Experimental evaluation of effects of damage evolution at coarse secondary phase particles on mechanical properties of aluminum alloys

Abstract

The present work is aimed at experimental evaluation of the effects of damage evolution at coarse intermetallic particles in Al-Li-Cu-Mg alloy, with particular attention on fracture toughness. To vary volume fraction of the particles, various thermo-mechanical treatments have been applied. Strength, ductility and fracture toughness decrease with the increase in volume fraction from 1.2 to 2.2%. Especially, σ0.2 and crack propagation resistance are significantly affected. In-situ SEM observation of the fracture toughness test has also been used to characterize damage evolution at the particles and crack propagation behavior. Some types of particles remain intact even adjacent to the fracture surface, while others are extensively fractured far ahead of a crack-tip. A combination of the crack-tip singularity and micromechanics is used to estimate in-situ strengths of the particles. It is concluded that the particle strengths have strong dependence on diameter, and that, due to the existence of the damaged particles, crack deflection and formation of secondary cracks occur in the case of high particle volume fraction. Fracture mechanical analysis reveals that reduction in the crack propagation resistance is attributed to the amplification of mode I crack driving force due to the existence of microcracks. Finally, measured fracture toughness is interpreted utilizing a computer simulation constructed in the previous study. Although individual microstructural factors have only modest effects, competitive and synergistic effects are attributable to the measured large drop in the fracture toughness.