Dynamic recovery during compression testing of monocrystalline NaCl at elevated temperatures

Abstract

Single crystals of NaCl in a 〈100〉 orientation were tested under compression in the constant-strain-rate mode at 873 and 973 K. The evolution of the dislocation microstructure was studied at both temperatures over the entire range of strains (up to 0.2) using standard etch-pitting techniques. At 873 K the flow stress σ slowly increases monotonically with strain even though the free dislocation density ϱ (the dislocations within subgrains) passes through a maximum at a plastic strain of about 0.02. At 973 K, ϱ behaves similarly, although nearly steady state flow is eventually observed; in this stage the substructure as well as the flow stress remain invariant with further straining. It is argued that the initial increase in ϱ is comparable with that which occurs on the loading of a sample during a creep test. However, since the loading rate during a constant-strain-rate test is typically much slower than it is during a creep test, the increase in ϱ at small strains is easier to follow. On establishment of the maximum dislocation density compatible with the (approximate) steady state flow stress at elevated temperatures, recovery produces coarsening of the dislocation network. The attendant reduction in ϱ at nearly constant σ is then entirely analogous to the decrease in ϱ that occurs during primary creep in conventional creep tests.