Background/Motivation: Chemical doping or alloying is often used to improve the electrical and mechanical properties of materials. The controlled preparation of alloy nanomaterials is rendered difficult owing to the increased surface area- to-volume ratio which can lead to the expulsion of impurities (self-purification). In the present paper, we explore using molecular dynamics (MD) simulations the extent of alloying that persists when bimetallic transition metal nanowires are mechanically stretched to the limit of a linear chain of atoms. For a given bimetallic alloy composition, the studied nanowires have varying segregation profiles of Pd, the common metal to the three bimetallics studied here. Analyses of the changes in crystal structure and segregation characteristics associated with the wire deformation are carried out and are used to deduce the alloying effect on the nanowire mechanical properties. In addition, the effect of lattice size mismatch on the nanowire mechanical properties is also evaluated.

Computational details: MD simulation of infinitely long, cylindrical bimetallic alloy nanowires (Pd-Pt, Pd-Rh, Pd-Cu and Pd-Ni), with an approximate diameter of 1.4 nm and varying compositions (25 and 50% Pd) are used to investigate their mechanical properties. The nanowires are subjected to uniaxial tensile strain along the [001] axis. The empirical quantum Sutton-Chen potential function is used to describe the inter-atomic potential between the various transition metal atoms. A loose-coupling thermostat (Berendson) is selected for finite-temperature control of the simulated system, with a time constant of 25% of the total relaxation time during each strain increment. In the present simulation, these nanowires are subjected to varying strain rates of 0.05%, 0.5%, and 5.0% ps-1, at simulation temperatures of 50 and 300 K, in order to study the effects of different strain rates and thermal conditions on the deformation characteristics and mechanical properties of alloy nanowires.

Results and Discussion: Our simulation results indicate that although qualitatively there are no differences in the deformation and rupture mechanism of the various alloy nanowires compared to the single metal counterpart, significant quantitative differences in mechanical properties exist. At low temperature and strain rate, where crystal order and stability are highly preserved, the calculated stress-strain response of alloy nanowires showed clear periodic, stepwise dislocation-relaxation recrystallization behavior whereas fully amorphous deformation takes place at high strain rates (5.0% ps-1), with amorphous melting detected at 300 K. Due to higher entropy of the nanowire at higher temperature and strain rate, periodic stress-strain behavior becomes less clearly defined, and superplasticity behavior is observed. We find that the deformation of nanowire at low strain rates results in the development of a single-walled helical substructure at 300 K and lower strain rates, when the superplasticity characteristic is significantly enhanced.

The atomistic aspect associated with the metal alloy nanowire thinning during mechanical stretching are explored in detail. In particular, the segregation profiles of the alloy nanowires were found to undergo significant changes with extension, especially near the neck region. Our simulation results reveal a gradual evolution of the arrangement of the different atomic elements in the narrowest region of the chain, where impurities may be expelled to the surface or enclosed during elongation. The nanowire stress-strain response as well as yielding characteristics showed strong dependence on composition of the alloy nanowires. The calculated Young's modulus and Poisson ratio are about 50%-75% of the bulk value, with the actual value depending on the alloy composition. The results are expected to throw light on the higher mechanical stability exhibited by alloys nanowire of certain composition range over pure component nanowires.