Les dislocations dans la forsté rite déformée à haute Tempé rature

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Abstract
Dislocations in forsterite deformed at a high temperature Creep experiments have been performed at high temperatures on forsterite single crystals at moderate stress (8–100 MPa, 1400–1650°C). Samples have been deformed by compression in three simple orientations. Dislocation microstructures are reported here for these three groups of samples. Previous observations were made using a very efficient decoration technique. TEM results confirm the reliability of this technique. Moreover they give a complete identification of Burgers vectors and activated slip systems. For the first orientation, most of the dislocations are ‘a’ edges in glide bands. Tilt walls are also present. The ‘a’ edges are not very straight. For the third orientation, most of the dislocations are ‘c’ screws. Twist walls are numerous. The ‘c’ screws are very straight. The second orientation is intermediate between the first and the third ones. It shows the two types of microstructures in highly variable proportions. These facts shed some light on the somewhat contradictory results on diffusion and creep in forsterite. It seems that the creep mechanism is climb-controlled glide in the first case (‘a’ edges and tilt walls) and glide of screws, controlled by lattice friction or cross slip in the third case (‘e’ screws and twist walls). This model explains very well why the numerous works performed with orientations intermediate between the first and the third one have led to large confusion: both mechanisms are comparable in such situations. The consequences of this interpretation are very important as far as the extrapolation from single crystals to aggregates is concerned. The orientation of an average grain is necessarily an ‘intermediate’ one in the case of the aggregate. Therefore, creep should be climb controlled at low stress (dominant slip direction ‘a’) and controlled by ‘c’ screws at high stress or very high temperature.