Acoustic emission characterization of the microplastic mechanisms of ductile fracture in aluminum 6061

Ductile fracture of metals through transgranular microvoid coalescence is recognized as a complex mechanism involving many dynamic and competitive interactions of a wide range of material properties, processes and behavior. A partial list of these would include microstructure, yield strength, strain hardening, stress concentration, particle size and distribution, strain localization and accumulation, and characteristics of the particle-matrix interface. While the understanding of the intricacies surrounding this phenomenon is advancing steadily, it remains founded on the basis that ductile fracture is the end result of three fundamental processes: void nucleation, growth and coalescence. Significant progress has been made in the characterization and modeling of the growth phase of this process; however, the micromechanisms driving nucleation and coalescence are less well understood. An improvement in material design from the standpoint of fracture resistance can be achieved through better comprehension of these void nucleation mechanisms. This research utilizes the acoustic emission phenomena of an aluminum alloy subjected to uniaxial load application in an attempt to characterize the nature of the microplastic events occurring in strain regimes well below the bulk yield response of the material. A distinct relationship between the characteristic acoustic emission response as a function of imposed triaxial constraint is identified for varied precipitation aging times. A fractographic evaluation of the final microvoid morphology suggests a stronger contribution from particle structures to void nucleation under elevated triaxial stress-state conditions