Dislocation creep

Plastic deformation can occur by a collective motion of atoms as crystal dislocations. Evidence for dislocation creep in Earth is abundant although other mechanisms such as diffusional creep (see Chapter 8) dominate under some conditions. The rate of deformation due to dislocation motion is proportional to dislocation density and velocity (the Orowan equation). In most cases dislocation density increases with applied stress and dislocation velocity also increases with stress leading to a non-linear relationship between stress and strain rate. However, steady-state dislocation density is achieved only after a certain time or strain and therefore a significant period of transient creep is often observed in dislocation creep. The dislocation velocity for its glide motion is controlled by a variety of resistance forces including the intrinsic resistance caused by the crystal lattice (the Peierls stress, reorganization of dissociated (partial) dislocations), the interaction with impurity (solute) atoms, and mutual interaction. Dislocation motion out of its glide plane (climb) is controlled by the diffusion of atoms. In both glide and climb, the motion of dislocation often occurs in a step-wise manner through the motion of kinks and jogs respectively whose density is in most cases controlled by the thermochemical equilibrium. Consequently, the velocity of dislocations is often sensitive to thermochemical environment such as oxygen and water fugacity. Creep due to dislocation motion involves a number of processes many of which must occur sequentially. As a result, the slowest of these processes usually controls the overall rate of deformation. […]