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

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 h9(Al2Cu), Al3Zr, and T-1(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 degrees C, the density and size of T-1 phase keep increasing through the process and reach peak in about 18 h. T-1 phase tends to have a certain orientation relationship of (0001)T-1 //{{111}(Al), < 1010 >(T1) // < 110 >(Al) 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 T-1 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 T-1 phase reaches a threshold.