23 research outputs found
Metastable nanomaterials and nanocomposites obtained by high-pressure torsion powder consolidation
Nanostructuring can dramatically improve the mechanical and functional properties of metallic materials and composites and to achieve the goal of a nanostructured bulk material innovative techniques have to be used.
High Pressure Torsion (HPT), a method of Severe Plastic Deformation (SPD) is a novel processing route for powder consolidation, featuring a complete absence of a sintering treatment – bulk samples are the direct result of the SPD process. HPT further shows two advantages: First, the starting material (powder mixtures) can be processed at any concentration. Even immiscible compositions were successfully processed for different systems (e.g. Cu-Fe, Cu-Co). Second, the severe deformation gives rise to supersaturated solid solutions (“far from equilibrium”) yielding interesting material properties, different from the pure or alloyed elements’ ones.
These supersaturated solid solutions, upon adequate annealing, show phase separations yielding nanoscaled composites. This gives a tool in one’s hands to systematically tune certain material properties. The nanostructures that were formed in such a way show high strength and ductility but also interesting functional properties. To give an example, annealing of above mentioned binary systems changes their magnetic properties regarding coercivity, remanence and magnetoresistance.
Thus, combining the right choice of processed powders (composition, size, shape), HPT-processing parameters (applied strain, processing temperatures) and subsequent annealing treatment (time, temperature) results in desired, tailored microstructures of optimized properties.
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 757333
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
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
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
Manufacturing of Textured Bulk Fe-SmCo Magnets by Severe Plastic Deformation
Exchange-coupling between soft- and hard-magnetic phases plays an important
role in the engineering of novel magnetic materials. To achieve exchange
coupling, a two-phase microstructure is necessary. This interface effect is
further enhanced if both phase dimensions are reduced to the nanometer scale.
At the same time, it is challenging to obtain large sample dimensions. In this
study, powder blends and ball-milled powder blends of Fe-SmCo are
consolidated and are deformed by high-pressure torsion (HPT), as this technique
allows us to produce bulk magnetic materials of reasonable sizes. Additionally,
the effect of severe deformation by ball-milling and severe plastic deformation
by HPT on exchange coupling in Fe-SmCo composites is investigated. Due to
the applied shear deformation, it is possible to obtain a texture in both
phases, resulting in an anisotropic magnetic behavior and an improved magnetic
performance.Comment: 12 pages, 6 figures, 1 tabl
Intermixing of Fe and Cu on the atomic scale by high-pressure torsion as revealed by DC- and AC-SQUID susceptometry and atom probe tomography
The capability of high-pressure torsion on the preparation of supersaturated
solid solutions, consisting of Cu-14Fe (wt.%), is studied. From microstructural
investigations a steady state is obtained with nanocrystalline grains. The
as-deformed state is analyzed with atom probe tomography, revealing an enhanced
solubility and the presence of Fe-rich particles. The DC-hysteresis loop shows
suppressed long range interactions in the as-deformed state and evolves towards
a typical bulk hysteresis loop when annealed at 500{\deg}C. AC-susceptometry
measurements of the as-deformed state reveal the presence of a
superparamagnetic blocking peak, as well as a magnetic frustrated phase,
whereas the transition of the latter follows the Almeida-Thouless line,
coinciding with the microstructural investigations by atom probe tomography.
AC-susceptometry shows that the frustrated state vanishes for annealing at
250{\deg}C