Grain boundary sliding

This occurs in polycrystalline material under external stress at high homologous temperature (above ~0.4[1]) and low strain rate and is intertwined with creep.

Therefore it is not surprising that Nabarro Herring and Coble creep is dependent on grain boundary sliding.

We can simulate this type of boundary with a sinusoidal curve, with amplitude h and wavelength λ. Steady-state creep rate increases with rising λ/h ratios.

[1] It has been shown that Lifshitz grain boundary sliding contributes about 50-60% of strain in Nabarro–Herring diffusion creep.

[7] This mechanism is the primary cause of ceramic failure at high temperatures due to the formation of glassy phases at their grain boundaries.

[7] The sliding motion is accommodated by the diffusion of vacancies from induced stresses and the grain shape changes during the process.

This type of mechanism is synonymous to Nabarro Herring and Coble creep but describes the grain at superplastic conditions.

[13][7] Thus, the minimum creep rate becomes: The total strain under creep conditions can be denoted as εt , where: εt  =   εg + εgbs +εdc εg =   Strain associated with intragranular dislocation processes εgbs =   Strain due to Rachinger GBS associated with intragranular sliding εdc   =   Strain due to Lifshitz GBS associated with diffusion creep During practice, experiments are normally performed in conditions where creep is negligible, therefore equation 1 will reduce to: εt  =   εg + εgbs Therefore the contribution of GBS to the total strain can be denoted as: Ⲝ =   εgbs / εt First, we need to illustrate the three perpendicular displacement vectors: u, v, and w, with a grain boundary sliding vector: s. It can be imagined as the w displacement vector coming out of the plane.

Subsequently this was observed in other systems as well including in Zn-Al alloys using electron microscopy,[16] and octachloropropane using in situ techniques.

This is beneficial for relatively low temperature operations as it impedes dislocations motion or diffusion due to high volume fraction of grain boundaries.

[17] Grain shape plays a large role in determining the sliding rate and extent.

Ideally, single crystals will completely suppress this mechanism as the sample will not have any grain boundaries.

To provide a substantial engineering basis for real-world construction, the modeling of high-strength steel is very important.

An example would be for commercial fine-grained Al-Mg alloys, unusually weak grain boundary sliding is observed during the initial stage of superplastic deformation.

The deformation was orchestrated by increased precipitation depletion zone fractions, particle segregation on the longitudinal grain boundaries, dislocation activity, and subgrains.

Researchers found that the predominant mechanism for failure in these tungsten filaments was grain boundary sliding accommodated by diffusional creep.

During operation, the tungsten wire is stressed under the load of its own weight and because of the diffusion that can occur at high temperatures, grains begin to rotate and slide.

[21] To combat this grain boundary sliding, researchers began to dope the tungsten filament with aluminum, silicon and most importantly potassium.

[22] These bubbles interact with all defects in the filament pinning dislocations and most importantly grain boundaries.

[22] Thus, this initially counter-intuitive approach to strengthening tungsten filaments began to be widely used in almost every incandescent lightbulb to greatly increase their lifetime.

A simple schematic of grain boundary sliding in a polycrystalline sample (adapted from [ 9 ] ). When a tensile load is applied to the materials, the grains stretch along that direction. This leads to the creation of voids/cavities and a loss of coherency. To prevent void formation, the grains slide relative to each other to fill in these unfavorable gaps.
A simple schematic showing how experimentalists observe grain boundary sliding between two adjacent grains. Initially, a polycrystalline material is scratched with a marker line (shown here as a thick dashed line). If these two grains slide relative to one another, there will be an offset in this marker line occurring at the grain boundary. This can be observed using various microscopy techniques.