Molecular dynamics simulation study of phase transformations in transition bimetallic nanowires

Molecular dynamic simulations were carried out to study the thermal characteristics of Pd-Rh and Pd-Cu nanowires of approximately 2.3 nm diameter using the quantum Sutton-Chen potential function to model the metal-metal interactions. Monte Carlo simulations employing the bond order simulation model were used to generate the initial configurations. Melting temperatures for these bimetallic nanowires of varying composition were estimated based on variations in thermodynamic properties such as potential energy and specific heat capacity. These melting temperatures were found to be much lower than those of bulk alloys of same composition and at least 100-200 K higher than same-diameter nanoclusters. Density distributions along the wire cross-section and axis as well as components of velocity auto-correlation function and shell based diffusion coefficients were used to identify the mechanism of nanowire melting. It is found to be surface initiated, the onset of which is triggered by predominantly cross-sectional or in-plane atomic movement. This two-dimensional melting mechanism differs from that observed in nanoclusters (3-D) where atomic movement is isotropic. The differences in melting mechanism manifest themselves in the form of differences in the simulated phase transition diagrams of wires and clusters. Nanowire and nanocluster melting mechanisms are associated with two competing processes (i.e., solid-solid and solid-liquid transition). Structural transitions (fee to hcp) in the simulated phase diagram identified using bond orientational order parameters reveal the existence of low-temperature hcp phases prior to the melting transition. The composition dependence of existence of hcp structures is influenced by the competition between surface melting (solid-liquid) and fcc-hcp (solid-solid) transition. The temperature range of existence of these structures varies with bimetallic composition and is influenced by the melting mechanism and nanomaterial geometry as well as the relative strengths of metal-metal interactions in the bimetallic. Investigations into the thermal stability of low temperature solid phases of these bimetallic nanowires were carried out by simulating alternative starting configurations such as hypothetical hcp and glassy annealed structures for 50% Pd composition. The simulated phase diagrams of the corresponding bulk systems agree well with experimentally reported ones.