Plastic Deformation of Alkali Halide Crystals at High Pressure: Work-Hardening Effects

Abstract

The influence of hydrostatic pressure on the plastic deformation of alkali halide single crystals at large strain has been observed. The rates of work hardening in stages I and II are approximately independent of pressure in LiF, NaCl, and KCl, but both are doubled at 4.3 kbar in KI. In stage II the fractional increase of plastic flow stress at high pressure in an interrupted compression test, δσ/σ, is accounted for by the pressure-induced increase in elastic constants in all the alkali halides examined. Evidently, the strength in stage II is controlled by elastic interactions. The stress at the onset of stage III, σIII, in the deformation of NaCl at 4.3 kbar is about half that at 1 atm; it is reduced slightly in KCl and is unchanged in KI. Deformation proceeds at lower stresses in stage III in NaCl and KCl under 4.3 kbar, and if compression at 1 atm in this stage is interrupted and reinitiated at 4.3 kbar, pronounced work softening is observed. The dependence of flow stress on pressure in stage III at constant structure, (δσ/σ)s, is small and less than that expected from the pressure-induced change of elastic constants. The negative pressure dependence of σIII and the small (δσ/σ)s in stage III may be explained in terms of thermally activated cross slip of screw dislocations when stacking faults on the primary slip plane cause a strong local dilation of the lattice.