Max Planck Institute for Iron Research GmbH
Recent publications
The composition of the metastable L21 Heusler phase in Fe2AlX with (X = Nb, Ta) alloys has been determined by atom probe tomography (APT). It was found that the composition of L21 is off-stoichiometric in both systems, however closer to the stoichiometric composition in the Fe-Al-Ta than in the Fe-Al-Nb(-B) alloy. L21 dissolves faster and therefore the formation of the stable C14 Laves phase proceeds quicker in the Fe-Al-Nb(-B) alloy. Doping with boron does not lead to the formation of borides and only a slight enrichment of boron in the Fe-Al matrix at the grain boundary, which is covered with C14 precipitates is observed.
Metallic materials, especially steel, underpin transportation technologies. High-manganese twinning induced plasticity (TWIP) austenitic steels exhibit exceptional strength and ductility from twins, low-energy microstructural defects that form during plastic loading. Their high-strength could help light-weighting vehicles, and hence cut carbon emissions. TWIP steels are however very sensitive to hydrogen embrittlement that causes dramatic losses of ductility and toughness leading to catastrophic failure of engineering parts. Here, we examine the atomic-scale chemistry and interaction of hydrogen with twin boundaries in a model TWIP steel by using isotope-labelled atom probe tomography, using tritium to avoid overlap with residual hydrogen. We reveal co-segregation of tritium and, unexpectedly, oxygen to coherent twin boundaries, and discuss their combined role in the embrittlement of these promising steels.
La(Fe,Mn,Si)13-based magnetocaloric materials are one of the most promising material families for the realisation of near-room temperature magnetic refrigeration. The functional and mechanical properties of these materials crucially depend on their chemistry, which is difficult to control at interfaces between microstructural units. Atom probe tomography was employed to reveal the local elemental distribution at the α-Fe/1:13 phase boundary and the 1:13/1:13 grain boundary. Strong Mn segregation (and Fe depletion) at the α-Fe/1:13 phase boundary suggests the potential effect of phase boundary area on the Curie temperature of the material. A local off-1:13 stoichiometry layer at the 1:13/1:13 grain boundary may adversely affect the magnetocaloric performance. Routes to mitigate the negative effects of interfaces on the functional and mechanical performance of these materials are discussed, in order to achieve durable and efficient operation of magnetic cooling devices.
Layered double hydroxides (LDHs), whose formation is strongly related to OH‐ concentration, have attracted significant interest in various fields. However, the effect of the real‐time change of OH‐ concentration on LDHs’ formation has not been fully explored due to the unsuitability of the existing synthesis methods for in situ characterization. Here, the deliberately designed combination of NH3 gas diffusion and in situ pH measurement provides a solution to the above problem. The obtained results revealed the formation mechanism and also guided us to synthesize a library of LDHs with the desired attributes in water at room temperature without using any additives. After evaluating their oxygen evolution reaction performance, we found that FeNi‐LDH with a Fe/Ni ratio of 25/75 exhibits one of the best performances so far reported.
Layered double hydroxides (LDHs), whose formation is strongly related to OH‐ concentration, have attracted significant interest in various fields. However, the effect of the real‐time change of OH‐ concentration on LDHs’ formation has not been fully explored due to the unsuitability of the existing synthesis methods for in situ characterization. Here, the deliberately designed combination of NH3 gas diffusion and in situ pH measurement provides a solution to the above problem. The obtained results revealed the formation mechanism and also guided us to synthesize a library of LDHs with the desired attributes in water at room temperature without using any additives. After evaluating their oxygen evolution reaction performance, we found that FeNi‐LDH with a Fe/Ni ratio of 25/75 exhibits one of the best performances so far reported.
We discuss how to include our recently proposed thermopotentiostat technique [Deissenbeck et al. Phys. Rev. Lett. 2021, 126, 136803] into any existing ab initio molecular dynamics (AIMD) package. Using thermopotentiostat AIMD simulations in the canonical NVTΦ ensemble at a constant electrode potential, we compute the polarization bound charge and dielectric response of interfacial water from first principles.
