/low-angle grain boundaries can be resolved at a pixel level. Our approach constitutes a versatile and robust tool, complementing other fast indexing methods for microstructure characterization.

The present work takes a new look at the high temperature strength of single crystal (SX) Ni‐base superalloys. It compares high temperature constant strain rate (CSR) testing, creep testing, and out‐of‐phase thermomechanical fatigue (OP TMF) testing, which represent key characterization methods supporting alloy development and component design in SX material science and technology. The three types of tests are compared using the same SX alloy, working with precisely oriented <001>‐specimens and considering the same temperature range between 1023 and 1223 K, where climb controlled micro‐creep processes need to be considered. Nevertheless, the three types of tests provide different types of information. CSR testing at imposed strain rates of 3.3 × 10−4 s−1 shows a yield stress anomaly (YSA) with a YSA stress peak at a temperature of 1073 K. This increase of strength with increasing temperature is not observed during constant load creep testing at much lower deformation rates around 10−7 s−1. Creep rates show a usual behavior and increase with increasing temperatures. During OP‐TMF loading, the temperature continuously increases/decreases in the compression/tension part of the mechanical strain‐controlled cycle (±0.5%). At the temperature, where the YSA peak stress temperature is observed, no peculiarities are observed. It is shown that OP‐TMF life is sensitive to surface quality, which is not the case in creep. A smaller number of cycles to failure is observed when reducing the heating rate in the compression/heating part of the mechanical strain‐controlled OP‐TMF cycle. The results are discussed on a microstructural basis, using results from scanning and transmission electron microscopy, and in light of previous work published in the literature.

Recent advances in hardware technology as well as sophisticated methods for post-processingProcessing of Electron Backscatter DiffractionX-ray diffraction (EBSD) and Energy DispersiveX-ray characterization X-Ray Spectroscopy (EDS) data have opened up new possibilities for detailed quantitative microstructure characterizationMicrostructure characterization of polycrystalline Ni-based superalloysNi- based superalloys. However, combining EBSD and EDS scans to reconstruct the true morphology of primary γ′ particles remains challenging, as some important microstructural features exist at a scale below the EDS method’s lateral resolution limit, which leads to undesired artifacts at γ/γ′ interfaces. We present an automated computer vision architectureAutomated computer vision architecture capable of resolving the meso-scale features of polycrystalline γ/γ′ microstructuresMicrostructure with a level of detail that has not previously been demonstrated. Our methodology involves the following steps: 1. The combination of multiple elemental EDS maps. 2. Edge-preserving filtering of EDS maps using a non-local-means algorithm. 3. Unsupervised machine learningMachine learning phase segmentation based on k-means clustering and 4. An automated artifact correction for the combination of EDS and EBSD information based on morphological conditions. In this manner, digital micrographs are reconstructed in a way that allows for quantitative determination of meaningful numeric metrics by utilizing methods from the field of algorithmic geometry. Various microstructural entities such as discrete primary γ′ particles, mixed γ/γ′ grains, or γ grains can be characterized separately, including properties of related boundaries. Geometric characteristics can be quantified in terms of the local arrangement and cluster behavior of particle groups, as well as their spacings. The present work contributes to the development of digital workflows for precise and automatic microstructure characterizationMicrostructure characterization.

The present work uses the rotation vector baseline electron back scatter orientation imaging method (RVB-EBSD) to study the evolution of small misorientations between the gamma- and gamma'-phase in Ni-base single crystal superalloys (SXs) during creep. For this purpose, two material states of the SX ERBO1 (CMSX4 type) were characterized after creep deformation at 850 degrees C and 600 MPa to final strains of 1% and 2%. Obtaining reliable phase boundary misorientation (PBM), kernel average misorientation (KAM) and orientation spread (OS) data represents a challenge for electron backscatter diffraction (EBSD), not only because the method operates at its limits of lateral and angular resolution, but also because it is difficult to differentiate between the two phases merely based on Kikuchi diffraction. The two phases differ in chemical composition which gives rise to different EBSD background intensities. These can be exploited to differentiate between the two phases. In the present work, crystallographic and chemical information are combined to demonstrate that orientation imaging can be used to document the formation of dislocation networks at gamma/gamma'-interfaces and the filling of gamma-channels by dislocations. These findings are in good agreement with reference results from diffraction contrast scanning transmission electron microscopy. It is also shown that misorientations evolve between small groups of equally oriented gamma/gamma'-neighborhoods, on a size scale above characteristic gamma/gamma'-dimensions (>0.5 mu m) and below distances associated with dendritic mosaicity (<200 mu m). The methodological aspects as well as the new material specific results are discussed in the light of previous work published in the literature.

