5 Werkstofftechnik
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Organisationseinheit der BAM
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Regarding feedstocks for the additive manufacturing (AM) of ceramics, two features are most critical in classical powder based AM processes: a high particle packing density (typically >50% TD) must be achieved with very fine particles (typically submicron) in order to ensure sufficient sintering activity. Three innovative approaches will be introduced to overcome this problem:
1. Layer wise slurry deposition: The use of water based ceramic slurries as feedstock for the additive manufacture of ceramics has many advantages which are not fully exploit yet. In the layerwise slurry deposition (LSD) process a slurry with no or low organic content is repetitively spread as thin layers on each other by means of a doctor blade. During the deposition, the ceramic particles settle on the previously deposited and dried material to form thin layers with a high packing density (55-60%). The LSD therefore shares aspects both of tape casting and slip casting. The LSD differentiates from the classical powder-based AM layer deposition, which typically achieves with a flowable coarse grained powder a low packing density (35-50%) only, consequently hindering the ability of sintering ceramic parts to full density. The LSD is coupled with the principles of selective laser sintering (SLS) or binder jetting, to generate novel processes which take advantage of the possibility of achieving a highly dense powder-bed.
2. Laser induced slip casting: Contrary to the LSD process, which requires drying of each individual layer, the direct interaction of ceramic slurries with intense laser radiation, for the laser induced slip casting (LIS), is a promising approach for the additive manufacture of voluminous parts.
3. Gas flow assisted powder deposition: By the application of a vacuum pump a gas flow is realized throughout the powder bed. This gas flow stabilizes the powder bed and results into an enhanced flowability and packing density of the powder during layer deposition.
The presentation will provide a detailed discussion of potentialities and issues connected to the mentioned technologies and will describe the most recent developments in their application to technical ceramics.
Gas turbine components, made of nickel-based alloys, undergo material damage due to high temperatures and mechanical stresses. These components need periodic replacement to avoid efficiency loss and failure. Repair of these parts is more cost-effective than replacement. State-of-the-art repair technologies, including different additive manufacturing (AM) and brazing processes, are considered for efficient restoration. Materials properties mismatches and/or internal defects in repaired parts may expedite crack initiation and propagation, reducing fatigue life. To understand the crack growth behavior in joining zones and predict the remaining life of repaired components, fatigue crack growth (FCG) tests were conducted on specimens of nickel-based alloys joined via brazing, pre-sintered preforms and AM. The FCG experimental technique was successfully adapted for joined specimens and results indicate that the investigated braze material provides a lower resistance to crack growth. In AM-sandwich specimens, the crack growth rates are significantly reduced at the interface of AM and cast material.
Unlike conventional alloys, which typically consist of one main element, high-entropy alloys (HEAs) contain five or more principal elements, which broaden chemical complexity and with it a realm of synergistic mechanisms. The AlMo0.5NbTa0.5TiZr HEA initiated a subclass of Al-containing refractory (r)HEAs that has recently drawn attention [2]. The alloy has a superalloy-resembling B2/bcc nanostructure, which inspired its name refractory high entropy superalloy (RSA). With high-temperature (HT) compressive strengths beyond conventional Ni-based superalloys, this nanostructure could be used for improved HT structural applications. However, in the application-relevant HT regime the Al-Zr-rich B2 phase decomposes to form a hexagonal Al-Zr-based intermetallic (Al4-xZr5; x: 0..1) [3,4]. This work explores the fascinating yet fatal micromechanisms associated to this phase transformation, in the context of creep, annealing and oxidation experiments performed between 800 and 1200 °C.
The material was produced by arc-melting and heat treatment in argon, which lead to grain boundaries decorated with up to 7%. Interrupted constant-load creep tests were performed under vacuum (at 10-4 Pa), at 900–1100 °C with external tensile stresses of 30–120 MPa. Oxidation experiments were separately conducted for 24 hours at 800 and 1000 °C in both dry (21% O2 + 79% N2) and humid (8% O2 + 74% N2 + 18% H2O) air. After the experiments, the samples were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy to reveal degradation mechanisms. Crystallographic texture, orientation relationships and stabilization of an oxygen-containing iso structure (Al4-xZr5(Ox-y); y: 0..x) of the Al-Zr-rich intermetallic are found and discussed.
