Effect of Hydrostatic Pressure on Dislocation Mobility in Lithium Fluoride

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Abstract
The effects of hydrostatic pressures up to 13 kbar on the stress dependence of the velocity of screw dislocations in single crystals of lithium fluoride at room temperature have been determined. The shear stresses to move the dislocations were introduced by means of a three-point bending device incorporating a load-cell and operated entirely within a fluid medium at constant pressure and temperature. The range of velocities observed was from 10−5 to 10−2 cm/sec. Both high-purity and impurity-doped crystals were studied. At all the pressures examined, the dislocation velocity was found to vary with the applied shear stress τ in accordance with the empirical relationship v = v*·e−A/τ (where v* is a terminal velocity and A is a drag stress) suggested by Gilman for one-atmosphere behavior. However, the stress required to move a dislocation at a given velocity in increases rapidly with pressure, by a factor of approximately 3 at 10 kbar. Correspondingly, the dislocation velocity at constant applied shear stress decreases by several orders of magnitude. Based on the magnitudes of the activation volume measured for the high-purity and doped crystals, the effects are interpreted in terms o the mobility of jogs as the mechanism controlling dislocation velocity at pressure. The measured decrease in dislocation mobility has been used to compute the expected change in stress—strain behavior in LiF as a function of pressure.