EFFECT OF Er MICRO-ALLOYING ON MECHANICAL PROPERTIES AND MICROSTRUCTURES OF 2055 Al-Li ALLOY

Al-Li alloys are considered as the ideal structural materials for aerospace industry because of their low density, high specific strength and specific elastic modulus as well as low fatigue crack growth rate and good low temperature performance. 2055 Al-Li alloy among new Al-Li alloys developed recently is a super-high strength Al-Li alloy. An important method to improve the performance of Al-Li alloys is to add micro-alloying elements. Er-microalloying in Al alloy has been investigated much, but the study on Al-Li alloy is still seldom reported. In this work, the effect of 0.2%Er and 0.4%Er addition on the microstructure and mechanical properties of 2055 Al-Li alloy sheet with T8 aging (6% cold rolling pre-deformation and aging at 160 degrees C) were investigated. The results show that the addition of 0.2% Er significantly decreases the strength by about 50 MPa, but enhances the elongation; the strength is further decreased by about 100 MPa with the addition of 0.4%Er. The precipitate types in Er micro-alloyed Al-Li alloy are not changed with the addition of Er, and the aging precipitates are still T1 (Al2CuLi) and theta' (Al2Cu) phases. In the Er-microalloyed Al-Li alloy, the incubation time of T1 precipitate is longer, and its precipitation rate is decreased, accordingly the aging response is slowed. Meanwhile, under peak-aging condition, the fraction of T1 precipitates, especially theta' precipitates in the Er-microalloyed Al-Li alloy is decreased, which results in a decrease of strength. As Er is added to the Al-Li alloy, Er-contained particles Al8Cu4Er are formed during solidification process, and their amount is increased with the addition increasing. These particles cannot be completely dissolved into the alloy matrix during homogenization process. After solution treatment following cold rolling, they are not yet dissolved into the solid solution. These particles contain Cu and Er simultaneously, and the concentration of dissolved Cu in solid solution is therefore decreased. With increasing Er addition, the Cu concentration in solid solution is further decreased. The precipitation rate of T1 is consequently decreased, slowing the aging response of the Er-microalloyed Al-Li alloy. And this factor also decreases the fraction of T1 and theta' precipitates and lowers the alloy strength.