Microstructure evolution of AISI 316 L in torsion at high temperature

Hot monotonic, double-deformation and reversed torsion deformation of AISI 316L stainless steel was investigated with a constant equivalent strain rate of 0.006 s(-1) in the temperature range 850-1100 degreesC. The effect of grain size and strain rate was also investigated and compared with monotonic tension. A tendency for the grain boundaries to align with the major shear directions was found for all deformation conditions in the temperature range 900-1000 degreesC, while the equiaxed grain structure was retained at 850 degreesC. The grain shape was predominantly rhomboidal for monotonic torsion, but adopted a square shape for fully reversed torsion, but in both cases dihedral angles at the triple junctions and 90degrees elbow grain boundaries were observed. The dislocation structure was found to be highly heterogeneous, with dislocation lines predominantly present as low angle grain boundaries local to high angle grain boundaries, aligned parallel and perpendicular to the high angle grain boundaries. The mechanism by which the grain boundaries aligned to the shear axes appeared to be strain induced grain boundary migration (SIGBM) associated with these heterogeneities in dislocation density. The dislocation density was insufficient to drive recrystallisation, allowing the effect of SIGBM to be seen. However, at higher strains, dislocation density increased sufficiently that static recrystallisation occurred local to the high angle grain boundaries. The mechanisms of the grain shape change are discussed.