Using Misorientation and Weighted Burgers Vector Statistics to Understand Intragranular Boundary Development and Grain Boundary Formation at High Temperatures.

During plastic deformation, strain weakening can be achieved, in part, via strain energy reduction associated with intragranular boundary development and grain boundary formation. Grain boundaries (in 2D) are segments between triple junctions, that connect to encircle grains; every boundary segment in the encircling loop has a high (>10°) misorientation angle. Intragranular boundaries terminate within grains or dissect grains, usually containing boundary segments with a low (<10°) misorientation angle. We analyze electron backscatter diffraction (EBSD) data from ice deformed at −30°C (Th≈ ${T}_{h}\approx $ 0.9). Misorientation and weighted Burgers vector (WBV) statistics are calculated along planar intragranular boundaries. Misorientation angles change markedly along each intragranular boundary, linking low‐ (<10°) and high‐angle (10–38°) segments that exhibit distinct misorientation axes and WBV directions. We suggest that these boundaries might be produced by the growth and intersection of individual intragranular boundary segments comprising dislocations with distinct slip systems. There is a fundamental difference between misorientation axis distributions of intragranular boundaries (misorientation axes mostly confined to ice basal plane) and grain boundaries (no preferred misorientation axis). These observations suggest during progressive subgrain rotation, intragranular boundaries remain crystallographically controlled up to large misorientation angles (>>10°). In contrast, the apparent lack of crystallographic control for grain boundaries suggests misorientation axes become randomized, likely due to the activation of additional mechanisms (such as grain boundary sliding) after grain boundary formation, linking boundary segments to encircle a grain. Our findings on ice intragranular boundary development and grain boundary formation may apply more broadly to other rock‐forming minerals (e.g., olivine, quartz). Plain Language Summary: When grains of minerals are placed under high stresses and temperatures, they develop internal (intragranular) boundaries due to the accumulation of crystal defects ("dislocations"). Over time, these boundaries introduce more local rotation ("misorientation") into the crystal structure. Eventually, the amount of rotation becomes large enough that an intragranular boundary evolves into a distinct, new grain boundary. Thus, grains become progressively subdivided during deformation. This process of subdivision allows minerals to weaken, promoting their large‐scale flow. To understand intragranular boundary development and grain boundary formation at temperatures close to the melting point, we examined the microstructure of ice samples deformed at −30°C. We found widespread variation in the crystalline structure of individual intragranular boundaries, which cannot easily be explained by previous models for boundary evolution. We propose that intragranular boundaries grow and develop by the progressive coalescence of smaller boundary sections. Furthermore, intragranular boundaries evolve via rotation around specific crystal axes. Intragranular boundaries can transform into grain boundaries, which encloses an area (grain), after rotating beyond a threshold misorientation angle. The grain boundaries are not tied to any specific rotation axis. This implies the introduction of a secondary mechanism (such as sliding along grain boundaries), which serves to disperse crystal orientations. Key Points: We propose that intragranular boundaries form via the intersection of boundary segments containing dislocations with distinct slip systemsSubgrain rotation can operate to large misorientation angles (∼38°) with rational, low‐order misorientation axes (a and m axes)Boundary misorientation axes become randomized from additional mechanisms (e.g., grain boundary sliding) following grain boundary formation [ABSTRACT FROM AUTHOR]