Uncovering the Mystery of Harper-Dorn Creep in Metals

In the present investigation, the occurrence of Harper - Dorn creep in Pb was studied under the condition of large strains. In performing the study, two Pb grades, commercial-purity Pb (99.95 pct) and high-purity Pb (99.999 pct), were creep tested at 573 K. The mechanical data showed that 99.95 Pb, unlike 99.999 Pb, did not exhibit accelerated creep rates at stresses <0.1 MPa (the region of Harper - Dorn creep). Also, the data on 99.999 Pb revealed that the creep curves associated with Harper - Dorn creep exhibited periodic accelerations and that the stress exponent was approximately 3, not unity as previously reported for Pb and other materials. An examination of the substructure developed during creep revealed that the sub-structural details in 99.95 Pb were markedly different from those characterizing Harper - Dorn creep in 99.999 Pb. In 99.95 Pb, the substructural observations included the presence of very large subgrains and triple junctions of subgrains. By contrast, in 99.999 Pb, the substructural features included the nucleation of new grains and the presence of twins. Combining the present data on Pb and those reported recently for Al has led to three important conclusions. First, the accelerated creep rates noted in the region of Harper - Dorn creep are produced by a creep process for which the dominant restoration mechanism is dynamic recrystallization. Second, the linear-stress dependence of creep rate previously reported for Harper - Dorn creep is most probably a direct consequence of short-term measurements involving small strains (total creep strains <= 0.01). Finally, Harper - Dorn creep will be observed in a large-grained metal at very low stresses if its purity level is high. This condition favors dynamic recrystallization as a restoration mechanism.