Structure of Cross-Slip Bands and Brittle-to-Ductile Transition in Ionic Crystals

The causal relation between cross-slip structure and brittle-to-ductile transition has been examined using NaCl single crystals with the [100] axis. At around room temperature, the crystals exhibit cleavage fracture after a few percent elongation, although their yield stress is lower than 2 MPa. When the temperature is raised, brittle-to-ductile transition takes place at around 400 and 500 K with the respective strain rates of 5.5×10−6 and 5.5×10−5 s−1. The transition temperatures correspond well to those for the abundant operation of cross slip. The cross-slip lines are not parallel to a specific crystallographic plane, but are widely distributed in angles of 10°∼40° from the slip line of the primary plane (10\1) on the (001) surface. Even in high magnification images observed using an UHV-AFM, cross-slip lines appear to be wavy, indicating the cross slip on indefinite planes. Dislocations emitted from a crack tip can extend sideways along a moving crack front by the cross slip on indefinite planes, and reduce local stress intensities to impede the crack advance by crack tip shielding due to dislocations.