Finished Research Projects

Finished Research Projects

Limitations in the characterization of the partitioning of multiphase alloys, which takes place at the submicron scale, lead to a microstructure optimization of these alloys typically based on the evaluation of the averaged response referred to the macroscopic stress-strain-curves. We introduce a novel experimental-numerical methodology to strengthen the integrated understanding of the microstructure and mechanical properties of these alloys. [more]
Scandium-containing aluminium alloys are currently attracting interest as candidates for high-performance aerospace structural materials due to their outstanding combination of strength, ductility and corrosion resistance. Strengthening is achieved by precipitation of Al3Sc-particles upon ageing heat treatment. [more]
In this project, we investigate the phase transformation and twinning mechanisms in a typical interstitial high-entropy alloy (iHEA) via in-situ and interrupted in-situ tensile testing ... [more]
Single crystalline copper beams with thicknesses between 0.7 and 5 μm are manufactured with a focused ion beam technique and bent in a nanoindenter. The yield strengths of the beams show a mechanical size effect (smaller-is-stronger). [more]
In this project, we develop a new class of high-entropy alloys (HEAs) which is interstitially alloyed and unifies all known metallic strengthening mechanisms in one material. This results in joint activation of twinning- and transformation-induced plasticity (TWIP and TRIP) by tuning the matrix phase’s instability in a metastable TRIP-assisted dual-phase HEA. [more]
In this project, we employ a metastability-engineering strategy to design bulk high-entropy alloys (HEAs) with multiple compositionally equivalent high-entropy phases. [more]
We review microstructures and properties of metal matrix composites produced by severe plastic deformation of multiphase alloys. Typical processings are wire drawing, ball milling, roll bonding, equal-channel angular extrusion, and high-pressure torsion of multiphase materials. [more]
Among the high number of multi-principal-element alloys that are referred to as high-entropy alloys (HEAs) in the literature, only a limited number solidify as single-phase solid solutions. [more]
For this project three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 um) but with the same martensite content (30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. [more]
In this project we investigate the hydrogen distribution and desorption behavior in an electrochemically hydrogen-charged binary Ni-Nb model alloy. The aim is to study the role of the delta phase in hydrogen embrittlement of the Ni-base alloy 718. [more]
For this project two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 um by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). [more]
In this project we work on correlative atomic structural and compositional investigations on Co and CoNi-based superalloys as a part of SFB/Transregio 103 project “Superalloy Single Crystals”. The task is to image the boron segregation at grain boundaries in the Co-9Al-9W-0.005B alloy. [more]
The potential of high-entropy alloys (HEAs) to exhibit an extraordinary combination of properties by shifting the compositional regime from the corners towards the centers of phase diagrams has ledto worldwide attention by material scientists. [more]
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. [more]
In this project nanoprecipitates are designed via elastic misfit stabilization in Fe–Mn maraging steels by combining transmission electron microscopy (TEM) correlated atom probe tomography (APT) with ab initio simulations. Guided by these predictions, the Al content of the alloys is systematically varied... [more]
Carbon partitioning between ferritic and austenitic phases is essential for austenite stabilization in the most advanced steels such as those produced by the quenching and partitioning (Q&P) process. [more]
Despite the immanent advantages of metals and alloys processed by additive manufacturing (e.g. design freedom for complex geometry) and unexpected merits (e.g. superior mechanical performance) of AM processes, there are several remaining issues that need to be addressed in order to practically apply AM alloys to various industries. One of the most important issues is the mechanical behavior of AM alloys under hydrogen environments, since it is easily encountered in the industrial fields and has generally detrimental effects on metals and alloys. [more]
Within this project we investigate chemical fluctuations at the nanometre scale in polycrystalline Cu(In,Ga)Se2 and CuInS2 thin-flims used as absorber material in solar cells. [more]
