Retrogression characteristics of a novel Al-Cu-Li-X alloy

Retrogression characteristics of a novel Al-Cu-Li-X alloy of 2A97 were studied by hardness testing,transmission electron microscopy (TEM),and differential scanning calorimetry(DSC).The retrogression treatments of aging at 155℃ for 12 h followed by aging at220 and 240℃ were chosen by determining the peak temperature of δ' precipitation at 230℃ by DSC.The retrogression treatment at a lower temperature of 220℃ causes the precipitation and coarsening of δ' and θ' phases in the matrix,resulting in an increase in hardness.Retrogression at a higher temperature of 240℃ causes the dissolution and coarsening of δ' and θ' precipitates in the matrix and on the grain boundaries,resulting in a decrease in hardness.Microstructural changes upon retrogression including the appearance of equilibrium precipitates such as T1,T2,δ',and θ are confirmed by the selected area electron diffraction and the bright and dark field image analysis.