Misorientation analysis and the formation and orientation of subgrain and grain boundaries

Abstract

In contrast to the behaviour of individual grains, both inter- and intra-granular boundaries within rocks have received much less attention. However, many geological processes, particularly during deformation (e.g., yielding, dislocation creep, recrystallisation, superplasticity and various fracture mechanisms), and petrophysical properties depend to some extent on the nature of boundaries present in a rock. In this contribution, we consider the role of intergranular and intragranular crystal boundaries. A precise characterisation of such boundaries depends on defining the crystallographic and dimensional orientations of the boundary and the misorientation between the adjacent regions (i.e. grains, subgrains, etc.) separated by the boundary. Although several theoretical descriptions of boundary configuration are available, practical precision is lacking and approximations are necessary. We describe two specific approximations for boundary formation and orientation obtained using the SEM electron channelling technique. The first is a geometrical interpretation of electron channelling patterns (ECP) in terms of the likely formation and orientation of the intervening boundary. The second considers the misorientation between adjacent regions across a boundary. This involves a model which assumes a simple geometrical relationship between crystal slip systems responsible for the rotation and misorientation between adjacent regions, and the formation and orientation of the resulting boundary. These approximations are capable of: (a) identifying trends in the dispersion of crystallographic directions during deformation; (b) identifying active slip systems; (c) calculating the relative Schmid Factors for each crystal slip system (and therefore the most likely system to be activated); (d) modelling synthetic misorientations and predicting the crystal slip systems and boundary configurations to be expected; and (e) comparing real data with synthetic models. Our analyses are illustrated via natural examples of dynamic recrystallisation in quartzite and a theoretical simulation of the behaviour of an individual quartz grain during deformation.