arXiv subjects

Andrea Bachmaier

Publications and source records attributed to Andrea Bachmaier.

26 records · Page 2Linked to original sources

arXiv2020

Magnetic dilution by severe plastic deformation

Mixtures of Fe and Cu powders are cold-compacted and subsequently deformed with severe plastic deformation by high-pressure torsion, leading to bulk samples. The dilution of Fe in the Cu matrix is investigated with SQUID-magnetometry, whereas the magnetic properties change as a function of Fe-content from a frustrated regime to a thermal activated behaviour. The magnetic properties are correlated with the microstructure, investigated by synchrotron X-ray diffraction and atom probe tomography. Annealing of the as-deformed states leads to demixing and grain growth, with the coercivity as a function of annealing temperature obeying the random anisotropy model. The presented results show that high-pressure torsion is a technique capable to affect the microstructure even on atomic length scales.

cond-mat.mtrl-sci↗

arXiv2020

Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials

Bulk nanocrystalline materials of small and medium ferromagnetic content were produced using severe plastic deformation by high-pressure torsion at roomtemperature. Giant magnetoresistive behavior was found for as deformed materials, which was further improved by adjusting the microstructure with thermal treatments. The adequate range of annealing temperatures was assessed with in-situ synchrotron diffraction measurements. Thermally treated CuCo materials show larger giant magnetoresistance after annealing for 1 h at 300C, while for CuFe this annealing temperature is too high and decreases the magnetoresistive properties. The improvement of magnetoresistivity by thermal treatments is discussed with respect to the microstructural evolution as observed by electron microscopy and ex situ synchrotron diffraction measurements.

cond-mat.mtrl-sci↗

arXiv2020

Tuneable Magneto-Resistance by Severe Plastic Deformation

Bulk metallic samples were synthesized from different binary powder mixtures consisting of elemental Cu, Co, and Fe using severe plastic deformation. Small particles of the ferromagnetic phase originate in the conductive Cu phase, either by incomplete dissolution or by segregation phenomena during the deformation process. These small particles are known to give rise to granular giant magnetoresistance. Taking advantage of the simple production process, it is possible to perform a systematic study on the influence of processing parameters and material compositions on the magneto-resistance. Furthermore, it is feasible to tune the magnetoresistive behavior as a function of the specimens chemical composition. It was found that specimens of low ferromagnetic content show an almost isotropic drop in resistance in a magnetic field. With increasing ferromagnetic content, percolating ferromagnetic phases cause an anisotropy of the magnetoresistance. By changing the parameters of the high pressure torsion process, i.e., sample size, deformation temperature, and strain rate, it is possible to tailor the magnitude of giant magneto-resistance. A decrease in room temperature resistivity of approx. 3.5% was found for a bulk specimen containing an approximately equiatomic fraction of Co and Cu.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic Binary Supersaturated Solid Solutions Processed by Severe Plastic Deformation

Samples consisting of one ferromagnetic and one diamagnetic component which are immiscible at the thermodynamic equilibrium (Co-Cu, Fe-Cu, Fe-Ag) are processed by high-pressure torsion at various compositions. The received microstructures are investigated by electron microscopy and synchrotron X-ray diffraction, showing a microstructural saturation. Results gained from microstructural investigations are correlated to magnetometry data. The Co-Cu samples show mainly ferromagnetic behavior and a decrease in coercivity with increasing Co-content. The saturation microstructure of Fe-Cu samples is found to be dual phase. Results of magnetic measurements also revealed the occurrence of two different magnetic phases in this system. For Fe-Ag, the microstructural and magnetic results indicate that no intermixing between the elemental phases takes place.

cond-mat.mtrl-sci↗

arXiv2020

Impact of interfaces on the radiation response and underlying defect recovery mechanisms in nanostructured Cu-Fe-Ag

Newest developments in nuclear fission and fusion technology as well as planned long-distance space missions demand novel materials to withstand harsh, irradiative environments. Radiation-induced hardening and embrittlement are a concern that can lead to failure of materials deployed in these applications. Here the underlying mechanisms are accommodation and clustering of lattice defects created by the incident radiation particles. Interfaces, such as free surfaces, phase and grain boundaries, are known for trapping and annihilating defects and therefore preventing these radiation-induced defects from forming clusters. In this work, differently structured nanocomposite materials based on Cu-Fe-Ag were fabricated using a novel solid-state route, combining severe plastic deformation with thermal and electrochemical treatments. The influence of different interface types and spacings on radiation effects in these materials was investigated using nanoindentation. Interface-rich bulk nanocomposites showed a slight decrease in hardness after irradiation, whereas the properties of a nanoporous material remain mostly unchanged. An explanation for this different material behavior and its link to recovery mechanisms at interfaces is attempted in this work, paving a concept towards radiation resistant materials.

cond-mat.mtrl-sci↗

arXiv2018

Mechanical properties of electrodeposited amorphous/crystalline multilayer structures in the Fe-P system

Amorphous/crystalline multilayer structures of Fe-P alloys were deposited electrochemically using the single bath technique. Hall-Petch behavior of microhardness with respect to sublayer thickness was observed down to a sublayer thickness of 15 nm. For thinner sublayers, a hardness plateau was obtained. The transition at a sublayer thickness of 15 nm coincides with the loss of the multilayer structure as observed in transmission electron microscopy. The transition is a possible result of a change in the amorphous to crystalline sublayer thickness ratio and the interface roughness development during the deposition process. Additionally, crack deflection at the interfaces was observed for the layered structures with small sublayer thickness in microbending experiments.

cond-mat.mtrl-sci↗

arXiv2016

New insights on the formation of supersaturated solid solutions in the Cu Cr system deformed by high pressure torsion

In the Cu Cr system, the formation of supersaturated solid solutions can be obtained by severe plastic deformation. Energy dispersive synchrotron diffraction measurements on as deformed Cu Cr samples as a function of the applied strain during deformation confirm the formation of supersaturated solid solutions in this usually immiscible system. Due to evaluation of the diffraction data by a newly developed energy dispersive Rietveld program, lattice parameter and microstructural parameters such as domain size and microstrain are determined for as deformed as well as annealed samples. The obtained information is used to deepen the understanding of the microstructural evolution and the formation of supersaturated solid solutions during severe plastic deformation. Complimentary transmission electron microscopy investigations are furthermore performed to characterize the evolving microstructure in detail. After annealing at elevated temperatures, the formed solid solutions decompose. Compared to the as deformed state, an enhanced hardness combined with a high thermal stability is observed. Possible mechanisms for the enhanced hardness are discussed.

cond-mat.mtrl-sci↗

arXiv2016

Phase separation of a supersaturated nanocrystalline Cu Co alloy and its influence on thermal stability

The thermal decomposition behavior, the microstructural evolution and its influence on the mechanical properties of a supersaturated Cu Co solid solution with ~100 nm average grain size prepared by severe plastic deformation is investigated under non-isothermal and isothermal annealing conditions. Pure fine grained Cu and Co exhibit substantial grain growth upon annealing, whereas the Cu Co alloy is thermally stable at the same annealing temperatures. The annealed microstructures are studied by independent characterization methods, including scanning electron microscopy, electron energy loss spectroscopy and atom probe tomography. The phase separation process in the Cu Co alloy proceeds by the same mechanism, but on different length scales: a fine scaled spinodal type decomposition is observed in the grain interior, simultaneously Co and Cu regions with a larger scale are formed near the grain boundary regions. Subsequent grain growth at higher annealing temperatures results in a microstructure consisting of the pure equilibrium phases. Such mechanisms can be used to tailor nano structures to optimize certain properties.

cond-mat.mtrl-sci↗