Unraveling mechanism of structural transformation from D022-Al3Y to L12-Al3(Y, Zr) phase by inward diffusion of Zr and anti-phase boundary in the Al-Zr-Y alloy at elevated temperature

The Y element preferentially forms the brittle D0-type Al₃Y phase rather than the L1₂-Al₃Y phase, challenging to utilize the Al₃Y phase as an L1₂-structural phase. However, a few studies have reported the presence of L1₂-Al₃(M, Y) phases induced by the multi-addition of Y with L1₂ phase-stable elements. It is still unclear how the multi-addition of Y with L1₂ phase-stable element facilitates to precipitate preferentially L1₂ phase rather than D0-type Al₃Y phase, to date. This study investigated the mechanism of irreversible structural transformation of the Al₃Y phase from equilibrium D0-type phase to metastable L1₂ phase by economically viable L1₂ phase-stable Zr element in Al-Zr-Y alloy at elevated temperature, using transmission electron microscopy. The results demonstrate that the inward diffusion of the Zr element into the D0-type phase stabilizes the Al₃Y phase to the L1₂-Al₃(Zr, Y) phase as aging progresses, indicating an irreversible structural transformation from the D0-type phase to the L1₂ phase induced by Zr diffusion as well as anti-phase boundary (APB). The APB helps pipe diffusion of Zr to the adjacent D0₂₂ phase, thereby accelerating the kinetic process of this structural transformation better than inward diffusion of Zr atoms without APB. The movement of APB provides the driving force for local structural transformation from D0-type Al₃Y to L1₂-Al₃(Zr, Y) phase by rearrangement of the atomic configuration of the D0₂₂ structure at the vicinity of the APB layer. This rearrangement of atomic configuration synergistically acts for the phase transformation with the Zr pipe diffusion. The above statement is supported by the presence of additional L1₂ atomic layers between the APB and D0-type phase observed at the structural transition point, providing direct evidence for a local irreversible structural transition.