Creep of Cu-Zr metallic glasses and metallic glass composites: A molecular dynamics study

Creep is the time-dependent deformation of a material at elevated temperature and under stress-conditions below yield. This slow, time-delayed deformation can ultimately lead to the failure of an engineering component, especially in high-temperature applications. But even well before material failure, the elongation of an engineering component, e.g. a turbine blade, during service life can have catastrophic consequences. Thus, knowledge of the mechanism of creep deformation is of utmost importance for choosing a material for a specific engineering application. While the phenomenon of creep is already well understood in metals and a large body of work exists on how to improve their creep resistance, this behavior is not exclusive to crystalline materials. Partly crystalline materials such as polymers and ceramics and even glasses can exhibit significant creep deformation as well. For the conventional soda-lime glass the possibility of creep seems irrelevant in its typical application window, but with the development of new glassy materials, such as metallic glasses, that are amorphous metals quenched from the melt and potential candidates for a wide application range of temperatures and stresses, the assessment of the creep behavior of amorphous materials has been taken beyond purely scientific interest. In this thesis molecular dynamics simulations are used to understand the creep behavior of a homogeneous Cu_{64}Zr_{36} metallic glass as well as glass-crystal composites. First, we treat the case of the homogeneous glass, and establish the temperature and stress parameter range necessary to observe creep in molecular dynamics simulations. Second, we will study the influence of the glass-crystal interface properties on the creep rates. The latter also critically depends on how realistic the computer composite model is.Third, we study a different microstructure of amorphous-crystalline composites which belong to the nanoglass family. We show how the glassy grain-boundary phase present in a nanoglass can be altered to have a reinforcing effect, both in the low temperature regime and under creep conditions

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

Creep of Cu-Zr metallic glasses and metallic glass composites: A molecular dynamics study

Creep is the time-dependent deformation of a material at elevated temperature and under stress-conditions below yield. This slow, time-delayed deformation can ultimately lead to the failure of an engineering component, especially in high-temperature applications. But even well before material failure, the elongation of an engineering component, e.g. a turbine blade, during service life can have catastrophic consequences. Thus, knowledge of the mechanism of creep deformation is of utmost importance for choosing a material for a specific engineering application. While the phenomenon of creep is already well understood in metals and a large body of work exists on how to improve their creep resistance, this behavior is not exclusive to crystalline materials. Partly crystalline materials such as polymers and ceramics and even glasses can exhibit significant creep deformation as well. For the conventional soda-lime glass the possibility of creep seems irrelevant in its typical application window, but with the development of new glassy materials, such as metallic glasses, that are amorphous metals quenched from the melt and potential candidates for a wide application range of temperatures and stresses, the assessment of the creep behavior of amorphous materials has been taken beyond purely scientific interest. In this thesis molecular dynamics simulations are used to understand the creep behavior of a homogeneous Cu_{64}Zr_{36} metallic glass as well as glass-crystal composites. First, we treat the case of the homogeneous glass, and establish the temperature and stress parameter range necessary to observe creep in molecular dynamics simulations. Second, we will study the influence of the glass-crystal interface properties on the creep rates. The latter also critically depends on how realistic the computer composite model is.Third, we study a different microstructure of amorphous-crystalline composites which belong to the nanoglass family. We show how the glassy grain-boundary phase present in a nanoglass can be altered to have a reinforcing effect, both in the low temperature regime and under creep conditions

Creep Deformation of a Cu-Zr Nanoglass and Interface Reinforced Nanoglass-Composite Studied by Molecular Dynamics Simulations

Using molecular dynamics simulations, we compare the creep properties of a homogeneous Cu64Zr36 metallic glass, a nanoglass with the same nominal composition, and a nanoglass-crystal composite, where the amorphous grain boundary phase has been reinforced with the high-temperature stable Cu2Zr Laves phase. While the nanoglass architecture is successful at preventing shear band formation, which typically results in a brittle failure mode at room temperature and conventional loading conditions, we find that the high fraction of glass-glass grain boundary phase therein is not beneficial to its creep properties. This can be amended by reinforcing the glass-glass interphase with a high-temperature stable crystalline substitute

Creep Deformation of a Cu-Zr Nanoglass and Interface Reinforced Nanoglass-Composite Studied by Molecular Dynamics Simulations

