Effects of pre-strain and stress level on stress relaxation ageing behaviour of 2195 Al–Li alloy: Experimental and constitutive modelling

Abstract

In the present work, the stress relaxation behaviour, mechanical properties and microstructural evolution of 2195 Al–Li alloy with different pre-strains (0%, 2% and 4%) have been experimentally investigated under different initial stress levels at 180 °C for 16 h. It is found that pre-strain can extend the duration of the first stress relaxation stage and significantly promote the stress reduction. Meanwhile, a new “multi-step stage” stress relaxation feature, containing a particular linear relaxation stage, is observed for the 2195 Al–Li alloy during stress relaxation ageing (SRA) process. In addition, the tensile strength of the alloy increases with the increase of pre-strains and initial stresses, while the corresponding elongation is opposite. The evolution of precipitates, including G.P. Zones, θ″, θ′ and T1 phases, have been characterized by transmission electron microscope (TEM). The number of T1 phases increases with the increase of the pre-strains and initial stresses, while the number of θ′ phases gradually decreases. Based on precipitation kinetics, stress relaxation mechanism and dislocation theory, a physically-based SRA constitutive model considering the evolution of dislocation density and relative volume fraction of T1 and θ′ precipitates is established. The results calculated by the constitutive model are in good agreement with the experimental data.