Dislocation multiplication in silicon at the onset of plasticity observed by in situ synchrotron x-ray topography

In situ observations by synchrotron x-ray topography were performed on init ially dislocation-free silicon single crystals deformed in creep conditions at temperatures between 975 K and 1075 K, and tensile stresses equal to 22 MPa or 44 MPa, in order to study the multiplication of dislocations during the very first stages of plastic deformation. It could be seen that the first dislocations, created at Vickers micro-inde nts or at residual surface damage, did not develop in a strictly planar way : prismatic half loops, gliding simultaneously on two {111} planes. were co mmonly observed. Cross-slip events appeared to be quite frequent, in the bu lk as well as at free surfaces. Groups of similar dislocations soon exhibit ed irregular shapes with cusps and trailing dipoles, which was taken as an indication that they developed jogs during their motion. Several configurat ions of dislocation sources formed by the mobile dislocations were observed and are described in detail. The formation of new sources usually involved cross-slip. The role of jogs formed by forest-cutting seems important. The efficiency of sources was strongly limited by the lack of stability of the cross-slipped segments, which acted as poles for the Frank-Read mechanism.