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

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.

KEYWORDS:

• Al–Li alloys
• creep aging
• precipitation
• aging kinetics
• heat treatment

Acknowledgement

The authors would like to thank the help from all labmates at the Integrated Computational Materials Engineering (ICME) lab, Beijing Institute of Technology, China.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The research work is supported by National Natural Science Foundation of China (Project number:52073030) and National Natural Science Foundation of China-Guangxi Joint Fund(U20A20276). The funding support from the University of Science and Technology Beijing (USTB) through the International People-to-people Exchange program in Science, Technology and Civilisation (2022KFYB007) is also acknowledged.