From metallic glasses to nanocrystals: Molecular dynamics simulations on the crossover from glass-like to grain-boundary-mediated deformation behaviour

Nanocrystalline metals contain a large fraction of high-energy grain boundaries, which may be considered as glassy phases. Consequently, with decreasing grain size, a crossover in the deformation behaviour of nanocrystals to that of metallic glasses has been proposed. Here, we study this crossover using molecular dynamics simulations on bulk glasses, glass-crystal nanocomposites, and nanocrystals of Cu64Zr36 with varying crystalline volume fractions induced by long-time thermal annealing. We find that the grain boundary phase behaves like a metallic glass under constraint from the abutting crystallites. The transition from glass-like to grain-boundary-mediated plasticity can be classified into three regimes: (1) For low crystalline volume fractions, the system resembles a glass-crystal composite and plastic flow is localised in the amorphous phase; (2) with increasing crystalline volume fraction, clusters of crystallites become jammed and the mechanical response depends critically on the relaxation state of the glassy grain boundaries; (3) at grain sizes $\geq$ 10 nm, the system is jammed completely, prohibiting pure grain-boundary plasticity and instead leading to co-deformation. We observe an inverse Hall-Petch effect only in the second regime when the grain boundary is not deeply relaxed. Experimental results with different grain boundary states are therefore not directly comparable in this regime.Comment: 19 pages, 17 figure

Solid-state amorphization of Cu nanolayers embedded in a Cu64Zr36 glass

Solid-state amorphization of crystalline copper nanolayers embedded in a Cu64Zr36 metallic glass is studied by molecular dynamics simulations for different orientations of the crystalline layer. We show that solid-state amorphization is driven by a reduction of interface energy, which compensates the bulk excess energy of the amorphous nanolayer with respect to the crystalline phase up to a critical layer thickness. A simple thermodynamic model is derived, which describes the simulation results in terms of orientation-dependent interface energies. Detailed analysis reveals the structure of the amorphous nanolayer and allows a comparison to a quenched copper melt, providing further insights into the origin of excess and interface energy.Comment: 16 pages, 18 figure

Kooperation oder Konfrontation? Der Aufstieg Chinas in der globalen politischen Ökonomie

Welche Folgen zeitigt der rasante Aufstieg Chinas? Im Anschluss an Erkenntnisse der Chinaforschung und politökonomischer Ansätze wird die Volksrepublik China als eine Variante der nachholenden (staats-)kapitalistischen Entwicklung analysiert, die an einer wirtschaftlichen und geopolitischen Aufwertung interessiert ist. Dabei verkörpert die inter- und transnationale Einbettung des neuen chinesischen Kapitalismus in ein instabiles Weltsystem trotz des Bemühens der Staatsführung um eine verantwortungsvolle Großmachtpolitik einen spannungsreichen Prozess. Zwar bleibt die chinesische Volkswirtschaft abhängig von den alten Zentren des globalen Kapitalismus. Daraus folgt jedoch nicht umstandslos eine reibungslose Zusammenarbeit. Im Gegenteil weisen aktuelle Währungsdispute mit den USA im Zuge des globalen Wirtschaftseinbruchs auf politisch vermittelte Standortkonflikte hin. Perspektiven einer konfliktarmen Integration Chinas müssen daher relativiert werden - wie zusätzlich die Führungsrolle des Landes im ostasiatischen Regionalisierungsprozess illustriert -, ohne voreilige Schlussfolgerungen hinsichtlich militärischer Zusammenstöße und eines Übergangs zu einer neuen globalen chinesischen Hegemonie zu übernehmen. Abgezielt wird darauf, die Vorteile einer politökonomischen Betrachtungsweise bei der Beantwortung einer Frage herauszustellen, die üblicherweise in einer Perspektive der Internationalen Beziehungen, das heißt fokussiert auf zwischenstaatliche Beziehungen, behandelt wird. -- What has been the impact of China's rapid rise? Referring to findings of current research on China as well as theories of political economy, this paper analyzes the People's Republic of China as a variety of a (state-led) capitalist catch-up development process that is focused on economic and geopolitical upgrading. Despite efforts by China's political leadership to act as a responsible superpower, the international and transnational embedding of the new Chinese capitalism is fraught with tension. The Chinese economy remains dependent on the old centers of global capitalism, but this has not paved the way for smooth cooperation. On the contrary, the latest currency disputes between China and the US during the global economic slump are indicative of politically charged conflicts between the world's relevant economic regions. Thus, the prospect of China being integrated harmoniously seems to be overly optimistic - as is also demonstrated by China's efforts to play a leading role in the East Asian regionalization process. At the same time, one has to be careful not to draw (the wrong) conclusions about the inevitability of military conflict and a transition towards a new global Chinese hegemony. The paper aims to highlight the advantages of a political economy approach in answering a question that has primarily been addressed from International Relations perspectives, i.e. focused on relations between states.

