Publikationen im Open Access gefördert durch die UBhttps://kobra.uni-kassel.de:443/handle/123456789/20130930440222024-03-19T11:55:11Z2024-03-19T11:55:11ZHigh-Strength Dissimilar Welds Between a NiTi Shape Memory Alloy and Titanium Obtained by Intermixing Niobium Using Pulsed Laser Beam WeldingWiegand, MichaelSommer, NiklasMarks, LindaBöhm, Stefanhttps://kobra.uni-kassel.de:443/handle/123456789/155692024-03-18T15:10:04Z2023-11-17T00:00:00ZJoining of NiTi shape memory alloys with commercially pure titanium (Ti) is of great interest for manifold industrial applications. However, dissimilar fusion welding of these materials is associated with the formation of extremely brittle intermetallic compounds, e.g., Ti2Ni and TiNi3, which drastically limit the mechanical properties of NiTi/Ti joints. The present investigation seeks to improve both chemical compatibility and mechanical performance through intermixing of a niobium foil as filler material into a NiTi/Nb/Ti butt-joint configuration by means of pulsed laser beam welding. Different pulse durations are applied to tailor intermixing and evaluate the deviating chemical compositions of the weld metal. It is demonstrated that intermixing of niobium significantly increases the weldability of the material combination NiTi/Ti compared to autogenous welding. However, the proportion of intermixed filler material exerts a substantial impact on the microstructural evolution and mechanical properties. Ultimate tensile strengths of up to 679 MPa with fracture occurring in the titanium base metal at an elongation at break of 18.7 pct is obtained by a reduction of the pulse duration, which represents a major improvement over previous studies focusing on fusion welding of NiTi to titanium and its alloys.
2023-11-17T00:00:00ZWiegand, MichaelSommer, NiklasMarks, LindaBöhm, StefanJoining of NiTi shape memory alloys with commercially pure titanium (Ti) is of great interest for manifold industrial applications. However, dissimilar fusion welding of these materials is associated with the formation of extremely brittle intermetallic compounds, e.g., Ti2Ni and TiNi3, which drastically limit the mechanical properties of NiTi/Ti joints. The present investigation seeks to improve both chemical compatibility and mechanical performance through intermixing of a niobium foil as filler material into a NiTi/Nb/Ti butt-joint configuration by means of pulsed laser beam welding. Different pulse durations are applied to tailor intermixing and evaluate the deviating chemical compositions of the weld metal. It is demonstrated that intermixing of niobium significantly increases the weldability of the material combination NiTi/Ti compared to autogenous welding. However, the proportion of intermixed filler material exerts a substantial impact on the microstructural evolution and mechanical properties. Ultimate tensile strengths of up to 679 MPa with fracture occurring in the titanium base metal at an elongation at break of 18.7 pct is obtained by a reduction of the pulse duration, which represents a major improvement over previous studies focusing on fusion welding of NiTi to titanium and its alloys.Anthropogenic and natural disturbances increase local genetic diversity in an early spring geophyte (Ficaria verna Huds)Guicking, DanielaKeßler, SarahStörmer, ElizaBersch, MaggieDuchoslav, Martinhttps://kobra.uni-kassel.de:443/handle/123456789/155642024-03-18T11:50:04Z2023-12-26T00:00:00ZThe tetraploid Ficaria verna is a common spring geophyte in central Europe and is considered invasive in the USA and Canada. It is considered an almost seed-sterile taxon, relying on vegetative reproduction by underground tubers and aerial bulbils. Recent studies have revealed high levels of population genetic diversity in F. verna, raising the question of how genetic diversity is maintained and which factors may be responsible for the observed patterns. Polymorphic nuclear microsatellite markers were established to define multi-locus genotypes (MLGs), to analyze fine-scale spatial genetic structure (SGS) using grid and cross-sampling schemes, and to quantify genetic diversity within and between nine populations with different disturbance regimes in central Germany. In total, 115 MLGs were identified among a total of 347 samples. The G/N ratio varied between 0.16 and 0.70 among populations, and in each population several unique MLGs occurred. Genotypes were highly intermingled within populations, suggesting a “guerrilla” dispersal strategy. Significant SGS (negative regression slope of kinship coefficients against inter-individual distances) was found in four out of nine populations in fine-scale cross-sampling (up to 4 m) and in only one population in grid sampling (up to 14.6 m). No single MLG was found in more than one population, while many alleles were shared between populations. Within-population clonal and allelic diversity increased with greater exposure to both anthropogenic and natural disturbances. Regular gap openings, facilitated propagule establishment, and propagule dispersal by water and mowing machines are likely important factors explaining the positive effects of disturbance on local genetic diversity of F. verna.
