Abnormal grain growth in metallic materials approached by solid-state wetting

Abstract

To explain the selective abnormal grain growth (AGG) in metallic materials, the mechanism of sub-boundary enhanced solid-state wetting has been suggested. According to the mechanism, if a grain contains sub-boundaries of very low energy, the probability of the grain to grow by solid-state wetting along triple junctions becomes so high that grains containing sub-boundaries undergo selective AGG. If this concept is applied to the selective AGG of Goss grains in Fe-3%Si steel, abnormally-growing Goss grains are expected to have sub-boundaries exclusively. Therefore, the initial stage of secondary recrystallization of Fe-3%Si steel was investigated using a transmission electron microscope with a focus on the existence of sub-boundaries in the abnormally-growing Goss grains. Randomly-chosen 10 abnormally-growing Goss grains were observed to have the sub-boundaries consisting of aligned edge dislocations. In contrast, no sub-boundaries were observed in the matrix grains. The misorientation angles of observed sub-boundaries were in the range of 0.1 ~ 0.5°, which was estimated based on the spacing between the dislocations. This means that sub-boundary with very low boundary energy is a specific feature of Goss grains in Fe-3%Si steel. Therefore, in an effort to understand why sub-boundaries are formed exclusively in Goss grains after primary recrystallization. In order for Goss grains to have sub-boundaries exclusively, they should undergo only recovery without recrystallization during annealing for the primary recrystallization whereas other grains undergo recrystallization. For this, Goss grains should have the lowest stored energy after cold rolling. Goss and rotated cube orientations have the same and the lowest Taylor factor among the grains formed after cold rolling, implying that they are expected to have the lowest stored energy. The stored energy between Goss and rotated cube orientations was compared by the crystal plasticity finite element calculation, which showed that the stored energy of Goss orientation was lower than that of the rotated cube orientation after plane strain deformation. For this reason, Goss grains stored lower energy than other oriented-grains would undergo only recovery during primary recrystallization, and therefore they exclusively have sub-boundaries. Based on the sub-boundary enhanced solid-state wetting for the mechanism of selective AGG of Goss grains, puzzling microstructures of Goss grains such as pancake-shaped growth and irregular grain boundaries evolved after secondary recrystallization were examined. During secondary recrystallization of Fe-3%Si steel, Goss grains near the surface grow faster in the direction parallel to the surface than in the direction vertical to the surface, resulting in pancake-shaped growth. The growth advantage in the direction parallel to the surface was investigated. The surface had a higher percentage of low energy boundaries with respect to the Goss grains such as low angle and coincidence site lattice boundaries than the center, which would provide the growth advantage of Goss grains along the surface from the viewpoint of solid-state wetting. This difference between the surface and the center was induced by the texture inhomogeneity through the thickness formed after primary recrystallization which would be inherited from the texture after hot rolling. Interpass aging, which refers to short time aging treatment between passes of the single stage cold rolling, is famous for improving magnetic property of grain oriented electrical steel. Irregular or smooth grain boundaries are formed depending on whether interpass aging is adopted or not during cold rolling. Interpass aging induces the primary texture of high intensity of {111}<112> and {411}<148> orientations, which have a Σ9 relation relative to Goss grains and therefore are favorable for Goss grains to grow by solid-state wetting. Under this condition Goss grains come in contact each other by wetting without leaving any small grains in between, resulting in irregular boundaries. One of the reasons for the difficulty in identifying the AGG mechanism in metallic materials would be that the details of AGG cannot be traced out sequentially. If we could trace out the sequential microstructural change during AGG, the detailed information as to the AGG could be obtained. To trace out the sequential microstructural change during AGG, we tried to make ex-situ observations as to how AGG occurs sequentially in aluminum alloy. The sequential microstructural evolution of abnormal grain growth in the aluminum alloy was traced by ex-situ observation using electron-backscattered diffraction during secondary recyrstallization. Near the growth front of abnormally-growing grains, many three- or four-sided grains with a negative curvature, which had the same orientation as abnormally-growing grains, was penetrating the surface, indicating that abnormal grain growth occurs dominantly by repeated events of triple junction wetting. Also, abnormally-growing grains were shown exclusively to have sub-boundaries. AGG in the nanostructured Invar alloy fabricated by electrodeposition was investigated by electron backscattered diffraction. The observation showed that most of grains growing abnormally during annealing at 380 °C have Σ3 boundaries. The observation could be best explained by the mechanism of solid-state wetting, where the Σ3 boundary provides the low energy boundary, which increases the probability of solid-state wetting, leading to exclusive growth.