An investigation of Harper–Dorn creep at large strains

Harper–Dorn creep in Al was investigated under the condition of large strains (more than 0.1). In performing the investigation, two Al grades were creep tested at 923 K. The first grade is of 99.9995% purity (99.9995 Al) whereas the second grade is of 99.99% purity (99.99 Al). The results show that 99.99 Al, unlike 99.9995 Al, does not exhibit the Harper–Dorn creep at low stresses; and that the creep curves associated with Harper–Dorn creep in 99.9995 Al exhibit regular, periodic accelerations. An examination of the substructure developed during creep reveals that the details of the substructure in 99.99 Al are significantly different from those characterizing Harper–Dorn creep in 99.9995 Al. In 99.99 Al, an extensive, regular array of equiaxed subgrains is formed. By contrast, in 99.9995 Al, a wide range of substructural features including new grains, boundaries with high and low dislocation densities on opposite sides of the boundary, and localized regions of very high dislocation density, are observed. Consideration of the present data leads to three important findings. First, dynamic recovery is the dominant restoration mechanism during the creep of 99.99 Al, while dynamic recrystallization is the dominant restoration mechanism during the creep of 99.9995 Al. Second, the occurrence of Harper–Dorn creep in Al requires that two conditions be satisfied: high purity Al and very low dislocation density in the annealed samples (10 3–3×10 4 cm −2). Third, while the results verify the Newtonian nature of Harper–Dorn creep at small strains (less than 0.01), they indicate that under the condition of large strains (more than 0.1), the stress exponent is more than 2.