Effects of Pressure on Plastic Deformation of Polycrystalline Solids: Some Geological Applications
- PDF / 2,476,780 Bytes
- 12 Pages / 414.72 x 648 pts Page_size
- 84 Downloads / 231 Views
mechanism may not necessarily be generalized to other mechanisms. Therefore it is critical to identify the mechanisms of deformation in any experimental studies. There are a large number of deformation mechanisms that may operate in any polycrystalline solids [1]. Among them I will discuss the following three mechanisms for which we have some important results on the effects of pressure (or phase transformations). They include: (i) the Peierls mechanism, (ii) dislocation creep (power law creep) and (iii) diffusion creep (or superplasticity). These mechanisms operate under a wide range of physical conditions [1] and hence may play important roles in various geological processes in the deep interior of terrestrial planets [2]. Since most of experimental studies involve technical developments, I will first review some important experimental techniques so far developed with discussions on their limitations. EXPERIMENTAL TECHNIQUES OF HIGH PRESSURE DEFORMATION A range of apparatus has been used to investigate plastic properties of solids under high pressures (and high temperatures). A detailed review of these apparatus that have been widely used in Earth science community was provided in [3]. They include a gas-medium high P (pressure)-T (temperature) deformation apparatus (the Paterson apparatus) and a solid-medium, piston-cylinder type deformation apparatus (the Griggs apparatus). However, the maximum pressure under which these apparatus work is less than - 4 GPa and such an effect as the effect of pressure-induced phase transformations in typical silicate minerals (e.g., (Mg,Fe)2SiO4, (Mg,Fe)Si0 3 ) cannot be investigated with these apparatus. Since mid-70's, various attempts have been made to extend the pressure range beyond such a limit. Here I will summarize these techniques. Diamond Anvil Cell Kinsland and Bassett [4] and Sung et al. [5] were the among the first to use a diamond anvil cell (DAC) as a deformation apparatus. Meade and Jeanloz [6, 7, 8] applied this technique to investigate the creep strength of some ionic solids including silicate minerals. Poirier et al. [9] used a similar technique to investigate the plastic flow of ice, where they replaced a diamond with a sapphire to increase the sample size. Since a high pressure is generated by uniaxial motion of two pistons, non-hydrostatic stress is readily created, if a sample is in direct contact with pistons. This causes a radial flow of sample, which is prevented by the strength of a sample and by the strength of a gasket. When the contribution from the gasket strength can be neglected, the pressure gradients in a sample space measured by the lateral variation in ruby fluorescence peak shifts can be used to infer the strength of samples, viz., dP dr
2a( h
where a is the "strength" of a sample, r is radial distance from the center of the sample and h is the sample thickness. Poirier et al. [9] extended this technique by measuring sample strain by monitoring the positions of fine particles during experiments. Thus, this technique provides a measure of t
Data Loading...
