亚埃分辨Z衬度直接成像结合第一性原理计算研究Mg-Zn-RE合金内的原子团簇及其形成机制

First-order phase transformations are achieved through the formation and subsequent growth of "nuclei" of the new phase. However, little is known about the atomic structure of the nuclei and how they develop from randomly distributed solute atoms in multi-component systems. Such knowledge is very valuable for understanding the phase transformation theory, which in turn advances the theory and its application. However, direct atomic imaging of tiny nuclei in bulk materials is extremely difficult to achieve, because they usually have non-periodic structures consisting of several atomic species and are buried in the matrix.The single atom imaging ability of aberration-corrected Z-contrast electron microscopy makes it possilbe to solve such scientific conundrum. We will carry out combination studies of aberration-corrected Z-contrast electron microscopy and first-principles calculations in order to reveal the atomic structure of fcc nuclei/clusters in Mg-Zn-RE alloys, and the underlying mechanism controlling their formation, evolution and growth. The key issues are to determine the atom or atoms that trigger the appearance of the clusters, and to clarify the atomic processes finally. This research is devoted to advance the first-order phase transformation theory, especially the dynamics of the formation of nuclei/clusters to the atomic level, which thus provides theoretical guidelines for alloy design, treatment and optimization.

一级相变是通过新相"晶核"的形成和长大实现的。但是，对多组元合金体系内"晶核"的原子构型，以及它们如何由随机分布的溶质原子聚集演变而来等科学问题,迄今仍知之甚少。这些信息对于相变理论的认知、发展和实际应用都至关重要。然而，由于多组元体系内的微小"晶核"通常由几种原子组成，尚处于化学无序态，而且还包埋在基体晶格中，很难对它们进行原子分辨的直接成像。亚埃分辨Z衬度电子显微学技术的单原子成像能力使得解决该难题成为可能。拟利用像差校正Z衬度成像，结合第一性原理计算，研究Mg-Zn-RE合金中具有面心立方（fcc）结构特征的"晶核"或"团簇"的原子构型，进而阐明其形成、演变和生长的微观机制。核心问题是哪个或哪些原子的迁移激发了"团簇"的形成，原子尺度的过程又是怎样进行的。预计本项目的研究结果将推进对一级相变形核阶段细节的原子尺度认知和理论发展，并为合金设计、制备与优化提供理论指导。

合金化是强化工程合金材料的重要途径（沉淀强化和固溶强化）。在原子尺度上认识每种合金元素在工程合金中的行为和作用机制是科学调控材料微观结构，实现材料性能优化的根本基础，是材料科学的核心科学问题之一。单个合金原子的直接成像，确定原子的准确晶体学位置，并识别其元素类型，正是解决这个科学问题的关键。本项目以Mg-TM-RE合金为研究对象，成功实现了单个合金化TM和RE原子的直接成像与识别，并且在亚埃分辨成像的基础上结合第一性原理计算深入认识了点缺陷聚集形成微小团簇的物理机制。这为在原子尺度揭示合金中各元素的交互行为和机制，提供关键结构信息，是深入理解强化相的形核与生长机理，位错与固溶合金元素的交互作用等的重要基础。