Investigation of stress effect on creep, precipitation and dislocation evolution of Al–Li alloy during creep age forming

The 3rd generation Al–Li alloy is a good candidate for large panel fabrication adopting creep age forming (CAF) because of its balanced synergy of high strength, good toughness, corrosion resistance and light weight. In this study, the creep-ageing behaviour of a 2xxx series 3rd generation Al–Li alloy in T8 state has been experimentally investigated under different stresses ranging from 300 to 430 MPa at 143 °C for up to 20 h. The corresponding evolution of precipitate size, its distribution and dislocation density during CAF has been analysed using transmission electron microscopy (TEM) and X-ray diffraction (XRD) tests. With the increase of the applied stress, the dominant deformation mechanism transformed from diffusion creep to dislocation creep at around 400 MPa, with corresponding stress exponent n = 2.9 in the former and n = 8.4 in the latter. A strong stress dependent characteristic has been observed in creep deformation and evolutions of dislocation density and precipitate size during creep-ageing. The creep strain increased dramatically above 400 MPa and the accumulated creep strain at 430 MPa was 7 times higher than that at 400 MPa after 20 h creep-ageing. Much higher dislocation density was observed at the applied stress above 400 MPa according to XRD measurements, which accelerates the precipitation kinetics. It was also found that the dominance of T₁ over θ′ precipitates increased with applied stress and ageing time during creep-ageing at 143 °C.