Transient creep in high-purity aluminum at ultra-low strain rate and room temperature by constant stress and changing-stress experiments

Creep of high-purity aluminum (5N Al) at room temperature and ultra-low strain rate was investigated by a high sensitive helicoid-spring specimen technique under conditions of constant and changing stress. Creep deformation consists of transient creep stages, and no secondary creep stage was observed. Li's equation showed a good fit to the experimental curves. During nominal steady-state creep, the stress exponent is equal to one regardless of initial state of specimens. However, the nominal steady-state creep rate for water quenched 5N Al is one order less than that for the static recovered specimens due to work hardening. With increase in stress, creep strengthening (the creep rate progressively decreasing in subsequent segments) was observed, which is due to different hardening remains because changing-stress creep experiment was conducted in the transient creep stage. Those phenomena of work hardening indicate creep deformation is controlled by recovery and work-hardening mechanism. During transient creep, every decrease in stress is associated with the large and long anelastic backflow. The anelastic transient strain for stress reduction is equivalent to elastic deformation corresponding to the applied stress, while transient strain is 2.5 times greater than the equivalent elastic deformation regardless of whether stress increases or is constant. The transient effect was suggested to be due to a mix of anelastic behavior caused by the internal redistribution of stress and inelastic behavior.