On the shear strain associated with T₁ precipitation in Al-Cu-Li(-Mg-Ag) alloys

The formation mechanism and the distribution arrangements of T₁ precipitate plates in peak-aged samples of an Al-4Cu-1.2Li-0.3Mg-0.4Ag (wt.%) alloy are studied using atomic-resolution high-angle annular dark-field (HAADF) and annular bright-field (ABF) scanning transmission electron microscopy (STEM). The formation of a T₁ plate of a single unit-cell thickness from the α-Al matrix is found to be associated with a large shear strain that is equivalent to the formation and glide of two different Shockley partial dislocations on two alternate {111}_α planes. The T₁ precipitate plates tend to cluster into various configurations, including T-shaped and V-shaped configurations comprising two plates of two different T₁ variants, and X-shaped configurations consisting of three or four plates of two different variants. The signature of the shear strain is clearly visible in these configurations. A single T₁ plate of one unit-cell thickness is elastically distorted in a T-shaped configuration, and the two T₁ plates of the same variant in an X-shaped configuration are displaced with respect to each other. Different types of stair-rods are also formed in the intersection of the paired T₁ plates in the V-shaped configurations.