Grain Boundary Sliding is a material deformation mechanism where grains slide against each other. This occurs in polycrystalline material under external stress at high homologous temperature and low strain rate. Homologous temperature describes the operating temperature relative to the melting temperature of the material. There are mainly two types of grain boundary sliding: Rachinger sliding, and Lifshitz sliding. Grain boundary sliding usually occurs as a combination of both types of sliding. Boundary shape often determines the rate and extent of grain boundary sliding. Many people have developed estimations for the contribution of grain boundary sliding to the total strain experienced by various groups of materials, such as metals, ceramics, and geological materials. Grain boundary sliding contributes a significant amount of strain, especially for fine grain materials and high temperatures. It has been shown that Lifshitz grain boundary sliding contributes about 50-60% of strain in Nabarro-Herring diffusion creep. This mechanism is the primary cause of ceramic failure at high temperatures due to the formation of glassy phases at their grain boundaries.
Rachinger Sliding
Rachinger sliding is purely elastic; the grains retain most of their original shape. The internal stress will build up as grains slide until the stress balances out with the external applied stress. For example, when a uniaxial tensile stress is applied on a sample, grains move to accommodate the elongation and the number of grains along the direction of applied stress increases.
Lifshitz Sliding
Lifshitz sliding only occurs with Nabarro-Herring and Coble creep. The sliding motion is accommodated by the diffusion of vacancies from induced stresses and the grain shape changes during the process. For example, when a uniaxial tensile stress is applied, diffusion will occur within grains and the grain will elongate in the same direction as the applied stress. There will not be an increase in number of grains along the direction of applied stress.
Nanomaterials
Nano-crystalline materials, or nanomaterials, have fine grains which helps suppress lattice creep. This is beneficial for relatively low temperature operations as it impedes dislocations motion or diffusion due to high volume fraction of grain boundaries. However, fine grains are undesirable at high temperature due to the increased probability of grain boundary sliding.
Prevention
Grain shape plays a large role in determining the sliding rate and extent. Thus, by controlling the grain size and shape, the amount of grain boundary sliding can be limited. Generally, materials with coarser grains are preferred, as the material will have less grain boundaries. Ideally, single crystals will completely suppress this mechanism as the sample will not have any grain boundaries. Another method is to reinforce grain boundaries by adding precipitates. Small precipitates located at grain boundaries can pin grain boundaries and prevent grains from sliding against each other. However, not all precipitates are desirable at boundaries. Large precipitates may have the opposite effect on grain boundary pinning as it allows more gaps or vacancies between grains to accommodate the precipitates, which reduces the pinning effect.
