Experimental deformation of coarse‐grained rock salt to high strain

Abstract

The processes and deformation mechanisms (e.g., dislocation creep, pressure solution, grain boundary sliding, and recrystallization) of rock salt are still a matter of debate. In order to fill this gap, high strain constriction experiments at 345°C, atmospheric pressure and a strain rate of ~10−7 s−1 have been conducted on natural halite cuboids (60 × 60 × 45 mm) from the Morsleben mine of Northern Germany. Most samples were almost single crystals and contain a small amount of smaller grains (10–26%). The grain boundaries are decorated with fluid inclusions. The experiments were stopped at different final strains (εy = z of ~10, 20, 30, and 40%) corresponding to a maximum strain (εx) range of 20–170%. The halite is deformed by dislocation creep, and the size of developed subgrains corresponds to the applied stress. The combined Schmid factor and subgrain boundary analysis indicate that slip was largely accommodated by the {110} < 110 > slip systems, with possible minor contribution by slip on the {100} < 110 > slip systems. Some of the deformed samples show evidence of grain boundary migration. In addition, subgrains with small misorientations form that result in large cumulative misorientations within a single grain (>40°). However, no subgrain rotation recrystallization is observed (i.e., misorientation angles are <10°). All the experiments show strain hardening, suggesting that recrystallization by grain boundary migration was not extensive and did not reset the microstructure. The experiments show that high finite strain in coarse‐grained relatively dry rock salt can be accommodated by dislocation creep, without extensive dynamic recrystallization.

Olivine Grain Size Distributions in Faults and Shear Zones: Evidence for Nonsteady State Deformation

“…At higher temperatures grain size reduction may take place by either brittle or ductile mechanisms depending both on the stress level and a range of other factors, including temperature (De Bresser et al, 2001;Stipp et al, 2002), water content (H. Jung & Karato, 2001), crystal orientation (Linckens et al, 2016), strain rate (Stipp et al, 2002), and the presence or absence of multiple phases (Cross et al, 2015;Doherty et al, 1997;Drury & Urai, 1990). The grain size evolution at elevated temperatures is controlled by a competition between grain size-reducing processes and grain growth (e.g., Cross et al, 2015).…”

“…This also implies that the entire grain size distribution (rather than the mean value) needs to be taken into account when relating microstructures to mechanisms (Rozel et al, 2011). distribution even in rocks deformed at depth greater than the traditional brittle-ductile transition is often controlled by fragmentation and growth processes that are to some extent separated in time and reflects an initial period of grain size reduction at high stress conditions, followed by recovery and growth at lower stresses (Druiventak et al, 2012;Linckens et al, 2016;Trepmann et al, 2013). Nonsteady state grain size distributions may be more common than hitherto thought and carry valuable information about deformation history beyond the time-independent perspective.…”

Nanoindentation Studies of Plasticity and Dislocation Creep in Halite

“…Three different dislocation creep flow laws extrapolated to room temperature are shown for comparison. The black line, a fit to the raw data, is shifted according to Equation (12), either in the vertical or, equivalently, the horizontal direction. The two shifted black curves agree well with the predictions of the flow laws, particularly that of Wawersik and Zeuch [6].…”

“…The development of flow laws for various creep mechanisms in halite, and characterization of their associated microstructures, allows for reliable extrapolation of laboratory-derived behavior to natural conditions. Many studies have explored dislocation creep in polycrystalline halite [4][5][6][7][8][9][10][11][12]. The rate-limiting process in the dislocation creep regime, up to temperatures of 200 • C, is believed to be either dislocation climb [4,5,13] or cross-slip of screw dislocations [6,10,[14][15][16], but definitive evidence has not been obtained for either mechanism [2,10].…”

“…ε is the strain rate, A is a constant, σ is differential stress, n is the stress exponent, Q the activation energy, R the gas constant, and T absolute temperature. Dislocation creep in polycrystalline halite is characterized by n = 5-7 (typically n ≈ 5) and Q = 50-130 kJ/mol (typically Q is in the range of 50-70 kJ/mol) [4][5][6][7][8][9][10][11][12]17,18]. Dislocation-creep experiments have been typically conducted at elevated temperatures in the range 60-400 • C, and confining pressures up to 200 MPa to avoid fracturing the samples.…”