Harper–Dorn creep in metals at intermediate temperatures revisited: Constant structure test of pure Al

Creep tests of 99.9% pure aluminum were conducted at 723 K in the stress range from 0.12 to 2.0 MPa using the helicoid spring specimen technique. The testing technique was modified to allow for stress reduction during the test. The results show that the steady-state creep rate after a stress reduction becomes lower than that obtained from a monotonic experiment at the same stress level. The stress exponent derived from the constant stress experiment corresponded to the Harper–Dorn creep regime, whereas the stress exponent derived from the stress change experiment was between 1.2 and 3.0 increasing with increasing stress. The stress exponent should be considered an apparent parameter only, dependent on the experimental procedure, and cannot be related directly to the properties of any underlying deformation mechanism.