This is the stable version of the full-notch creep test ontology (OntoFNCT) that ontologically represents the full-notch creep test. OntoFNCT has been developed in accordance with the corresponding test standard ISO 16770:2019-09 Plastics - Determination of environmental stress cracking (ESC) of polyethylene - Full-notch creep test (FNCT).
The OntoFNCT provides conceptualizations that are supposed to be valid for the description of full-notch creep tests and associated data in accordance with the corresponding test standard. By using OntoFNCT for storing full-notch creep test data, all data will be well structured and based on a common vocabulary agreed on by an expert group (generation of FAIR data) which is meant to lead to enhanced data interoperability. This comprises several data categories such as primary data, secondary data and metadata. Data will be human and machine readable. The usage of OntoFNCT facilitates data retrieval and downstream usage. Due to a close connection to the mid-level PMD core ontology (PMDco), the interoperability of full-notch creep test data is enhanced and querying in combination with other aspects and data within the broad field of materials science and engineering (MSE) is facilitated.
The class structure of OntoFNCT forms a comprehensible and semantic layer for unified storage of data generated in a full-notch creep test including the possibility to record data from analysis and re-evaluation. Furthermore, extensive metadata allows to assess data quality and reliability. Following the open world assumption, object properties are deliberately low restrictive and sparse.
This contribution presents the results of an experimental study on the LCF behavior of an austenitic 316L stainless steel produced by laser powder bed fusion featuring a low defect population, which allows for an improved understanding of the role of other typical aspects of a PBF‑LB microstructure. The LCF tests were performed between room temperature and 600 °C. A hot‑rolled 316L variant was tested as a reference. The mechanical response is characterized by strain-life curves, a Coffin‑Manson‑Basquin fitting, and cyclic deformation curves. The damage and deformation mechanisms are studied with X-ray computed tomography, optical and electron microscopy. The PBF‑LB/M/316L exhibits lower fatigue lives at lower strain amplitudes. The crack propagation is mainly transgranular. The solidification cellular structure seems to be the most relevant underlying microstructural feature determining the cyclic deformation behavior.
Several studies have been shown that the electron beam can be used to create nanomaterials from microparticle targets in situ in a transmission electron microscope (TEM). Here, we show how this method has to be modified in order to synthesize plasmonic gold nanoparticles (NPs) on insulating silicon oxide substrate by employing a scanning electron microscope with a comparatively low acceleration voltage of 30 kV. The synthesized NPs exhibit a random distribution around the initial microparticle target: Their average size reduces from 150 nm to 3 nm with growing distance to the initial Au microparticle target. Similarly, their average distance increases. The synthesized NP assemblies therefore show distinctly different plasmonic behaviour with growing distance to the target, which allows to study consequences of random hybridization of surface plasmon in disordered system, such as Anderson localization. To reveal the surface plasmons and their localization behaviour we apply electron energy loss spectroscopy in the TEM.
KupferDigital mechanical testing datasets: Stress relaxation and low-cycle fatigue (LCF) tests
(2024)
The KupferDigital project deals with the development of a data ecosystem for digital materials research on the basis of ontology-based digital representations of copper and copper alloys. This document provides exemplary mechanical testing datasets for training the developed KupferDigital infrastructures. Different types of cast copper alloys were provided for this research and their mechanical testing (stress relaxation and low-cycle fatigue) was performed in the accredited materials testing laboratory, while the test results were reported according to the DIN/ISO standards and attached with the maximum possible metadata about the sample history, equipment, and calibration. The attached content file consisted of the obtained primary raw testing data as well as the secondary datasets of these tests containing the detailed metadata of mechanical testing methods. Such test data files are processed by the KupferDigital digital tools to be converted to standardized machine-readable data files.
