Diffusional flow is a regime typically below dislocation creep and occurs at high temperatures due to the diffusion of point defects in the material.  Diffusional flow can be further broken down into more specific mechanisms: Nabarro–Herring creep, Coble creep, and Harper–Dorn creep.
While most materials will exhibit Nabarro-Herring creep and Coble creep, Harper-Dorn creep is quite rare, having only been reported in a select few materials at low stresses including aluminium, lead, and tin.
The equation for Nabarro-Herring creep is dominated by vacancy diffusion within the lattice, whereas Coble creep is dominated by vacancy diffusion within the grain boundaries. The equation for these mechanisms is shown below where   is the applied shear stress, Ω is the atomic volume, k is the Boltzmann constant ,d is the grain size, T is the temperature, and  is the effective diffusion coefficient.
The effective diffusion coefficient, =  (the volumetric diffusion constant) for Nabarro-Herring creep which dominates at high temperatures, and  (where  is the grain boundary width and  is the diffusion coefficient in the boundary) for Coble creep which dominates at low-temperatures.
From these equations it becomes clear that the boundary between boundary diffusion and lattice diffusion is heavily dependent on grain size. For systems with larger grains, the Nabarro-Herring lattice diffusion region of the deformation mechanism map will be larger than in maps with very small grains. Additionally, the larger the grains, the less diffusional creep and thus the power-law creep region of the map will be larger for large grained materials. Grain boundary engineering is thus an effective strategy to manipulate creep rates.