Evolution of precipitates in AlLiCu and AlLiCuSc alloys after age-hardening treatment

The structure of precipitates formed during ageing at 200 °C of AlLiCu and AlLiCuSc alloys containing 2.5 wt.% Li, 2.0 wt.% Cu and 0.2 wt.% Sc was studied using conventional and high resolution electron microscopy (HREM). Both alloys show similar hardness maximum and the main hardening phases were identified as Θ′ (Al2Cu), T1 (Al2LiCu) and δ′ (Al3Li). The first one is formed in the shape of thin plates of thickness of a few atomic layers. The HREM allowed to identify the structure of plates, its crystallographic relationship with the matrix and nucleation of δ′ phase at the edges of plates preserving their coherency with the matrix. The T1 was identified in thicker areas of foils using electron diffraction method. The radius of δ′ increases rapidly in the first stage of ageing, what corresponds to the increase of hardness, while the thickness of T1 plates increases steadily during ageing. The following crystallographic relationship between the matrix and T1 precipitates was observed: [001]α∥[20]T1 and (110)α∥(100)T1.