Two types of S phase precipitates in Al-Cu-Mg alloys

Transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) have been used to study S phase precipitation in an Al-4.2 Cu-1.5 Mg-0.6 Mn-0.5 Si (AA2024) and an Al-4.2 Cu-1.5 Mg-0.6 Mn-0.08 Si (AA2324) (wt.%) alloy. In DSC experiments on as solution treated samples two distinct exothermic peaks are observed in the range 250-350 °C, whereas only one peak is observed in solution treated and subsequently stretched or cold worked samples. Samples heated to 270 and 400 °C at a rate of 10 °C/min in the DSC have been studied by TEM. The selected area diffraction patterns show that S phase precipitates with the classic orientation relationship form during the lower temperature peak, and for the solution treated samples, the higher temperature peak is caused by the formation of a second type of S phase precipitates which has an orientation relationship that is rotated by ∼4° to the classic one. The effects of Si and cold work on the formation of the second type of S precipitates have been discussed.