Polycrystals were cyclically deformed at room temperature under constant stress amplitudes. Cyclic softening after initial hardening was found in specimens with small particles (4nm), but no cyclic softening took place in specimens with large particles (11nm). Transmission electron microscopic observations revealed that dislocations were uniformly distributed under all applied stress amplitudes in the specimens which contained large particles, whereas clearly developed slip bands formed in the cyclically softened specimens bearing small particles. Two-step aging caused a decrease in the width of precipitate-free zones in the large-particle specimen, resulting in an improvement in fatigue life.

Effects of Al3Sc Particle Size and Precipitate-Free Zones on Fatigue Behavior and Dislocation Structure of an Aged Al–Mg–Sc Alloy. C.Watanabe, R.Monzen, K.Tazaki: International Journal of Fatigue, 2008, 30[4], 635-41