Pascal Thome

Interests

No activities entered.

Courses

Scholarly Contributions

Journals/Publications

• Chauniyal, A., Thome, P., & Stricker, M. (2024). Employing Constrained Nonnegative Matrix Factorization for Microstructure Segmentation. Microscopy and Microanalysis, 30(4). doi:10.1093/mam/ozae056
  More info

  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, (
• Gamanov, S., Dlouhy, A., Eggeler, G., Thome, P., & Bürger, D. (2024). Evolution of local misorientations in the γ/γ’-microstructure of single crystal superalloys during creep studied with the rotation vector baseline (RVB) EBSD method. Microscopy Research and Technique, 87(3). doi:10.1002/jemt.24453
  More info

  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 γ- and γ′-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°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 γ/γ′-interfaces and the filling of γ-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 γ/γ′-neighborhoods, on a size scale above characteristic γ/γ′-dimensions (>0.5 μm) and below distances associated with dendritic mosaicity (
• Sirrenberg, M., Guth, S., Parsa, A., Thome, P., Mills, M., Eggeler, G., Babinský, T., Bürger, D., & Dlouhý, A. (2024). The High Temperature Strength of Single Crystal Ni-base Superalloys – Re-visiting Constant Strain Rate, Creep, and Thermomechanical Fatigue Testing. Advanced Engineering Materials, 26(19). doi:10.1002/adem.202400368
  More info

  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 -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.
• Sirrenberg, M., Guth, S., Thome, P., Parsa, A., Eggeler, G., Babinský, T., & Bürger, D. (2024). Out-of-phase thermomechanical fatigue of a single crystal Ni-base superalloy. Materials Science and Engineering: A, 910. doi:10.1016/j.msea.2024.146851
  More info

  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 diffusion-controlled 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).
• Lopez-Galilea, I., Hecker, L., Epishin, A., Ruttert, B., Thome, P., Weber, S., Theisen, W., & Bürger, D. (2023). Super-Solidus Hot Isostatic Pressing Heat Treatments for Advanced Single Crystal Ni-Base Superalloys. Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 54(5). doi:10.1007/s11661-022-06884-y
  More info

  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.
• Pfetzing-Micklich, J., Fox, F., Thome, P., George, E., & Eggeler, G. (2023). Increasing the friction stress decreases the size dependence of strength in a family of face-centered cubic high- and medium-entropy alloy micropillars. Materials Science and Engineering: A, 885. doi:10.1016/j.msea.2023.145548
  More info

  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 °C). They exhibit a size-dependent critical resolved shear stress (CRSS ∝ 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 μm to larger pillar sizes of 30 μm. 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.
• Thome, P., Schneider, M., Yardley, V., Payton, E., & Eggeler, G. (2023). Local Maxima in Martensite Start Temperatures in the Transition Region between Lath and Plate Martensite in Fe-Ni Alloys. Materials, 16(4). doi:10.3390/ma16041549
  More info

  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.
• Thome, P., Schneider, M., Yardley, V., Payton, E., & Eggeler, G. (2022). Crystallographic Analysis of Plate and Lath Martensite in Fe-Ni Alloys. Crystals, 12(2). doi:10.3390/cryst12020156
  More info

  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.
• Scholz, F., Cevik, M., Hallensleben, P., Thome, P., Eggeler, G., & Frenzel, J. (2021). A 3d analysis of dendritic solidification and mosaicity in ni-based single crystal superalloys. MDPI: Materials, 14(17). doi:10.3390/ma14174904
  More info

  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 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 mosa-icity.
• Cao, L., Thome, P., Somsen, C., Cailletaud, G., Eggeler, G., & Agudo Jácome, L. (2020). On the influence of crystallography on creep of circular notched single crystal superalloy specimens. Materials Science and Engineering: A, 782. doi:10.1016/j.msea.2020.139255
  More info

  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 {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.
• He, J., He, J., Scholz, F., Scholz, F., Horst, O., Horst, O., Thome, P., Thome, P., Frenzel, J., Frenzel, J., Eggeler, G., Eggeler, G., Gault, B., & Gault, B. (2020). Corrigendum to ‘On the Re segregation at the low angle grain boundary in a single crystal Ni-base superalloy’ Scripta Materialia Volume 185, August 2020, Pages 88-93 (Scripta Materialia (2020) 185 (88–93), (S1359646220302475), (10.1016/j.scriptamat.2020.03.063)). Scripta Materialia, 187. doi:10.1016/j.scriptamat.2020.06.039
  More info

