Creep is a diffusion controlled mechanism where grain boundaries slide past each other.
- Creep is dependant on time, temperature and the load/force applied. If all these factors are increased then creep will be more severe.
- Creep also accelerates fatigue.
- To design against creep, limit the number of grain boundaries.
It is possible to see from the large poly-crystal that the separate crystals can move when force is applied: it is like wobbly teeth.
Compare the creep process for a single crystal with one grain and a poly-crystal with lots of grains and thus grain boundaries. There is no slip/creep in the single crystal whereas you can feel and see the slip in the large poly-crystal.