Deformation-Mechanism Maps; Creep

[Astronuc]

Deformation-Mechanism Maps, The Plasticity and Creep of Metals and Ceramics, by Harold J Frost, Dartmouth College, USA, and Michael F Ashby, Cambridge University, UK.

Creep is a slow plastic flow of a material, and plastic flow is a kinetic process.

In general, the strength of the solid depends on both strain and strain-rate, and on temperature. It is determined by the kinetics of the processes occurring on the atomic scale: the glide-motion of dislocation lines; their coupled glide and climb; the diffusive flow of individual atoms; the relative displacement of grains by grain boundary sliding (involving diffusion and defect-motion in the boundaries); mechanical twinning (by the motion of twinning dislocations) and so forth. These are the underlying atomistic processes which cause flow. But it is more convenient to describe polycrystal plasticity in terms of the mechanisms to which the atomistic processes contribute. We therefore consider the following deformation mechanisms, divided into five groups.

Collapse at the ideal strength —(flow when the ideal shear strength is exceeded).
Low-temperature plasticity by dislocation glide—(a) limited by a lattice resistance (or Peierls' stress); (b) limited by discrete obstacles; (c) limited by phonon or other drags; and (d) influenced by adiabatic heating.
Low-temperature plasticity by twinning.
Power-law creep by dislocation glide, or glide-plus-climb —(a) limited by glide processes; (b) limited by lattice-diffusion controlled climb (“high-temperature creep”); (c) limited by core*diffusion controlled climb (“low-temperature creep”); (d) power-law breakdown, (the transition from climb-plus-glide to glide alone); (e) Harper-Dorn creep; (f) creep accompanied by dynamic recrystallization.
Diffusional Flow—(a) limited by lattice diffusion (“Nabarro-Herring creep”); (b) limited by grain boundary diffusion (“Coble creep”); and (c) interface-reaction controlled diffusional flow.

http://engineering.dartmouth.edu/defmech/

I'll elaborate later, but others are certainly welcome and encouraged to contribute.

 

[Mapes]

One of my thesis committee members, an outstanding metallurgist, taught me: "There is no such thing as elasticity, only negligible plasticity." Creep occurs in all materials at all temperatures; nothing is immune. Of course, the deformation is generally undetectable at less than a considerable fraction -- perhaps one-third or one-half -- of the absolute melting temperature.

 

[Astronuc]

The text does make such a point - "Although it is often convenient to think of a polycrystalline solid as having a well defined yield strength, below which it does not flow and above which flow is rapid, this is true only at absolute zero."