Session Q25: Focus Session: Cooperative Phenomena in Plasticity II

Satisfying Detailed Balance in Mesoscale Lattice Models of Amorphous Plastic Deformation

A number of lattice and finite element models have been proposed to model the correlations and localization transitions that occur during driven plastic flow in glasses and granular media. Green's function formulations are used to redistribute stress from local shear transformation events. However the ad hoc kinetic assumptions used in such models often lead to violations of detailed balance. These violations preclude the association of a given configuration of the model with a well-defined energy. In this way these models are very different from atomistic models of deformation, where energy is explicitly defined and forces are derived from the underlying energy description. Here we discuss the origin of these violations of detailed balance, and discuss how mesoscale lattice models of shear transformation activity that satisfy detailed balance can, in principle, be constructed.   

Modeling the toughness of metallic glasses

Metallic glasses are a new type of alloy whose atoms form an amorphous structure in contrast to most metals. They have many favorable properties such as excellent wear resistance and high tensile strength, but are prone to breakage in some circumstances, depending on their method of preparation. This talk will describe the development of a quasi-static projection method in an Eulerian finite-difference framework, for simulating a physical model of a metallic glass based on the shear transformation zone theory. The simulations are capable of resolving the multiple timescales that are involved, and provide an explanation of the experimentally observed differences in breakage strength.   

Statistical modeling of athermal micromechanical yielding in amorphous solids and mean-field approaches

We investigate the micromechanical properties of non-equilibrium yielding under stress of an amorphous solid, modeled in terms of shear-transformation zones. First, we investigate in detail the possible mean-field limits of such models and their capacity to capture the character of the original micromechanical model. For this purpose, we utilize exact as well as numerical solutions. Second, we study the dynamical features of such models when short-time relaxation effects (induced by beta-relaxation modes) are incorporated in simple ways: we demonstrate that shear-banding and stick-slip serrated plastic flow are generic outcomes of such features. Finally, we discuss the relation of this mechanism with the self-induced stochastic resonance effects in non-linear dynamical systems.   

Reversible Avalanches and Criticality in Amorphous Solids

Despite its importance for basic science and industry, the physical process that causes a solid to change its shape permanently under external deformation is still not well understood. In this paper we use molecular dynamics simulations of amorphous solids under oscillatory shear to study this phenomenon, and show that at a critical strain amplitude, the size of the cooperative atomic motion that allows for a permanent deformation diverges. We compare this non-equilibrium critical behavior to that of a ``front depinning'' transition. This viewpoint, based on fluctuations and statistics, is complementary to the dynamical ``transition to chaos'' which was previously identified at the same strain amplitude. Below this irreversibile-depinning transition, we observe large avalanches which are completely repetitive with each shear strain cycle. This suggests that while avalanches on their own do not cause irreversible deformation, it is likely that the irreversibility transition and the ``depinning-like'' transition are two aspects of the same phenomena. One implication is that the transition could be detected before the onset of irreversible flow by an analysis of the power spectra of avalanches. \\[4pt] Work done in collaboration with Ido Regev, Karin Dahmen, John Weber, and Turab Lookman.   

Experimental evidence for both progressive and simultaneous shear during quasistatic compression of a bulk metallic glass

A simple mean field model for avalanche behavior during quasistatic compressive deformation of a bulk metallic glass demonstrates that two distinct types of slip events occur during serrated plastic flow. These events are distinguished by their stress drop rate profiles. A symmetric stress drop rate profile characterizes the small serrations (with stress drop rates of 400-1000 MPa/s). These small slips correspond to the progressive or incremental propagation of a shear band. A sharply peaked stress drop rate profile characterizes the large slips (with stress drop rates of 1000-100,000 MPa/s). These large slips correspond to simultaneous shear with uniform sliding over the entire shear plane. Both small and large slip events occur throughout plastic deformation.   

A model for flash heating in sheared fault gouge

We develop a model for sheared gouge layers that accounts for the local increase in temperature at the grain contacts during sliding. We use the shear transformation zone (STZ) theory, a statistical thermodynamic theory, to describe irreversible macroscopic plastic deformations due to local rearrangements of the gouge particles. We track the temperature evolution at the grain contacts using a one dimensional heat diffusion equation. Our model predicts a logarithmic rate dependence of the steady state shear stress in the quasi-static regime. In the dense flow regime the frictional strength decreases rapidly with increasing slip rate due to thermal softening at the granular interfaces. The transient response following a step in strain rate includes a direct effect and a following evolution effect, both of which depend on the magnitude and direction of the velocity step. In addition to frictional heat, the energy budget includes an additional energy sink representing the fraction of external work consumed in increasing local disorder. The model links low-speed and high-speed frictional response of gouge layers, and provides an essential ingredient for multiscale modeling of earthquake ruptures with enhanced coseismic weakening.   

Effect of dislocation structure on the strain rate dependence of the flow stress in a 2D discrete dislocation dynamics model

It is well known for almost three decades that crystal plasticity in metals, such as Cu, is strongly rate dependent at strain rates higher than 10\textasciicircum 3/s. This rate sensitivity is typically attributed to dislocation drag effects, but there appears to be a large range of possible high-rate-sensitivity exponents, depending on the sample and the experimental group. Thus, one may hypothesize that the dislocation structure has a strong influence on these effects. We elucidate the origins of rate effects in crystal plasticity and their connection with relaxed, before applying stress, dislocation structures by investigating simple bending in a model of discrete dislocation plasticity in two dimensions. We find that the high-strain-rate sensitivity changes significantly as a function of strain, different material treatment (annealed or not) and properties of dislocation sources (surface vs. bulk nucleation). We characterize in detail the emerging patterning in the dislocation structure and we provide predictions for future experiments on the dependence of the rate sensitivity on dislocation-related characteristics.   

