National Renewable Energy Laboratory, Golden, Colorado 80401, USA.
Phys Rev Lett. 2012 Mar 16;108(11):115901. doi: 10.1103/PhysRevLett.108.115901. Epub 2012 Mar 13.
The development of short-to-medium-range order in atomic arrangements has generally been observed in noncrystalline solid systems such as metallic glasses. Whether such medium-range order (MRO) can exist in materials at well above their melting or glass-transition temperature has been a long-standing important scientific issue. Here, using ab initio molecular dynamics simulations, we show that a novel, persistent MRO exists in liquid Al-Cu alloys near the composition of CuAl3. The correlated atomic motions associated with the MRO give rise to a substantially enhanced viscosity in the vicinity of the composition. The component of the MRO liquid state gradually decreases with increasing temperature, and it disappears above a crossover temperature T(LLC). The continuous liquid-liquid crossover through a percolationlike transition leads to a pronounced heat capacity peak at T(LLC).
短程到中程有序在原子排列中的发展通常在非晶态固体系统中观察到，如金属玻璃。在远高于其熔点或玻璃化转变温度的材料中是否存在这种中程有序（MRO）一直是一个重要的科学问题。在这里，我们使用从头算分子动力学模拟表明，在 CuAl3 附近成分的液态 Al-Cu 合金中存在一种新颖的、持久的 MRO。与 MRO 相关的原子运动导致在该成分附近的粘度大大增加。MRO 液态的组成部分随着温度的升高逐渐降低，并且在交叉温度 T（LLC）以上消失。通过类渗流的连续液-液交叉导致在 T（LLC）处出现明显的热容峰。
