Enhancing strength and ductility of Al–Cu–Li alloys by microalloying both Er and Zr to promote complete transformation from δ′ (Al3Li) to T1 (Al2CuLi) precipitates

Abstract

The impact of erbium (Er) on grain refinement in Al–Cu–Li alloys is limited by the thermodynamic instability of Al3Er inoculants. Nevertheless, the concurrent addition of both Er and Zr to Al–Cu–Li alloys results in a marked refinement of the average grain size. Er exerts a significant influence on the precipitation sequence within Al–Cu–Li alloys, promoting a complete transition from δ′ (Al3Li) and θ′ (Al2Cu) to T1 (Al2CuLi) precipitates during aging at 120 °C, thereby substantially enhancing both strength and ductility. Through high-resolution transmission electron microscopy (HRTEM), the underlying mechanism of the T1 nucleation by Er microalloying has been elucidated. It has been demonstrated that the presence of Er leads to the formation of solute enrichment regions (SERs) comprising Cu, Zn, Mg and Er. These SERs serve as catalysts for the development of Cu-rich atomic planes along the {111}Al, providing a direct precursor structure for T1 nucleation. Consequently, the synergistic microalloying of Zr and Er not only effectively refines the grain structure but also facilitates the elimination of the δ′ phase. The addition of Er facilitates the complete transformation of the δ′ phase into the strong and ductile T1 phase, resulting in a significant enhancement of the alloy's strength and ductility. Following 100 h of aging at 120 °C, the ultimate tensile strength has increased from 373 to 457 MPa, and the elongation after fracture has risen from 2.8% to 4.9%.

Keywords

• Al-Cu-Li alloy
• Nucleation
• Precipitation kinetics
• Er microalloying
• Phase transition