Grain boundary sliding plays an important role in plastic deformation at various scales, including macro-, micro-, and nano-scale. It is the fundamental mechanism by which grain boundaries are able to slip past each other, with relatively little basal plane resistance, resulting in a change in shape of a material. At higher tension, the grain boundaries may be disrupted, leading to grain fragmentations and ultimately mechanical failure. Thus, understanding the forces involved in grain boundary sliding and their correlations with macroscopic plastic behavior is essential for developing materials with enhanced properties and durability.
Grain boundary sliding has been widely studied in metals since the 1950s, with major breakthroughs in the theories of solid-state plasticity and characteristics of the grain boundaries. It was found that the grain boundary sliding is activated by differential stress along the grain boundary plane, and is strongly dependent on the shear stiffness of the individual grain boundary. Additionally, it was determined that the shear stress required for initiating grain boundary sliding varies from layer to layer and varies with the misalignment of the boundary planes. Furthermore, grain boundary sliding is affected by the presence of point defects, such as dislocations, on either side of the boundary, as well as the grain boundary energy.
The process of grain boundary sliding is complex and depends on several factors such as the temperature, material properties, total specific surface energy, grain boundary misalignment, and the local geometry of the boundary plane. Generally, the forces required to deform a material are directly related to the amount of interaction that is taking place between the grain boundary planes. The higher the interaction, the more resistant the boundaries become to sliding. Alternatively, if there is a lower interaction, the boundary planes become more easily deformed.
In addition to these factors, the presence of other crystalline lattice defects and radiation damage can also affect the overall plastic behavior by influencing the grain boundary motion. For example, when grain boundaries meet, they can produce a complex network of interconnected channels between neighboring grains that lead to asymmetric grain boundary sliding, even when the amount of differential stress is uniform. This type of grain boundary sliding is referred to as “grain boundary diffusion” and has been found to cause significant mechanical degradation in certain materials.
At the micro- and nano-scale, grain boundary sliding can be observed under the electron microscope associated with simple and equal-channel angular pressing (ECAP) techniques. In ECAP, a large number of micro-deformation processes occur within a single sample and are responsible for severe grain boundary sliding along the various deformation bands. This type of grain boundary sliding has been found to play a major role in the work hardening of microstructured materials, such as nanocrystalline metals and alloys.
In conclusion, grain boundary sliding is an important mechanism of plastic deformation at many length scales, giving rise to a number of unique mechanical properties. The forces involved in grain boundary motion and their correlation with macroscopic behavior can provide valuable insights into designing better materials with enhanced properties and greater durability. The presence of atomistic defects at grain boundaries can further complicate the process of grain boundary sliding, leading to important micro-structural changes and influencing the overall mechanical properties of the material. Therefore, the knowledge of the deformation behavior of grain boundaries is essential for predicting and designing materials with improved performance.