The present work studies the out-of-phase thermomechanical fatigue (OP-TMF) behavior of the precisely oriented [001] single crystal Ni-base superalloy ERBO/1 (CMSX-4 type). The OP-TMF tests are performed between minimum and maximum temperatures of 1023 and 1223 K and a cyclic mechanical strain amplitude of +/- 0.5 %. In accordance with previous findings, the present study confirmed that isothermal low cycle fatigue (LCF) tests at 1023 and 1223 K show significantly longer fatigue lives than the OP-TMF tests, specimens with rougher surfaces fail earlier than polished specimens and deformation bands and cracks form in the specimen surface. Two new results were obtained: First, when comparing OP-TMF cycles with fast cooling (tensile part of cycle) and fast vs. slow heating (compressive part of cycle) one finds that the slower heating/compressive cycle is more damaging. Second, analytical scanning electron microscopy of the flanks of an OP-TMF crack allows to study the diffusioncontrolled growth kinetics of the oxide and the underlying zone, which is depleted by the oxide forming elements. These results are discussed in the light of the microstructural, mechanical and chemical results of the present study considering previous work on OP-TMF of superalloy single crystals (SXs).

In the binary Fe-rich Fe-Ni system, martensite start temperatures MS decrease from 500 to 200 K when Ni concentrations increase from 20 to 30 at.%. It is well known that alloys with Ni concentrations below 28.5 at.% exhibit lath martensite (LM) microstructures (athermal transformation, small crystals, accommodation by dislocations). Above this concentration, plate martensite (PM) forms (burst-like transformation, large crystals, accommodation by twins). The present work is based on a combination of (i) ingot metallurgy for the manufacturing of Fe-Ni alloys with varying Ni-concentrations, (ii) thermal analysis to measure phase transformation temperatures with a special focus on MS, and (iii) analytical orientation imaging scanning electron microscopy for a quantitative description of microstructures and crystallographic features. For Ni-concentrations close to 28.5 at.%, the descending MS-curve shows a local maximum, which has been overlooked in prior works. Beyond the local maximum, MS temperatures decrease again and follow the overall trend. The local maximum is associated with the formation of transition martensite (TM) microstructure, which exhibits LM and PM features. TM forms at higher MS temperatures, as it is accommodated by simultaneous twinning and dislocation slip. An adopted version of the Clausius-Clapeyron equation explains the correlation between simultaneous accommodation and increased transformation temperatures.

