Dislocation Velocities, Dislocation Densities, and Plastic Flow in Lithium Fluoride Crystals

Velocities of individual dislocations have been measured in LiF, covering a range of twelve orders of magnitude in velocity, from 10−7 cm/sec to 105 cm/sec. The velocity is extremely sensitive to applied stress at low velocities, and for each crystal there exists a minimum stress for dislocation motion, below which dislocations do not move. The edge components of dislocation loops move considerably faster than the screw components. The upper limit for dislocation velocity appears to be the velocity of sound in the crystal. The effects of temperature, impurities, and radiation damage on dislocation velocity are described. These variables affect the dynamic resistance to motion encountered by a moving glide dislocation. The growth of total dislocation density, the growth of individual glide bands, and the distribution of glide dislocations during plastic deformation are described. The yield stress of LiF is determined by the resistance to motion encountered by a glide dislocation in moving through an otherwise dislocation-free region of the crystal. The yield stress is independent of the dislocations present in an undeformed crystal, and the state of pinning and geometrical arrangement of such dislocations do not affect the yield stress. Stress-strain curves have been calculated from the data on dislocation mobility and dislocation density, and the calculated and measured curves are compared. At low strains the flow stress can be predicted from measured dislocation properties.