Fatigue of single crystalline silicon: Mechanical behaviour and TEM observations

We report on tension–compression cyclic loading of single-slip oriented silicon single crystals between 1073 and 1173 K. In the temperature and strain rate domains investigated, dislocation glide is still controlled by lattice friction. Along cycling, a maximum stress is attained through two hardening stages, a logarithmic, followed by a linear one. At variance to what is observed in metals (fcc and bcc), the maximum stress decreases with increasing strain amplitude. This effect may be a consequence of the combined strain localization and the strain-rate sensitivity of Si. The built-up dislocation structures, observed in transmission electron microscopy (TEM) show three basic types of arrangements. During the linear hardening, dislocations dipoles gather in corrugated walls, separated by zones of lower density. Once the maximum stress is attained, strain localizes in walls and channels that retain the initial chevron shape, but in which the walls are much thinner. Well-characterized “ladder-structure” currently observed in persistent slip-bands are here very rare. Characteristic lengths of the observed patterns are given and briefly discussed in the light of current theories of cyclic deformation. For this purpose, convergent-beam electron diffraction patterns were taken across a channel to account for potential internal-stress effects.