Creep behavior and deformation-induced grain growth in electrodeposited (ED) nanocrystalline (nc) Ni with a grain size of about 20 nm were studied over more than five orders of magnitude of strain rate (10 À9 s À1 to 2 3 10 À4 s À1 ) at 393 K (0.23 T m , where T m is the melting point). In addition, the activation energy for the creep in ED nc-Ni was determined by using the temperature change procedure. The results show that the creep behavior of the material is characterized by (a) a stress exponent that increases continuously from about 4.5 to about 30 with increasing applied stress; (b) an apparent activation energy for creep in the range of 126 to 141 kJ/mol; (c) an activation volume of about 20 b 3 , where b is the Burgers vector; and (d) a grain size that upon loading, grows, attaining a constant value once steady-state creep is approached. The mechanical characteristics cannot be accounted for by current deformation processes. Analysis of the creep data along with consideration of available information leads to the suggestion that the creep behavior of nc-Ni arises from a deformation process that is based on the concept of dislocation-accommodated boundary sliding. By quantitatively developing this concept, a rate-controlling deformation process is formulated. It is shown that the predictions of this process are in good agreement with experimental results and trends.

Thermal stability in bulk ultrafine-grained (UFG) 5083 Al that was processed by gas atomization followed by cryomilling, consolidation, and extrusion, and that exhibited an average grain size of 305 nm, was investigated in the temperature range of 473 to 673 K (0.55 to 0.79 T m , where T m is the melting temperature of the material) for different annealing times. Appreciable grain growth was observed at temperatures Ͼ573 K, whereas there was limited grain growth at temperatures Ͻ573 K even after long annealing times. The values of the grain growth exponent, n, deduced from the grain growth data were higher than the value of 2 predicted from elementary grain growth theories. The discrepancy was attributed to the operation of strong pinning forces on boundaries during the annealing treatment. An examination of the microstructure of the alloy suggests that the origin of the pinning forces is most likely related to the presence of dispersion particles, which are mostly introduced during cryomilling. Two-grain growth regimes were identified: the low-temperature region (Ͻ573 K) and the high-temperature region (Ͼ573 K). For temperatures lower than 573 K, the activation energy of 25 Ϯ 5 kJ/mol was determined. It is suggested that this low activation energy represents the energy for the reordering of grain boundaries in the UFG material. For temperatures higher than 573 K, an activation energy of 124 Ϯ 5 kJ/mol was measured. This value of activation energy, 124 Ϯ 5 kJ/mol, lies between that for grain boundary diffusion and lattice diffusion in analogous aluminum polycrystalline systems. The results show that the strength and ductility of bulk UFG 5083 Al, as obtained from tensile tests, correlate well with substructural changes introduced in the alloy by the annealing treatment.

No 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.

No abstract