Stick-slip dynamics and recent insights into shear banding in metallic glasses

Despite extensive research, the understanding of the fundamental processes governing yielding and plastic flow in metallic glasses remains poor. This is due to experimental difficulties in capturing plastic flow as a result of a strong localization in space and time by the formation of shear bands at low homologous temperatures. Unveiling the mechanism of shear banding is hence key to developing a deeper understanding of plastic deformation in metallic glasses. We will compile recent progress in studying the dynamics of shear-band propagation from serrated flow curves. We will also take a perspective gleaned from stick-slip theory and show how the insights gained can be deployed to explain fundamental questions concerning the origin, mechanism, and characteristics of flow localization in metallic glasse

Glass fracture surface energy calculated from crystal structure and bond-energy data

We present a novel method to predict the fracture surface energy of oxide glasses, {\gamma}, using readily available crystallographic structure data of their isochemical crystal and tabled diatomic chemical bond energies, D0. The method assumes that {\gamma} equals the fracture surface energy of the most likely cleavage plane of the crystal. Calculated values were in excellent agreement with those calculated from measured glass density and D0 in an earlier work. This finding demonstrates a remarkable equivalence between crystal cleavage planes and glass fracture surfaces

Constrained incipient phase transformation in Ni-Mn-Ga films: A small-scale design challenge

Ni-Mn-Ga shape-memory alloys are promising candidates for large strain actuation and magnetocaloric cooling devices. In view of potential small-scale applications, we probe here nanomechanically the stress-induced austenite–martensite transition in single crystalline austenitic thin films as a function of temperature. In 0.5 µm thin films, a marked incipient phase transformation to martensite is observed during nanoindentation, leaving behind pockets of residual martensite after unloading. These nanomechanical instabilities occur irrespective of deformation rate and temperature, are Weibull distributed, and reveal large spatial variations in transformation stress. In contrast, at a larger film thickness of 2 μm fully reversible transformations occur, and mechanical loading remains entirely smooth. Ab-initio simulations demonstrate how an in-plane constraint can considerably increase the martensitic transformation stress, explaining the thickness-dependent nanomechanical behavior. These findings for a shape-memory Heusler alloy give insights into how reduced dimensions and constraints can lead to unexpectedly large transformation stresses that need to be considered in small-scale actuation design

Giant segregation transition as origin of liquid metal embrittlement in the Fe-Zn system

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 significantly increases with decreasing temperature, while the required Zn content in the alloy for triggering the segregation transition decreases. Direct evidence of this Zn segregation transition is obtained using high-resolution scanning transmission electron microscopy. We trace the origin of the segregation transition and its temperature dependence back to the low cohesive energy of Zn and a miscibility gap in Fe-Zn GB, arising from the magnetic ordering effect, which is demonstrated by ab initio calculations. We show that the massive Zn segregation resulting from the segregation transition greatly assists with liquid wetting and reduces the work of separation along the GB. These findings reveal the fundamental origin of GB weakening and therefore liquid metal embrittlement in the Fe-Zn system.Comment: Original work, Letter, 14 pages including supplementary material (SM), 8 figures (3 in SM), 2 tables in S

Small town research in Germany - status quo and recommendations

Urban studies in Germany are traditionally oriented towards large cities. The structures, meanings and functions of small towns are not sufficiently perceived and differentiated in scientific or political debates. Adequate research on small towns requires systematic, comparative, inter- and transdisciplinary approaches. Traditional attributions should be questioned critically and small towns should be examined empirically in their diversity and differentiation. This involves paying attention to external influences and heterogeneous internal structures as well as to regional functions and interdependencies. The availability and generation of statistical data, which also make small-scale analyses possible, are just as necessary as more comprehensive studies, which go beyond limited case studies. Finally, also research funding and academic teaching should address small towns more systematically than it has been the case in the past. This position paper presents recommendations for research, university teaching, official statistics and research funding in the field of small town research. The Ad-hoc Working Group focused on small town research in Germany and German-language literature, respectively

Kleinstadtforschung

Stadtforschung ist in Deutschland traditionell großstadtorientiert. Kleinstädte werden weder in wissenschaftlichen noch in politischen Auseinandersetzungen in ihren Strukturen, Bedeutungen und Funktionen hinreichend wahrgenommen und differenziert betrachtet. Eine adäquate Erforschung von Kleinstädten erfordert systematische, vergleichende sowie inter- und transdisziplinäre Ansätze. Traditionelle Zuschreibungen sollten kritisch hinterfragt und Kleinstädte in ihrer Vielfalt und Differenziertheit empirisch untersucht werden. Dabei geht es sowohl um die äußeren Prägungen und heterogenen inneren Strukturen als auch um regionale Funktionen und Verflechtungen. Die Verfügbarkeit bzw. Generierung von statistischen Daten, die auch kleinräumige Analysen ermöglichen, sind dabei ebenso notwendig wie umfassendere Studien, die über begrenzte und anlassbezogene Einzelfalluntersuchungen hinausgehen. Schließlich sollten auch die Forschungsförderung und die akademische Lehre Kleinstädte systematischer als bislang adressieren. Dieses Positionspapier enthält Empfehlungen für Wissenschaft, Lehre, amtliche Statistik und Forschungsförderung aus der Perspektive der Kleinstadtforschung.Urban studies in Germany are traditionally oriented towards large cities. The structures, meanings and functions of small towns are not sufficiently perceived and differentiated in scientific or political debates. Adequate research on small towns requires systematic, comparative, inter- and transdisciplinary approaches. Traditional attributions should be questioned critically and small towns should be examined empirically in their diversity and differentiation. This involves paying attention to external influences and heterogeneous internal structures as well as to regional functions and interdependencies. The availability and generation of statistical data, which also make small-scale analyses possible, are just as necessary as more comprehensive studies, which go beyond bounded and case studies. Finally, research funding and academic teaching should address small towns more systematically than has been the case in the past. This position paper presents recommendations for research, university teaching, official statistics and research funding in the field of small town research

Author Correction: Non-local effect of impurity states on the exchange coupling mechanism in magnetic topological insulators

A Correction to this paper has been published: https://doi.org/10.1038/s41535-021-00314-