Grain Boundary Structure
The grain boundaries play an important role in determing the properties of all polycrystalline materials. Grain boundary structure has great influence on diffusion, slip transition and grain boundary sliding (GBS). The energy of grain boundaries is an important thermodynamic parameter with respect to segregation, precipitation, grain boundary mobolity and the ideal fracture strength. GBS is believed to be the dominant strain producing mechanism during superplastic deformation. In contrast to experiments, in computer simualtions it is easy to systematically vary all the macroscopic geometrical parameters describing a planar, bicrystalline grain bounadry. In this manner, computer modeling can be used to give fundamental insight into the physics of grain boundaries. The combination of computer simulation studies with related experiments is capable of providing insight which, from experiments alone, would be very difficult to obtain.
Molecular statics and molecular dynamics are widely used in performing atomistic simulations. Molecular statics is used in determining the equilibrium positions of atoms in a crystal, by minimizing the total energy of the crystal at 0 K. Molecular dynamics, on the other hand, can be used to study the time-related phenomena for crystals at a defined temperature and/or stress. To run a molecular statics/dynamics simulation, the interatomic potential which predescibes the interations between atoms, is a necessary input. An accurate and relatively simple model of the energetics is required so that a sufficiently large set of atoms can be considered in the computational crystal. The Embedded Atom methos (EAM) appears to be a good choice for metallic systems. EAM takes into account the many-body effects yet does not require sighnificantly more computational efforts than traditional pair potentials.
Atomistic simulations have been conducted to systematically study the grain boundary structures and ennergies associted with the equlibrium (0 K) structures, high temperature sturctures and the energy profile during grain boundary sliding (GBS). The results show that the EAM-based molecular statics (energy minimization) calulations have correctly predicted the tilt CSL grain boundary structures of aluminium. The garin boundary energy is related to the interpannar spacing of the grain boundary plane, and the width of the grain boundaries is about 10 Angstrom. The GBS process has been studied by the energy calculations of the GBS-induced distorted structures. Energy barriers are found to be the great resistence to the GBS, and the grain boundary cleavage or voids are energetically unfavourable to form due to the higher energies associated with the free surface.
Grain Boundary Structure | |
Energy Distribution in Grain Boundary Structures |  |
Energy Evolutions in GBS process |  |
P. Dang and N. Chandra, "Atomistic Simulation of Grain Boundary Structure, Energy and Sliding" , Acta Metall. , Submited, March, 1997.
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