Low quench sensitivity of superplastic 8090 Al-Li thin sheets

Quench sensitivity of the superplastic 8090 Al-Li thin sheets was investigated from the strengthening point of view for the various precipitates formed in the alloy. Specimens subjected to different cooling rates (water quenching, silicon oil cooling, and air cooling) from solution treatment (or superplastic forming) temperature were examined by tensile tests, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Experimental results show that δ′ is the predominant phase in the alloy under all cooling conditions. Because δ′ could be formed in a similar amount and size after the T6 treatment, and because the amount of S′ precipitates and T₂ particles were extremely low, the superplastic 8090 Al-Li alloy would, thus, always exhibit low quench sensitivity. Based on DSC and high resolution electron microscopy (HREM) studies, new interpretations of the first two exotherms of the DSC traces were made. Strong evidence shows that the first exothermic peak is attributed to the further precipitation of δ′ phase, and the second one is contributed from the Al-Cu-Mg GPB zone formation.