High-Temperature Phase Transformations in Al-Li-Cu-Mg-Zr-Sc Alloy Studied via In Situ Electron Microscopy

A homogenization of billets from Al-Cu-Li-Mg-Sc-Zr alloys should be accomplished at high annealing temperatures exceeding 500 degrees C. This type of aluminum alloy is susceptible to the depletion of surface layers from Li. Therefore, choosing a suitable homogenization temperature and duration is a crucial step in assuring a homogeneous distribution of alloying elements and optimal exploitation of the potential of the alloy. In situ heating in an electron microscope was performed on a twin-roll-cast Al-Cu-Li-Mg-Sc-Zr alloy to understand the peculiarities of the homogenization process. Four types of primary phase particles rich in Cu, Li, Mg, and Fe were identified in the as-cast material. They appear as coarse particles at the boundaries of eutectic cells. Their partial dissolution occurs at temperatures above 450 degrees C. They are almost fully dissolved at 550 degrees C, except for complex phases containing Fe and Cu. Small dimensions of eutectic cells in the range of 10 mu m assure a homogeneous distribution of the main alloying elements within the matrix after 20 min of annealing at 530 degrees C. Direct comparison with the same material prepared by mold casting indicates that such short annealing times result in the dissolution of the main primary phase particles but do not assure a homogeneous distribution of the alloying elements in the whole volume of the specimen.