When ductile metals (such as copper, nickel and aluminum) come into contact with some liquid metals (such as bismuth and gallium), intergranular failure occurs at abnormally low stress level, which is called liquid metal embrittlement (LME). Decades of research have been devoted to understanding the underlying micro-mechanisms. The development of spherical aberration corrected scanning transmission electron microscopy (AC-STEM) solved the structural problems of general grain boundaries (GBs), which led to re-evaluation of some classical intergranular dissociation systems at atomic scale. Al-Ga system is a typical example of LME intergranular structure, and the atomic structure at the front of Ga penetration is still unstable. In-situ transmission electron microscope showed that liquid g a rapidly permeated along Al multi-grain boundaries, forming multiple adsorption layers of Ga. A lot of research has been done from theory to experiment, and different segregation behaviors have been predicted by theoretical calculation of different methods. Previous studies have shown that the Al-Ga interface may contain multiple layers, one of which is fixed on Al surface, and the other is Ga layer with poor top bonding. This multi-layer structure is determined by the GB structure, but the atomic structure of the interface is still elusive, and it is not clear whether there are ordered Ga layers in ordinary GB.

The researchers of Fuzhou University revealed the atomic structure of Ga enriched on the common grain boundary of Al, and clarified the composition of disordered Ga layer with GB as the core, which was further verified by dynamic Monte Carlo and molecular dynamics simulation. Related papers were published in Scripta Materialia, titled "Interface Structure Supporting al-ga Liquid Metal Inscription: Disorder and Ordered Gradient".

Paper link:

https://doi . org/ 10. 10 16/j . script Amat . 202 1. 1 14 149

Punch a 3mm disc from aluminum polycrystalline foil (purity 99.99%, grain size about 5μm, thickness 100μm) with a disc punch. The thickness of the disk was reduced from the grinder to about 25μm, and then immediately transferred to the hot plate of 1 10, and Ga particles (purity 99.999%) were placed on the reverse side.

It is found that both non-infiltrated gallium and gallium-containing Al-Ga gratings have high bending degrees, which indicates that the penetration of gallium has not changed the degree of freedom of Al-Ga gratings. A new ordered gradient morphological structure was found on the grain boundary of Ga infiltration into aluminum. The composite layer at the infiltration front consists of at least three layers, of which two layers are attached to the surface of each aluminum grain in turn, and the third layer is in the middle, and the Ga content is in decreasing order. The mixed MC/MD simulation verifies this interesting separation behavior.

Fig. 1 HAADF image of Al sample with non-permeability (a) and Ga permeability (b), GBs is indicated by white arrow; (c)Ga is enlarged through GB HAADF image; The distribution diagram of (d, e) elements confirmed the enrichment of gallium on grain boundaries. (f) Perform EDS line scanning on GB in (c).

Fig. 2 (a) Curved aluminum grain boundary; (b) HAADF diagram of grain boundary edge after infiltration; (c) the results of line scanning; HRTEM diagram of disordered gallium layer; (e； F) hybrid MC/MD simulation model

Fig. 3 (a) shows multiple layers separated in GB; (b) enlarged HAADF diagram and line scanning results; (c) simulating the atomic structure of the grain boundary infiltrated into Ga; (d) Two-dimensional average density distribution and disordered parameter distribution of d)Ga atoms.

Multilayer structure can transition from highly ordered bimolecular layer to disordered layer, and Ga adsorption layer integrates these two structural features into a composite layer. These results show that the structure with multi-layer adsorption (two or more layers) may lead to interfacial debonding. The fundamental reason of LME is not the interface order, but the weak interaction between atoms in GB core. Generally speaking, this paper reveals the complex but ubiquitous interface separation structure in aluminized GBS, which has an orderly gradient and enriches the researchers' understanding of surface structure. (Text: Breaking the Wind)