Long-term annealing of high purity aluminum single crystals: New insights into Harper-Dorn creep

Single crystals of 99.999% and 99.9999% pure aluminum were annealed at high elevated temperatures (0.98Tm) for relatively long times of up to one year, the longest in the literature. Remarkably, the dislocation density remains relatively constant at a value of about 109 m−2 over a period of one year. The stability suggests some sort of “frustration” limit. This has implications towards the so-called “Harper-Dorn creep” that generally occurs at fairly high temperatures (e.g. > 0.90Tm) and very low stresses. It is possible that ordinary five-power-law creep occurs within the tradition Harper-Dorn regime with very low initial dislocation densities in aluminum. Higher initial dislocation densities, such as with this annealing study, may lead to Harper-Dorn (Newtonian) creep.