In this project, we work on a generic solution to design advanced high-entropy alloys (HEAs) with enhanced magnetic properties. By overturning the concept of stabilizing solid solutions in HEAs, we propose to render the massive solid solutions metastable and trigger spinodal decomposition. The motivation for starting from the HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behaviour using multiple components.
Magnetic properties of the Fe15Co15Ni20Mn20Cu30 HEA samples in different processing conditions. a, b) Temperature dependence of magnetization of the homogenized a) and 600 oC / 240 h annealed b) HEA samples after zero-field cooling in applied magnetic fields of 50-0.05 T and after a 0.5 T field cooling in an applied magnetic field of 0.01 T. c) Hysteresis loops investigated up to 2 T of the HEA with different annealing time at 300 K. d) Experimental Curie temperatures as a function of annealing time. The morphological evolution of the alloy’s nanostructure as a function of annealing time is shown in terms of APT reconstructions of volume portions with dimensions of 40×40×200 nm3. The APT reconstructions also show 50 at. % iso-composition surfaces of Cu. e) DFT calculated Curie temperatures as a function of annealing time. The blue shaded area indicates the impact of strain induced volume dilatation on the Curie temperature, i.e., the difference between the Curie temperature calculated for a hypothetical free-standing unconstrained single-phase bulk-like Fe-Co alloy and that for the experimentally measured strained volume of the same region in the HEA. For the 6 h and 24 h annealed samples, light blue squares and spheres indicate the Curie temperature values calculated for compositions corresponding to three different APT tips while the final value is the arithmetic mean over the three compositions. The oval shapes for these two annealing times mark the fluctuations in the Curie temperatures that occur due to the scatter in local composition among the different APT specimens.
Magnetic properties of the Fe15Co15Ni20Mn20Cu30 HEA samples in different processing conditions. a, b) Temperature dependence of magnetization of the homogenized a) and 600 oC / 240 h annealed b) HEA samples after zero-field cooling in applied magnetic fields of 50-0.05 T and after a 0.5 T field cooling in an applied magnetic field of 0.01 T. c) Hysteresis loops investigated up to 2 T of the HEA with different annealing time at 300 K. d) Experimental Curie temperatures as a function of annealing time. The morphological evolution of the alloy’s nanostructure as a function of annealing time is shown in terms of APT reconstructions of volume portions with dimensions of 40×40×200 nm3. The APT reconstructions also show 50 at. % iso-composition surfaces of Cu. e) DFT calculated Curie temperatures as a function of annealing time. The blue shaded area indicates the impact of strain induced volume dilatation on the Curie temperature, i.e., the difference between the Curie temperature calculated for a hypothetical free-standing unconstrained single-phase bulk-like Fe-Co alloy and that for the experimentally measured strained volume of the same region in the HEA. For the 6 h and 24 h annealed samples, light blue squares and spheres indicate the Curie temperature values calculated for compositions corresponding to three different APT tips while the final value is the arithmetic mean over the three compositions. The oval shapes for these two annealing times mark the fluctuations in the Curie temperatures that occur due to the scatter in local composition among the different APT specimens.
Since its first emergence in 2004, the HEA concept has aimed at stabilizing single- or dual-phase multi-element solid solutions through high mixing entropy. Here, we change this strategy and render such massive solid solutions metastable, to trigger spinodal decomposition for improving the alloys’ magnetic properties. The motivation for starting from a HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behavior using multiple components. The key idea is to form Fe-Co enriched regions which have an expanded volume (relative to unconstrained Fe-Co), due to coherency constraints imposed by the surrounding HEA matrix. As demonstrated by theory and experiments, this leads to improved magnetic properties of the decomposed alloy relative to the original solid solution matrix. In a prototype magnetic FeCoNiMnCu HEA, we show that the modulated structures, achieved by spinodal decomposition, lead to an increase of the Curie temperature by 48% and a simultaneous increase of magnetization by 70% at ambient temperature as compared to the homogenized single-phase reference alloy.
Low dimensional electronic systems, featuring charge density waves and collective excitations, are highly interesting from a fundamental point of view. These systems support novel types of interfaces, such as phase boundaries between metals and charge density waves.
Oxides find broad applications as catalysts or in electronic components, however are generally brittle materials where dislocations are difficult to activate in the covalent rigid lattice. Here, the link between plasticity and fracture is critical for wide-scale application of functional oxide materials.
In this project, we employ a metastability-engineering strategy to design bulk high-entropy alloys (HEAs) with multiple compositionally equivalent high-entropy phases.
The wide tunability of the fundamental electronic bandgap by size control is a key attribute of semiconductor nanocrystals, enabling applications spanning from biomedical imaging to optoelectronic devices. At finite temperature, exciton-phonon interactions are shown to exhibit a strong impact on this fundamental property.
Enabling a ‘hydrogen economy’ requires developing fuel cells satisfying economic constraints, reasonable operating costs and long-term stability. The fuel cell is an electrochemical device that converts chemical energy into electricity by recombining water from H2 and O2, allowing to generate environmentally-friendly power for e.g. cars or houses…
The project Hydrogen Embrittlement Protection Coating (HEPCO) addresses the critical aspects of hydrogen permeation and embrittlement by developing novel strategies for coating and characterizing hydrogen permeation barrier layers for valves and pumps used for hydrogen storage and transport applications.
In this project we conduct together with Dr. Sandlöbes at RWTH Aachen and the department of Prof. Neugebauer ab initio calculations for designing new Mg – Li alloys. Ab initio calculations can accurately predict basic structural, mechanical, and functional properties using only the atomic composition as a basis.
Efficient harvesting of sunlight and (photo-)electrochemical conversion into solar fuels is an emerging energy technology with enormous promise. Such emerging technologies depend critically on materials systems, in which the integration of dissimilar components and the internal interfaces that arise between them determine the functionality.
