Creep deformation grain boundary sliding

Many ceramic materials also show creep deformation at elevated temperature. Among the several creep mechanisms studied or proposed for ceramic materials, sintered silicon nitride with metal oxide additives is considered to show creep with grain boundary sliding mechanism. At the final stage of the... 

Fine-grained materials, when subjected to high temperatures and low applied stresses, deform by mutual accommodation of grains assisted by grain boundary sliding and transport of matter (diffusion). Under conditions where lattice diffusion dominates, the diffusional creep rate is reasonably well characterized by the Nabarro-Herring creep process. (For a review of this and other classical creep mechanisms, see Refs. 5 and 6.) Here the strain rate is expressed as... 

Fig. 6.63 Seven grains are shown in a two-dimensional hexagonal array before creep deformation. Following diffusion, the grains elongate in one direction and decrease perpendicularly to the tensile axis. A void formation develops between the grains, but grain-boundary sliding, which may accompany this process, removes these voids...

Kerr and Chawla observed the microstructure of 96.5Sn-3.5Ag solder after creep testing using TEM techniques (Ref 71). Dislocations were observed to extend between AgjSn particles within the ternary eutectic regions. In the same study, the fracture surfaces of creep rupture samples (in shear) indicated that grain boundary sliding did not contribute significantly to creep deformation. 

In grain boundary sliding, the crystals are moving above each other. However, in a solidified metal the crystals fit in only one constellation. Thereby, the crystals move on complex 3-dimensional curves rotating around various axes. During this movement, the crystals deform either by diffusion similar to the diffusion creep or by dislocation glide due to local stress concentration. 

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