Interface-controlled creep in metallic glass composites

In this work we present molecular dynamics simulations on the creep behavior of $\rm Cu_{64}Zr_{36}$ metallic glass composites. Surprisingly, all composites exhibit much higher creep rates than the homogeneous glass. The glass-crystal interface can be viewed as a weak interphase, where the activation barrier of shear transformation zones is lower than in the surrounding glass. We observe that the creep behavior of the composites does not only depend on the interface area but also on the orientation of the interface with respect to the loading axis. We propose an explanation in terms of different mean Schmid factors of the interfaces, with the amorphous interface regions acting as preferential slip sites.Comment: 11 pages, 13 figure

Influence of Crystalline Nanoprecipitates on Shear-Band Propagation in Cu-Zr Based Metallic Glasses

The interaction of shear bands with crystalline nanoprecipitates in Cu-Zr-based metallic glasses is investigated by a combination of high-resolution TEM imaging and molecular-dynamics computer simulations. Our results reveal different interaction mechanisms: Shear bands can dissolve precipitates, can wrap around crystalline obstacles, or can be blocked depending on size and density of the precipitates. If the crystalline phase has a low yield strength, we also observe slip transfer through the precipitate. Based on the computational results and experimental findings, a qualitative mechanism map is proposed that categorizes the various processes as a function of the critical stress for dislocation nucleation, precipitate size, and distance.Comment: 16 pages, 15 figure

Mapping and analysis of a distribution process in a make-to-order supply chain

Background: Supply chains become more and more complex. Mapping can be a good way to understand a company’s supply chain and its processes. IKEA’s Direct Delivery Customer (DDC) process has a rather unique characteristic at IKEA. By using a make-to-order strategy are sofas, sofa covers and custom made worktops delivered to customers’ home from the supplier. The material flow goes through IKEA’s Customer Distribution Center (CDC). The DDC material flow has historically been very small and has therefore not received much attention. The volume of this material flow is continuously increasing. Problem description: The CDC terminal in Torsvik perceives problems as lack of space and much manual administrative work due to the growing DDC material flow. The trend indicates that the impact of the perceived problems becomes more and more serious as the flow continues to grow. To fully understand the process and enable process improvements, it is requested to thoroughly map the process. Purpose: The purpose of this study is to map the DDC process and investigate how it is managed. Another purpose is to identify problems in the process and suggest improvements. Objectives: 1. Explain why a product is classified as DDC and why transshipment is made at the CDC terminal in Torsvik. 2. Create understanding of the DDC process by mapping from customer order to the point when the products are delivered at customer’s home. 3. Identify and describe problems that occur in the part of the DDC process managed by the CDC terminal in Torsvik. 4. Suggest short-term and long-term improvements in the DDC process for the CDC terminal in Torsvik. Method: The study is based on a system approach since synergy effects are expected between the different parts in the studied process. Improvement efforts require a system view to avoid sub optimization. The study was performed as a case study with single case design. Primary qualitative methods were used, as semi-structured, unstructured interviews and participating observations. Quantitative methods for collecting data also occurred, but to a minor extent. The authors developed their own research procedure. After the literature review the research procedure was refined, in order to explain how the literature review would be used to answer the objectives. The refined research procedure contains the 6 steps; exploration, current state, identify and describe problems, analysis of problems and recommendations. Conclusion: The mapping resulted in a description of the DDC process current state. Nine problems were identified through the mapping and analysis. Seven of these problems; lack of gate area, long lead time, no one responsible for the entire process, low efficiency in administrative work, missing goods, large amount of handovers, and custom related problems were further analyzed in terms of source, consequences and customer impact and potential solutions. From these seven problems it became clear that five depends upon low IT-support, one way or another. In this case study it became clear that a growing material flow creates a more complex information flow. This information flow often requires to be managed with IT-support, which was missing in this case. The analysis resulted in five short-term and two long-term recommendations. The short-term recommendations are; reduce lead time, share process maps, conduct a workshop with intention to increase efficiency in administrative work, inspection of loaded goods and modify the limit of orders in the IT-system. The long-term recommendations are; investigate possibility to implement IT-support and centralize process responsibility

