An investigation of Harper-Dorn creep at large strains

Harper-Dorn creep in Al was investigated under the condition of large strai ns (more than 0.1). In performing the investigation, two Al grades were cre ep tested at 923 K. The first grade is of 99.9995% purity (99.9995 Al) wher eas 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 sub structure developed during creep reveals that the details of the substructu re 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 equi axed subgrains is formed. By contrast, in 99.9995 Al, a wide range of subst ructural features including new grains, boundaries with high and low disloc ation 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 domi nant restoration mechanism during the creep of 99.99 Al, while dynamic recr ystallization is the dominant restoration mechanism during the creep of 99. 9995 Al. Second, the occurrence of Harper-Dom creep in Al requires that two conditions be satisfied: high purity Al and very low dislocation density i n the annealed samples (10(3)-3X10(4) cm(-2)). Third, while the results ver ify the Newtonian nature of Harper-Dom 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. (C) 2001 Acta Materialia Inc. Publis hed by Elsevier Science Ltd. All rights reserved.