Due to the thermodynamics of vacancy formation there are very few vacancies
present at low temperatures and hence diffusion is also negligible.

As the temperature is increased, the formation of vacancies becomes more favourable,
and so at higher temperatures more vacancies are present in the crystal structure

When σ is zero, there is no preferential direction in which the atoms will diffuse,
therefore the grain does not deform

As the σ is increased the driving force for diffusion increases towards the top of the grain
and so the rate of deformation increases

σ/σ_y

T/T_m