Atomistic simulation of alumina grain boundary structure and diffusion

含铝或含铬合金上的氧化鳞片提供了高效和廉价的防腐保护。然而，氧化垢形成的关键原子尺度机制尚不清楚或存在争议，这限制了人们的认识和理解，并阻碍了最佳合金和氧化垢的计算设计。氧化垢形成机理的原子模拟是困难的。首先，需要精确的势能面表示，以便能够模拟晶界中复杂的原子重排，并包括电荷转移对能量的贡献。其次，模拟单元需要包含数千个原子来模拟真实的几何形状，模拟需要运行足够长的时间来观察原子的重排和扩散。虽然密度泛函理论（DFT）足够精确，但它无法模拟那么多的原子。另一方面，利用经典的原子间电位可以模拟足够的原子，但这些电位通常不够精确或可转移，即使包括电荷转移模型。直到最近，一类精确的原子间势，通常被称为机器学习势，才变得突出，它们以接近dft的精度描述势能面。在这一类中，我们支持的方法是原子簇扩展（ACE），我们在其中加入了电荷转移。它是快速和系统的改进。利用这种方法，我们建议为铝-氧系统开发准确的和可转移的原子间电位。我们将利用这些势来模拟氧化垢的晶界扩散，并从我们的模拟中提取原子尺度机制和扩散系数，这对理解氧化垢的生长速度是决定性的。我们的项目将是我们所知的第一个解决高温下氧化物晶界原子尺度扩散机制和速率问题的项目，为保护尺度的原子尺度设计奠定基础。

Oxide scales on Al- or Cr-containing alloys provide efficient and inexpensive corrosion protection. However, key atomic scale mechanisms in oxide scale formation are unknown or disputed, which limits insight and understanding and blocks the computational design of optimal alloys and oxide scales. Atomistic simulation of mechanisms in oxide scale formation is difficult. Firstly, an accurate representation of the potential energy surface is required that enables the simulation of complex atomic rearrangements in grain boundaries and includes the contribution of charge transfer to the energy. Secondly, the simulation cells need to comprise thousands of atoms to sample realistic geometries and simulations need to be run for sufficiently long times to observe atomic rearrangements and diffusion. While density functional theory (DFT) is sufficiently accurate, it is unable to simulate that many atoms. On the other hand, simulations with sufficient atoms are possible by employing classical interatomic potentials, but these potentials are generally not sufficiently accurate or transferable, even when including a model of charge transfer. Only recently a class of accurate interatomic potentials, often termed machine-learning potentials, became prominant, which describe a potential energy surface with near-DFT precision. Within this category the approach we favour is the Atomic Cluster Expansion (ACE), in which we are incorporating charge transfer. It is fast and systematically improvable. With this approach we propose to develop accurate and transferable interatomic potentials for the aluminium-oxygen system. We will employ these potentials for the simulation of diffusion in grain boundaries of the oxide scale and extract from our simulations the atomic scale mechanisms and diffusion coefficients that are decisive for understanding the growth rates of oxide scales. Our project will be the first to our knowledge which tackles the problem of atomic scale diffusion mechanisms and rates in oxide grain boundaries at high temperature, laying the foundations for atomic scale design of protective scales.