TY - GEN A1 - Morozova, Iuliia A1 - Kehm, Christian A1 - Obrosov, Aleksei A1 - Yang, Yitong A1 - Miah, Kamal Uddin Mohammad A1 - Uludintceva, Elena A1 - Fritzsche, Sebastian A1 - Weiß, Sabine A1 - Michailov, Vesselin T1 - On the Heat Treatment of Selective-Laser-Melted 316L T2 - Journal of Materials Engineering and Performance N2 - The effect of heat treatment at various temperatures (650, 850, 1050, and 1100°C) and dwell times (10 min and 1 h) on the metallurgical and microstructural evolution as well as on the related tensile properties of stainless steel 316L processed by selective laser melting (SLM) has been systematically evaluated. The metallurgical and microstructural features such as defects, stability of the columnar–cellular structure and substructure, second phase particles, and phase transformation imparted by SLM and heat treatment have been discussed. It has been shown that the processing conditions specific to SLM significantly alter the kinetics of phase evolution compared to standard welding techniques which affects the accuracy of the prediction. The influence of these characteristics on tensile properties and hardness was elucidated. It was disclosed that with increasing heat treatment temperature there was a gradual increase in elongation but a decrease in strength related to the dislocation density and the development of the microstructure. KW - 316L KW - grain structure KW - phase evolution KW - SLM KW - tensile properties Y1 - 2023 U6 - https://doi.org/10.1007/s11665-022-07404-0 SN - 1544-1024 VL - 32 (2023) IS - 10 SP - 4295 EP - 4305 ER - TY - GEN A1 - Morozova, Iuliia A1 - Królicka, Aleksandra A1 - Obrosov, Aleksei A1 - Yang, Yitong A1 - Doynov, Nikolay A1 - Weiß, Sabine A1 - Michailov, Vesselin T1 - Precipitation phenomena in impulse friction stir welded 2024 aluminium alloy T2 - Materials Science and Engineering: A N2 - Microhardness variations across the friction stir welded (FSW) and impulse friction stir welded (IFSW) AA2024–T351 joints have been elucidated by the transformations of the S–Al2CuMg phase with a special focus on a distinguished hardness peak within the heat-affected zone (HAZ) of the impulse welds. The increase in hardness within the stir zone (SZ) originated from the partial re-precipitation of the initial Guinier-Preston-Bagaryatsky zones (GPB) and metastable S needles, previously dissolved.) Formation and growth of stable S precipitates via coalescence accounted for the softening through the thermo-mechanically affected zone (TMAZ). The peak strengthening within the HAZ of the IFSW joints was mainly caused by the dense needle-shaped S particles, which can be explained by a mutual influence of the process specific temperature and strain cycles. Dislocations and subgrain boundaries introduced to the material due to plastic deformation facilitated the nucleation of strengthening S precipitates in the HAZ. It demonstrates that the impact of deformation should be considered by the characterization of the precipitation development in the HAZ. KW - AA2024 KW - impulse friction stir welding KW - microhardness KW - s precipitation KW - thermal cycle KW - deformation Y1 - 2022 U6 - https://doi.org/10.1016/j.msea.2022.143617 SN - 0921-5093 SP - 1 EP - 11 ER - TY - GEN A1 - Jensch, Felix A1 - Eissing, Katharina A1 - Richard, Williams A1 - Trautmann, Marcus A1 - Yang, Yitong A1 - Dubinin, Sergej A1 - Härtel, Sebastian T1 - Improving the structural integrity of challenging to manufacture LPBF components with toolpath correction T2 - Materials Research Proceedings N2 - This work deals with the influence of optimised exposure strategies on the distortion and microstructure of components susceptible to overheating and warpage. Therefore, different distortion-prone specimen geometries of 316L were fabricated with the standard parameters, as well as with exposure strategies optimised by machine learning, which were generated using the AMAIZE software package. The manufactured samples were analysed with regard to distortion. The results of the distortion analysis were then linked with the results of the digital tomography from AMAIZE. Furthermore, components were manufactured that tend to overheat due to their geometry and orientation on the substrate plate. The influence of overheating during the LPBF process on the microstructure and porosity was investigated along the build-up direction by means of an EBSD analysis and a porosity analysis. With the presented approach