Following the new paradigm of materials development, design, and optimization, digitalization is the main goal in materials sciences and engineering (MSE) which imposes a huge challenge. In this respect, the quality assurance of processes and output data as well as the interoperability between applications following FAIR principles are to be ensured. For storage, processing, and querying of data in contextualized form, Semantic Web technologies (SWT) are used since they allow for machine-actionable and human-readable knowledge representations needed for data management, retrieval, and (re)use.
The project ‘platform MaterialDigital’ (PMD, https://materialdigital.de) aims to bring together and support interested parties from both industrial and academic sectors in a sustainable manner in solving digitalization tasks and implementing digital solutions. Therefore, the establishment of a virtual material data space and the systematization of the handling of hierarchical, process-dependent material data are focused. Core points to be dealt with are the development of agreements on data structures and interfaces implemented in distinct software tools and to offer users specific support in their projects. Furthermore, the platform contributes to a standardized description of data processing methods in materials research. In this respect, selected MSE methods are semantically represented which are supposed to serve as best practice examples with respect to knowledge representation and the creation of knowledge graphs used for material data.
Accordingly, this presentation shows the efforts taken within the PMD project towards the digitalization in MSE such as the development of the mid-level PMD core ontology (PMDco, https://github.com/materialdigital/core-ontology). Furthermore, selected results of a PMD partner project use case addressing data and knowledge management from synthesis, production, and characterization of materials are shown.
Highly unrelaxed structural states of metallic glasses have often advantageous mechanical properties. Since metallic glasses continuously relax with time (age) or inherently are well relaxed after processing, methods to uniformly rejuvenate the material are needed. One approach that has received attention is the so-called cryogenic-cycling method, during which a metallic glass is repeatedly immersed into liquid nitrogen. In some cases, cryogenic cycling is truly efficient in increasing the stored excess enthalpy of metallic glasses, but it does not seem to be universally applicable to all alloys and structural states. The origins for these differences remain unclear due to our limited understanding of the underlying structural evolution. In order to shed more light onto the fundamental structural processes of cryogenic cycling, we pursue in-situ x-ray photon correlation spectroscopy (XPCS) to trace the atomic-scale structural dynamics of a Zr-based metallic glass in two different structural states (ribbon and bulk metallic glass). This method allows calculating the relaxation times as a function of time throughout the thermal cycling. It is found that the investigated glasses exhibit heterogeneous structural dynamics at 300 K, which changes to monotonic aging at 78 K. Cryogenic cycling homogenizes the relaxation time distribution for both structural states. This effect is much more pronounced in the ribbon, which is the only structural state that rejuvenates upon cycling. We furthermore reveal how fast atomic-scale dynamics is correlated with long-time average structural relaxation times irrespective of the state, and that the ribbon exhibits unexpected additional fast atomic-scale relaxation in comparison to the plate material. Overall, a picture emerges that points towards heterogeneities in fictive temperature as a requirement for cryogenic energy storage.
Additive manufacturing (AM) offers significantly greater freedom of design compared to conventional manufacturing processes since the final parts are built layer by layer. This enables metal AM, also known as metal 3D printing, to be utilized for improving efficiency and functionality, for the production of parts with very complex geometries, and rapid prototyping. However, despite many technological advancements made in recent years, several challenges hinder the mass adoption of metal AM. One of these challenges is mechanical anisotropy which describes the dependency of material properties on the material orientation. Therefore, in this work, stainless steel 316L parts produced by laser-based powder bed fusion are used to isolate and understand the root cause of anisotropy in AM parts. Furthermore, an efficient and accurate multiscale numerical framework is presented for predicting the deformation behavior of actual AM parts on the macroscale undergoing large plastic deformations. Finally, a novel constitutive model for the plastic spin is formulated to capture the influence of the microstructure evolution on the material behavior on the macroscale.