Meaning
Plastic deformation of crystalline materials occurring at low stresses and high temperatures is governed by grain boundary diffusion. In fine-grained metals and ceramics, coble creep dominates the deformation behavior under sustained mechanical loads at elevated thermal levels. The atomic flow occurs along the interfaces between crystal grains rather than through the bulk lattice.
This movement leads to gradual elongation of the grains in the direction of the applied tensile stress.
Physical Mechanism
Grain boundaries present paths of lower activation energy for atomic diffusion than the interior of a grain. When a polycrystalline material is subjected to stress at high temperatures, vacancies migrate along these boundaries from regions of tension to regions of compression. Atoms move in the opposite direction, which results in the gradual sliding of grains past one another.
Temperature Influence
Elevated temperatures accelerate the rate of atomic movement along the boundaries. Because coble creep is thermally activated, its deformation rate depends on boundary diffusion which increases with temperature.
Mechanical Failure
Engineering alloys exposed to prolonged stress in high-temperature environments eventually experience mechanical failure due to grain boundary cavitation. As coble creep progresses, small voids nucleate along the grain boundaries perpendicular to the applied tensile force. These voids coalesce over time to form microcracks that rapidly spread and lead to sudden structural fracture.
