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

Thermodynamics of mono and di-vacancies in barium titanate

The thermodynamic and kinetic properties of mono and di-vacancy defects in cubic (para-electric) barium titanate are studied by means of density-functional theory calculations. It is determined which vacancy types prevail for given thermodynamic boundary conditions. The calculations confirm the established picture that vacancies occur in their nominal charge states almost over the entire band gap. For the dominating range of the band gap the di-vacancy binding energies are constant and negative. The system, therefore, strives to achieve a state in which under metal-rich (oxygen-rich) conditions all metal (oxygen) vacancies are bound in di-vacancy clusters. The migration barriers are calculated for mono-vacancies in different charge states. Since oxygen vacancies are found to readily migrate at typical growth temperatures, di-vacancies can be formed at ease. The key results of the present study with respect to the thermodynamic behavior of mono and di-vacancies influence the initial defect distribution in the ferroelectric phases and therefore the conditions for aging.Comment: 9 pages, 4 figures, 4 table

Formation and switching of defect dipoles in acceptor doped lead titanate: A kinetic model based on first-principles calculations

The formation and field-induced switching of defect dipoles in acceptor doped lead titanate is described by a kinetic model representing an extension of the well established Arlt-Neumann model [Ferroelectrics {\bf 76}, 303 (1987)]. Energy barriers for defect association and reorientation of oxygen vacancy-dopant (Cu and Fe) complexes are obtained from first-principles calculations and serve as input data for the kinetic coefficients in the rate equation model. The numerical solution of the model describes the time evolution of the oxygen vacancy distribution at different temperatures and dopant concentrations in the presence or absence of an alternating external field. We predict the characteristic time scale for the alignment of all defect dipoles with the spontanenous polarization of the surrounding matrix. In this state the defect dipoles act as obstacles for domain wall motion and contribute to the experimentally observed aging. Under cycling conditions the fully aligned configuration is perturbed and a dynamic equilibrium is established with defect dipoles in parallel and anti-parallel orientation relative to the spontaneous polarization. This process can be related to the deaging behavior of piezoelectric ceramics.Comment: 10 pages, 7 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

Influence of elastic strain on the thermodynamics and kinetics of lithium vacancy in bulk LiCoO2

The influence of elastic strain on the lithium vacancy formation and migration in bulk LiCoO2 is evaluated by means of first-principles calculations within density functional theory (DFT). Strain dependent energies are determined directly from defective cells and also within linear elasticity theory from the elastic dipole tensor (Gij) for ground state and saddle point configurations. We analyze finite size-effects in the calculation of Gij, compare the predictions of the linear elastic model with those obtained from direct calculations of defective cells under strain and discuss the differences. Based on our data, we calculate the variations in vacancy concentration and mobility due to the presence of external strain in bulk LiCoO2 cathodes. Our results reveal that elastic in-plane and out-of-plane strains can significantly change the ionic conductivity of bulk LiCoO2 by an order of magnitude and thus strongly affect the performance of Li-secondary batteries

Influence of Br−^{-}/S2−^{2-} site-exchange on Li diffusion mechanism in Li6_6PS5_5Br -- a computational study

We investigate the influence of Br$^-$/S$^{2-}$ site-exchange on lithium diffusion in the agyrodite-type solid electrolyte Li$_6$PS$_5$Br by ab-initio molecular dynamics simulations. Based on the calculated trajectories a new mechanism for the internal lithium reorganization within the Li-cages around the $4d$ sites is identified. This reorganization mechanism is highly concerted and cannot be described by one single rotation axis only. Simulations with Br$^-$/S$^{2-}$ defects reveal that Li$^._i$ interstitials are the dominant mobile charge carriers, which originate from Frenkel pairs. These are formed because Br$^._\text{S}$ defects on the $4d$ sites cause the transfer of one or even two Li$^._i$ to the neighboring 12 cages. The lithium interstitials then carry out intercage jumps via interstitial and interstitialcy mechanisms. With that, one single Br$^._\text{S}$ defect enables Li diffusion over an extended spatial area explaining why low degrees of site-exchange are sufficient to trigger superionic conduction. The vacant sites of the Frenkel pairs, namely V$'_\text{Li}$, are mostly immobile and bound to the Br$^._\text{S}$ defect. To a lesser degree also S$'_\text{Br}$ defects induce disturbances in the lithium distribution and act as sinks for lithium interstitials restricting the Li$^._i$ motion to the vicinity of the S$'_\text{Br}$ defect

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

Computersimulationen zu Struktur und Wachstum von Bornitrid

Dünne Filme aus kubischem Bornitrid (c-BN) sind aufgrund ihrer herausragenden thermomechanischen, chemischen und elektronischen Eigenschaften von besonderem Interesse für die Materialforschung. Bornitrid in der kubischen Phase ist das nach Diamant härteste bekannte Material. Anders als Kohlenstoff verhält es sich gegenüber ferrithaltigen Metallen chemisch inert und zeigt auch bei hohen Temperaturen eine vergleichsweise geringere Oxidationsneigung. Wegen seiner hohen thermischen Stabilität und der Möglichkeit, dünne Schichten bei niedrigen Temperaturen zu synthetisieren, ist Bornitrid als Hard-Coating-Material für Werkzeuge besonders geeignet. Mögliche Anwendungen für mikroelelektronische Hochleistungsbauelemente ergeben sich aus der hohen Wärmeleitfähigkeit und der weiten Bandlücke (Eg ~ 6 eV). Bornitrid kann mit Beryllium und Silizium n- bzw. p-Typ dotiert werden und läßt sich mit einer Oxidschicht passivieren. Die optische Transparenz im sichtbaren und Infrarotbereich macht das Material zudem für die Oberflächenvergütung optischer Bauelemente geeignet.

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