Creep Mechanisms in a Fine-Grained Al-5356 Alloy at Low Stress and High Temperature

Monotonic creep tests were carried out on fine-grained Al-5356 alloy with grain size d_g = 5 ± 0. 5 mm by the helicoid spring specimen technique at homologous temperatures ranging from 0.63 to 0.74 and applied stresses of 0.13 to 1.42MPa. At stresses lower than about 0.50 MPa, Bingham-type viscous creep with activation energy Q = 80 ± 25 kJ/mol, characterized by a threshold stress which decreases with increasing temperature, was predominant. At a stress above about 0.50 MPa, grain boundary sliding with a stress exponent n = 2 and Q = 85 ± 25 kJ/mol obviously contributed to the measured creep data. Stress redistribution was evaluated, and it did not greatly influence the stress exponent. The creep mechanisms were elucidated with respect to standard creep models supported by the substructures studied by transmission electron microscopy. Viscous creep (n = 1) was identical to be Harper-Dorn creep controlled by dislocation core diffusion. The motion of jogs on edge dislocations dependent on dislocation core diffusion was observed to control the creep. Grain boundary sliding accommodated by slip with n = 2 was noted, while hardening and the recovery of dislocations at grain boundaries were suggested to control the creep. Microstructural observations along with a determination of parametric variations in the creep rates were useful for identifying the underlying deformation mechanisms.