Low-cycle fatigue in silicon: comparison with fcc metals

Undoped FZ silicon single crystals were cyclically loaded in tension-compression under plastic strain amplitude control in a temperature and strain rate domain where the glide of dislocations is still controlled by the lattice friction: 1073-1173 K and (1.5-6) x 10(-4)s(-1); the plastic strain amplitude being varied from 6 x 10(-4) to 10(-2). Cyclic hardening curves display logarithmic then linear hardening and pass through a maximum before the peak stress per cycle stabilizes. The maximum and stabilized stresses decrease when temperature increases. Microscopical observations suggest that strain localization takes place near the maximum cyclic stress and beyond. But, contrary to what is observed in fcc metals, the maximum or saturation stress decreases when the strain amplitude per cycle increases. Several types of dislocation arrangements, rather different from those found in copper, but looking more similar to Ni fatigued at low temperatures, were revealed by transmission electron microscopy. Before mechanical saturation, edge dislocation dipoles gather in thick stripes forming more or less corrugated walls, depending on the amplitude per cycle, when viewed normal to the primary slip plane. Once the maximum stress is reached, it seems that parts of the microstructure cease to participate in the imposed plastic strain, while others concentrate it. In active areas, thick walls condense into much thinner ones, forming chevrons when viewed normal to the slip plane. In areas which seem to be inactive, dipolar walls anneal out leaving a rather homogeneous distribution of prismatic loops. Thin PSBs with a well-characterized 'ladder-structure' are very rare. Characteristic lengths of the observed patterns are given and briefly discussed using current theories of cyclic deformation.