Grain boundary sliding accommodated by diffusional flow: Diffusional accommodation: Using diffusional creep mechanisms, the material can diffuse along grain boundaries or through grains to allow for compatibility.Elastic distortion: When the sliding distance is small, the grains can deform elastically (and sometimes recoverably) to allow for compatibility.Dislocation movement: Dislocations can move through the material by processes such as climb and glide to allow for compatibility.Various accommodation mechanisms have been proposed to account for this issue. When polycrystalline grains slide relative to each other, there must be simultaneous mechanisms that allow for this sliding to occur without the overlapping of grains (which would be physically impossible). There will not be an increase in number of grains along the direction of applied stress. 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. The sliding motion is accommodated by the diffusion of vacancies from induced stresses and the grain shape changes during the process. Lifshitz sliding only occurs with Nabarro–Herring and Coble creep. 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. The internal stress will build up as grains slide until the stress balances out with the external applied stress. Rachinger sliding is purely elastic the grains retain most of their original shape. 10 Experimental Study: Superplastic Forming Technique via GBS.9 Modeling effects of GBS in high strength steel.5 Estimating the contribution of GBS to the overall Strain. 4 Deformation rate from grain boundary sliding.This mechanism is the primary cause of ceramic failure at high temperatures due to the formation of glassy phases at their grain boundaries. It has been shown that Lifshitz grain boundary sliding contributes about 50-60% of strain in Nabarro–Herring diffusion creep. Grain boundary sliding contributes a significant amount of strain, especially for fine grain materials and high temperatures. 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. Additionally, when λ/h ratios are high, it may impede diffusional flow, therefore diffusional voids may form, which leads to fracture in creep. On the other hand, it will be controlled by grain boundary diffusion (Coble Creep). At high λ and high homologous temperatures, grain boundary sliding is controlled by lattice diffusion (Nabarro-Herring mechanism). Steady-state creep rate increases with rising λ/h ratios. We can simulate this type of boundary with a sinusoidal curve, with amplitude h and wavelength λ. During high temperature creep, wavy grain boundaries are often observed. Therefore it is not surprising that Nabarro Herring and Coble creep is dependent on grain boundary sliding. Keep in mind that at high temperatures, many processes are underway, and grain boundary sliding is only one of the processes happening. Grain boundary sliding is a motion to prevent intergranular cracks from forming. Boundary shape often determines the rate and extent of grain boundary sliding. Grain boundary sliding usually occurs as a combination of both types of sliding. There are mainly two types of grain boundary sliding: Rachinger sliding, and Lifshitz sliding. Homologous temperature describes the operating temperature relative to the melting temperature of the material. This occurs in polycrystalline material under external stress at high homologous temperature (above ~0.4 ) and low strain rate and is intertwined with creep. Grain boundary sliding (GBS) is a material deformation mechanism where grains slide against each other.
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