"abnormal transient creep" in fine-grained Al-5356 alloy observed at low strain rates by high-resolution strain measurement

Transient creep at very low strain rates (less than 10^-10s ^-1) is still unclear. The traditional uniaxial creep testing is less useful due to unsatisfied resolution strain ∼(10^-6). To study transient creep behavior at such low strain rates, a high-resolution strain measurement using the helicoid spring specimen technique was employed in a fine-grained Al-5356 alloy at temperatures ranging from 0.47 T_m to 0.74T_m (T_m: melting point). To clarify transient creep mechanism at such low strain rates, transmission electron microscopy (TEM) was used in microstructure observation of crept specimens. The abnormal transient creep, high temperature strengthening at T>T_p(T_p: the phase transformation temperature, 0.58T_m) or intermediate temperature softening at 0.4T_m<T≤T_p and double-normal type (creep curves including double work-hardening stages) at T=T_p, were firstly observed. The substructure observation in a crept specimen at T=0.58T_m and É=1*10^-4 shows pile-up dislocations including many small jogs with equal interval, and dislocations emitted from grain boundaries. The β-Al₃Mg₂ phase dissolves under the condition of testing temperatures higher than 523 K, which causes solid-solution quantity of Mg atoms to increase. Therefore, the "abnormal transient creep" may be related to the difference of solid solution strengthening caused by phase change during the creep tests.