Published online by Cambridge University Press: 10 February 2011
Experimental studies on the effects of pressure on plastic properties are reviewed with an emphasis on those on geological materials. High pressures may affect plastic properties through two different ways: (i) through the effects of pressure on plastic deformation of a material with a given crystal structure and (ii) through the effects of pressure-induced phase transformations on plastic deformation. It is emphasized that the manner in which pressure affects plastic properties of solids depends on the microscopic mechanisms of plastic deformation and therefore it is essential to identify the mechanisms of deformation involved in a particular experimental condition. Three mechanisms are considered: (i) the Peierls mechanism, (ii) dislocation creep (power-law creep) and (iii) diffusion creep (or superplasticity). Creep strength of materials changes with pressure through the change in shear modulus and in the activation enthalpy for the Peierls mechanism. Structural phase transformations can dramatically change the creep strength determined by the Peierls mechanism. Changes can be either positive (increase in strength in a high pressure phase) or negative (decrease in strength in a high pressure phase). Creep strength controlled by dislocation creep usually increases exponentially with pressure through the increase in the activation enthalpy with pressure. Phase transformations can change the creep strength either positively or negatively for this mechanism depending on the manner in which dislocation mobility changes with crystal structure. Creep strength corresponding to diffusion creep changes with pressure in a similar fashion as dislocation creep.
High pressures in the deep interior of Earth and other terrestrial planets could lead to a significant increase in viscosity with depth, resulting in high temperatures due to the feedback between viscosity and temperature. Also phase transformations in constituent minerals that occur in certain depth ranges can cause dramatic changes in plastic properties. Both of them are likely to have important influence on the style of convection in the mantle of terrestrial planets and thus affect their geological history and tectonic activities.