Beta relaxations and their correlations to plasticity in metallic glasses and soft disordered systems

Focusing on metallic glasses as model systems, we show that mechanical properties and deformation mechanisms of glassy materials are closely related to a kind of dynamical process inherent in glasses, i.e., the so-called Johari-Goldstein beta relaxations. Microscopically, we demonstrate that beta relaxations and the basic deformation units of glasses have the same activation energy, and this activation energy correlates with the deformability of metallic glasses. Macroscopically, we illustrate that metallic glasses with pronounced beta relaxations around room temperature could have outstanding tensile plasticity, and the transition from brittle to ductile in tension and the beta relaxations follow a same temperature-time relationship. We will also show how to tune the beta relaxations by the understanding of chemical influence to get desirable properties [1]. Atomic signatures of beta relaxations in metallic glasses will be addressed based on recent computer simulations. \\[4pt] [1] See also review papers, H.B. Yu, W.H. Wang, H.Y. Bai and K. Samwer, National Science Review 1, 429-461 (2014); H.B. Yu, W.H. Wang and K. Samwer, Materials Today 16,183 (2013)   

Pressure induced metallization with absence of structural transition in layered MoSe$_{2}$

Layered transition-metal dichalcogenides 2H-MX$_{2}$ (M $=$ Mo, W, and etc, X $=$ S, Se, and Te) are emerging as exciting material systems with unique electronic properties and atomically thin geometries. Here, we systematically investigating the high pressure behavior of 2H$_{\mathrm{c}}$-MoSe$_{2}$ up to 60 GPa via a diamond anvil cell, we identified MoSe$_{2}$ as a promising candidate for lattice and electronic engineering. In sharp contrast to MoS$_{2}$, the crystal structure of MoSe$_{2}$ evolves from an anisotropic two-dimensional layered network to a highly isotropic three-dimensional solid without any structural transition. The role of the chalcogenides anions in stabilizing either 2H$_{\mathrm{a}}$ or 2H$_{\mathrm{c}}$ layered patterns is underscored by our layer sliding calculations. MoSe$_{2}$ possesses highly tunable optical and electrical transport properties as a function of pressure, which is essentially determined by the narrowing of its band gap followed by closure at around 40 GPa. Our ab-initio calculations further support the semiconductor to metal transition.   

Frictional sliding at a compressed polycrystalline 50 nm grain size Al-Al interface

We present the results of large-scale NEMD simulations for a polycrystalline Al-Al interface sliding at a relative velocity of 60 m/s and a pressure of 15 GPa with a boundary temperature of 300K. The sample consisted of annealed grains, 125 grains on either side of the initial sliding interface, with dimensions of 2x(236) nm in the normal direction and 236 nm in the periodic sliding and transverse directions. Simulation times were of order 20 ns and the sample had 1.8B atoms interacting with an Al-EAM potential. The initial grain structure evolves to a complex dynamic steady state grain morphology that is very different from the initial grain structure and is characterized by large plastic strains and strain rates in a deformation region of thickness 150 nm at the interface in the normal direction. The steady state shows a sequence of grain growth and refinement and a highly strained graded microstructure. This behavior is similar to that seen in simulations for 13 and 20 nm grains and a mesoscale model that takes into account the large plastic strains and strain rates, and the size of the deformation region is able to reproduce the values of the frictional force per unit area.   

ME-$\mu $SR MgO study: search for O[-1] earthquake-like precursors

We analyze O$^{-1}$ earthquake-like precursor effects [1,2] by studying the $\mu $SR signals of MgO using Muon-Spin Resonance and Maximum Entropy (ME) [3,4]. Due to its abundance in the earth crust, MgO is ideal for studying these features: O$^{-1}$ (or positive hole) formation results from a break in an oxygen anion pair under elevated temperature or high pressure conditions [2]. For a 3N-MgO single crystal above RT, a small percentage (\textless 1{\%}) is predicted to be in an O$^{-1}$ state, instead of typical O$^{-2}$ ions. ME analysis of transverse field (100 Oe) MgO data show asymmetrical $\mu $SR peaks at $\sim$ 1.4 MHz. [4] Small T-dependent deviations from a Lorentzian (Lor) signal could be effects of O$^{-1}$ states in MgO. We have fitted ME transforms with three Lorentzians to obtain a reasonable description of the 1.4-MHz peak. The T dependences of this 3-Lor set and their B-field dependences at 17$^{\circ}$C are reported and discussed, and their relation to precursor earthquake-like O-valency effects. Research supported by RSCA-SJSU, SETI, WiSE-SJSU and AFC San Jose. \\[4pt] [1] FT Freund, Nat Hazards Earth Sys Sci \textbf{7} (2007) 1-7.\\[0pt] [2] FT Freund \textit{et al,} Phys Chem Earth \textbf{31} (2006) 389.\\[0pt] [3] C Boekema and MC Browne, MaxEnt 2008, AIP Conf Proc {\#}1073 p260.\\[0pt] [4] S Lee \textit{et al,} HUIC Educ, Math {\&} Eng Tech Conf, Uo HI (2013).