Abnormal grain growth in metallic materials

Abstract

The mystery of selective abnormal grain growth of Goss oriented grain in Fe-3%Si steel has long remained unsolved since its discovery by Goss in 1933. Here, attempts to solve this puzzle are reviewed with a focus on sub-boundary enhanced solid-state wetting along the triple junction of polycrystalline metals. Especially, the grains with sub-boundaries of very low energy exclusively have a high probability to grow by solid-state wetting and as a result can grow abnormally. The microstructure at the initial stage of abnormal grain growth in Fe-3wt%Si steel was investigated using synchrotron X-ray microdiffraction to identify the sub-boundaries, which had been suggested to induce the selective AGG of Goss-oriented grains. The splitting of Laue diffraction peaks caused by sub-boundaries was observed only in Goss grains and not in matrix grains in the entire samples. The sub-boundaries were measured to have misorientation angles below 0.6° which is very low. The characteristics and morphologies of sub-boundaries in Goss grains were analyzed based on the splitting directions and positions of Laue diffraction peaks. Based on the mechanism of sub-boundary enhanced solid state wetting and the experimental results that shows the existence of sub-boundaries in abnormally growing Goss grains in Fe-3%Si steel, abnormal grain growth of non-Goss grains was induced by forming sub-boundary through low deformation in Fe-3%Si steel. The primary recrystallization specimens of Fe-3%Si was locally deformed by micro-indenter and annealed in order to form sub-boundary at matrix grains. After the specimens were subjected to secondary recrystallization, abnormal grain growth of non-Goss grains occurs. It was found that all of these abnormally grown grains which have totally different orientation with Goss grains had sub-boundary inside them. It is suggested that abnormal grain growth of grains which are desired to grow could be controlled and texture could made by forming sub-boundaries based on sub-boundary enhanced solid-state wetting mechanism. The effect of an external electric field on recrystallization and grain growth during annealing was investigated in pure iron and Fe-2.9%Si alloy. The elimination behavior of island grains inside abnormally grown grains by the external electric field was also investigated in Fe-3%Si steel and Al 5052 alloy. It was observed that recrystallization, grain growth and the elimination of island grains were accelerated when the external electric field was applied during annealing. It is suggested that the crystal defects and nearby atoms could easily overcome the activation energies for the recrystallization and the grain boundary migration because the excess electric charges at surface of metal induced by the external electric field could excite the energy state of the crystal defects and nearby atoms.