Diffusion Creep - Flow Laws

Flow Laws

Each plastic deformation of a material can be described by a formula in which the strain rate depends on the differential stress (σ or σ_D), the grain size (d) and an activation value in the form of an Arrhenius equation:

In which A is the constant of diffusion, Q the activation energy of the mechanism, R the gas constant and T the absolute temperature (in kelvins). 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 sensitive to grain size of a material: materials with larger grains can deform less easily by Coble creep than materials with small grains.