In diffusion creep, why does grain size dependence is different for N-H creep rate and Coble creep rate ?

For N-H creep, creep rate is inversely proportional to d2 , whereas for Coble creep, creep rate is inversely proportional to d3 . What can be the possible explanation for d2 or d3 ?

22 February 2019 1,836 4 View