The Nabarro–Herring creep rate, , can be derived by considering an individual rectangular grain (in a single or polycrystal).[1] Two opposing sides have a compressive stress applied and the other two have a tensile stress applied. The atomic volume is decreased by compression and increased by tension. Under this change, the activation energy to form a vacancy is altered by . The atomic volume is and the stress is . The plus and minus indication is an increase or decrease in the activation energy due to the tensile and compressive stresses, respectively. The fraction of vacancy concentrations in the compressive () and tensile () regions are given as:
- ,
In these equations is the vacancy formation energy, is the Boltzmann constant, and is the absolute temperature. These vacancy concentrations are maintained at the lateral and horizontal surfaces in the grain. These net concentrations drive vacancies to the compressive regions from the tensile ones which causes grain elongation in one dimension and grain compression in the other. This is creep deformation caused by a flux of vacancy motion.
The vacancy flux, , associated with this motion is given by:
where is the vacancy diffusivity. This is given as:
where is the diffusivity when 0 vacancies are present and is the vacancy motion energy. The term is the vacancy concentration gradient. The term is proportional to the grain size and . If we multiply by we obtain:
where is the volume changed per unit time during creep deformation. The change in volume can be related to the change in length along the tensile axis as . Using the relationship between and the NH creep rate is given by:
This equation can be greatly simplified. The lattice self-diffusion coefficient is given by:
As previously stated, NH creep occurs at low stresses and high temperatures. In this range . For small , . Thus we can re-write as:
where is a constant that absorbs the approximations in the derivation.
Alternatively, this can be derived in a different method where the constant has different dimensions. In this case, the NH creep rate is given by:[5]
Coble creep is closely related to Nabarro–Herring creep and is controlled by diffusion as well. Unlike Nabarro–Herring creep, mass transport occurs by diffusion along the surface of single crystals or the grain boundaries in a polycrystal.[1] For a general expression of creep rate, the comparison between Nabarro–Herring and Coble creep can be presented as follows:[6]
More information Mechanism, favorable conditions ...
Mechanism | favorable conditions | Description | A | n | p |
Nabarro–Herring creep |
High temperature, low stress and small grain size |
Vacancy diffusion through the crystal lattice |
10–15 |
1 |
2 |
Coble creep |
Low stress, fine grain sizes and temperature less than those for which NH creep dominates |
Vacancy diffusion along grain boundaries |
30–50 |
1 |
3 |
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G is the shear modulus. The diffusivity is obtained form the tracer diffusivity, . The dimensionless constant depends intensively on the geometry of grains. The parameters , and are dependent on creep mechanisms. Nabbaro–Herring creep does not involve the motion of dislocations. It predominates over high-temperature dislocation-dependent mechanisms only at low stresses, and then only for fine-grained materials. 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] Lower temperatures favor Coble creep and higher temperatures favor Nabbaro–Herring creep because the activation energy for vacancy diffusion within the lattice is typically larger than that along the grain boundaries, thus lattice diffusion slows down relative to grain boundary diffusion with decreasing temperature.[2]