Necklace formation during dynamic recrystallization: Mechanisms and impact on flow behavior

The aim of the current study was to investigate the nucleation mechanisms of new grains during DRX and the deformation behavior of a necklace structure. The investigations were conducted on Ni3Al, because Ni3Al develops a distinct necklace structure during dynamic recrystallization DRX. Local orientation measurements were conducted to determine misorientations between new recrystallized grains and their parent grains. DRX was set off by strain induced bulging of prior grain boundaries. Additionally, the formation of new grains by recrystallization twinning was observed. With progressing DRX the orientation coherency of DRX grains with the matrix grains diminished rapidly, and the texture tended to randomize. The strain rate sensitivity indicated superplastic flow in the recrystallized volume. The deformation behavior changed significantly, when these soft regions formed a contiguous 3D network along the original grain boundaries. A new model for the flow curve is proposed that accounts for the percolation character of necklace structures. (C) 1997 Acta Metallurgica Inc.