Material: Nickel-base superalloy ERBO/1 (more details: Parsa, A. B., et al. Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys. Adv. Eng. Mater. 2015, 17 (2), 216-230, https://doi.org/10.1002/adem.201400136) Casting: Bridgman seed technique; Withdrawal rate: 180 mm/h, Thermal gradient 13.3 K/mm (more details: Hallensleben, P., et al. On the evolution of cast microstructures during processing of single crystal Ni-base superalloys using a Bridgman seed technique, Mat. Des. 2017, 128, 98–111, https://doi.org/10.1016/j.matdes.2017.05.001) Sample: Cross sectional slices extracted perpendicular to the growth direction of a single crystal superalloy cylinder (diameter 12mm, length 120 mm). Preperation: Each slice was individually mounted, grinded, polished and etched 6 seconds with an etching solution consisting of 100ml H2O, 100ml HCl, 100ml HNO3 and 3g MoO3. Image acquisition: Optical microscope of type Axio (Carl Zeiss GmbH) equipped with a high-resolution CCD-camera of type Leica DFC320 and stepper-motor driven sample stage of type Tango Desktop (Märzhäuser) -------------------------------------- The published data is a compilation of 20 serial sectioned optical micrographs resolving the dendritic microstructure of the sample described above. They show a central region of the specimen at different heights of the cylindrical sample. The names of those micrographs correspond to the z-coordinate in millimeters, i.e. micrograph "012.tif" was extracted at 12mm. For each micrograph, an object detector based on a neural network was used to identify the dendrite core positions. Afterwards, registration algorithms were used to determine the growth directions together with branching and extinction events of all dendrites. Neighboring dendrites were identified by calculating a triangulation for each micrograph. This quantitative data was transformed into a microstructure database stored as a .JSON file using the "Relational Geometric Ontology" approach described in XXX. The structure of the .JSON data is described in the data in brief paper YYY. Some methodological aspects are demonstrated in the MethodX paper ZZZ.

Size effects ('smaller is stronger') are important aspects of the mechanical behavior of materials. Experimentally, they are usually probed by performing compression tests on micropillars of different sizes (cross-sectional areas). To overcome limitations associated with comparing different crystal structures and to better handle the influence of melting points in different metals, single crystal face centered cubic (fcc) micropillars of equiatomic binary, ternary, and quaternary medium-entropy alloy (MEA) subsystems of the quinary CrMnFeCoNi high-entropy alloy (HEA) were tested. All eight alloys investigated were single-phase solid solutions having the fcc crystal structure. Their melting temperatures varied only over a narrow range (1189-1462 degrees C). They exhibit a size-dependent critical resolved shear stress (CRSS proportional to d-n, where d is the pillar diameter, n is a power law exponent). The results show that, all else being equal, size effects scale inversely with friction stress. In contrast, there is no systematic dependence on configurational entropy, contrary to speculations in some earlier papers that solid solution strengthening would increase as the number of alloying elements increases. 'Bulk' CRSS values were estimated by extrapolating the measured CRSS values of pillars with diameters of approximately 1-8 mu m to larger pillar sizes of 30 mu m. Good agreement was found with available CRSS values of bulk single crystals. It is concluded that it is possible to obtain bulk CRSS values more reliably from micropillar tests than from the Taylor factor corrected yield strengths of bulk polycrystals.

Super-solidus hot isostatic pressing (SSHIP) heat treatment has first been developed and applied to the third-generation Ni-base single crystal superalloy CMSX-10 K. This new type of heat treatment aims at significantly reducing the total time required for the solution heat treatment and at enhancing mechanical properties as compared to conventional heat treatment routes. The SSHIP is an innovative, economical and sustainable approach that can be applied to all types of Ni-base SX superalloys. It is especially interesting for alloys with a high content of refractory elements and a large volume fraction of eutectic microstructure in the as-cast state.

In the present work, we use an advanced EBSD method to analyze the two prominent types of martensite microstructures that are found in the binary Fe-Ni system, lath martensite (27.5 at.% Ni) and plate martensite (29.5 at.% Ni). We modify, document, and apply an analytical EBSD procedure, which was originally proposed by Yardley and Payton, 2014. It analyzes the distributions of the three KSI-angles (ξ1, ξ2, and ξ3, KSI after Kurdjumov and Sachs), which describe small angular deviations between crystal planes in the unit cells of martensite and austenite—which are related through specific orientation relationships. The analysis of the angular distributions can be exploited to obtain high-resolution, color-coded micrographs of martensitic microstructures, which, for example, visualize the difference between lath and plate martensite and appreciate the microstructural features, like midribs in large plate martensite crystals. The differences between the two types of martensite also manifest themselves in different distributions of the KSI-angles (wider for lath and narrower for plate martensite). Finally, our experimental results prove that local distortions result in scatter, which is larger than the differences between the orientation relationships of Kurdjumov/Sachs, Nishiyama/Wassermann, and Greninger/Troiano.

