Crystallographic characterization of Al₁₈Mg₃Ti₂ intermetallic phase in Al–Zn–Mg–Cu–Zr–Ti alloy

We studied the crystallographic characteristics of the Al₁₈Mg₃Ti₂ intermetallic phase in a solution-treated Al–7.6Zn–2.6Mg–2.0Cu–0.1Zr–0.07Ti (wt.%) alloy utilizing transmission electron microscopy (TEM). Cubic spinel Al₁₈Mg₃Ti₂ intermetallic phase features {111} faceted interfaces with the Al matrix. Selected-area electron diffraction pattern, stereographic projection, and atomic projection studies indicated four possible orientation relationships between the Al₁₈Mg₃Ti₂ phase and the Al matrix, which are categorized by a misorientation angle (θ) at {111} planes of each phase (i.e., θ = ± m, 70.5°±m, where m = 12–14°). High resolution TEM analyses confirmed that the growth of the Al₁₈Mg₃Ti₂ intermetallic phase is based on the ledge mechanism on the {111} interfaces along with multiple {111} twinning. This growth mechanism of the Al₁₈Mg₃Ti₂ phase contributes to the formation of faceted morphologies with {111} interfaces. The interface between the Al₁₈Mg₃Ti₂ phase and Al matrix provides a heterogeneous nucleation site for the L1₂-precipitate and a Zn-rich phase.