From dimpled to intergranular fracture: the role of aging in ductility degradation of Al–Cu–Li alloy

Abstract

This study investigates the effect of aging on the mechanical properties and fracture behavior of the AA2055 (Al-Cu-Li) aluminum alloy. Specimens were subjected to artificial aging at 165 °C for 5, 50, and 150 h, corresponding to under-aged, peak-aged (maximum strength), and over-aged conditions, as defined by the aging curve. In the peak-aged condition, the alloy exhibits yield strength of 495 MPa, ultimate tensile strength of 530 MPa, and elongation to failure of ~ 6.2 %. Ductility drops sharply upon aging from the under-aged state (~24 %) to the peak-aged condition and remains low even after over-aging. Fractographic analysis reveals that the under-aged alloy fails by ductile transgranular fracture with dimple formation. Prolonged aging leads to matrix strengthening due to precipitation of the T1 (Al2CuLi) phase, concurrent with grain boundary degradation, which promotes strain localization and a transition to brittle intergranular fracture. The reduction in ductility from ~ 24 % to ~ 6 % with increasing aging time from 5 to 150 h is attributed to the evolution of the nanostructure of precipitates and a shift in the dominant fracture mechanism – from ductile (dimple) to intergranular.