Understanding the ductility versus toughness and bendability decoupling of large elongated and fine grained Al 7475 - T6 alloy

Abstract

7XXX aluminium alloys are widely used in aerospace components due to their high strength to weight ratio. However, first generation 7XXX alloys has a limited fracture toughness and crack propagation resistance. New generation of 7XXX alloys succeeds increasing fracture toughness, by playing on both alloying elements composition and purity level. One of the toughest alloy is the 7475 Al alloy, with a fracture toughness (KIc) that reaches up to 60 MPa√m in T6 conditions. Friction stir processing (FSP) is applied to 7475 Al rolled plates in order to generate a fine-grained microstructure. Heat treatments (HT) are performed in order to set rolled and FSPed microstructures into T6 peak-aged condition. While the T6 FSPed microstructure shows a significant fracture strain improvement in uniaxial tensile loading, this microstructure does not improve the crack propagation resistance compared to the T6 rolled material, neither on compact tension nor on bending specimens. Which mechanisms are responsible for this effect? A decoupling between ductility and toughness in 7XXX aluminium alloys is unexpected and the understanding of this phenomenon is of prime importance for mechanical design. This work reveals the competing effects of the FSPed microstructure on crack initiation and crack propagation. FSPed material delays the crack initiation under loading but the FSPed fine grains favour fully intergranular crack propagation. In comparison, the energy needed for the crack to cut through or circle around the rolled elongated grains is much higher in the rolled material. Indeed, elongated rolled grains act as crack arresters when the crack direction is perpendicular to the grain's main axis. In this case, failure of the rolled material leads to extensive transgranular propagation, increasing the energy absorption and rising the KIc values to higher levels. This work illustrates the formation and propagation of cracks in the rolled and FSPed 7XXX alloys microstructures in order to explain the damage sequence and the competition between transgranular and intergranular failure.