Grain- and interphase boundary diffusion in eutectic AlCoCrFeNi2.1 compositionally complex alloy

Multi-principal-element alloys (MPEAs) attracted increasing attention over the past decade, boosted by a rigorous formulation of the concept of high entropy alloys (HEAs) by Yeh et al. [1] and motivated by contemporary extended research [2]. The HEAs showed an improved performance compared with conventional structural materials with HEAs showing exciting mechanical properties, especially at cryogenic temperatures [3] and presenting noticeable corrosion and wear resistance [4], [5]. While the single-phase solid solution HEAs, especially equiatomic HEAs, are typically constrained by a trade-off between strength and ductility, with typically low strength and good ductility found only for single-phase FCC alloys [3], off-equiatomic HEAs with heterogeneous microstructures can largely improve the combination of strength and ductility via suitable thermomechanical processing [6], [7]. Though, precipitation strengthening and a combination of hard and soft phases in compositionally complex alloys provide further instruments to tune the strength–ductility trade-off. ... 

Heng Zhang, G. Mohan Muralikrishna, Anoosheh Akbari, Harald Rösner, Bonnie J. Tyler, Sergiy V. Divinski, Gerhard Wilde

Combined measurements of composition-dependent tracer-, impurity- and intrinsic diffusion coefficients and atomic correlation factors from a binary diffusion couple

A fundamental understanding of atomic diffusion is crucial for technological advances in the application of multi-component alloys. The augmented tracer-interdiffusion couple approach provides the composition-dependent tracer diffusion coefficients (mobilities) along the whole diffusion path. This study introduces a novel methodology for accessing the vacancy flux calculations in a diffusion couple, substantiating a rich variety of the diffusion parameters accessible by a single diffusion couple experiment aside of the interdiffusion coefficients. We demonstrate that the composition-dependent thermodynamic factors and Manning’s factors can be estimated using this approach. Furthermore, the composition-dependent correlation factors of the diffusing elements in a diffusion couple are estimated for the first time. A modified tracer-interdiffusion couple approach is applied to estimate the composition-dependent impurity diffusion coefficients by placing suitable radiotracers at the Matano plane. The Onsager coefficients are estimated for the whole concentration range under investigation. Under the vacancy flux in the Ni–Fe diffusion couple, Cr, Co, and Mn atoms are biased towards the Ni-rich side, though the vacancy flux-driven drift of the Mn atoms is most pronounced.

 Neelamegan Esakkiraja, Jasper Berndt, Stephan Klemme, Gerhard Wilde, Aloke Paul, Sergiy V. Divinski a

Tellurium self-diffusion in crystalline Ge2Sb2Te5 phase change material

Ge2Sb2Te5 is the most commonly used material for phase change random access memory. In this work, a chemically homogeneous 200 nm thick layer of amorphous Ge2Sb2Te5 was grown on a single crystal Si wafer using DC magnetron sputtering and applying a stoichiometric target at room temperature. A metastable NaCl-type structure having a face-centered-cubic lattice was obtained by subsequent annealing at 473 K for 30 min. The crystal structure and microstructure were analyzed by X-ray diffraction and transmission electron microscopy. Te self-diffusion was measured by secondary ion mass spectroscopy applying a highly enriched natural 122Te isotope. The Te self-diffusion coefficients follow an Arrhenius law in the temperature range between room temperature and 353 K with an activation enthalpy of (125.0 ± 5) kJ/mol. The diffusion data are discussed in terms of either grain boundary diffusion contributions or, alternatively, in relation to volume diffusion enhanced by structural vacancies. In comparison to the amorphous counterpart, the Te self-diffusion rates in crystalline Ge2Sb2Te5 are only marginally lower and exceed the volume diffusivities of Te in crystalline Te by more than four orders of magnitude, indicating that the structural vacancies seem to determine the measured diffusion rates.

Qingmei Gong, Haihong Jiang, Martin Peterlechner, Sergiy V. Divinski, Gerhard Wilde a

Hierarchy of defects in near- 5 tilt grain boundaries in copper studied by length-scale bridging electron microscopy

Grain boundaries (GBs) are material imperfections that significantly impact material properties. Understanding how their atomic structure deviates from ideal symmetric orientations is crucial for establishing fundamental structure–property relationships. In this study, we utilized aberration-corrected scanning transmission electron microscopy, geometric phase analysis and nanobeam electron diffraction (NBED) to examine the structure of a series of near- tilt grain boundaries in copper and to explore the formation of GB defects and their associated strain field evolution on different length scales. Globally, the GB appears flat with no noticeable defects, as confirmed by NBED strain mapping. On the atomic-scale, however, various types of GB defects are observed. When a slight deviation in the misorientation is introduced, a patterning emerges featuring characteristic structural units from the and tilt boundaries. This pattern can be interpreted as secondary GB dislocations, a conclusion that is supported by GB structure prediction. Since these defects are confined to within the GB core, their associated strain field does not extend into the adjacent bulk grains. The structural landscape of the GB becomes more complex when GB plane inclination is also present, such as a wavy morphology or staircase-like architecture. The wavy morphology shows an unusual V-shape of the expansion and compression zones of the GB facet junctions that continue to extend into the bulk crystals for several nanometers. Our investigation into GB structure, particularly its inherent defects, is a prerequisite towards gaining atomic-scale insights into their potential impact on material properties.

