Interactions between zirconium and manganese dispersoid-forming elements on their combined addition in Al–Cu–Li alloys

The present work aims to clarify the interactions that can occur between Zr and Mn in their role as dispersoid-forming elements in Al–Cu–Li alloys when they are jointly added to rolled products. A combination of Zr and Mn are used because their opposite microsegregation patterns can potentially increase the uniformity of the dispersoid pinning pressure, and help prevent recrystallization. It is shown, in the AA2198 base alloy studied, that their combined addition in fact reduced recrystallization resistance. The source of this apparent contradiction has been investigated in detail by examining the behaviour of the dispersoids they form, Al3Zr and Al20Cu2Mn3, from the as-cast ingot to the final rolled product. The Al3Zr dispersoids were found to be subtly affected by the presence of Mn, which reduced their number density. The main mechanism responsible is the incorporation of low levels of Zr in Mn-rich particles, which slightly reduces the Zr supersaturation in the matrix. Estimates of the level of matrix Zr loss, and modelling of dispersoid precipitation, have been used to demonstrate the high level of sensitivity of this effect on the Al3Zr dispersoid-free band widths present and resultant recrystallization resistance when the alloy is rolled to sheets.