Bulge Test Characterization of Static Softening and Dynamic Instabilities in Foils of an Al-Based Alloy

ABSTRACTEffects of recovery and recrystallization on subsequent stress – strain behavior were studied in foils of non-age hardenable Al based alloy. A bulge testing device for mechanical testing of thin free standing films and foils, enabling the detection of large strains, was employed. The bulge tester was used at RT both for the predeformation of the foil in the biaxial mode and also for the study of softening after subsequent one-step annealing. Three stages of strength drop as a function of the annealing temperature were observed between RT and 590°C. The contribution of different annealing processes to the softening was established using transmission electron microscopy. It was found that below 200°C only redistribution of dislocations inside dislocation cells and refinement of the cell structure occurred. Between 200°C and 380°C the formation of a subgrain structure was observed. The softening process is terminated as partial recrystallization takes place at higher annealing temperatures. Dynamic interaction of solutes with dislocations was revealed during prestraining as well as poststraining of foils. This effect resulted in the appearance of a negative strain rate sensitivity and dynamic instabilities after appropriate prestraining and annealing conditions. The above results show that bulge testing of Al foils allows to study the successive annealing stages by measuring the most important macroscopic parameters of plastic deformation (yield stress, work hardening rate, ductility, strain rate sensitivity, etc.). These stages could be related to the microstructure evolution.