Different microscopic processes can control creep in different ranges of stress, temperature and grain size.

Dislocation creep

At moderate-to-high stress, dislocations can continue to move by glide and by thermally activated climb, in which atoms or vacancies diffuse so that a dislocation moves out of its original slip plane. The creep rate is then strongly stress dependent.

Diffusional creep

At lower stresses and high homologous temperature, atoms can diffuse from compressed regions of a grain toward tensile regions, allowing the grain to change shape without extensive dislocation motion. Bulk-lattice diffusion and grain-boundary diffusion produce different grain-size dependences.

Grain-boundary sliding

Neighboring grains can also slide relative to one another. Accommodation by diffusion or dislocation motion is needed to prevent gaps and overlaps, so mechanisms interact rather than acting in complete isolation.

A deformation-mechanism map plots normalized stress against homologous temperature $T/T_m$ and indicates which mechanism is expected to dominate. Grain size is another important parameter.

These maps explain why one alloy may be strong at room temperature yet creep rapidly at elevated temperature, and why changing grain size can improve one regime while worsening another. Creep-resistant microstructures are designed to obstruct the rate-controlling mechanism while remaining stable during long exposure.