Effect of confining pressure on dilatation, recrystallization, and flow of rock salt at 150°C

TLDR: In this paper, deformation experiments performed on natural rock salt at a constant strain rate of 3.5×10−7 s−1, 150°C, and confining pressures Pc of 3-30 MPa were conducted.

Abstract: Microstructural evidence for fluid-assisted dynamic recrystallization (FADRX) is widespread in naturally deformed rock salt. However, the principal experimental evidence for FADRX in salt has been obtained from stress relaxation experiments, and it is unclear whether the process occurs during steady state dislocation creep and what its effect might be. Here we report deformation experiments performed on natural rock salt at a constant strain rate of 3.5×10−7 s−1, 150°C, and confining pressures Pc of 3–30 MPa. Samples deformed at Pc = 3 MPa showed continuous work hardening and minor dilatation. Microstructurally, they exhibited intergranular cracking plus slip band and subgrain structures indicative of dislocation glide/creep. Electron backscatter diffraction analysis revealed a high frequency of boundaries with low-angle misorientations in the range 5°–10°. In contrast, samples deformed at Pc ≥ 6.5 MPa showed work hardening followed by steady state flow at strains >6–7%. These samples compacted slightly, and crystal plastic deformation was accompanied by extensive FADRX, with a predominance of high-angle boundaries (30°–50°) over low-angle boundaries. We infer that FADRX is suppressed by dilatation at low pressures as a result of grain boundary disruption. At pressures high enough to prevent dilatation, however, FADRX acts as a “recovery” mechanism counteracting work hardening. The results offer a possible explanation for rheological variability seen in previous experiments conducted at pressures up to 30 MPa. A simple rate model for diffusion- and interface-controlled FADRX indicates that FADRX should become increasingly important toward natural halokinetic conditions, although the effect on flow stresses is likely to be small.