Are there any known or theoretical substances that don't experience creep? Creep is very slow elasticity that most materials have, that cause pipes to sag over decades.  Are there any materials, in theory or in practice, that are creepless?
 A: Creep in many engineering materials requires dislocation travel. Since dislocation travel is assisted by diffusion, and since diffusion processes in metals kick in exponentially at temperatures above about ~1/2 the melt temperature (in degrees absolute), creep can be prevented by choosing alloys with the highest possible melting points, called superalloys (typically iron with lots of chromium, nickel, vanadium, etc.) and by not exposing the material to high temperatures.
This also means that creep can be inhibited by choosing a material that has no dislocations in it. Since grain boundaries create dislocations, a material that has no grain boundaries will be creep resistant. Single-crystal, directionally-cast gas turbine blades are made this way to prevent creep.
Creep is also caused by progressive dislocation pileup at grain boundaries which allows the grains themselves temporarily to decouple themselves and rotate slightly in response to creep stress. This can be inhibited in iron-based alloys by adding in certain alloying constituents, called carbide formers, which selectively segregate out at grain boundaries to stabilize them.
Even in the presence of active dislocations, creep can be prevented by "pinning" the dislocations so they cannot move much. This is done by adding materials to the metal which precipitate out of solid solution upon cooling from the melt or during heat-treatment as nanoscale particles within the crystalline matrix of the metal (precipitation hardening).
You can also prevent creep in polycrystalline materials by choosing one that has no dislocation movement mechanisms in it that are active at the temperature of interest. This means ceramic materials will be very resistant to creep even at relatively high temperatures.