2023-12-26T00:00:00ZGuicking, DanielaKeßler, SarahStörmer, ElizaBersch, MaggieDuchoslav, MartinThe tetraploid Ficaria verna is a common spring geophyte in central Europe and is considered invasive in the USA and Canada. It is considered an almost seed-sterile taxon, relying on vegetative reproduction by underground tubers and aerial bulbils. Recent studies have revealed high levels of population genetic diversity in F. verna, raising the question of how genetic diversity is maintained and which factors may be responsible for the observed patterns. Polymorphic nuclear microsatellite markers were established to define multi-locus genotypes (MLGs), to analyze fine-scale spatial genetic structure (SGS) using grid and cross-sampling schemes, and to quantify genetic diversity within and between nine populations with different disturbance regimes in central Germany. In total, 115 MLGs were identified among a total of 347 samples. The G/N ratio varied between 0.16 and 0.70 among populations, and in each population several unique MLGs occurred. Genotypes were highly intermingled within populations, suggesting a “guerrilla” dispersal strategy. Significant SGS (negative regression slope of kinship coefficients against inter-individual distances) was found in four out of nine populations in fine-scale cross-sampling (up to 4 m) and in only one population in grid sampling (up to 14.6 m). No single MLG was found in more than one population, while many alleles were shared between populations. Within-population clonal and allelic diversity increased with greater exposure to both anthropogenic and natural disturbances. Regular gap openings, facilitated propagule establishment, and propagule dispersal by water and mowing machines are likely important factors explaining the positive effects of disturbance on local genetic diversity of F. verna.Molecular-frame photoelectron angular distributions during double core-hole generation in O₂ and N₂ moleculesRezvan, Dmitrii V.Novikovskiy, Nikolay MikhailovichHaubenreißer, Daniel MartinLagutin, Boris M.Demekhin, Philipp V.https://kobra.uni-kassel.de:443/handle/123456789/155632024-03-18T10:50:04Z2023-09-15T00:00:00ZAngular distributions of photoelectrons emitted upon double core-hole (DCH) generation in nitrogen and oxygen molecules are studied theoretically in the frame of a molecular reference. The respective electronic structure calculations are performed by the single center method for photoelectron kinetic energies up to 40 eV in the relaxed-core Hartree–Fock approximation. The molecular frame photoelectron angular distributions are computed for single-site and two-site DCH creation processes and further analyzed for different orientations of the molecular axis with respect to the electric field vector of linearly polarized incident light and for localized or delocalized emitting atomic site scenarios. The present theoretical results provide reliable predictions for future experiments with high-repetition free-electron lasers.
2023-09-15T00:00:00ZRezvan, Dmitrii V.Novikovskiy, Nikolay MikhailovichHaubenreißer, Daniel MartinLagutin, Boris M.Demekhin, Philipp V.Angular distributions of photoelectrons emitted upon double core-hole (DCH) generation in nitrogen and oxygen molecules are studied theoretically in the frame of a molecular reference. The respective electronic structure calculations are performed by the single center method for photoelectron kinetic energies up to 40 eV in the relaxed-core Hartree–Fock approximation. The molecular frame photoelectron angular distributions are computed for single-site and two-site DCH creation processes and further analyzed for different orientations of the molecular axis with respect to the electric field vector of linearly polarized incident light and for localized or delocalized emitting atomic site scenarios. The present theoretical results provide reliable predictions for future experiments with high-repetition free-electron lasers.Task-Level Checkpointing and Localized Recovery to Tolerate Permanent Node Failures for Nested Fork–Join Programs in ClustersReitz, LukasFohry, Claudiahttps://kobra.uni-kassel.de:443/handle/123456789/155622024-03-18T10:00:05Z2024-03-13T00:00:00ZExascale supercomputers consist of millions of processing units, and this number is still growing. Therefore, hardware failures, such as permanent node failures, become increasingly frequent. They can be tolerated with system-level Checkpoint/Restart, which saves the whole application state transparently and, if needed, restarts the application from the saved state; or with application-level checkpointing, which saves only relevant data via explicit calls in the program. The former approach requires no additional programming expense, whereas the latter is more efficient and allows to continue program execution after failures on the intact resources (localized shrinking recovery). An increasingly popular programming paradigm is asynchronous many-task (AMT) programming. Here, programmers identify parallel tasks, and a runtime system assigns the tasks to worker threads. Since tasks have clearly defined interfaces, the runtime system can automatically extract and save their interface data. This approach, called task-level checkpointing (TC), combines the respective strengths of system-level and application-level checkpointing. AMTs come in many variants, and so far, TC has only been applied to a few, rather simple variants. This paper considers TC for a different AMT variant: nested fork–join (NFJ) programs that run on clusters of multicore nodes under work stealing. We present the first TC scheme for this setting. It performs a localized shrinking recovery and can handle multiple node failures. In experiments with four benchmarks, we observed execution time overheads of around 44 % at 1536 workers, and negligible recovery costs. Additionally, we developed and experimentally validated a prediction model for the running times of the scheme.
2024-03-13T00:00:00ZReitz, LukasFohry, ClaudiaExascale supercomputers consist of millions of processing units, and this number is still growing. Therefore, hardware failures, such as permanent node failures, become increasingly frequent. They can be tolerated with system-level Checkpoint/Restart, which saves the whole application state transparently and, if needed, restarts the application from the saved state; or with application-level checkpointing, which saves only relevant data via explicit calls in the program. The former approach requires no additional programming expense, whereas the latter is more efficient and allows to continue program execution after failures on the intact resources (localized shrinking recovery). An increasingly popular programming paradigm is asynchronous many-task (AMT) programming. Here, programmers identify parallel tasks, and a runtime system assigns the tasks to worker threads. Since tasks have clearly defined interfaces, the runtime system can automatically extract and save their interface data. This approach, called task-level checkpointing (TC), combines the respective strengths of system-level and application-level checkpointing. AMTs come in many variants, and so far, TC has only been applied to a few, rather simple variants. This paper considers TC for a different AMT variant: nested fork–join (NFJ) programs that run on clusters of multicore nodes under work stealing. We present the first TC scheme for this setting. It performs a localized shrinking recovery and can handle multiple node failures. In experiments with four benchmarks, we observed execution time overheads of around 44 % at 1536 workers, and negligible recovery costs. Additionally, we developed and experimentally validated a prediction model for the running times of the scheme.