Ni-based single crystal superalloys contain microstructural regions that are separated by low-angle grain boundaries. This gives rise to the phenomenon of mosaicity. In the literature, this type of defect has been associated with the deformation of dendrites during Bridgman solidification. The present study introduces a novel serial sectioning method that allows to rationalize mosaicity on the basis of spatial dendrite growth. Optical wide-field micrographs were taken from a series of cross sections and evaluated using quantitative image analysis. This allowed to explore the growth directions of close to 2500 dendrites in a large specimen volume of approximately 450 mm3. The application of tomography in combination with the rotation vector base-line electron back-scatter diffraction method allowed to analyze how small angular differences evolve in the early stages of solidification. It was found that the microstructure consists of dendrites with individual growth directions that deviate up to ≈4° from the average growth direction of all dendrites. Generally, individual dendrite growth directions coincide with crystallographic <001> directions. The quantitative evaluation of the rich data sets obtained with the present method aims at contributing to a better understanding of elementary processes that govern competitive dendrite growth and crystal mosaicity.

Author: F. Scholz, M. Cevik, P. Hallensleben, P. Thome, G. Eggeler, J. Frenzel Affiliation: Ruhr University Bochum Date: 08/2021 Material: Nickel-base superalloy ERBO/1 (more details: Parsa, A. B., et al. Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys. Adv. Eng. Mater. 2015, 17 (2), 216-230, https://doi.org/10.1002/adem.201400136) Casting: Bridgman seed technique; Withdrawal rate: 180 mm/h, Thermal gradient 13.3 K/mm (more details: Hallensleben, P., et al. On the evolution of cast microstructures during processing of single crystal Ni-base superalloys using a Bridgman seed technique, Mat. Des. 2017, 128, 98–111, https://doi.org/10.1016/j.matdes.2017.05.001) Sample: Cross sectional slices extracted perpendicular to the growth direction of a single crystal superalloy cylinder (diameter 12mm, length 120 mm). Image acquisition: Optical microscope of type Axio (Carl Zeiss GmbH) equipped with a high-resolution CCD-camera of type Leica DFC320 and stepper-motor driven sample stage of type Tango Desktop (Märzhäuser) Image pre-processing: Preparation of wide-field image collages using the stitching procedures implemented in software package Imagic ims (https://imagic.ch/en/imagic-ims, 07/2021) Image post-processing: Image registration with CorelDraw X7 (: https://www.coreldraw.com/en/, 07/2021) using a contour reference mask Quantitate analysis: Dendrite positions were extracted using the software package ImageJ (https://imagej.de.softonic.com/, 07/2021). -------------------------------------- The five optical micrographs cross sections represent image data which were obtained by tomographic characterization of as-cast single crystal nickel-base superalloy prepared by a seeded Bridgman technique. The material has been studied in the frame of the collaborative research center SFB/TR 103. All details on the applied Bridgman technique are described in the literature (Hallensleben, P., et al., Mat. Des. 2017, 128, 98–111, https://doi.org/10.1016/j.matdes.2017.05.001 and Hallensleben, P., et al., Crystals 2019, 9 (3), 149, https://doi.org/10.3390/cryst9030149). The tomographic image slices were prepared by successive electro discharge machining using incremental steps of 1mm. The image series represents the evolution of dendritic microstructures during the early stages of crystal growth from the back melted seed. The five wide-field micrographs were used to retrieve dendrite positions (enclosed as CSV data for each cross section) to evaluate crystal mosaicity on the basis of dendrite growth directions. All information and a detailed interpretation of tomographic are available in (Scholz, F., PhD-thesis, Ruhr University Bochum, https://doi.org/10.13154/294-8079). We hope that our image data will be useful for other types of solidification research. Please provide a notification by personal mail on the re-use of our raw data. Thank you. All images and dendrite position data were evaluated in the following study concerning dendrite growth behavior, low angle misorientation defects, dendrite arrangements and spacings: Scholz, F.; Cevik, M.; Hallensleben, P.; Thome, P.; Eggeler, G.; Frenzel, J. A 3D Analysis of Dendritic Solidification and Mosaicity in Ni-based Single Crystal Superalloys, Materials 2021, 14 (17), 4904 (https://doi.org/10.3390/ma14174904).

