Structure of cross-slip bands and brittle-to-ductile transition in ionic crystals

The causal relation between cross-slip structure and brittle-ro-ductile tra nsition has been examined using NaCl single crystals with the [100] axis. A r around room temperature, the crystals exhibit cleavage fracture after a f ew percent elongation, although their yield stress is lower than 2 MPa. Whe n the temperature is raised, brittle-to-ductile transition takes place at a round 400 and 500 K with the respective strain rates of 5.5 x 10(-6) and 5. 5 x 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 degrees similar to 40 degrees from the slip line of the primary plan e (10(1) over bar) on the (001) surface. Even in high magnification images observed using an UHV-AFM, cross-slip lines appear to be wavy, indicating t he cross slip on indefinite planes. Dislocations emitted from a crack tip c an extend sideways along a moving crack front by the cross slip on indefini te planes, and reduce local stress intensities to impede the crack advance by crack tip shielding due to dislocations.