Coarsening of the dislocation structure after stress reduction during creep of NaCl single crystals

Abstract

Abstract The dislocation structure during compressive creep of NaCl single crystals has been investigated by means of the etch pit technique. The steady-state subgrain size L is about 27 times the average distance of dislocations in the subgrain interior, (pi)−1/2, and scales inversely proportional to the compressive stress normalized by the shear modulus. ‘Nests’ of etch pits of relatively high density are found in small subgrain sections at a distance of ≍ 3 L and are interpreted as indicating dissolving subgrain boundaries. After a load reduction during creep at 923 K, corresponding to a decrease in stress from 1.20 to 0.50 MN/m2, L and pi −1/2 increase and reach their new steady-state values within 3 to 6% transient strain. The distribution of subgrain intercepts broadens during transient creep, indicating that subgrain boundary migration is involved in the coarsening process. There is no significant difference between the stress exponents of the steady-state creep rate determined from load reduction tests and from constant load tests.