Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States.
Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States.
J Chem Theory Comput. 2022 Jul 12;18(7):4447-4455. doi: 10.1021/acs.jctc.2c00303. Epub 2022 Jun 7.
Although the equilibrium composition of many alloy surfaces is well understood, the rate of transient surface segregation during annealing is not known, despite its crucial effect on alloy corrosion and catalytic reactions occurring on overlapping timescales. In this work, CuNi bimetallic alloys representing (100) surface facets are annealed in vacuum using atomistic simulations to observe the effect of vacancy diffusion on surface separation. We employ multi-timescale methods to sample the early transient, intermediate, and equilibrium states of slab surfaces during the separation process, including standard MD as well as three methods to perform atomistic, long-time dynamics: parallel trajectory splicing (ParSplice), adaptive kinetic Monte Carlo (AKMC), and kinetic Monte Carlo (KMC). From nanosecond (ns) to second timescales, our multiscale computational methodology can observe rare stochastic events not typically seen with standard MD, closing the gap between computational and experimental timescales for surface segregation. Rapid diffusion of a vacancy to the slab is resolved by all four methods in tens of nanoseconds. Stochastic re-entry of vacancies into the subsurface, however, is only seen on the microsecond timescale in the two KMC methods. Kinetic vacancy trapping on the surface and its effect on the segregation rate are discussed. The equilibrium composition profile of CuNi after segregation during annealing is estimated to occur on a timescale of seconds as determined by KMC, a result directly comparable to nanoscale experiments.
尽管许多合金表面的平衡组成已经得到很好的理解，但在退火过程中瞬态表面偏析的速率却不得而知，尽管它对重叠时间尺度上发生的合金腐蚀和催化反应有至关重要的影响。在这项工作中，使用原子模拟方法退火了代表（100）表面晶面的 CuNi 双金属合金，以观察空位扩散对表面分离的影响。我们采用多时间尺度方法来采样在分离过程中表面的早期瞬态、中间和平衡状态，包括标准 MD 以及三种进行原子、长时间动力学的方法：并行轨迹拼接（ParSplice）、自适应动力学蒙特卡罗（AKMC）和动力学蒙特卡罗（KMC）。从纳秒（ns）到秒的时间尺度，我们的多尺度计算方法可以观察到标准 MD 通常看不到的罕见随机事件，从而缩小了表面偏析的计算和实验时间尺度之间的差距。空位在几十纳秒内迅速扩散到薄片中，这四种方法都可以分辨出来。然而，空位在亚表面的随机重新进入只在两个 KMC 方法的微秒时间尺度上看到。表面上的动力学空位捕获及其对偏析速率的影响进行了讨论。通过 KMC 确定，在退火过程中发生偏析后，CuNi 的平衡组成剖面估计在秒的时间尺度上发生，这是一个直接与纳米尺度实验可比的结果。
