Spatio-temporal characteristics of plastic instability in AA5182-O during biaxial deformation

Highlights: • Unique plastic instabilities were observed in hydraulic bulge test of AA5182 by DIC. • A critical strain was required to trigger serrated flow in biaxial deformation. • Negative strain rate sensitivity of AA5182 was observed in biaxial deformation. • Dynamic strain aging is suggested to be the mechanism for the serrated flow. - Abstract: Spatio-temporal characteristics of plastic instability denoted by locally extreme values in strain rate contours computed with digital image correlation are investigated in biaxial deformation of AA5182-O sheet. Temporal characteristics are similar to those of Portevin–Le Châtelier (PLC) bands in uniaxial tension. Spatial characteristics, however, dramatically differ from PLC band morphologies in uniaxial tension. Initiation occurs in a localized deformation ring (LDR) that does not include the pole, followed by a rapid transition to a circular region that includes pole, i.e. a localized deformation circle (LDC). A critical strain is required to trigger serrated flow at the pole, and a negative strain rate sensitivity of the flow stress at the pole suggests that the underlying microstructural mechanism is dynamic strain aging. Plastic instability hops between the LDR and the LDC until a critical strain is reached at which it appears in a localized deformation band, with no circular symmetry, that hops around the pole. Future model development will require modifications to existing theoretical frameworks to account for the unique spatial characteristics of plastic instability during biaxial deformation of AA5XXX alloys.