Using molecular dynamics simulations, we compare the creep properties of a homogeneous Cu64Zr36 metallic glass, a nanoglass with the same nominal composition, and a nanoglass-crystal composite, where the amorphous grain boundary phase has been reinforced with the high-temperature stable Cu2Zr Laves phase. While the nanoglass architecture is successful at preventing shear band formation, which typically results in a brittle failure mode at room temperature and conventional loading conditions, we find that the high fraction of glass-glass grain boundary phase therein is not beneficial to its creep properties. This can be amended by reinforcing the glass-glass interphase with a high-temperature stable crystalline substitute

Elastostatic loading of metallic glass-crystal nanocomposites: Relationship of creep rate and interface energy

We study the creep behavior of Cu64Zr36 glass-crystal nanocomposites under elastostatic loading conditions in molecular dynamics simulations. By manipulating the glass-crystal interfaces of a precipitation-annealed glass containing Laves-type crystallites, we show that the creep behavior can be tuned. Specifically, we find that for the same microstructure the creep rate scales exponentially with the excess energy in the interfaces, which we raise artificially by disturbing the local short-range order in the atomistic model. The behavior shows analogies to Coble creep in crystalline metals, which depends on grain boundary diffusivity and implicitly on grain boundary energies

Reinforcement of nanoglasses by interface strengthening

Nanoglasses consist of glassy grains connected by an amorphous interface. While internal interfaces in nanoglasses help to prevent brittle failure, they are usually not beneficial to the glasses overall strength. In this molecular dynamics study, we manipulate the glass–glass interfaces of a Cu–Zr nanoglass, such that they are replaced by stronger crystalline interphases. Analogous to grain boundary strengthening in crystalline materials, we show that it is possible to reinforce the nanoglass without compromising its ductility

SEM2: Introducing mechanics in cell and tissue modeling using coarse-grained homogeneous particle dynamics

Modeling multiscale mechanics in shape-shifting engineered tissues, such as organoids and organs-on-chip, is both important and challenging. In fact, it is difficult to model relevant tissue-level large non-linear deformations mediated by discrete cell-level behaviors, such as migration and proliferation. One approach to solve this problem is subcellular element modeling (SEM), where ensembles of coarse-grained particles interacting via empirically defined potentials are used to model individual cells while preserving cell rheology. However, an explicit treatment of multiscale mechanics in SEM was missing. Here, we incorporated analyses and visualizations of particle level stress and strain in the open-source software SEM++ to create a new framework that we call subcellular element modeling and mechanics or SEM2. To demonstrate SEM2, we provide a detailed mechanics treatment of classical SEM simulations including single-cell creep, migration, and proliferation. We also introduce an additional force to control nuclear positioning during migration and proliferation. Finally, we show how SEM2 can be used to model proliferation in engineered cell culture platforms such as organoids and organs-on-chip. For every scenario, we present the analysis of cell emergent behaviors as offered by SEM++ and examples of stress or strain distributions that are possible with SEM2. Throughout the study, we only used first-principles literature values or parametric studies, so we left to the Discussion a qualitative comparison of our insights with recently published results. The code for SEM2 is available on GitHub at https://github.com/Synthetic-Physiology-Lab/sem2

Highly Porous Silicon Embedded in a Ceramic Matrix: A Stable High-Capacity Electrode for Li-Ion Batteries

We demonstrate a cost-effective synthesis route that provides Si-based anode materials with capacities between 2000 and 3000 mAh·g_Si^–1 (400 and 600 mAh·g_composite^–1), Coulombic efficiencies above 99.5%, and almost 100% capacity retention over more than 100 cycles. The Si-based composite is prepared from highly porous silicon (obtained by reduction of silica) by encapsulation in an organic carbon and polymer-derived silicon oxycarbide (C/SiOC) matrix. Molecular dynamics simulations show that the highly porous silicon morphology delivers free volume for the accommodation of strain leading to no macroscopic changes during initial Li–Si alloying. In addition, a carbon layer provides an electrical contact, whereas the SiOC matrix significantly diminishes the interface between the electrolyte and the electrode material and thus suppresses the formation of a solid–electrolyte interphase on Si. Electrochemical tests of the micrometer-sized, glass-fiber-derived silicon demonstrate the up-scaling potential of the presented approac