The design of complex multi‐component superalloys has always been challenging due to the interaction of multiple elements and stringent requirements for various properties. In this study, an integrated approach to designing the high‐component (>7) γ′‐strengthened Co‐based superalloys with well‐balanced properties is developed by combining the diffusion‐multiples and machine‐learning models. A “cross‐component” prediction is achieved by the machine‐learning models, where two types of novenary superalloys are screened out for aero‐engine and industrial gas turbine blades, respectively, based on the experimental database mainly consisting of 6‐7 elements. The method is verified to be effective or slightly more favorable than the Calculation of Phase Diagram (CALPHAD) in predicting the γ′ solvus temperature (Tγ′) and phase constituent of the high‐component alloys when reasonable data of low‐component alloys is just provided. Furthermore, the oxidation resistance and hardness of polycrystal superalloys as well as the compressive strength of single crystal superalloys are tested. Finally, some factors affecting the accuracy of “cross‐component” prediction are discussed. Expanding the compositional range and supplementing the critical interaction data of multiple elements in the database are beneficial for improving the accuracy of the “cross‐component” prediction. This article is protected by copyright. All rights reserved.
One of the main challenges for the synthesis and application of the promising hard-magnetic compound CeFe 11 Ti is the formation of Laves phases that are detrimental for their thermodynamic stability and magnetic properties. In this paper, we present an ab initio based approach to modify the stability of these phases in the Ce-Fe-Ti system by additions of 3d and 4d elements. We combine highly accurate free-energy calculations with an efficient screening technique to determine the critical annealing temperature for the formation of Ce(Fe,X) 11 Ti. The central findings are the dominant role of the formation enthalpy at T = 0 K on chemical trends and the major relevance of partial chemical decompositions. Based on these insights, promising transition metals to promote the stability of the hard-magnetic phase, such as Zn and Tc, were predicted. The comparison with suction casting and reactive crucible melting experiments for Ce-Fe-Ti-X (X = Cu, Ga, Co, and Cr) highlights the relevance of additional phases and quaternary elements.
In order to accelerate the adoption of γ′-strengthened CoNi-based wrought superalloys in engineering applications, two alloys that were previously designed using a framework combining data from a multicomponent diffusion-multiple and machine learning. In this study, we evaluated the comprehensive properties of these alloys, one with spherical γ′ morphology and the other with cuboidal, including the alloy density, phase transformation temperatures, microstructural stability, oxidation resistance and mechanical properties. The properties were discussed with respect to alloying effects and in regard to other CoNi-based and Ni-based wrought superalloys. The results show that the designed alloys have relatively low density, decent microstructural stability, good oxidation resistance and mechanical properties. This study will provide guidance for further design and optimization of γ′-strengthened CoNi-based wrought superalloys.
A unique strategy for the synthesis of a supramolecular metallogel employing zinc ions and adipic acid in DMF medium has been established at room temperature. Rheological analysis was used to investigate the mechanical characteristics of the supramolecular Zn(II)-metallogel. Field emission scanning electron microscopy and transmission electron microscopy were used to analyse the hexagonal shape morphological features of the Zn(II)-metallogel. Interestingly, the electrical conductivity is observed in the electronic device with Zn(II)-metallogel based metal-semiconductor (MS) junctions. All aspects of the metallogel's electrical properties were investigated. The electrical conductivity of the metallogel-based thin film device was 7.38 × 10 −5 S m −1. The synthesised Zn(II)-metallogel based device was investigated for its semi-conductive properties, such as its Schottky barrier diode nature.
Creep strength in polycrystalline Ni-based superalloys is influenced by the formation of a rich variety of planar faults forming within the strengthening γ′ phase. The lengthening and thickening rate of these faults – and therefore the creep rate – depends on an intriguing combination of dislocation interactions at the γ-γ′ interface and diffusional processes of the alloying elements at the core of the fault tip. The effect of alloy composition on this process is not fully understood. In this work we use correlative high resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy to study the deformation faults in two different Ni-based superalloys with carefully designed ratios of disordering-to-ordering-promoting elements (Co-Cr against Nb-Ta-Ti). The results show that the additions of ordering-promoting elements reduce the diffusional processes required for the faults to lengthen and thicken thus reducing the creep rates found for the higher Nb-Ta-Ti alloy. These insights provide a path to follow in the design of improved grades of creep-resistant polycrystalline alloys beyond 700 ∘C.
Metal powders in laser-powder-bed-fusion (L-PBF) often exhibit cohesive flow resulting from interparticle adhesion. Nanoparticle dry-coating can improve powder flowability and promote powder layer densification. A Co25Cr25Fe25Ni25 metal powder (20-90 µm) is dry-coated with TiN particles with a diameter of 16 nm at low concentrations of up to 69 ppm. The dynamic angle of repose decreases and bulk powder density increases compared to the uncoated state from 49 ° and 4.67 g/cm³ to 29 ° and 4.81 g/cm³ with dry-coating of TiN, respectively. UV/VIS spectroscopy showed negligible alterations by TiN additions on the powder light absorption. The powder modifications strongly affected their corresponding processability in L-PBF and reduced the melt pool signatures of the in situ detected confocal single-color pyrometer signal as well as ex situ measured melt pool depth and width. With increasing flowability, a significant decrease in thermal emission and melt pool size was observed. The results demonstrate the impact of powder flowability and bulk powder density on the quality of L-PBF parts when particle interactions are actively modified.
