T 1 (Al 2 CuLi) Precipitation in Aluminium–Copper–Lithium Alloys

T ₁ precipitation has been studied by electron microscopy in aluminium alloys containing (wt.-%) Cu 3.5, Li 1.5 and Cu 2.5, Li 2.0. The T ₁ phase forms as extremely thin hexagonal-shaped plates with a {111} habit plane and, depending on the degree of super-saturation, nucleates either at GP zones or at dissociated dislocations by a stacking-fault mechanism. The precipitate is bounded by 1/6 < 211 > Shockley partial dislocations and growth of the phase occurs by separation of these partial dislocations.