Simultaneously improved the strength and ductility of cast Al-3Li-2Cu alloy by Ti addition and heat treatment

This work systematically investigated the impact of minor Ti addition and heat treatment on grain structure, precipitates, precipitate-free zone (PFZ), and the performance of the Al-3Li-2Cu alloy. Minor Ti addition significantly refines the as-cast alloy, leading to a transition from columnar grains to equiaxed grains. The primary Al3Ti phases, with a low mismatch (0.99%) to the matrix, act as heterogeneous nucleation sites for alpha-Al, while the presence of Ti solutes restricts grain growth. As a result, the average grain size can be refined to 35.2 mu m by adding 0.2 wt% Ti. Besides, minor Ti addition affects the corresponding precipitation behavior of the alloy. Detailed TEM observations reveal that the addition of Ti inhibits the growth of S'(Al3Li) precipitates and the accompanying S'-PFZ, which should be related to the interaction between Ti solutes and vacancies. Besides, reducing the aging temperature exerts a similar effect. Finally, the 0.2 wt% Ti-modified alloy exhibits excellent mechanical properties after aging at 175 degrees C for 8 h. It demonstrates a high yield strength (272 MPa), a high ultimate tensile strength (399 MPa), and an acceptable elongation (5.9%), which represent an increase of 8.8%, 34.8%, and 293.3%, respectively, compared to the alloy without Ti addition. The simultaneous improvement in strength and ductility is attributed to the presence of fine and equiaxed grains, high-density and homogenous S' precipitates, and a narrow S'-PFZ. In-depth discussions are provided to elucidate the underlying mechanisms governing the microstructure and mechanical properties.