Christian Liebscher

Max Planck Institute for Iron Research GmbH | MPIE · Department of Structure and Nano-/ Micromechanics of Materials

A giant Zn segregation transition is revealed using CALPHAD-integrated density-based modeling of segregation into Fe grain boundaries (GBs). The results show that above a threshold of only a few atomic percent Zn in the alloy, a substantial amount of up to 60 at.% Zn can segregate to the GB. We found that the amount of segregation abruptly increase...

Grain boundaries, the two-dimensional defects between differently oriented crystals, tend to preferentially attract solutes for segregation. Solute segregation has a significant effect on the mechanical and transport properties of materials. At the atomic level, however, the interplay of structure and composition of grain boundaries remains elusive...

The embrittlement of metallic alloys by liquid metals leads to catastrophic material failure and severely impacts their structural integrity. The weakening of grain boundaries by the ingress of liquid metal and preceding segregation in the solid are thought to promote early fracture. However, the potential of balancing between the segregation of co...

A giant Zn segregation transition is revealed using CALPHAD-integrated density-based modelling of Zn segregation into Fe grain boundaries (GBs). The results show that above a threshold of only a few atomic percent Zn in the alloy, a substantial amount of up to 60 at.% Zn can segregate to the GB. We also found that the amount of segregation signific...

Alloying a material and hence segregating solutes to grain boundaries is one way to tailor a material to the demands of its application. Direct observation of solute segregation is necessary to understand how the interfacial properties are altered. In this study, we investigate the atomic structure of a high-angle grain boundary both in pure copper...

When solid-state redox-driven phase transformations are associated with mass loss, vacancies are produced that develop into pores. These pores can influence the kinetics of certain redox and phase transformation steps. We investigated the structural and chemical mechanisms in and at pores in a combined experimental-theoretical study, using the redu...

Metallic glasses (MGs) are known to be structurally heterogeneous at the nanometer (nm) scale. In addition, elastic property mapping has indicated the presence of at least an order-of-magnitude larger length scales, of which the origin continues to remain unknown. Here we demonstrate the existence of an elastic decorrelation length of the order of...

Characterizing crystal structures and interfaces down to the atomic level is an important step for designing novel materials. Modern electron microscopy routinely achieves atomic resolution and is capable to resolve complex arrangements of atoms with picometer precision. Here, we present AI-STEM, an automatic, artificial-intelligence based method,...

Alloying a material and hence segregating solutes to grain boundaries is one way to tailor a material to the demands of its application. Direct observation of solute segregation is necessary to understand how the interfacial properties are altered. In this study, we investigate the atomic structure of a high angle grain boundary both in pure copper...

The embrittlement of metallic alloys by liquid metals leads to catastrophic material failure and severely impacts their structural integrity. The weakening of grain boundaries by the ingress of liquid metal and preceding segregation in the solid are thought to promote early fracture. However, the potential of balancing between the segregation of co...

Grain boundaries, the two-dimensional (2D) defects between differently oriented crystals, control mechanical and transport properties of materials. Our fundamental understanding of grain boundaries is still incomplete even after nearly a century and a half of research since Sorby first imaged grains. Here, we present a systematic study, over 9 orde...

Here, we study the hydride formation in a metastable Ti–33Zr–22Hf–11Ta (at.%) refractory high entropy alloy (RHEA). Deviating to non-equiatomic compositions of RHEAs promotes the formation of transformation-induced plasticity where the body-centered cubic phase transforms to hexagonal close-packed (HCP) phase. It is found that the phase transformat...

In the last few decades, it has been recognized that a single grain boundary (GB) may exist in several different stable and metastable states, which differ in their atomic structure. However, experimental insights at the atomic structure level are rarely reported. In this study, two different microstates of incoherent Σ3 [111] (112¯) GBs from two d...

Solid-state redox-driven phase transformation is associated with mass loss, accommodated by vacancies that develop into pores. These influence the kinetics of the redox reactions and phase transformation. We have investigated the underlying structural and chemical mechanisms in and at pores in a combined experimental-theoretical study, using the re...