for optimising the exposure strategy with AMAIZE, it could be shown that a successful production of distortion- prone components with a porosity of less than 1 % is possible in the first trial. KW - LPBF-Process, Machine Learning, Microstructural Investigation Y1 - 2024 UR - https://d21zja6o12zyp0.cloudfront.net/9781644903131.pdf U6 - https://doi.org/10.21741/9781644903131-12 SN - 2474-395X VL - 41 SP - 110 EP - 119 PB - Materials Research Forum LLC ER - TY - GEN A1 - Yang, Yitong A1 - Weiß, Sabine T1 - In-situ investigation on cyclic bending deformation of oligocrystalline 316LVM steel for coronary stent application with EBSD T2 - International Conference on Strength of Materials 2022 N2 - The objective of this work is to investigate the strain localization and slip activation of the microstructure of oligo-crystalline 316LVM steel struts evolving during different cyclic bending deformation stages (number of cycles) and loading conditions (maximum stress and neutral). A micro-scale three-point bending fixture was designed and incorporated into a micro tensile/compression machine inside a scanning electron microscope (SEM) to perform in-situ electron backscatter diffraction (EBSD) measurements during different phases of cyclic fatigue testing. The following results were obtained: 1) The quantitative strain could be compared after each stage of deformation. 2) The rotation of the grains orientations is observed during the cyclic deformation while the morphology did not change significantly. The results contribute to the understanding of the evolution of the microstructure at low strain variations under bending fatigue conditions, and can support the prediction of the fatigue life of 316LVM stainless steel oligocrystalline microdevices like coronary artery stents. KW - Oligo-crystalline microstructure, in-situ bending fatigue test, EBSD, 316LVM austenitic steel, coronary stent, Y1 - 2022 UR - https://hal.science/hal-03829658v1 PB - HAL open science ER - TY - GEN A1 - Yang, Yitong A1 - Eisentraut, Mark A1 - Weiß, Sabine A1 - Bolz, Sebastian T1 - Comparison of the data from two EBSD system to verify the accuracy of the technique T2 - AK Treffen Mikrostrukturcharakterisierung im REM”, Ruhr-Uni Bochum, 30. November – 1. Dezember 2023 Y1 - 2023 UR - https://www.researchgate.net/publication/377636122_Comparison_of_the_data_from_two_EBSD_system_to_verify_the_accuracy_of_the_technique_AK-Treffen_Mikrostrukturcharakterisierung_im_REM_Ruhr-Uni_Bochum CY - Bochum ER - TY - GEN A1 - Emdadi, Aliakbar A1 - Yang, Yitong A1 - Bolz, Sebastian A1 - Stryzhyboroda, Oleg A1 - Tovar, Michael A1 - Gein, Sergej A1 - Hecht, Ulrike A1 - Weiß, Sabine T1 - Mechanisms of necklace recrystallization in a BCC Fe-Al-Ta alloy with strengthening Laves phase precipitates T2 - Scripta Materialia N2 - A necklace structure composed of fine grains formed by dynamic recrystallization was uncommonly observed at the pre-existing grain boundaries during the hot compression of a BCC Fe-25Al-1.5Ta alloy containing C14 - (Fe, Al)2Ta Laves phase precipitates. Two possible mechanisms for necklace formation were proposed; particle-stimulated nucleation and grain boundary bulging, depending on whether the original grain boundaries are occupied by C14 particles, or they are free of them. Recrystallization was initiated preferentially around the clusters of large particles at the boundaries containing particles. In contrast, the bulging of the original grain boundaries by strain-induced boundary migration was observed as a preliminary stage for necklace formation at the particle-free boundaries. The necklace structure expanded into the deformed volume in such a way that low-angle subgrain boundaries decorating the necklace layers transformed into grains with increasing deformation strain. Y1 - 2023 UR - https://www.sciencedirect.com/science/article/pii/S1359646223004281 U6 - https://doi.org/10.1016/j.scriptamat.2023.115705 SN - 1359-6462 SN - 1872-8456 VL - 237 ER - TY - GEN A1 - Lasko, Galina A1 - Yang, Yitong A1 - Weiss, Sabine A1 - Schmauder, Siegfried A1 - Dogahe, Kiarash T1 - FEM Simulations of Fatigue Crack Initiation in the Oligocrystalline Microstructure of Stents T2 - Materials N2 - For over two decades, vascular stents have been widely used to treat clogged vessels,serving as a scaffold to enlarge the narrowed lumen and recover the arterial flow area. High-purityoligocrystalline austenitic steel is usually applied for the production of