Cooling rates dependence of medium-range order development inCu64.5Zr35.5metallic glass

Abstract

The atomic structure of metallic glasses (MGs) plays an important role in their properties. Numerous molecular dynamics (MD) simulations have revealed icosahedral short-range order (ISRO) as a dominant motif in Cu-Zr metallic glasses. However, the cooling rates utilized in most of the MD simulations (usually on the order of ${10}^{10--13}\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{s}$) can be too high to allow the structure to relax into the actual structures. By performing a long sub-${T}_{g}$ annealing of the $\mathrm{C}{\mathrm{u}}_{64.5}\mathrm{Z}{\mathrm{r}}_{35.5}$ alloy model at 700 K up to $2.0\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{s}$ using MD simulations, we systematically address the evolution of medium-range order (MRO) as the cooling rates in MD simulations approach the experimental cooling rates (usually ${10}^{3--6}\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{s}$). By reducing the effective cooling rates to as low as $2.8\phantom{\rule{0.16em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}{10}^{7}\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{s}$, we found a significant enhancement of the ISRO and Bergman-type MRO. Comparing to the widely used face-, edge-, or vertex-sharing icosahedra, we propose that the Bergman-type MRO is a much more unambiguous metric to characterize the MRO in Cu-Zr MGs. By analyzing the network formed by interpenetrating icosahedra using the graphical theory, we show that the degree of interpenetration of the icosahedra centers increases with decreasing cooling rates. The network becomes aggressively assortative, indicating that higher degree nodes tend to cluster and form backbones in the MG. All these results show that the networks in the models prepared using lower cooling rates strongly deviate from a stringlike morphology.