液态金属脆化机理的电子显微学研究

Intrinsic ductile metals are prone to catastrophic brittle fracture at extremely low stress when exposed to certain liquid metals. Solving the mechanism of liquid metal embrittlement (LME) is of technological importance. Recently, the science community made progress in achieving this goal in one classic LME system. Researchers confirmed the general existence of interfacial phases in the form of Bi bilayers within Ni general grain boundaries (gbs). They claimed that the Bi infused Ni samples became totally brittle because that their boundaries were weakly bonded by two Bi layers. However, the bilayer configuration only explains why the boundaries would become brittle. It doesn’t touch the root cause of LME: why the Bi bilayers are stable and how they are formed within general gbs? Thus, we are going to demystify the underlying LME mechanism by reconstructing the three-dimensional atomic configurations of multiple interfacial phases in a bunch of classic LME systems. A ‘grain boundary crystallography analysis’ platform will be developed on the TEM interface to assist this complicated process. The common existence of bilayers in other LME systems and the general characters of boundary structures will be examined. The boundary structures of liquid metal infused boundaries will be compared with those of boundaries that block liquid metals, shedding light on the LME mechanism.

液态金属脆化（LME）会导致金属结构材料的灾难性失效，其机理一直是材料科学领域的待解难题。近些年来，研究者在液态Bi侵蚀的Ni晶界上发现Bi双层晶界相，并指出此时晶界因依靠两层Bi-Bi弱键连接而脆化。双层晶界相的发现是LME机理研究的重大突破，但双层构型只是定性地解释了脆化的结果，并未真正阐明LME的根源。解决LME机理的关键在于探索脆化晶界上晶界相的形成规律和稳定机理。因此，本项目拟以透射电镜为主要研究工具，在几个经典LME体系中（Bi侵蚀Ni/Cu，Bi侵蚀Fe，Hg侵蚀Cu）寻找双层晶界相的普适性，解析这些晶界相的三维原子构型，再对比液态金属难侵蚀材料的晶界结构，从而构建晶界相结构与晶界脆化的关联，揭示液相金属脆化的共性规律和根源。

液态金属会导致金属结构材料发生灾难性的脆性转变，近年来显微技术的进步把科学界对致脆机理的认识提升到了原子尺度。一个典型的案例是，研究者在液态Bi渗入的Ni晶界上发现有序的Bi双层晶界相，晶界成键变成Bi-Bi弱键而促发了脆化。然而，对于有序双层构型的普适性、液态金属致脆的普遍原理等科学问题的理解还不够深入。本课题以几种常见的液态金属致脆体系为研究对象，结合球差校正电镜技术、第一性原理计算和蒙特卡洛模拟，深入剖析界面相的原子构型，获得了以下结果：①在经典的Al-Ga体系中发现了一种新颖的界面相结构，晶界核心为1-2层无序Ga，两侧各存在与Al晶粒相连的有序Ga单层。具有有序度结构梯度（有序过渡到无序）的多层Ga界面相在Al普通晶界上普遍存在。该研究将有序度作为一种新元素添加到界面相结构中，在原子层次上丰富了界面结构的认识。②针对在第三代核反应堆中有应用背景的铁基金属-PbBi体系，发现了晶内和晶界两种不同的裂纹扩展通道，扩展路径依赖于材料体系。更重要的是，在纯铁模型系统中发现PbBi在晶界上形成了双层有序界面相，这和Ni-Bi体系一致。结合最近在Ni-S体系中揭示的无序多层吸附相结构，我们通过一系列研究回答了前述两个基本科学问题：①有序双层界面相不是普适的；②液态金属致脆根源在于界面上形成多层（≥2）吸附相，多层吸附相内部的弱键触发了脆性转变。液态金属致脆体系为界面学科提供了多彩的界面构型，丰富了科学界对普通晶界的认识。