A duplex stainless steel was processed by high-pressure torsion (HPT) and then examined by optical microscopy. The results reveal unusual flow patterns including double-swirl strains, shear vortices, and the presence of significant local turbulence. Similar flow behavior was also visible in disks of high-purity aluminum and a Zn-22%Al eutectoid alloy. These complex flow patterns and the presence of double-swirls are consistent with the presence of a Kelvin-Helmholtz instability during HPT processing where this may arise if there are local shear velocity gradients between adjacent positions within the HPT disks.