Industrial scale single crystal (SX) Ni-base superalloys contain numerous low angle grain boundaries inherited from the solidification process. Here, we demonstrate that low angle grain boundaries in a fully heat-treated SX model Ni-base superalloy are strongly segregated with up to 12 at% Re. Some Re-rich dislocations forming this grain boundary are found located inside gamma, others close to a gamma/gamma' interface. Although these segregated Re atoms lose their solid-solution strengthening effect, they may enhance the creep resistance by pinning the low angle grain boundaries and slowing down dislocation reactions. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In the present work, three Ni-based single-crystal superalloys (SXs) were investigated, a Re-containing alloy ERBO/1 (CMSX-4 type) and two Re-free SXs referred to as ERBO/15 and ERBO/15-W, which differ in W content. The microstructural evolution of the three alloys during heat treatment and their creep behavior is investigated. When one applies one heat treatment to all three alloys, one obtains different γ/γ′-microstructures. Subjecting these three alloys to creep in the high-temperature low-stress creep regime, ERBO/15 outperforms ERBO/1. In order to separate the effects of alloy chemistry and microstructure, the kinetics of the microstructural evolution of the three alloys was measured. The results were used to establish similar microstructures in all three alloys. Comparing ERBO/15 with ERBO/15-W, it was found that in ERBO/15-W particles grow faster during the first precipitation heat treatment and that ERBO/15-W creeps significantly faster. At constant microstructures, ERBO/15 and ERBO/1 show similar creep behavior. In the high-temperature and low-stress creep regime, ERBO/15 shows lower minimum creep rates but ERBO/1 features a slower increase of creep rate in the tertiary creep regime. It was also found that in the high-temperature low-stress creep regime, ERBO/1 shows a double minimum creep behavior when particles are small.

The present work contributes to a better understanding of the effect of stress multiaxiality on the creep behavior of single crystal Ni-base superalloys. For this purpose we studied the creep deformation and rupture behavior of double notched miniature creep tensile specimens loaded in three crystallographic directions [100], [110] and [111] (creep conditions: 950 °C and 400 MPa net section stress). Crystal plasticity finite element method (CPFEM) was used to analyze the creep stress and strain distributions during creep. Double notched specimens have the advantage that when one notch fails, the other is still intact and allows to study a material state which is close to rupture. No notch root cracking was observed, while microstructural damage (pores and micro cracks) were frequently observed in the center of the notch root region. This is in agreement with the FEM results (high axial stress and high hydrostatic stress in the center of the notched specimen). Twinning was observed in the notch regions of [110] and [111] specimens, and <112> {111} twins were detected and analyzed using orientation imaging scanning electron microscopy. The present work shows that high lattice rotations can be detected in SXs after creep fracture, but they are associated with the high strains accumulated in the final rupture event.

In the present work we present the Rotation Vector Base Line Electron Back Scatter Diffraction (RVB-EBSD) method, a new correlative orientation imaging method for scanning electron microscopy (OIM/SEM). The RVB-EBSD method was developed to study crystal mosaicity in as-cast Ni-base superalloy single crystals (SX). The technique allows to quantify small crystallographic deviation angles between individual dendrites and to interpret associated accommodation processes in terms of geometrically necessary dislocations (GNDs). The RVB-EBSD method was inspired by previous seminal approaches which use cross correlation EBSD procedures. It applies Gaussian band pass filtering to improve the quality of more than 500 000 experimental patterns. A rotation vector approximation and a correction procedure, which relies on a base line function, are used. The method moreover features a novel way of intuitive color coding which allows to easily appreciate essential features of crystal mosaicity. The present work describes the key elements of the method and shows examples which demonstrate its potential.