Accelerating the precipitation kinetics of nano-sized T1 and S’ phases in Al-Cu-Li alloys by hot-deformation and creep-aging

Abstract

ABSTRACT It is well known that a Cu/Li ratio over 4.0 leads to significant precipitation hardening in the third generation of Al–Li alloys, which have been widely used in aerospace and aeronautic industries. However, increasing Cu levels not only cancels some gains in weight reduction but also results in reductions in ductility when the traditional age-hardening process is adopted. In this study, we show that higher strength and ductility can be achieved simultaneously by replacing traditional artificial aging with new creep aging and tailoring the entire process from casting to age hardening without changing any nominal chemical compositions. Surprisingly, the precipitation kinetics of AA2060 alloy is significantly enhanced through the combination of hot extrusion, creep aging, and pre-strain compared to traditional artificial aging. For example, at 160°C, it follows the transformation path of supersaturated solid solution (SSSS)→GP zones + δ′+T1 precursors which will further transform into T1 +S’; at 200°C, SSS→ GP zones, and they transform into T1 + S’ immediately. The precipitation of θ′ and δ′ phases is significantly inhibited at elevated temperatures due to the competing relationship with T1 and S’ phases at a peak-aged state. Contrary to traditional knowledge in Al–Li alloys, we have found that the greater extrusion ratio at 33.0 and higher aging temperature at 200°C can give a much higher peak aging strength with average ultimate tensile strength 659 MPa.