Uncovering the Mystery of Harper–Dorn Creep in Metals

Abstract

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 substructural 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.

Effect of electric current on the creep deformation of lead

“…After a short transient stage due to elastoplastic deformation, the impression creep reaches a steady state similar to the compression creep test with sudden jumps in the impression curves during the steady state impression creep. The sudden jumps in the impression curves are similar to that for the impression creep of single crystal Pb without the passing of electric current [13] and the double-shear creep of pure Pb (99.999 wt%) [14]. As suggested by Chiang and Li [13], these sudden jumps were due to local recrystallization underneath the punch to release the internal stresses created by the impression; and the recrystallization did not have any significant effect on the steady state impression velocity (creep rate) for the steady state impression creep.…”

“…Mohamed and his co-workers [14] in their recent study of the double-shear of pure Pb found that the occurrence of the sudden jumps in the creep curves depends on the purity of Pb. They showed that there were periodic jumps for the double-shear creep of pure Pb with purity of 99.999 wt% and no jumps for the doubleshear creep of pure Pb with purity of 99.9 wt%.…”

The role of grain boundaries in creep strain accumulation

“…More recently, commercially pure lead (99.95%) and high-purity lead (99.999%) of large, uniform grain sizes were creep tested at around 573 K [25]. However, with the 99.999% Pb, periodic accelerations giving faster overall creep rates as a result of dynamic recrystallization provided n values of 3 rather than 1.…”

“…Once again, with the 99.95% Pb, periodic accelerations in creep rate were not found, giving lower measured creep rates. On this basis, it was concluded that the linear stress dependence thought to characterize Harper-Dorn creep is probably a direct consequence of making short-term measurements involving total creep strains less than 0.01 [25].…”

Grain Boundary Sliding in High‐Temperature Deformation of Directionally Solidified hcp Zn Alloys and Implications for the Deformation Mechanism of Earth's Inner Core

“…The overall viscosity adds inversely since the mechanisms operate in parallel. In one study, because of the large grain sizes and small stresses, van Orman (2004) suggested Harper-Dorn creep as the deformation mechanism, obtaining a Journal of Geophysical Research: Solid Earth 10.1002/2017JB014881 very low viscosity, 10 11 Pa s, but the existence of Harper-Dorn creep has been questioned (Blum et al, 2002;Karato, 2008), especially in materials that are not ultra pure (Cheng et al, 1999). They found that the bulk shear strength is about 60% less than previous estimates (Hemley et al, 1997), suggesting that lower stresses can result in dislocation creep and hence a lower viscosity.…”