Investigation of nucleation processes during dynamic recrystallization of ice using cryo-EBSD

Abstract

Nucleation mechanisms occurring during dynamic recrystallization play a crucial role in the evolution of microstructures and textures during high temperature deformation. In polycrystalline ice, the strong viscoplastic anisotropy induces high strain heterogeneities between grains which control the recrystallization mechanisms. Here, we study the nucleation mechanisms occurring during creep tests performed on polycrystalline columnar ice at high temperature and stress (T=-5°C;σ=0.5 MPa) by post-mortem analyses of deformation microstructures using cryogenic electron backscatter diffraction. The columnar geometry of the samples enables discrimination of the nuclei from the initial grains. Various nucleation mechanisms are deduced from the analysis of the nuclei relations with the dislocation sub-structures within grains and at grain boundaries. Tilt sub-grain boundaries and kink bands are the main structures responsible for development of polygonization and mosaic sub-structures. Nucleation by bulging at serrated grain boundaries is also an efficient nucleation mechanism near the grain boundaries where strain incompatibilities are high. Observation of nuclei with orientations not related to the 'parent' ones suggests the possibility of 'spontaneous' nucleation driven by the relaxation of the dislocation-related internal stress field. The complexity of the nucleation mechanisms observed here emphasizes the impact of stress and strain heterogeneities on dynamic recrystallization mechanisms.This article is part of the themed issue 'Microdynamics of ice'.

Keywords: dynamic recrystallization; electron backscatter diffraction; ice; nucleation.

Figure 1.

Pre- (a,c) and post- (b,d) deformation texture analyses from [0001] c-axis measurements performed with the AITA (20 μm step size), with the associated pole figure for [0001] axis for both sample no. 1 (a,b) and no. 2 (c,d). The colourwheel related to c-axis orientations is shown at the bottom right of the undeformed microstructures. Selected areas for EBSD analyses are shown within white rectangles on the deformed microstructures. The white pixels in post-deformation microstructures are non-indexed. (Online version in colour.)

Figure 2.

(a) KAM representation of an EBSD map (20 μm step size). The grain boundaries are plotted in black (misorientation angle higher than 7°). (b) Misorientation angle analysis along profile (b); the red line shows the cumulative misorientation and the blue line the misorientation between neighbouring pixels, associated pole figures for the right and left tilt SGB pointing in different hemispheres. (c) Misorientation angle evolution along various profiles across grain P7, at increasing distances from the grain boundary. Profiles are extracted perpendicularly to the c-axis direction, in order to show the sub-structure evolution as a function of the distance to the grain boundary. (Online version in colour.)

Figure 3.

(a) Second order KAM of an EBSD map showing mosaic sub-structure (20 μm step size). The profile shows the cumulative misorientation along the dashed line relative to labelled point 0. The grain boundaries are plotted in black (misorientation angle higher than 7°). (b) Pole figures for each SGB. The total misorientation accommodated by the SGB is indicated at the bottom right of the pole figure. On the SGB P8-S3 pole figure, the misorientation axis resulting from the combination of the misorientations of SGBs P8-S1 and P8-S2 is plotted as a grey square. (Online version in colour.)

Figure 4.

(a) Mis2mean representation of a serrated grain boundary from an EBSD map (5 μm step size). The grain boundaries are plotted in black (misorientation angle higher than 7°). (b) Pole figures for SGB analyses are plotted for P9-S1, P10-S1 and P10-S2. (Online version in colour.)

Figure 5.

EBSD map (20 μm step size) of serrated GB with nuclei. The grain boundaries are plotted in black (misorientation angle higher than 7°). (a) Mis2mean representation. (b) GOS magnitude map colour-coding. (c) c-Axis 〈0001〉 pole figure using the GOS colourcode and a maker size proportional to the grain size. (Online version in colour.)

Figure 6.

(a) EBSD map (20 μm step size) using GOS colour-coding. The grain boundaries are plotted in black (misorientation angle higher than 7°). (b) c-Axis pole figure using GOS colour-coding and a maker size proportional to the grain size. (c) Map of the dashed rectangular area. The nuclei are coloured depending on the inferred ‘parent’ grain P15 or P16. The grain boundaries are colour-coded using the misorientation angle. The frequency histogram of the misorientation angle boundaries is added to the colour-bar and the initial misorientation between P15 and P16 is shown by the arrow. (Online version in colour.)

Figure 7.

(a) EBSD colour map and pole figure colour-coded depending on the ‘nature’ of the grain. The grains inherited from the initial sample appear in grey. The nuclei formed by bulging appear in black. The nuclei with new orientations appear in blue, cyan, green and red. The table provides the misorientation angle between the new small grains and the closest parent grains (grey). The grain boundaries are plotted in black (misorientation angle higher than 7°). (b) Theoretical coincidence site lattice (CSL) for Ice-Ih [62] and the misorientation axes between the blue nuclei and the five neighbouring grains. The associated misorientation angle is in bold and the misorientation angle between the rotation axis and the closest CSL rotation axis in italics. The red misorientation axis corresponds to the misorientation axis between the blue grain and the grain marked by the red star. (Online version in colour.)