Effect of microstructure and strength on the fracture behavior of AA2219 alloy

The plane strain fracture toughness, KJIc and crack growth toughness in terms of the non-dimensional tearing modulus, TR of Al-Cu alloy were evaluated following the J-integral method. An empirical relation is proposed for estimating the fracture toughness from the tensile properties. The estimated fracture toughness values from the empirical relation are found to be in good agreement with the measured fracture data. The microstructural as well as strength variations in the subject alloy were examined through different aging treatments (viz., under aging (UA), natural aging (NA), peak aging (PA) and over aging (OA)). The microstructural observations in different aging conditions were made through transmission electron microscopy (TEM) to understand the type of phase or intermediate stages of the phase present (GP zones, θ″, θ′ or θ). Fracture toughness testing has been carried out on 14 mm thick compact tension (CT) specimens and estimated the crack extension by unloading compliance technique. It was noted that the yield strength increases and fracture toughness decreases with the increase in the degree of aging from NA to PA condition. On overaging the alloy to a yield strength level as that of UA, it was not possible to retain the UA level of fracture toughness. The highest crack growth toughness is exhibited by the NA temper. The fractography analysis revealed that the fracture mode was predominantly transgranular dimpled rupture. Fracture initiation occurred by void nucleation at the second phase particles. The differences in the fracture toughness of the alloy in different aging conditions were shown to be dependent on the differences in the matrix deformation behavior and the strength differential.