stents. Despite the popularityand benefit of stenting, it still may cause serious clinical adverse issues, such as in-stent restenosisand stent fracture. Therefore, the study of the mechanical properties of stents and in particularthe prediction of their life cycles are in the focus of materials research. In our contribution, withinthe finite element method, a two-scale model of crack initiation in the microstructure of stents iselaborated. The approach is developed on the basis of the physically based Tanaka–Mura model(TMM), considering the evolution of shear bands during the crack initiation phase. The model allowsfor the analysis of the microstructure with respect to the life cycles of real materials. The effects ofdifferent loading conditions, grain orientation, and thickness of the specimen on Wöhler curves wereanalysed. It was found that the microstructural features of oligocrystals are very sensitive to differentloading conditions with respect to their fatigue behaviour and play a major role in fatigue crackinitiation. Different grain-orientation distributions result in qualitative and quantitative differencesin stress distribution and in the number of cycles for crack initiation. It was found that presence ofa neutral zone in the cut-out of the microstructure under three-point-bending loading conditionschanges the qualitative and quantitative patterns of stress distribution and affects the number ofcycles for crack initiation. It was found that under both tensile and bending loading conditions,thicker specimens require more cycles for crack initiation. The Wöhler curves for crack initiation inoligocrystalline microstructures of stents could be compared with the ones in the experiment, takinginto account that for high cyclic fatigue (HCF), typically, more than 70% of the cycles refer to crackinitiation. The developed numerical tools could be used for the material design of stents KW - SCHM 746/222-1 KW - WE 2671/11-1 Y1 - 2023 UR - https://www.mdpi.com/1996-1944/16/17/6003 U6 - https://doi.org/10.3390/ma16176003 VL - 16 IS - 17 ER - TY - GEN A1 - Emdadi, Aliakbar A1 - Yang, Yitong A1 - Szyndler, Joanna A1 - Jensch, Felix A1 - Ertugrul, Gökhan A1 - Tovar, Michael A1 - Härtel, Sebastian A1 - Weiß, Sabine T1 - Highly printable Fe₃Al intermetallic alloy T2 - Metals : open access journal N2 - Intermetallic Fe₃Al-based alloys reinforced with Laves-phase precipitates are emerging as potential replacements for conventional high-alloy steels and possibly polycrystalline Ni-based superalloys in structural applications up to 700 °C. Their impressive mechanical properties, however, are offset by limited fabricability and poor machinability due to their severe brittleness. High tool wear during finish-machining, which is still required for components such as turbine blades, remains a key barrier to their broader adoption. In contrast to conventional manufacturing routes, additive manufacturing offers a viable solution by enabling near-net-shape manufacturing of difficult-to-machine iron aluminides. In the present study, laser powder bed fusion was used to produce an Fe-25Al-1.5Ta intermetallic containing strengthening Laves-phase precipitates, and the porosity, microstructure and phase composition were characterized as a function of the process parameters. The results showed that preheating the build plate to 650 °C effectively suppressed delamination and macrocrack formation, even though noticeable cracking still occurred at the high scan speed of 1000 mm/s. X-ray tomography revealed that samples fabricated with a lower scan speed (500 mm/s) and a higher layer thickness (0.1 mm) contained larger, irregularly shaped pores, whereas specimens printed at the same volumetric energy density (40 J/mm3) but with different parameter sets exhibited smaller fractions of predominantly spherical pores. All samples contained mostly elongated grains that were either oriented close to <001> relative to the build direction or largely texture-free. X-ray diffraction confirmed the presence of Fe₃Al and C14-type (Fe, Al)₂Ta Laves phase in all samples. Hardness values fell within a narrow range (378–398 HV10), with only a slight reduction in the specimen exhibiting higher porosity. KW - Fe₃Al intermetallic KW - Iron aluminide KW - Additive manufacturing KW - Laser powder bed fusion (LPBF) KW - Porosity KW - Microstructure Y1 - 2026 U6 - https://doi.org/10.3390/met16010005 VL - 16 IS - 5 SP - 1 EP - 15 PB - MDPI CY - Basel ER -