Papers by Author: G.M. Pennock

Abstract: Calcite deformed by high temperature creep develops a heterogeneous microstructure consisting of deformed and recrystallised grains. The deformed grains either contain homogeneously distributed subgrains of similar size, or heterogeneously distributed small subgrains at grain boundaries (mantle subgrains) and relatively large subgrains in the core of grains (core subgrains). This paper demonstrates a method using electron backscattered diffraction (EBSD) to distinguish between the different types of grains and subgrains and to measure their sizes separately. In geological materials the average subgrain size, regardless of the subgrain type, is often used to estimate the deformation stress. However, this paper shows that mantle and core subgrain types only show a weak or no stress dependence.

Abstract: Subgrain rotation is a common mechanism of continuous dynamic recrystallization in minerals and some metals. The mechanism involves new grain boundary formation by progressive rotation of subgrains or subgrain boundary migration in regions with an orientation gradient. This paper reviews the status of our current knowledge of rotation recrystallization in minerals. In minerals a misorientation angle (θ) of 10˚ is often taken as the transition from subgrain boundary to grain boundary but recent studies on olivine indicate a much higher transition angle between 15-25˚. In contrast to a high transition angle, the onset of subgrain boundary mobility may occur at much lower angles between 3-10˚. In consequence, rotation recrystallization in minerals often involves an initial stage of subgrain rotation followed by subgrain growth once medium angle boundaries have formed. Current models assume that all subgrain boundaries increase in misorientation with strain. However, recent studies show that many different types of subgrain boundary develop in minerals. The formation of new high angle grain boundaries is only likely along some types of geometrically necessary boundary (GNB). The mineral halite (NaCl) is often quoted as the classic example of rotation recrystallization yet recent electron backscattered diffraction (EBSD) studies show that only limited grain sub-division occurs in NaCl polycrystals. This grain sub-division occurs on the scale of large subgrains that divide the old grain into a few domains and not by the rotation of the smaller equiaxed subgrains, as envisaged in current models. The small scale, equiaxed, mainly low angle network of subgrain boundaries that develop in many minerals may be incidental boundaries, as found in metals, or could be smaller length-scale GNBs. As minerals have high plastic anisotropy and a limited number of slip systems GNBs may dominate over incidental subgrain boundaries formed by trapping of statistically stored dislocations. New and extended models for rotation recrystallization are needed that consider i) incidental subgrain boundaries as well as different types of GNB, ii) the potential high mobility of medium angle (3-15˚) subgrain boundaries and iii) a link between the development of subgrain misorientation and texture development.

Abstract: NaCl is plastically anisotropic and forms a well developed substructure during deformation at 0.3-0.5Tm. EBSD was used to assess subgrain misorientations up to 0.5 true strain in dry NaCl. Equiaxed subgrains were ubiquitous but misorientations along segments of subgrain boundaries differed. Three types of subgrain boundary were identified: boundaries that surrounded equiaxed subgrains, boundaries that partly surrounded mantle subgrains, and extended subgrain boundaries, longer than the equiaxed subgrains. All of these subgrain features were recognised at low strains, <0.15. Misorientations of the majority of equiaxed subgrains were generally <2° at 0.5 strain, although segments could reach higher misorientations along kink-like boundaries. Mantle subgrains along grain boundaries tended to develop higher misorientations than in core subgrains. Long subgrain boundaries reached very high misorientations along segments of their length by 0.5 strain. Small new grains formed at triple points and more rarely within grains. Microstructures in NaCl are similar to those found in aluminium. Therefore, the dominant mechanism of high angle subgrain development at 0.5 strain and at 0.4Tm is probably an orientation splitting mechanism rather than equiaxed subgrain rotation.