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Coble creep is the second form of diffusion-controlled creep. In Coble creep the atoms diffuse along grain boundaries to elongate the grains along the stress axis. This causes Coble creep to have a stronger grain size dependence than Nabarro–Herring creep, thus, Coble creep will be more important in materials composed of very fine grains.
Grain boundary sliding (GBS) is a material deformation mechanism where grains slide against each other. 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.
They are also important to many of the mechanisms of creep. [2] On the other hand, grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve mechanical strength, as described by the Hall–Petch relationship.
Nabarro–herring creep, or volume diffusion, acts at high homologous temperatures and is grain size dependent with the strain-rate inversely proportional to the square of the grain size (creep rate decreases as the grain size increases). During Nabarro-Herring creep, the diffusion of vacancies occurs through the crystal lattice (microtectonics ...
The exponents n and m are values for the sensitivity of the flow to stress and grain size respectively. The values of A, Q, n and m are different for each deformation mechanism. For diffusion creep, the value of n is usually around 1. The value for m can vary between 2 (Nabarro-Herring creep) and 3 (Coble creep). That means Coble creep is more ...
Because of this, Nabarro–Herring creep does not have a dependence on grain boundary thickness, and has a weaker dependence on grain size . In Nabarro–Herring creep, the strain rate is proportional to as opposed to the dependence for Coble creep. When considering the net diffusional creep rate, the sum of both diffusional rates is vital as ...
The small size of individual metal grains provides high interfacial surface energy which is what prompts coarsening, the increase in grain size, and eventually metallic softening. [4] Nanocrystalline creep is considered to follow the Coble creep mechanism, the diffusion of atoms along grain boundaries at low stress levels and high temperatures ...
Nabarro–Herring creep is characterized by creep rates that increase linearly with the stress and inversely with the square of grain diameter. In contrast, in Coble creep atoms diffuse along grain boundaries and the creep rate varies inversely with the cube of the grain size. [2]