Stress relaxation and flow behavior of ultrafine grained AA 1050

• Flow behavior of UFG AA 1050 is investigated by PSC and stress relaxation tests.
• Effects of strain rate on saturated microstructure and flow stress are addressed.
• Time-dependent evolution of subgrain structure under relaxing stress is addressed.
• The slope of ln (-σ̇) versus ln (t) plot during stress relaxation is discussed.

Flow behavior of ultrafine grained (UFG) AA 1050 sheets processed by Accumulative Roll-Bonding (ARB) and its viscous nature are investigated by plane strain compression test (PSC) along with stress relaxation. Occurrence of dynamic recovery is validated by TEM observations as the microstructural explanation of the flow softening at the start of deformation of the 8-cycles specimen. Significant recovery of the UFG specimens during the stress relaxation test is also disclosed. It is discussed that neither the internal stress (σi) nor the density of mobile dislocations are constant during the test. The possible effects of these two factors as well as contribution of the grain boundary sliding (GBS) on the relaxation behavior of the specimens are analyzed. It is shown that these effects can explain why the absolute value of the slope |S| of ln (-σ̇) versus ln (t) plot is smaller than unity, while based on the physical meaning of the power law model, which fits well to the experiments, the |S| < 1 carries no sense. Moreover, the enhanced contribution of GBS due to grain refinement by ARB cycles seems to cause the decrease of the |S| by the number of ARB cycles.