Precipitation of T₁ phase in 2198 Al–Li alloy studied by atomic-resolution HAADF-STEM

Abstract

Aging treatment plays an important role in strengthening of 2198 Al–Li alloy. Through a serious of heat treatment processes, a large amount of precipitates emerge, mainly observed to be θ′(Al₂Cu), Al₃Zr, and T₁(Al₂CuLi), among which, T₁ turns to be the most important precipitate that contributes to the strengthening of 2198 Al–Li alloy. While the temperature of the aging process is 175 °C, the density and size of T₁ phase keep increasing through the process and reach peak in about 18 h. T₁ phase tends to have a certain orientation relationship of ${\left({0001} \right)_{{{\rm{T}}_1}}}//{\left\{ {111} \right\}_{{\rm{AI}}}}$, ${\left\langle {1010} \right\rangle _{{{\rm{T}}_{\rm{1}}}}}//{\left\langle {110} \right\rangle _{{\rm{AI}}}}$ and may have different kinds of multilayered structures. In most of the multilayered structures, the distance between two adjacent copper-rich laths is less than that in classical single-layered phase. Thus, it can be inferred that the microstructure of T₁ phase might change in the process of developing from single-layered structure to multilayered structure. In addition, the interactions between different phases become relatively frequent when the density of T₁ phase reaches a threshold.