Usually, the only requirement for the chemistry of the process gas in Laser Additive Manufacturing is a low oxygen content, i.e. a completely inert atmosphere. However, often a low oxygen content remains, leading to oxide inclusions in the produced alloy. In this project, we ask the question if the process atmosphere can be used intentionally to react with the feedstock material to produce materials with improved properties. [more]
In AM, parts are built from layer by layer fusion of raw material (eg. wire, powder etc.). Such layer by layer application of heat results in a time-temperature profile which is fundamentally different from any of the contemporary heat treatments.  Previous work in the group has established that this unique thermal profile can be exploited for microstructural modifications (eg. clustering, precipitation) during manufacturing. The aim of this work is to develop a fundamental understanding of such a strongly non-linear, peak-like thermal history on the precipitation kinetics. [more]
In this project, we directly image and characterize solute hydrogen and hydride by use of atom probe tomography combined with electron microscopy, with the aim to investigate H interaction with different phases and lattice defects (such as grain boundaries, dislocation, etc.) in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. [more]
This project investigates if particle strengthening is a viable mechanism for compositionally complex alloys (CCA) showing exceptional mechanical properties. Whether precipitates form analogously to conventional alloys, and, if so, with similar precipitation kinetics, still needs to be studied. Extending the concept of CCA to intentionally particle strengthened CCA (p-CCA) in a systematic way requires full microstructure analyses down to the atomic level. [more]
In this project we study a new strategy for the theory-guided bottom up design of beta-Ti alloys for biomedical applications using a quantum mechanical approach in conjunction with experiments. Parameter-free density functional theory calculations are used to provide theoretical guidance in selecting and optimizing Ti-based alloys... [more]
In this project we investigate the kinetics of the deformation structure evolution and its contribution to the strain hardening of a Fe–30.5Mn–2.1Al–1.2C (wt.%) steel during tensile deformation by means of transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction. [more]
Quench and Partioning (Q&P) steels are 3rd generation advanced high strengths (AHS) steels. They consist of a martensite-austenite microstructure created during a quenching process. However, due to the subsequent partitioning treatment the martensite is relatively soft and the austenite relatively stable against phase transformation which makes the alloy strong (tensile strength up to 1000 MPa) and ductile (uniform tensile elongation up to 20 %) at the same time. We aim at improving the microstructure by obtaining finer austenite dispersion. [more]
This project is about the understanding and optimization of the microstructure and properties of thin strip cast austenitic stainless steel (AISI 304, 1.4301). Concerning the processing steps the relevance of different thin strip casting parameters, in-line forming operations, and heat treatments for optimizing microstructure and properties have been studied. [more]
In this project an integrated simulation strategy for studying primary static recrystallization was developed and applied to a single-crystal nickel-base superalloy. By using a crystal plasticity finite element approach, the driving force for nucleation and grain growth around a Brinell-type indent was modeled. [more]
In this project we study - together with the department of Prof. Neugebauer and Dr. Sandlöbes at RWTH Aachen - the underlying mechanisms that are responsible for the improved room-temperature ductility in Mg–Y alloys compared to pure Mg. [more]
Hydrogen embrittlement of austenitic steels is of high interest because of the potential use of these materials in hydrogen-energy related infrastructures. In order to elucidate the associated hydrogen embrittlement mechanisms, the mapping of heterogeneities in strain, damage (crack/void), and hydrogen and their relation to the underlying microstructures is a key assignment in this field. [more]
Austenite reversion during tempering of a Fe-13.6Cr-0.44C (wt.%) martensite results in an ultra-high strength ferritic stainless steel with excellent ductility. [more]
We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600°C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc–bcc solid solution [more]
In this project we pursue recent developments in the field of austenitic steels with up to 18% reduced mass density. The alloys are based on the Fe-Mn-Al-C system. [more]
Carbon partitioning from martensite into austenite in the quenching and partitioning (Q&P) process has been suggested to be controlled by the constrained carbon equilibrium (CCE) criterion. [more]
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