Influência da distribuição inicial das partículas de segunda fase na microestrutura e comportamento mecânico de uma liga A-Zn-Mg-Cu processada por deformação severa

Abstract

In the literature, several studies have shown that the strength/weight ratio in aluminum alloys can be enhanced due to a significant increase in the mechanical strength by the application of severe plastic deformation techniques. Additionally, the material initial condition usually influences in the microstructural and mechanical behavior of these alloys throughout the deformation. Therefore, the present study aims to demonstrate, through practical experiments, the microstructural and mechanical behavior of an aluminum alloy processed by HPT (high pressure torsion) under different microstructural conditions before processing. The material was processed at room temperature with 1/8, 1, 5, 30, 50 and 100 HPT revolutions. To evaluate the alloy structural changes before and after processing, metallography procedures were performed followed by image acquisition via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). Analyses of the mechanical properties and their distribution throughout the samples were performed via microhardness tests. The results corroborate that the alloy initial microstructure before HPT processing directly interferes in the evolution of the structure and in the mechanical properties along deformation. For an initial microstructure with full amount of second phase particles, the results indicate an expressive grain refinement and elongation after large deformations (~100 turns). In addition, the material exhibits two distinct and consecutive hardening stages, unlike the conventional curves with only one stage, which are usually reported and accepted in the literature for aluminum alloys. This new strain hardening curve is attributed to a reduction in the grain boundary mobility due to the occurrence of solute segregation at the grain boundaries. On the other hand, for an initial solubilized microstructure with few second phase particles, the microstructure showed great refinement in the very first strain stages (~1 turn). Dynamic precipitation was observed throughout the processing leading to the occurrence of a hardness peak between 30-50 HPT turns. Subsequently, with the dissolution of these nanoparticles and predominance of recovery phenomena, the alloy showed a decrease in hardness values for strains up to ~100 HPT turns.