Low temperature heat capacity of severely deformed metallic glass

The low temperature heat capacity of amorphous materials reveals a low-frequency enhancement (boson peak) of the vibrational density of states, as compared with the Debye law. By measuring the low-temperature heat capacity of a Zr-based bulk metallic glass relative to a crystalline reference state, we show that the heat capacity of the glass is strongly enhanced after severe plastic deformation by high-pressure torsion, while subsequent thermal annealing at elevated temperatures leads to a significant reduction. The detailed analysis of corresponding molecular dynamics simulations of an amorphous Zr-Cu glass shows that the change in heat capacity is primarily due to enhanced low-frequency modes within the shear band region.Comment: 5 pages, 2 figure

Heterogeneities in Metallic Glasses: Atomistic Computer Simulations on the Structure and Mechanical Properties of Copper–Zirconium Alloys and Composites

The present thesis deals with molecular dynamics computer simulations of heterogeneities in copper–zirconium metallic glasses, ranging from intrinsic structural fluctuations to crystalline secondary phases. These heterogeneities define, on a microscopic scale, the properties of the glass, and an understanding of their nature and behaviour is required for deriving the proper structure–property relations. In terms of composite systems, we start with the amorphisation of copper nanolayers embedded in a metallic glass matrix. While copper is an fcc metal with a high propensity for crystallisation, amorphisation can in fact occur in such systems for thermodynamic reasons. This is due to interface effects, which are also known from heterogeneous interfaces in crystals or from grain boundary complexions, although in absence of lattice mismatch. In single-phase glasses, intrinsic heterogeneities are often discussed in terms of soft spots or geometrically unfavourable motifs (GUMs), which can be considered to be mechanically weaker, defective regions of the glass. We investigate the relation between these motifs and the boson peak, an anomaly in the vibrational spectrum of all glasses. We demonstrate a relation between the boson peak and soft spots by analysing various amorphous and partially amorphous samples as well as high-entropy alloys. Finally, we treat the plastic deformation of glasses, with and without crystalline secondary phases. We propose an explanation for the experimentally observed variations of propagation direction, composition, and density along a shear band. These variations of propagation direction are small in the case of single-phase glasses. A considerably greater influence on shear band propagation can be exerted by precipitates. We systematically investigate composites ranging from low crystalline volume fraction up to systems which resemble a nanocrystalline metal. In this context, we derive a mechanism map for composite systems and observe the breakdown of these mechanisms with increasing crystalline volume fraction during the transition towards the nanocrystalline state

Universality of grain boundary phases in fcc metals: Case study on high-angle [111] symmetric tilt grain boundaries

Grain boundaries often exhibit ordered atomic structures. Increasing amounts of evidence have been provided by transmission electron microscopy and atomistic computer simulations that different stable and metastable grain boundary structures can occur. Meanwhile, theories to treat them thermodynamically as grain boundary phases have been developed. Whereas atomic structures were identified at particular grain boundaries for particular materials, it remains an open question if these structures and their thermodynamic excess properties are material specific or generalizable to, e.g., all fcc metals. In order to elucidate that question, we use atomistic simulations with classical interatomic potentials to investigate a range of high-angle [111] symmetric tilt grain boundaries in Ni, Cu, Pd, Ag, Au, Al, and Pb. We could indeed find two families of grain boundary phases in all of the investigated grain boundaries, which cover most of the standard fcc materials. Where possible, we compared the atomic structures to atomic-resolution electron microscopy images and found that the structures match. This poses the question if the grain boundary phases are simply the result of sphere-packing geometry or if material-specific bonding physics play a role. We tested this using simple model pair potentials and found that medium-ranged interactions are required to reproduce the atomic structures, while the more realistic material models mostly affect the grain boundary (free) energy. In addition to the structural investigation, we also report the thermodynamic excess properties of the grain boundaries, explore how they influence the thermodynamic stability of the grain boundary phases, and detail the commonalities and differences between the materials.Comment: 18 pages, 13 figure