Structural changes and viscoplastic behavior of a generic embedded-atom model metal in steady shear flow

Universal behavior of soft-core fluids near the threshold of thermodynamic stability

We study, by using liquid-state theories and Monte Carlo simulation, the behavior of systems of classical particles interacting through a finite pair repulsion supplemented with a longer range attraction. Any such potential can be driven Ruelle-unstable by increasing the attraction at the expense of repulsion, until the thermodynamic limit is lost. By examining several potential forms, we find that all systems exhibit a qualitatively similar behavior in the fluid phase as the threshold of thermodynamic stability is approached (and possibly surpassed). The general feature underlying the approach to Ruelle instability is a pronounced widening of the liquid-vapor binodal (and spinodal) line at low temperatures, to such an extent that at the stability threshold a vanishing-density vapor would coexist with a diverging-density liquid. We attempt to rationalize the universal pathway to Ruelle instability in soft-core fluids by appealing to a heuristic argument.

Thermodynamic stability of soft-core Lennard-Jones fluids and their mixtures

Thermodynamic stability of model single component and binary mixture fluids is considered with the Fisher-Ruelle (FR) stability criteria, which apply in the thermodynamic limit, and molecular dynamics (MD) simulation for finite periodic systems. Two soft-core potential forms are considered, phi(6,1)(r)=4[1/(a+r(6))(2)-1/(a+r(6))] and phi(2,3)(r)=4[1/(a+r(2))(6)-1/(a+r(2))(3)], where r is the separation between the particle centers. According to FR these are unstable in the thermodynamic limit if a>a(c)=1/2 and a>a(c)=(7/32)(1/3), respectively. MD simulations with single-component particles show, however, that this transition on typical simulation times is more gradual for finite periodic systems with variation in a on either side of a(c). For a<a(c), asymmetric density fluctuations are stabilized by the periodic boundary conditions. Also for binary mixtures of (stable) Lennard-Jones and phi(2,3) particles, phase separation into regions richer in one component than the other was observed for a<a(c). Binary systems with interactions similar to a model polymer-colloid fluid in the "depletion" limit equilibrated particularly slowly for a>a(c), with the unstable component in the mixture breaking up into many long-lived microdroplets which conferred apparent equilibrium thermodynamic behavior (i.e., negligible N-dependence of the average potential energy per particle) in this period.

Universal scaling law for energy and pressure in a shearing fluid

Using nonequilibrium molecular-dynamics simulation, we study the shear-rate dependence of pressure and potential energy in a liquid metal subjected to shear. We show that both thermodynamic properties vary according to a power law gamma[over ];{beta} of the shear rate gamma[over ] , in which the exponent beta is a simple linear function of temperature and density. Moreover, we establish that the coefficients for this linear law are the same as those previously obtained for a Lennard-Jones fluid by Ge [Phys. Rev. E 67, 061201 (2003)]. This is a strong indication that these coefficients, as well as the linear law for beta , could be applicable to any atomic fluid. It is also an important step toward the determination of a nonequilibrium equation of state, which would predict the value of pressure and energy of a shearing fluid for any state point and any value of the applied shear rate.

The stability of many-body systems

We apply the pair interaction stability criteria of Fisher and Ruelle (1966 J. Math. Phys. 7 260) to establish the range of thermodynamic stability for a number of simple analytic potential forms used for condensed matter theory and modelling in the literature. We identify the ranges of potential parameters where, for a given potential, the system is thermodynamically stable, unstable and of uncertain stability. This was further explored by carrying out molecular dynamics simulations on the double Gaussian potential in the stable and unstable regimes. We show that, for example, the widely used exponential-6 and Born-Mayer-Huggins alkali halide potentials produce many-particle systems that are thermodynamically unstable.

Plastic response of a 2D Lennard-Jones amorphous solid: detailed analysis of the local rearrangements at very slow strain rate

We analyze in detail the atomistic response of a model amorphous material submitted to plastic shear in the athermal, quasi-static limit. After a linear stress-strain behavior, the system undergoes a noisy plastic flow. We show that the plastic flow is spatially heterogeneous. Two kinds of plastic events occur in the system: quadrupolar localized rearrangements, and shear bands. The analysis of the individual motion of a particle shows also two regimes: a hyper-diffusive regime followed by a diffusive regime, even at zero temperature.

Microscopic structure, dynamics, and wear at metal-metal interfaces in sliding contact

The "generic embedded atom model" (GEAM) has been investigated recently [Phys. Rev. E 69, 021509 (2004)]] to analyze the qualitative equilibrium and nonequilibrium properties of bulk metals in both undeformed and shear deformed states. In the present work, a natural extension of the GEAM is proposed and applied to characterize the microscopic structure, dynamics, and wear at clean commensurate metal(A) - metal(A) and metal(A) - metal(B) sliding interfaces. Nonequilibrium molecular dynamics simulation, used as a GEAM solver, reveals that the dynamics of dislocations, crystalline domains, and related flow behaviors (stress tensor, shear moduli) are coupled. The rotation of crystal domains is detected to trigger material mixing at the interface in early stages of sliding. Further, we study the dependence of structural changes in inhomogeneous metal interfaces on the relevant model parameters. A relation is established between shear moduli, effective shear rate, and shear stress across the interface.