Molecular dynamics simulation of grain boundary migration under dislocation during grain annexation

Many macroscopic properties of materials, such as mechanical properties, thermal stability, and fatigue resistance, mainly depend on their microstructure. To obtain materials with good performance, it is usually necessary to regulate the development of microstructure, such as promoting the movement of grain boundaries through heat treatment, thereby achieving the transformation of microstructure and obtaining the desired properties. Generally, most solid-state phase transitions, including grain annexation growth, recrystallization and phase transition processes, are dominated by grain boundary migration. At the atomic scale, grain boundary migration is determined by mechanisms at the atomic level. During heat treatment, as the temperature rises, the thermal motion of atoms intensifies, which involves the merging of atoms from one grain to another. This process is closely related to the structural reorganization of atoms at the grain boundaries, and the grain boundary migration associated with these dynamic processes often encounters lattice defects. However, in experiments, tracking the movement of individual atoms or columns, as well as the evolution of grain boundary structures related to migration, remains challenging, and the atomic mechanisms of grain boundary-defect interactions remain largely unclear.

The development of molecular dynamics (MD) has provided an opportunity for atomic-level research, allowing for the direct visualization of atomic grain boundary migration. Meanwhile, in-situ transmission electron microscopy (TEM) can track the structural details of complex grain boundaries and their evolution during migration. Although many studies have been dedicated to analyzing the migration mechanisms of atomic grain boundaries, most of them have focused on the grain boundary migration mechanisms induced by curvature or stress. Recently, the team from the Yuhua Advanced Materials Research Institute discovered the grain annexation phenomenon through video recordings obtained from in-situ TEM. Then, in combination with MD, they applied the "natural" driving force of grain annexation - dislocations to study the interaction between grain boundaries and dislocation defect structures, exploring the mechanism of grain annexation growth at the atomic level.

First, the two grain orientations of grain annexation were obtained from in-situ TEM, and the grain boundary angles of the two grains were measured to obtain the grain boundary angles and the coincident lattice factor Σ (Figure 1). Then, the MD method was adopted to construct the alloy and grain boundaries (Figure 2), and temperature simulation was conducted on them. The results show that the grain boundary migration in the crystal material is driven by the excess energy of dislocation arrangement. The grain boundaries migrate in an irregular manner and show a strong dependence on the existence of local dislocations.

Fig. 1  Grain annexation and growth during in situ observation of Ti alloys

Fig. 2 Simulation model used in the present work.

Fig. 3 Interaction between GB and dislocations

The combination of computational simulation and experimentation is a popular direction in future materials research. Through the rational design and computational simulation of materials, it has been proved that the process of grain merging is the synergistic effect of atoms and dislocations at the grain boundary edges, which can reveal the connection between the structure and performance of materials at the atomic level and predict new materials. In addition, this method is also applicable to other grain boundary dynamics problems, opening up new channels for the research of high-temperature titanium alloys. At the same time, this has also promoted the transformation of the material research model from "experience guiding experiments" to "rational design and computational simulation, experimental verification".