The application of proton exchange membrane fuel cells (PEMFCs) in light-duty vehicles (LDV) has dominated the fuel cell research field for the past decades. To reduce the LDV FC system cost to 30 $/kW by 2025 has driven the search for highly active cathode catalysts that could substitute the costly state-of-the-art Pt-alloy catalysts or reduce the...

The structure and chemistry of grain boundaries (GBs) are crucial in determining polycrystalline materials’ properties. Faceting and solute segregation to minimize the GB energy is a commonly observed phenomenon. In this paper, a deposition process to obtain pure tilt GBs in titanium (Ti) thin films is presented. By increasing the power density, a...

The phase behavior of grain boundaries can have a strong influence on interfacial properties. Little is known about the emergence of grain boundary phases in elemental metal systems and how they transform. Here, we observe the nanoscale patterning of a grain boundary by two alternating grain boundary phases with distinct atomic structures in elemen...

Ferritic materials consisting of a disordered matrix and a significant volume fraction of ordered intermetallic precipitates have recently gained attention due to their favorable properties regarding high-temperature applicability. Alloys strengthened by Heusler-type precipitates turned out to show promising properties at elevated temperatures, e.g...

The complex phase evolution sequence of a promising new high temperature structural alloy with composition Al20Cr20Fe35Ni20Ti5 (at.%) has been studied in situ using an electromagnetic levitation (EML) setup combined with time-resolved high energy X-ray diffraction (XRD), as well by scanning transmission electron microscopy (STEM) observations durin...

The properties of polycrystalline materials are related to their microstructures and hence a complete description, including size, shape, and orientation of the grains, is necessary to understand the behaviour of materials. Here, we use Scanning Precession Electron Diffraction (SPED) in the Transmission Electron Microscope (TEM) combined with a til...

This work presents the new template matching capabilities implemented in Pyxem, an open source Python library for analyzing four-dimensional scanning transmission electron microscopy (4D-STEM) data. Template matching is a brute force approach for deriving local crystal orientations. It works by comparing a library of simulated diffraction patterns...

Tungsten-tungsten carbide (W/W2C) composites are considered as possible structural materials for future nuclear fusion reactors. Here, we report on the effect of helium (He) implantation on microstructure evolution of polycrystalline W/W2C composite consolidated by field-assisted sintering technique (FAST), homogenously implanted at room temperatur...

Extensive experiments have shown that the transformation from the face-centered cubic to hexagonal close-packed ε phase usually occurs around coherent Σ3 boundaries. However, in this letter, we reveal a different transformation mechanism in a metastable dual-phase compositionally complex alloy via a systematic high-resolution scanning transmission...

Achieving high mechanical strength and ductility in age-hardenable Al7000 series (Al-Zn-Mg) alloys fabricated by selective laser melting (SLM) remains challenging. Here, we show that crack-free AlZnMgCuScZr alloys with an unprecedented strength-ductility synergy can be fabricated via SLM and heat treatment. The as-built samples had an architectured...

Grain boundaries are intrinsic and omnipresent microstructural imperfections in polycrystalline and nanocrystalline materials. They are short-circuit diffusion paths and preferential locations for alloying elements, dopants, and impurities segregation. They also facilitate heterogeneous nucleation and the growth of secondary phases. Therefore, grai...

This work presents the new template matching capabilities implemented in Pyxem, an open source Python library for analyzing four-dimensional scanning transmission electron microscopy (4D-STEM) data. Template matching is a brute force approach for deriving local crystal orientations. It works by comparing a library of simulated diffraction patterns...

The structure and chemistry of grain boundaries (GBs) are crucial in determining polycrystalline materials' properties. Faceting and solute segregation to minimize the GB energy is a commonly observed phenomenon. In this paper, a deposition process to obtain pure tilt GBs in titanium (Ti) thin films is presented. By increasing the power density, a...

InGaN/GaN quantum wells (QWs) with sub-nanometer thickness can be employed in short-period superlattices for bandgap engineering of efficient optoelectronic devices, as well as for exploiting topological insulator behavior in III-nitride semiconductors. However, it had been argued that the highest indium content in such ultra-thin QWs is kineticall...

The local variation of grain boundary atomic structure and chemistry caused by segregation of impurities influences the macroscopic properties of polycrystalline materials. Here, the effect of co-segregation of carbon and boron on the depletion of aluminum at a Σ5 (3 1 0 )[0 0 1] tilt grain boundary in a α − Fe-4 at%Al bicrystal is studied by combi...

