Understanding the mechanical properties of 2198 Al-Li-Cu alloy in relation with the intra-granular and inter-granular precipitate microstructure

Abstract

3rd generation Al-Li-Cu alloys such as AA2198 are known to present extremely high strength levels because of the precipitation state of the material. The final precipitation state is reached via a complex sequence that can involve notably solute clusters, GP zones, θ', T1 and δ' phases. In this work, we have studied the evolution of mechanical properties during heat treatment. Microhardness and tensile tests have shown a direct link with the evolution of the size of intra-granular T1 precipitates. The evolution of the fracture mode in relation with the precipitation state has been followed by the so-called short bar test that allows to load relatively thin plates in the short transverse direction, which is particularly sensitive to the inter-granular fracture mode. Along the heat treatment, from the naturally aged T351 state and along ageing at 155°C, we show a clear transition from a ductile transgranular fracture mode to a ductile inter-granular fracture mode, controlled by the precipitation of the T 1 phase at the grain boundaries. © 2010 IOP Publishing Ltd.