Structural changes associated with strain-induced grain boundary migration in Si-Fe

Strain-induced grain boundary migration was observed in several bicrystal sheet specimens of Si-Fe (3¼ pct Si) which were cold rolled 2 to 12 pct and then annealed at temperatures up to 1200°C. A chrome-acetic acid electroetching method was used to reveal the dislocation sites before and after grain boundary migration. Recovery effects were noted in the microstructure prior to boundary motion. Consequently, the residual strain energy in neighboring grains may determine if boundary migration with resulting increase of area occurs, and its direction of movement. Microstructural data indicate that the region initially traversed by the moving grain boundary has many structural defects in the form of low-angle boundaries and random dislocations of relatively high density. With increased distance of grain boundary migration, the density of these imperfections was found to decrease. Continued annealing at 1200°C, after boundary migration, lowered the density of random dislocations in the swept region to a limiting value of about 2×10⁶ lines per sq cm.