Steels with medium manganese (Mn) content (3∼12 wt-%) have emerged as a new alloy class and received considerable attention during the last decade. The microstructure and mechanical response of such alloys show significant differences from those of established steel grades, especially pertaining to the microstructural variety that can be tuned and the associated micromechanisms activated during deformation. The interplay and tuning opportunities between composition and the many microstructural features allow to trigger almost all known strengthening and strain-hardening mechanisms, enabling excellent strength-ductility synergy, at relatively lean alloy content. Previous investigations have revealed a high degree of microstructure and deformation complexity in such steels, but the underlying mechanisms are not adequately discussed and acknowledged. This encourages us to critically review and discuss these materials, focusing on the progress in fundamental research, with the aim to obtain better understanding and enable further progress in this field. The review addresses the main phase transformation phenomena in these steels and their mechanical behaviour, covering the whole inelastic deformation regime including yielding, strain hardening, plastic instability and damage. Based on these insights, the relationships between processing, microstructure and mechanical properties are critically assessed and rationalized. Open questions and challenges with respect to both, fundamental studies and industrial production are also identified and discussed to guide future research efforts.
Mechanically strong and ductile load-carrying materials are needed in all sectors, from transportation to lightweight design to safe infrastructure. Yet, a grand challenge is to unify both features in one material. We show that a plain medium-manganese steel can be processed to have a tensile strength >2.2 gigapascals at a uniform elongation >20%. This requires a combination of multiple transversal forging, cryogenic treatment, and tempering steps. A hierarchical microstructure that consists of laminated and twofold topologically aligned martensite with finely dispersed retained austenite simultaneously activates multiple micromechanisms to strengthen and ductilize the material. The dislocation slip in the well-organized martensite and the gradual deformation-stimulated phase transformation synergistically produce the high ductility. Our nanostructure design strategy produces 2 gigapascal-strength and yet ductile steels that have attractive composition and the potential to be produced at large industrial scales.
Interstitial solutes, such as carbon in steels, are effective solid‐solution hardening agents. These alloying elements are believed to occupy the octahedral interstices in body‐centered‐cubic (bcc) metals. Using deep‐sub‐angstrom resolution electron ptychography, here we provide the first experimental evidence to directly observe individual oxygen atoms, and discern in which interstitial sites they are located, in a concentrated bcc solid solution — the (TiNbZr)86O12C1N1 medium‐entropy alloy (MEA). In addition to oxygen interstitials residing in octahedral sites, we show the first unambiguous evidence of a switch in preference to the unusual tetrahedral sites at high oxygen concentrations. This shift away from octahedral occupancy is explained as resulting from the extra cost of strain energy when the requisite displacement of the host atoms is deterred in the presence of nearby octahedral interstitials. This article is protected by copyright. All rights reserved
Nanocomposite materials, consisting of two or more phases, at least one of which has a nanoscale dimension, play a distinctive role in materials science because of the multiple possibilities for tailoring their structural properties and, consequently, their functionalities. In addition to the challenges of controlling the size, size distribution and volume fraction of nanometer phases, thermodynamic stability conditions limit the choice of constituent materials. This study goes beyond this limitation by showing the possibility of achieving nanocomposites from a bimetallic system, which exhibits complete miscibility under equilibrium conditions. A series of nanocomposite samples with different compositions were synthesized by the co-deposition of 2000-atoms Ni-clusters and an atomic flux of Cu-atoms using a novel Cluster Ion Beam Deposition system. The retention of the metastable nanostructure is ascertained from atom probe tomography (APT), magnetometry, and magneto-transport studies. APT confirms the presence of nanoscale regions with ≈100 at. % Ni. Magnetometry and magneto-transport studies reveal superparamagnetic behavior and magneto-resistance stemming from the single-domain ferromagnetic Ni-clusters embedded in the Cu-matrix. Essentially, the magnetic properties of the nanocomposites can be tailored by the precise control of the Ni concentration. The initial results offer a promising direction for future research on nanocomposites consisting of fully-miscible elements. This article is protected by copyright. All rights reserved.
