Effect of trace amounts of added Sc on microstructure and mechanical properties of 2055 aluminum alloy

The effects of Sc additions and isothermal aging on the microstructure and mechanical properties of 2055 aluminum alloy have been investigated. The isothermal aging temperatures investigated were 175 degrees C and 200 degrees C. Results show that the aging temperature does not affect the type of precipitates in both Sc-free and Sc-added 2055 AI alloys. While remarkable changes in the size and number density of T-1 (Al2CuLi) precipitates have been observed. For samples aged at 175 degrees C for 8 h, the average length of T-1 precipitates was increased from 39.9 nm to 45.6 nm, the number density decreased from 5.77 x 10(22)m(-3) to 3.68 x 10(22)m(-3) after Sc addition. On the contrary, when samples were aged at 200 degrees C for 8 h, the length decreased from 59.89 nm to 43.32 nm and number density increased from 2.13 x 10(22)m(-3) to 3.00 x 10(22)m(-3) by adding trace Sc. Slight deterioration of tensile strength and yield strength with higher ductility was founded at the aging temperature of 175 degrees C for 8 h due to the larger size and lower number density of T-1 precipitates along with the formation of a W (AlCuSc) phase during the casting or homogenization processes. When aging temperature increased to 200 degrees C, tensile strength and yield strength were significantly improved by Sc addition without losing ductility. Highly dense fine T-1 precipitates take responsible for the strength improvement, and to some extent it compensated the strength loss caused by the formation of W phase. A vacancy-prison mechanism was adopted to explain the effect of Sc additions on the precipitation of T-1 phase as a function of aging temperature.