The melting properties of grain boundaries in crystalline materials are often of great relevance but complex to understand. Grain boundaries can be divided into two main groups: those in the solid state and those in the liquid state.
The solid state grain boundaries are high energy boundaries that are difficult to melt. They are formed by the relative misorientation of two adjacent grains. At these grain boundaries, atoms and bonds may be subjected to significant shear stresses when the material undergoes deformation. As a result, much higher shear rates are observed at these boundaries than anywhere else in the crystal.
The liquid state grain boundaries are the grain boundaries that are relatively easy to melt or soften. These boundaries are those between two crystalline layers or between two crystallites. These boundaries have a lower energy level than solid state boundaries because they are created by relative misorientation of the grain planes, which is by far less than that in the solid state.
The melting properties of grain boundaries are also influenced by their angle of misorientation, which may range from 0° to 180°. The crystal structure of the material affects the angle at which misorientation occurs and which grain boundaries become exposed to additional energy released during melting.
At low angles of misorientation, the grain boundary has a lower tendency to melt. However, at angles of less than 90°, the grain boundary is much more likely to not melt, due to the large degree of shear stress that would be required for melting.
At angles of misorientation greater than 90°, the grain boundaries have reduced shear stress, and the energy necessary for melting is lower. The energy released during melting is also correlated with the angle of misorientation, with the most energy being released at low angles and the least energy at high angles.
Grain boundaries in polycrystalline materials can be broken down further into categories such as polycrystalline twins, grain boundaries between different crystal structures, grain boundaries between two different crystalline phases, grain boundaries between two different lattice planes, and grain boundaries between two different grain orientations.
Of particular relevance to the melting properties of grain boundaries is the effect of defects such as impurities, dislocation loops, or dislocations. Impurities, for example, decrease the melting temperature of grain boundaries and size of the grain boundary region. Likewise, dislocation loops reduce the melting temperature and shear stress at grain boundaries.
The melting properties of grain boundaries are further modified by the presence of boundary sliding or boundary diffusion. Boundary sliding is the inefficient flow of atoms through grain boundaries, while boundary diffusion is the more efficient transfer of atoms across grain boundaries.
In conclusion, the melting properties of grain boundaries in crystalline materials are complex and governed by many factors. These include the angle of misorientation of the grains, the type of grain boundary, and the presence of impurities, dislocation loops, and dislocations. Understanding the melting behavior of grain boundaries is important in the optimization of material properties such as hardness and thermal stability.
