arXiv subjects

Thomas Niendorf

Publications and source records attributed to Thomas Niendorf.

3 recordsLinked to original sources

arXiv2026

Near-atomic investigation on the elemental redistribution during co-precipitation of nano-sized kappa phase and B2 phase in an Al-alloyed lightweight steel

In the present study, correlative transmission Kikuchi diffraction transmission electron microscopy (TKD-TEM) measurements, atom probe tomography (APT), and density functional theory (DFT) calculations are used to reveal the elemental redistribution during co-precipitation of nanosized kappa and B2 phases in an FCC matrix of an Al alloyed Fe-10Al-7Mn-6Ni-1C (wt.%) steel. Upon ageing at 800 C for 15 min, two co-nanoprecipitation modes are observed: B2 forming together with kappa and B2 forming separately from kappa in the FCC matrix. APT reveals that the B2 precipitate next to kappa (referred to as B2I) is close to an FeAl type phase, while the isolated B2 precipitate (referred to as B2II) is close to a NiAl type phase. The kappa precipitates maintain a nearly constant Al content of approximately 18.4 at.% regardless of their precipitation position. DFT confirms that kappa may accommodate limited Ni substitution at Fe sites without losing structural stability, and that Fe Ni atomic exchange between kappa and B2 is thermodynamically favorable at 800 C. This exchange drives the B2 phase to evolve from a NiAl type towards an FeAl type, improving the stability of both phases during co-precipitation. These results provide understanding of kappa B2 interactions and offer insights for designing nanosized intermetallic strengthened microstructures in Al alloyed lightweight steels.

cond-mat.mtrl-sci↗

arXiv2023

Additive manufacturing of Ni-Mn-Sn shape memory Heusler alloy -- Microstructure and magnetic properties from powder to printed parts

Ni-Mn-based Heusler alloys like Ni-Mn-Sn show an elastocaloric as well as magnetocaloric effect during the magneto-structural phase transition, making this material interesting for solid-state cooling application. Material processing by additive manufacturing can overcome difficulties related to machinability of the alloys, caused by their intrinsic brittleness. Since the magnetic properties and transition temperature are highly sensitive to the chemical composition, it is essential to understand and monitoring these properties over the entire processing chain. In the present work the microstructural and magnetic properties from gas-atomized powder to post-processed Ni-Mn-Sn alloy are investigated. Direct energy deposition was used for processing, promoting the evolution of a polycrystalline microstructure being characterized by elongated grains along the building direction. A complete and sharp martensitic transformation can be achieved after applying a subsequent heat treatment at 1173 K for 24 h. The Mn-evaporation of 1.3 at. % and the formation of Mn-oxide during DED-processing lead to an increase of the transition temperature of 45 K and a decrease of magnetization, clearly pointing at the necessity of controlling the composition, oxygen partial pressure and magnetic properties over the entire processing chain.

cond-mat.mtrl-sci↗

arXiv2017

Time-of-Flight Three Dimensional Neutron Diffraction in Transmission Mode for Mapping Crystal Grain Structures

The physical properties of polycrystalline materials depend on their microstructure, which is the nano-to-centimeter-scale arrangement of phases and defects in their interior. Such microstructure depends on the shape, crystallographic phase and orientation, and interfacing of the grains constituting the material. This article presents a new non-destructive 3D technique to study bulk samples with sizes in the cm range with a resolution of hundred micrometers: time-of-flight three-dimensional neutron diffraction (ToF 3DND). Compared to existing analogous X-ray diffraction techniques, ToF 3DND enables studies of samples that can be both larger in size and made of heavier elements. Moreover, ToF 3DND facilitates the use of complicated sample environments. The basic ToF 3DND setup, utilizing an imaging detector with high spatial and temporal resolution, can easily be implemented at a time-of-flight neutron beamline. The technique was developed and tested with data collected at the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Complex (J-PARC) for an iron sample. We successfully reconstructed the shape of 108 grains and developed an indexing procedure. The reconstruction algorithms have been validated by reconstructing two stacked Co-Ni-Ga single crystals and by comparison with a grain map obtained by post-mortem electron backscatter diffraction (EBSD).

physics.ins-det↗