Structural relevance among nano-phases along the common habit planes {111} and its implications for the design of cross-over Al alloys

Ω, T₁, Y, η' and η nano-phases in different series Al alloys are known to all precipitate along the major slip planes {111}_Al. Their precipitation in multi-element cross-over Al alloys could thus be very intricate. In this work, atomic resolution HAADF-STEM and first-principles calculations were combined to comprehensively investigate their atomic structures and energetics in a set of model Al alloys. Our results revealed that they all tend to initially form a 2D layered structure consisting of an isostructural (Al,Cu) O-unit along the {111}, and that their further evolution sensitively depends on alloy composition. Alloying with Mg and/or Ag stabilizes the O-units to form a single-layer Ω (SL-Ω or Ω') in Al-Cu-Mg(-Ag) alloys. Alloying with Li stabilizes O-units to form SL-T₁ in Al-Cu-Li alloys. Alloying with Mg and Zn stabilizes O-units to form SL-Y in Al-Zn-Mg-Cu alloys. For cross-over Al alloys with a low Mg+Zn and low Li content, O-units would grow into O-O pairs to induce the SL-Ω (Ω') → multi-layer (ML)-Ω transformation. Increasing the Li content helps SL-T₁ grow continuously or transform into ML-T₁ with an O-O'-O stacking. Both mechanisms take actions to suppress ML-Ω. For cross-over Al alloys with high Mg+Zn content, O-units grow into O-R pairs and further into R-R⁻¹ pairs, inducing the transformation of SL-Ω/T₁/Y → η' → η in strong preference over Ω and T₁ in Al-Zn-Mg-Cu(-Ag)(-Li) alloys. These results can fundamentally explain diverse experimental observations and provide many profound perspectives on the competitive precipitation in cross-over Al alloys.