Bulge Test Characterization of Static Softening and Dynamic Instabilities in Foils of an Al-Based Alloy
- PDF / 2,372,599 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 79 Downloads / 177 Views
Bulge Test Characterization of Static Softening and Dynamic Instabilities in Foils of an AlBased Alloy Miroslav Cieslar, Ayatollah Karimi and Jean-Luc Martin Department of Physics, Swiss Federal Institute of Technology (EPFL), CH–1015 Lausanne, Switzerland. ABSTRACT Effects 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.
INTRODUCTION Experimental techniques used for the study of stress-strain relations in thin films and foils differ from those used for bulk materials. The most commonly used conventional uniaxial tensile test of bulk materials is in general applicable for thin foils and films only with great difficulties. This is due to the extreme sensitivity of this method to any surface flaws and defects on the sample edges. Nanoindentation methods are a good alternative for the study of thin layers, which provides information not only on the hardness of the layer, but also on the creep and elastic properties. Nevertheless, these methods are limited only to surface layers on bulk materials or to thin films bound to a substrate. Bulge testing is a method suitable for the study of mechanical properties of free standing thin films and foils, which eliminates the problem of defects at the sample edges. Within the last decade, the bulge test became a well-established method permitting characterization of elastic and plastic deformation. The most often used analysis of experimental data is based on the spherical cap model and the assumption that both the stress and the strain state are equi-biaxial.
Data Loading...
