Microstructure Evolution and Mechanical Properties of AA2099 Al–Li Alloy with Tailored Li‐Containing Precipitates in Uniaxial Compression at Medium Temperature

Microstructure characteristics and mechanical behavior of AA2099 Al–Li alloy with no pre-existing Li-containing precipitates (AA2099-1 sample), pre-existing δ′ precipitates (AA2099-2 sample), pre-existing T1 phase (AA2099-3 sample) and pre-existing T2 phase (AA2099-4 sample) are systematically investigated via isothermal uniaxial compression at 250 °C in the present study. Experimental results demonstrate that at the onset of plastic deformation, dynamic precipitation of small-sized T1 phase occurs rapidly within AA2099-1 sample, while it will be hindered within AA2099-2 sample. The increasing plastic strain benefits to dynamic precipitation of small-sized T1 phase in AA2099-2 sample. Consequently, AA2099-1 and AA2099-2 samples possess similar and intermediate mechanical behaviors. In terms of AA2099-3 sample, the existence of large-sized T1 phase results in the maximum yielding stress. However, some regions within these large-sized T1 precipitates are suspected to be sheared by cross-slip, leading to the destruction of crystallographic structure and the formation of Al matrix intervals. This aspect is responsible for the gradual degradation in true stress-strain curve after peak stress. As for AA2099-4 sample, dynamic precipitation rarely happens during plastic deformation and the interaction between dislocation and the pre-existing T2 phase belongs to Orowan looping, resulting in the minimal mechanical response. Besides, AA2099-1 sample possesses the average minimum deviation angle (MDA) of ~ 16.5° between the loading direction and the  crystal direction, whereas AA2099-4 sample owns the average MDA of ~ 7.5°. The difference in MDA is mainly attributed to δ′ phase and T1 phase, which will separately accelerate and postpone the rotation of orientation towards the  crystal direction.