In this study, we report on face-centered cubic structured CoCrFeNi high-entropy alloy thin films with finely dispersed nano-oxide particles which are formed by internal oxidation. Analytical scanning transmission electron microscopy imaging found that the particles are Cr2O3. The oxide particles contribute to the hardening of the film increasing i...

Thermal stability of protective coatings is one of the performance-defining properties for advanced cutting and forming applications as well as for energy conversion. To investigate the effect of oxygen incorporation on the high-temperature behavior of (Ti,Al)N, metastable cubic (Ti,Al)N and (Ti,Al)(OxN1-x) coatings are synthesized using reactive a...

The phase behavior of grain boundaries can have a strong influence on interfacial properties. Little is known about the emergence of grain boundary phases in elemental metal systems and how they transform. Here, we observe the nanoscale patterning of a grain boundary by two alternating grain boundary phases with distinct atomic structures in elemen...

Lead chalcogenides are known for their thermoelectric properties since the first work of Thomas Seebeck on the discovery of this phenomenon. Yet, the electronic properties of lead telluride are still of interest due to the incomplete understanding of the metal-to-semiconductor transition at temperatures around 230 °C. Here, a temperature-dependent...

We present an unsupervised machine learning approach for segmentation of static and dynamic atomic-resolution microscopy data sets in the form of images and video sequences. In our approach, we first extract local features via symmetry operations. Subsequent dimension reduction and clustering analysis are performed in feature space to assign patter...

The mechanical behavior and deformation mechanisms of a body-centered cubic (BCC) Ti29Zr24Nb23Hf24 (at.%) high entropy alloy (HEA) was investigated in temperatures and strain rates from 700 to 1100 °C and 10⁻³ to 10 s⁻¹, respectively. The HEA exhibits a substantial increase in yield stress with increasing strain rate. The strain rate sensitivity (S...

The recent development of electron-sensitive and pixelated detectors has attracted the use of four-dimensional scanning transmission electron microscopy (4D-STEM). Here, we present a precession electron diffraction-assisted 4D-STEM technique for automated orientation mapping using diffraction spot patterns directly captured by an in-column scintill...

Scanning nano-beam electron diffraction with a pixelated detector was employed to investigate the orientation relationship of nanometer sized, irradiation induced G-phase (M6Ni16Si7) precipitates in an austenite matrix. Using this detector, the faint diffraction spots originating from the small G-phase particles could be resolved simultaneously as...

Three novel precipitation strengthened bcc alloys which exhibit a smooth microstructural gradient with composition have been fabricated in bulk form by induction casting. All three alloys are comprised of a mixture of disordered A2-(Fe, Cr) and L21-ordered (Ni, Fe)2AlTi type phases both as-cast and after long-term annealing at 900 ∘C. The ratio of...

In this work, we experimentally establish the isothermal nanofacet evolution at an asymmetric ∑5 tilt grain boundary in the Cu-Ag system using a diffusion couple approach. We investigate the nanofacet formation along the grain boundary in dependence of the Ag solute excess concentration. The initial grain boundary dissociates into asymmetric Ag-lea...

Titanium thin films were deposited on silicon nitride (SiNx) coated Si, NaCl, and sapphire substrates varying the deposition conditions using e-beam evaporation to investigate thin film growth modes. The microstructure and texture evolution in dependence of substrate, deposition rate, film thickness, and substrate temperature were studied using X-r...

Three novel precipitation strengthened bcc alloys which exhibit a smooth microstructural gradient with composition have been fabricated in bulk form by induction casting. All three alloys are comprised of a mixture of disordered A2-(Fe, Cr) and L2$_1$-ordered (Ni, Fe)$_{2}$AlTi type phases both as-cast and after long-term annealing at 900 $^{\circ}...

In this study, we report on face-centered cubic structured CoCrFeNi high-entropy alloy thin films with finely dispersed nano-oxide particles which are formed by internal oxidation. Analytical scanning transmission electron microscopy imaging found that the particles are Cr2O3. The oxide particles contribute to the hardening of the film increasing i...

