Cyclic deformation of silicon single crystals: mechanical behaviour and dislocation arrangements

Fatigue testing of single crystalline Si was performed in a temperature and strain rate domain where lattice friction is still effective: 1073–1173 K and 3×10−4 s−1. Tension–compression loading was applied to samples oriented for single slip under plastic strain amplitude control. For plastic strain amplitudes ranging from 6×10−4 to 10−2, cyclic stress–strain curves exhibit two different stages of hardening and pass through a maximum before saturation is reached. At variance from what is observed in fcc metals, the maximum and saturation stresses decrease when the strain amplitude per cycle is increased. Surface observations suggest that strain localisation takes place near the maximum cyclic stress and beyond. TEM observations revealed several kinds of typical dislocation arrangements, which are rather different from those observed in copper. Before the maximum stress, thick rectilinear walls made of edge dislocation dipoles are the main feature. When observed perpendicularly, those walls form more or less corrugated arrangements. After the maximum stress has been reached, plastic strain seems to concentrate in much thinner walls, while in inactive areas, the dislocation structure anneals, converting the dipolar walls in rows of prismatic loops. Active areas are much broader than the persistent slip bands with the well-known ladder structure that are observed in copper.