Plastic deformation and strain hardening in pure Nacl at low temperatures

The plastic deformation and strain hardening of NaCl crystals and f.c.c. crystals have much in common. The easy glide hardening rate for NaCl can be as low as that observed in copper. Random slip on the conjugate system can increase the hardening rate and cut short the easy glide extension. In Stage II of deformation, such secondary slip produces impenetrable obstacles to dislocations on the primary system in the form of long, narrow, and thin dislocation bands. With increasing strain, the primary dislocations, obstructed by the secondary dislocation bands, stress the latter, first in compression then in tension, as can be followed by the photoeleastic effect. The internal stresses of such birefringent secondary slip bands do not directly affect the now stress. The impenetrable bands, acting similarly to non-deformable fibres in a ductile matrix, govern the flow stress through the dimensions of the inter-band regions. Hardening occurs when the long-range stresses due to deformation incompatibilities are relieved by the formation of more secondary slip bands, converting such back-stresses to long-range stress-free slip obstacles. The estimated hardening rate compares favourably with measured hardening rates.