Scanning nano-beam electron diffraction with a pixelated detector was employed to investigate the orientation relationship of nanometer sized, irradiation induced G-phase (M$_6$Ni$_{16}$Si$_{7}$) precipitates in an austenite matrix. Using this detector, the faint diffraction spots originating from the small G-phase particles could be resolved simul...

The recent development of electron sensitive and pixelated detectors has attracted the use of four-dimensional scanning transmission electron microscopy (4D-STEM). Here, we present a precession electron diffraction assisted 4D-STEM technique for automated orientation mapping using diffraction spot patterns directly captured by an in-column scintill...

A comparative study on the isothermal oxidation behaviour of the two β0–containing titanium aluminide alloys GE 4822 and TNM-B1 in the HIPed condition has been conducted at temperatures between 600 °C and 900 °C for up to 1000 h in air. The two alloys exhibit clear differences in their oxidation behaviour, however, both suffer from undesirable embr...

The local variation of grain boundary atomic structure and chemistry caused by segregation of impurities influences the macroscopic properties of poylcrystalline materials. Here, the effect of co-segregation of carbon and boron on the depletion of aluminum at a $\Sigma 5\,(3\,1\,0\,) [0\,0\,1]$ tilt grain boundary in a $\alpha-$Fe-$4~at.~\%$Al bicr...

We investigate the strain-rate-dependent mechanical behavior and deformation mechanisms of a refractory high entropy alloy, Ti29Zr24Nb23Hf24 (at.%), with a single-phase body-centered cubic (BCC) structure. High-temperature compression tests were conducted at temperatures from 700 to 1100{\deg}C at strain rates ranging from 10-3 to 10 s-1. A sudden...

High-performance lightweight materials are urgently needed, given the pressing quest for weight reduction and the associated energy savings and emission reduction. Here, by incorporating the multi-principal element feature of compositionally complex alloys, we develop the concept of lightweight steels further and propose a new class of compositiona...

For the first time, multicomponent alloys belonging to a B2-ordered single phase were designed and fabricated by melting route. The design concept of high entropy alloys is applied to engineering the transition metal sublattice of binary B2 aluminide. The equiatomic substitution of transition metal elements in the Ni sublattice of binary AlNi follo...

open access links: https://link.springer.com/article/10.1007%2Fs11661-020-05947-2 https://rdcu.be/b6P8O https://doi.org/10.1007/s11661-020-05947-2 This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, for...

In this work, we experimentally establish the isothermal nanofacet evolution at an asymmetric tilt grain boundary in the Cu-Ag system using a diffusion couple approach. We investigate the nanofacet formation along the grain boundary in dependence of the Ag solute excess concentration. The initial grain boundary dissociates into asymmetric Ag-lean s...

Using both novel high-throughput screening via combinatorial thin film deposition and conventional bulk alloy synthesis techniques, a large region of the AlCrFeNiTi composition space has been probed for alloys that could serve as low cost alternatives to nickel-base superalloys for medium-to-high temperature structural applications. Phase formation...

The mechanical properties of high-entropy alloys (HEAs) are still not deeply understood. Detailed knowledge of the strengthening mechanism, especially, the atomistic origin of solid solution hardening and its interplay with dislocation plasticity is needed. Here, we report on the dislocation glide behavior of a FeCoCrNiMn face-centered cubic (FCC)...

High entropy alloys are generally considered to be single phase material. This state is, however, typically a non-equilibrium state after fabrication at high cooling rates. Phase constitution after fabrication or heat treatment is mostly known for isothermal annealing only and for casts as well as rapidly quenched alloys. Knowledge on early phase s...

We report on a multilayered structure comprising of rock-salt (rs) structured CrN layers of constant thickness and AlN layers of varying thicknesses, which surprisingly enables the growth of metastable zinc-blende (zb) AlN layers for certain layer-thickness combinations. The multilayer exhibits an atomic and electronic structure gradient as reveale...

The synthesis temperature dependent phase formation of Ni10Ti10Al25Fe35Cr20 thin films is compared to a bulk processed sample of identical composition. The as-cast alloy exhibits a dual-phase microstructure which is composed of a disordered BCC phase and AlNiTi-based B2- and/or L21-ordered phase(s). Formation of the BCC phase as well as an ordered...