In-Situ Investigation of Local Boundary Migration During Recrystallization

Abstract

A combination of electron channeling contrast (ECC) and electron backscatter diffraction pattern (EBSP) techniques has been used to follow in situ the migration during annealing at 323 K (50°C) of a recrystallizing boundary through the deformed matrix of high-purity aluminum rolled to 86 pct reduction in thickness. The combination of ECC and EBSP techniques allows both detailed measurements of crystallographic orientations to be made, as well as tracking of the boundary migration with good temporal resolution. The measured boundary velocity and the local boundary morphology are analyzed based on calculations of local values for the stored energy of deformation. It is found that the migration of the investigated boundary is very complex with significant spatial and temporal variations in its movement, which cannot directly be explained by the variations in stored energies, but that these variations relate closely to local variations within the deformed microstructure ahead of the boundary, and are found related to the local spatial arrangements and misorientations of the dislocation boundaries. The results of the investigation suggest that local analysis, on the micrometer length scale, is necessary for the further understanding of recrystallization boundary migration mechanisms.

A method to correct coordinate distortion in EBSD maps

“…The drift problems become more critical when EBSD techniques are used together with other techniques, such as electron channeling contrast (ECC) and transmission electron microscopy (TEM), to characterize the same microstructures [14][15][16][17]. Due to the different imaging principles and acquisition time, the same microstructure characterized by EBSD can easily be more distorted than that with ECC/TEM.…”

“…For ex-situ EBSD experiments, unavoidable sample misalignment after the sample has been removed from the microscope for processing and remounted in the microscope even further worsens the coordinate distortion. Direct quantitative comparison among the sequential EBSD maps, for example to measure local boundary migration distance during annealing [17], is rather difficult because of the distortion. As a result, although many in-situ and ex-situ studies have been conducted in the last decade, in most cases only qualitative analysis has been reported [18,[21][22][23].…”

Boundary migration during recrystallization: experimental observations

“…In between these fixed points, the migration of the recrystallizing boundary is quite heterogeneous: many small protrusions/retrusions form during the migration, and the migration velocity varies in space and time, with fast migration observed in particular during time interval #2. The misorientation analysis has shown that not much difference is observed between the migrating and non-migrating boundary segments [48]. By correlating the boundary traces with the deformed microstructure (figure 9e), it is evident that the coarsely spaced dislocation boundaries, marked by Greek letters in figure 9c, which have higher misorientations, have dominated the migration process and stimulated the formation of protrusions (see figure 10a).…”

“…The EBSD mapping step size for this study was coarse (2 µm), and effects of local deformation microstructures on the growth could therefore not be revealed. Very recently, new experiments have been conducted to follow the local boundary migration: EBSD was used to characterize the starting microstructure and the boundary migration was followed using ECC during in situ annealing inside the SEM [48]. Figure 9a and 9b show a subset of the starting microstructures, while figure 9d shows 7 boundary traces that have been analyzed in detailed.…”

“…Even when the local net curvature dragging/driving forces from protrusions/retrusions are included in a detailed quantification of boundary migration, a large scatter between the migration velocity, v and the modified driving force, FS + Fσ is found (see figure 10b), where a significant deviation from a simple straight line relationship as predicted by equation (4) is observed. Several reasons have been suggested to understand this result [48]. Among those, lack of knowledge about the appropriate value of mobility, M, which may be complex and may depend on many parameters, is suggested to be the main factor to be examined in future experiments.…”