Experiments carried out online at NSLS (FOR21) shows that during pressurisation at room temperature large deviatoric stresses (of the order of 1.5-2 GPa) are applied to the sample. As the pressure cell is the same for all of our experiments, it is likely that all samples experienced this cold compression deformation. Despite an isotropic compression, differential stresses occur in response of the marked mechanical anisotropy of the pressure cell which contains alumina pistons. The crushable alumina plugs are obviously not sufficient to avoid this phenomenon. The length of the crushable alumina plugs might be increased to minimize differential stresses during compression. Indeed, multianvil experiments (S2964, S2970 and S3024) have shown that a large density of defects is introduced in forsterite
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. 155 .
at this stage. Unfortunately, no TEM has been performed on samples just cold compressed in the D-DIA for comparison. The D-DIA experiments give us the possibility to anneal the sample prior to deformation to eliminate these defects. In experiments performed off-line in Bayreuth (DD12 and DD13) the specimens were annealed two hours at 1300°C before deformation began. Multianvil experiments have shown that the dislocation density decreases very rapidly during annealing at 1400°C (see Figure III.9b). In the present case a longer time at slightly lower temperature was chosen in order to minimize grain growth. Experiment DD14 showed that with starting material P0332, the grain size increased from 10 to ca. 60 µm during this annealing stage. TEM characterisations of sample recovered after this heating stage (DD10) confirms that the microstructure is recovered. Most grains are free of defects. Some grains still exhibit some dislocations but no subgrain boundaries have been observed. The differential stresses decrease very rapidly once the specimen is heated (experiment FOR21, Figure III.20) to reach about 500 MPa. This drop is probably due to the relaxation of elastic strain stored during pressurisation.
The deformation stage starts when the oil pressure in differential rams overcome friction forces in the main ram. Strain and strain rate can be deduced either from transducer measurements in off-line experiments or from in situ radiograph images of the sample. Strain rate is easily adjusted by driving differential rams with stepping motors. A few minutes are generally needed to find the appropriate motor speed for a given strain rate.
A few minutes are then necessary to reach a steady state regime. During deformation, the transducers placed between the guide blocks and the top or bottom anvils are crucial especially for off-line experiments. They indicate
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. 156 .
precisely at which moment the anvils move and thus at which moment the deformation starts. Even following sample length with radiograph images, this starting point is very difficult to determine with accuracy because it corresponds to a very small shortening of the sample (about 2 µm in 60s for a strain rate of 10-5s-1).
During deformation, the flow stress remains constant at a level of 500 MPa for a strain rate of 10-5s-1. The loading pressure in the main ramp is maintained automatically constant by a slight retraction of side anvils. The nominal pressure (and volume) of the sample is thus kept constant during deformation as seen in Figure III.20. Deformation continues as long as the differential rams move. In the present study the maximal total strain reached is about 25 %. Once the desired strain is reached, the deformation is stopped and the sample is immediately quenched.
Decompression is another important step. Both differential rams and the main ram have to be unloaded in such a way that the sample is not deformed at this stage. This is checked either following the sample length in situ by radiography and/or by checking that the transducers do not indicate any displacement of the independent anvils.
The cell developed for this study can be operated at 1300-1400°C over long periods with a very stable graphite furnace (no evolution of the input power). Up to now, no temperature distribution map has been determined on D-DIA sample, although it is an important parameter especially for establishing flow law. The maximal pressure that can be reached depends of the anvils and on the cell materials. Tungsten carbide anvils are usually chosen for their high stress resistance and their easy
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. 157 .
manufacturing. However, tungsten has a high atomic number and hence absorbs very much X-rays. These anvils are not very suitable for in situ measurements, especially for imaging the sample. Cubic boron nitride might be preferred for this application despite its lower strength. During this study, the maximal pressure reached with a pyrophyllite cubic cell was 7 GPa with WC anvils and 5 GPa with cBN anvils. The pressure efficiency of the cell can be enhanced using mullite or boron-epoxy instead of pyrophyllite as a pressure medium (Li Li, personal communication). The maximal nominal pressure might be slightly increased using sintered diamond anvils (Shimomura et al., 1992) or using tapered anvils (Wang et al., 1998). Price and manufacturing of anvils can be however considered as limiting parameters. Finally, we have observed in our experiments that the Pyrex sleeve behaves as a barrier against migration of water from the cell materials to the sample. Indeed, the annealing experiment DD14 performed from dehydrated sample (P0332: 369 H/106Si) shows a slightly smaller hydroxyl content of 347 H/106Si after two hours at 1300°C and 2 GPa.
D-DIA experiments can provide highly strained samples for which CPO can be measured by EBSD. Given the geometry of the experiment, these CPO are characteristic of compressed samples (DD12, DD31 and FOR21). These kinds of CPO are well adapted to identify the glide plane.
The normal to the slip plane tends to concentrate toward the shortening axis (e.g. Turner, 1948). However, the slip direction is not as easily determined as in CPO obtained in shear configuration.
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. 158 .