This paper examines the problem of the fully coupled magneto-electro-elastic (MEE) scattering of SH-waves incident upon a heterogeneous MEE scatterer which is embedded in an unbounded medium. The scatterer consists of a circular core and a circular encapsulator with eccentricity. All three regions: the core, encapsulator, and the surrounding matrix have distinct MEE properties and fully coupled constitutive relations. The generated coupled MEE fields coexist simultaneously in all these regions without resort to any simplifying assumptions. The precise description of the multifunctionality involves the solution of three fully coupled partial differential equations in three different regions. The associated Green’s function equations involve 9 independent components of Green’s functions. The behaviors of the regions are described by the generalized constitutive equations suitable for transversely isotropic MEE properties. Conventionally, wave function approach has been used to study the elastodynamic fields associated with the purely elastic axisymmetric problems; such a treatment encounters serious difficulties in the presence of eccentricity. As a rigorous analytical remedy the dynamic magneto-electro-mechanical equivalent inclusion method (DMEMEIM) will be developed in this work. To this end, the notions of eigenstress, eigenbody-force, eigenelectric, and eigenmagnetic fields will be introduced. As it will be shown, the employment of these notions in conjunction with the eigenfunction space of the pertinent coupled field equations provides a meticulous mathematical framework for the treatment of the proposed problem. The exact analytical formulation for the fully coupled total MEE scattering cross-section is derived. The ramifications of the MEE couplings as well as the wavenumber on the induced scattered fields are considered. As it will be seen, the magnetic field has a substantial effect on the total scattering cross-section. The interfacial stresses are remarkably affected not only by the eccentricity, but also by the magnetic parameters. Moreover, the dynamic electric displacement concentration factor (DEDCF), the dynamic stress concentration factor (DSCF), the electric potential, and the magnetic potential will be examined for different wavenumbers.
Water is an integral component in electrochemistry, in the generation of the electric double layer, and in the propagation of the interfacial electric fields into the solution; however, probing the molecular-level structure of interfacial water near functioning electrode surfaces remains challenging. Due to the surface-specificity, sum-frequency-generation (SFG) spectroscopy offers an opportunity to investigate the structure of water near working electrochemical interfaces but probing the hydrogen-bonded structure of water at this buried electrode-electrolyte interface was thought to be impossible. Propagating the laser beams through the solvent leads to a large attenuation of the infrared light due to the absorption of water, and interrogating the interface by sending the laser beams through the electrode normally obscures the SFG spectra due to the large nonlinear response of conduction band electrons. Here, we show that the latter limitation is removed when the gold layer is thin. To demonstrate this, we prepared Au gradient films on CaF2 with a thickness between 0 and 8 nm. SFG spectra of the Au gradient films in contact with H2O and D2O demonstrate that resonant water SFG spectra can be obtained using Au films with a thickness of ∼2 nm or less. The measured spectra are distinctively different from the frequency-dependent Fresnel factors of the interface, suggesting that the features we observe in the OH stretching region indeed do not arise from the nonresonant response of the Au films. With the newfound ability to probe interfacial solvent structure at electrode/aqueous interfaces, we hope to provide insights into more efficient electrolyte composition and electrode design.
It has long been a norm that researchers extract knowledge from literature to design materials. However, the avalanche of publications makes the norm challenging to follow. Text mining (TM) is efficient in extracting information from corpora. Still, it cannot discover materials not present in the corpora, hindering its broader applications in exploring novel materials, such as high-entropy alloys (HEAs). Here we introduce a concept of “context similarity" for selecting chemical elements for HEAs, based on TM models that analyze the abstracts of 6.4 million papers. The method captures the similarity of chemical elements in the context used by scientists. It overcomes the limitations of TM and identifies the Cantor and Senkov HEAs. We demonstrate its screening capability for six- and seven-component lightweight HEAs by finding nearly 500 promising alloys out of 2.6 million candidates. The method thus brings an approach to the development of ultrahigh-entropy alloys and multicomponent materials.
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218 members
Claudio Zambaldi
  • Department of Microstructure Physics and Alloy Design
L. Lymperakis
  • Department of Computational Materials Design
Xuyang Zhou
  • Department of Microstructure Physics and Alloy Design
Asif Bashir
  • Department of Interface Chemistry and Surface Engineering
Markus Valtiner
  • Department of Interface Chemistry and Surface Engineering
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Max-Planck-Straße 1, 40237 Düsseldorf, Düsseldorf, Germany
Head of institution
Dierk Raabe
Website
www.mpie.de/