Investigation of the unloading yield effect in 7075 Al alloys based on microstructural and digital image correlation analysis

Abstract

The unloading yield effect (UYE), which indicates that the reloading flow stress overshoots the monotonically loaded flow stress during uniaxial tensile test, was investigated by evaluating three aluminum alloy 7075 sheets with different precipitate states: solution heat-treated (SHT), aged for 20 min (A20, underaged), aged for 80 min (A80, peak aged) and aged for 240 min (A240, overaged). The precipitate states of A20 and A240 specimens were characterized by transmission electron microscope (TEM) and small-angle neutron scattering (SANS) analyses. The UYE was exhibited only in artificially aged specimens, but not in the SHT specimen. It was confirmed that the root cause of UYE is the interaction between the precipitate and dislocation during unloading. In addition, it was observed that strain instability is inevitably generated during stress overshoot owing to temporal geometric softening. The temporal strain instability diffused to the neighboring region in the case of A20 specimen, but that of A240 specimen directly influenced the premature localized necking or failure, resulting in a significant reduction in elongation.