Molecular dynamics simulation of temperature and strain rate effects on the elastic properties of bimetallic Pd-Pt nanowires

Abstract

Molecular dynamics simulation is used to investigate the mechanical properties of infinitely long, cylindrical bimetallic Pd-Pt nanowires, with an approximate diameter of $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and two different compositions (25% and 50% Pt). The nanowires are subjected to uniaxial tensile strain along the [001] axis with varying strain rates of $0.05%\phantom{\rule{0.2em}{0ex}}{\mathrm{ps}}^{\ensuremath{-}1}$, and $5.0%\phantom{\rule{0.2em}{0ex}}{\mathrm{ps}}^{\ensuremath{-}1}$, at simulation temperatures of 50 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, to study the effects of strain rates and thermal conditions on the deformation characteristics and mechanical properties of the nanowire. The deformation and rupture mechanism of these nanowires is explored in detail. Comparisons to the behavior exhibited by pure Pd and Pt nanowires of similar diameter are also made. The effect of lattice mismatch on the observed deformation modes in bimetallic nanowires is also discussed. Our simulations indicate that Pd-Pt alloy nanowires of various compositions, with little lattice mismatch between Pd and Pt atoms, undergo similar deformation and rupture upon uniaxial stretching. It is found that yielding and fracture mechanisms depend on the applied strain rate as well as atomic arrangement and temperature. At low temperature and strain rate, where crystal order and stability are highly preserved, the calculated stress-strain response of pure Pt and Pd as well as Pd-Pt alloy nanowires showed clear periodic, stepwise dislocation-relaxation behavior. Crystalline to amorphous transformation takes place at high strain rates $(5%\phantom{\rule{0.2em}{0ex}}{\mathrm{ps}}^{\ensuremath{-}1})$, with amorphous melting detected at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Deformation of nanowires at higher strain rates and low temperature, where the superplasticity characteristic is significantly enhanced, results in the development of a multishell helical structure. Mechanical properties of the alloy nanowires are significantly different from those of bulk phase and are dictated by the applied strain rate, temperature, alloy composition, as well as the structural rearrangement associated with nanowire elongation. We find that Young's modulus of both the single component as well as alloy nanowires depends on the applied strain rate and is about 70%--75% of the bulk value. Ductility of the studied nanowires showed a nonmonotonic variation with Pd composition at low strain rates and was significantly enhanced for wires developing and rearranging into a multishell helical structure which occurred at higher strain rates. The Poisson ratio of Pd rich alloys is 60%--70% of its bulk value, whereas that of Pt rich alloys is not significantly changed at the nanoscale. The calculated differences in the nanowire mechanical properties are shown to have significant effect on their applicability in areas such as sensing and catalysis.