V. Sorkin

Molecular dynamics study of melting of the bcc metal vanadium. II. Thermodynamic melting

We present molecular dynamics simulations of the thermodynamic melting transition of a bcc metal, vanadium, using the Finnis-Sinclair potential. Our motivation was to answer the question whether premelting phenomena, extensively studied for fcc metals, are also present in bcc metals. We studied the structural, transport, and energetic properties of slabs made of 27 atomic layers with a free surface. We investigated premelting phenomena at the low-index surfaces of vanadium, V(111), V(001), and V(011), finding that as the temperature increases, the V(111) surface disorders first, then the V(100) surface, while the V(110) surface remains stable up to the melting temperature. Also, as the temperature increases, the disorder spreads from the surface layer into the bulk, establishing a thin quasiliquid film in the surface region. We conclude that the hierarchy of premelting phenomena is inversely proportional to the surface atomic density, being most pronounced for the V( 111) surface which has the lowest surface density.

A local quasicontinuum method for 3D multilattice crystalline materials: Application to shape-memory alloys

The quasicontinuum (QC) method, in its local (continuum) limit, is applied to materials with a multilattice crystal structure. Cauchy–Born (CB) kinematics, which accounts for the shifts of the crystal motif, is used to relate atomic motions to continuum deformation gradients. To avoid failures of CB kinematics, QC is augmented with a phonon stability analysis that detects lattice period extensions and identifies the minimum required periodic cell size. This approach is referred to as Cascading Cauchy–Born kinematics (CCB). In this paper, the method is described and developed. It is then used, along with an effective interaction potential (EIP) model for shape-memory alloys, to simulate the shape-memory effect and pseudoelasticity in a finite specimen. The results of these simulations show that (i) the CCB methodology is an essential tool that is required in order for QC-type simulations to correctly capture the first-order phase transitions responsible for these material behaviors, and (ii) that the EIP model adopted in this work coupled with the QC/CCB methodology is capable of predicting the characteristic behavior found in shape-memory alloys.

Efficient algorithms for discrete lattice calculations

We discuss algorithms for lattice-based computations, in particular lattice reduction, the detection of nearest neighbors, and the computation of clusters of nearest neighbors. We focus on algorithms that are most efficient for low spatial dimensions (typically d=2,3) and input data within a reasonably limited range. This makes them most useful for physically oriented numerical simulations, for example of crystalline solids. Different solution strategies are discussed, formulated as algorithms, and numerically evaluated.

Influence of boundaries on high-pressure melting experiments

At low pressure, free surfaces play a crucial role in the melting transition, but to maintain pressure, the sample surface must be acted upon by some pressure-transmitting medium. To examine the effect of this medium on melting, we performed Monte Carlo simulations of a system of argon atoms in the form of a slab with upper and lower boundaries. We examined two cases: one with a soft and the other with a rigid medium at the boundaries. We found that in the presence of a rigid medium, melting resembles the mechanical lattice instability found in a surface-free solid. With a soft medium at the boundary, melting begins at the surface and at a lower temperature. The relevance of these results to experiment is discussed.

Salt fingers in double-diffusive systems

“Salt fingering”, the phenomenon of the double-diffusive convection has been studied experimentally. We prepared a system with vertical density variation, which contains salt and sugar solutions, separated one from the other. The “salt fingers” appear at the interface between these layers of salt and sugar solutions. The “salt fingers” have been observed by means of shadowgraph technique.

Visualization of Melting Simulations

We describe visualization aspects of our recent atomistic simulations of bulk and surface melting. A discussion of the use of visualization to elucidate the nature of the mixing between different layers as the temperature is increased is given. We implemented the AViz package, which was prepared in the Technion’s Computational Physics Group for use in atomistic simulations. This package enables, for example, the use of different colors for different intial locations in order to explore layer mixing and addlayer formation and the use of color to indicate the location of lattice defects at a glance.