Abstract: The effect of Cu on the evolution of the Al₃Zr phase in an Al-Cu-Zr cast alloy during solution treatment at 500 °C has been thoroughly studied by combining atomic resolution high-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy and first-principles calculations. The heat treatment initially produces a pure L1₂-Al₃Zr microstructure, allowing for about 13 % Cu to be incorporated in the dispersoid. Cu incorporation increases the energy barrier for anti-phase boundary (APB) activation, thus stabilizing the L1₂ structure. Additional heating leads to a Cu-induced “branched” path for the L1₂ structural transformation, with the latter process accelerated once the first APB has been created. Cu atoms may either (i) be repelled by the APBs, promoting the transformation to a Cu-poor D0₂₃ phase, or (ii) they may segregate at one Al-Zr layer adjacent to the APB, promoting a transformation to a new thermodynamically favored phase, Al₄CuZr, formed when these segregation layers are periodically arranged. Theoretical studies suggest that the branching of the L1₂ transformation path is linked to the speed at which an APB is created, with Cu attraction triggered by a comparatively slow process. This unexpected transformation behavior of the L1₂-Al₃Zr phase opens a new path to understanding, and potentially regulating the Al₃Zr dispersoid evolution for high temperature applications.

Key words: Aluminum alloys, Trialuminides, Anti-phase boundary, High-angle annular dark-field scanning transmission electron microscopy, (HAADF-STEM), First-principles calculations