  The authors regret that the acknowledgement for the funding in the initially version of the article was incomplete and should read “All authors acknowledge the financial support from DFG SFB TR 103 through project A1, A2, A4, B7”. The authors would like to apologise for any inconvenience caused.
• He, J., He, J., Scholz, F., Scholz, F., Horst, O., Horst, O., Thome, P., Thome, P., Frenzel, J., Frenzel, J., Eggeler, G., Eggeler, G., Gault, B., & Gault, B. (2020). On the rhenium segregation at the low angle grain boundary in a single crystal Ni-base superalloy. Scripta Materialia, 185. doi:10.1016/j.scriptamat.2020.03.063
  More info

  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 γ, others close to a γ/γ′ 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.
• Hallensleben, P., Scholz, F., Thome, P., Schaar, H., Steinbach, I., Eggeler, G., & Frenzel, J. (2019). On crystal mosaicity in single crystal Ni-based superalloys. Crystals, 9(3). doi:10.3390/cryst9030149
  More info

  In the present work, we investigate the evolution of mosaicity during seeded Bridgman processing of technical Ni-based single crystal superalloys (SXs). For this purpose, we combine solidification experiments performed at different withdrawal rates between 45 and 720 mm/h with advanced optical microscopy and quantitative image analysis. The results obtained in the present work suggest that crystal mosaicity represents an inherent feature of SXs, which is related to elementary stochastic processes which govern dendritic solidification. In SXs, mosaicity is related to two factors: inherited mosaicity of the seed crystal and dendrite deformation. Individual SXs have unique mosaicity fingerprints. Most crystals differ in this respect, even when they were produced using identical processing conditions. Small differences in the orientation spread of the seed crystals and small stochastic orientation deviations continuously accumulate during dendritic solidification. Direct evidence for dendrite bending in a seeded Bridgman growth process is provided. It was observed that continuous or sudden bending affects the growth directions of dendrites. We provide evidence which shows that some dendrites continuously bend by 1.7° over a solidification distance of 25 mm.
• Thome, P., Medghalchi, S., Frenzel, J., Schreuer, J., & Eggeler, G. (2019). Ni-base superalloy single crystal (SX) mosaicity characterized by the Rotation Vector Base Line Electron Back Scatter Diffraction (RVB-EBSD) method. Ultramicroscopy, 206. doi:10.1016/j.ultramic.2019.112817
  More info

  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.
• Hallensleben, P., Schaar, H., Thome, P., Jafarizadeh, A., Steinbach, I., Eggeler, G., Frenzel, J., & Jöns, N. (2017). On the evolution of cast microstructures during processing of single crystal Ni-base superalloys using a Bridgman seed technique. Materials and Design, 128. doi:10.1016/j.matdes.2017.05.001
  More info

  The present work takes a new look at a modified Bridgman process (Bridgman seed technique, BST) for the production of laboratory Ni-base single crystal (SX) superalloy cylinders of 12/120 mm diameter/length. This type of specimen is needed to perform inexpensive parametric studies for the development of new SX and for understanding the evolution of microstructures during SX casting. During melting, the seed partially melts back. The elementary segregation processes cause a so far unknown type of constitutional heating/cooling. Competitive growth eventually establishes a constant average dendrite spacing. In the present work it is documented how this dendrite spacing varies in one cylindrical ingot, and how it scatters when a series of SX ingots is produced. This type of information is scarce. The calculated temperature gradient across the solid/liquid interface (calculated by FEM) is in excellent agreement with predictions from the Kurz-Fisher equation which yields a dendrite spacing based on the experimental withdrawal rate and the microstructurally determined average dendrite spacing. The presence of small angle grain boundaries on cross sections which were taken perpendicular to the solidification direction can be rationalized on the basis of small deviations from the ideal growth directions of individual primary dendrites.

Proceedings Publications

• Arciniaga, L., Thome, P., Severs, K., & Tin, S. (2024). Using “Microstructure Informatics” to Understand Abnormal Grain Growth Factors in Powder Metallurgy Ni-Based Superalloys. In Superalloys 2024.
  More info

  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.
• Thome, P., Arciniaga, L., & Tin, S. (2024). “Microstructure Informatics” of Polycrystalline Ni-Base Superalloys Using Computer Vision Techniques to Understand Properties and Performance. In Superalloys 2024.
  More info

  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 characterizationof 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.
• Horst, O., Ibrahimkhel, S., Streitberger, J., Wochmjakow, N., Git, P., Scholz, F., Thome, P., Singer, R., Frenzel, J., Eggeler, G., & Körner, C. (2020). On the Influence of Alloy Composition on Creep Behavior of Ni-Based Single-Crystal Superalloys (SXs). In TMS 2020.
  More info

  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.