Hui Ding, Anoosheh Akbari, Enze Chen, Harald Rösner, Timofey Frolov, Sergiy Divinski, Gerhard Wilde, Christian H. Liebscher a e f

 

Effect of Peierls-like distortions on transport in amorphous phase change devices

Today, devices based on phase change materials (PCMs) are expanding beyond their traditional application in non-volatile memory, emerging as promising components for future neuromorphic computing systems. Despite this maturity, the electronic transport in the amorphous phase is still not fully understood, which holds in particular for the resistance drift. This phenomenon has been linked to physical aging of the glassy state. PCM glasses seem to evolve towards structures with increasing Peierls-like distortions. Here, we provide direct evidence for a link between Peierls-like distortions and local current densities in nanoscale phase change devices. This supports the idea of the evolution of these distortions as a source of resistance drift. Using a combination of density functional theory and non-equilibrium Green’s function calculations, we show that electronic transport proceeds by states close to the Fermi level that extend over less distorted atomic environments. We further show that nanoconfinement of a PCM leads to a wealth of phenomena in the atomic and electronic structure as well as electronic transport, which can only be understood when interfaces to confining materials are included in the simulation. Our results therefore highlight the importance and prospects of atomistic-level interface design for the advancement of nanoscaled phase change devices.

Nils Holle, Sebastian Walfort, Riccardo Mazzarello & Martin Salinga

Resistance Drift of Phase Change Materials Beyond the Power Law

Phase change materials (PCMs) are used in fast, non-volatile memory applications, where the information is encoded in the electrical contrast between a conductive crystalline and a resistive amorphous state. In principle, the resistance of a single device can be programmed in a near continuous manner by tuning the amorphous to crystalline volume ratio. This makes PCMs interesting for novel analog computing architectures. Here, a key challenge remains in either mitigating or even utilizing a characteristic of the amorphous state: Its resistance evolves with time. Because this so-called resistance drift is captured on typical experimental timescales by an otherwise featureless power law, it can be described by a variety of physical models, leaving the true underlying microscopic origin obscured. Using both electrical and ultrafast optical heating pulses, the resistance drift is resolved over 11 orders of magnitude in time down to the first nanoseconds after formation of the amorphous state. Clear deviations from the power law both on short timescales below 1 µs and, at elevated temperatures, also on longer timescales of seconds are observed. The observations are discussed in view of common drift models. Moreover a unifying energy landscape picture is offered as an interpretation of the experimental evidence.

Jakob Ballmaier, Sebastian Walfort, Martin Salinga

Importance of Density for Phase-Change Materials Demonstrated by Ab Initio Simulations of Amorphous Antimony

Phase change materials (PCMs) serve as useful components in electronics and photonics. Here we demonstrate that various kinds of material properties of a PCM are significantly influenced by the realized mass density. Using ab initio simulations, we investigate supercooled-liquid antimony and the subsequent transition to a glassy phase. We observe a transition in the supercooled-liquid phase from an undistorted high-temperature to an increasingly Peierls-like distorted low-temperature phase. This transition also manifests in both the electronic density of states and optical properties. The strong dependence of these properties on mass density leads the way to explorations of property design for nanoconfined devices beyond the usual compositional modifications.

Nils Holle, Sebastian Walfort, and Jakob Ballmaier Riccardo Mazzarello, Martin Salinga

The Photoinduced Response of Antimony from Femtoseconds to Minutes

As a phase change material (PCM), antimony exhibits a set of desirable properties that make it an interesting candidate for photonic memory applications. These include a large optical contrast between crystalline and amorphous solid states over a wide wavelength range. Switching between the states is possible on nanosecond timescales by applying short heating pulses. The glass state is reached through melting and rapid quenching through a supercooled liquid regime. While initial and final states are easily characterized, little is known about the optical properties on the path to forming a glass. Here we resolve the entire switching cycle of antimony with femtosecond resolution in stroboscopic optical pump-probe measurements and combine the experimental results with ab-initio molecular dynamics simulations. The glass formation process of antimony is revealed to be a complex multi-step process, where the intermediate transient states exhibit distinct optical properties with even larger contrasts than those observed between crystal and glass. The provided quantitative understanding forms the basis for exploitation in high bandwidth photonic applications.

Sebastian Walfort, Nils Holle, Julia Vehndel, Daniel T. Yimam, Niklas Vollmar, Bart J. Kooi, Martin Salinga