EVOLUTION OF PRECIPITATES IN ALLICU AND ALLICUSE ALLOYS AFTER AGE-HARDENING TREATMENT

The structure of precipitates formed during ageing at 200 degrees 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 m ain hardening phases were identified as Theta' (Al2Cu), T-1 (Al2LiCu) and delta' (Al3Li). The first one is formed in the shape of thin plate s of thickness of a few atomic layers. The HREM allowed to identify th e structure of plates, its crystallographic relationship with the matr ix and nucleation of delta' phase at the edges of plates preserving th eir coherency with the matrix. The T-1 was identified in thicker areas of foils using electron diffraction method. The radius of delta' incr eases rapidly in the first stage of ageing, what corresponds to the in crease of hardness, while the thickness of T-1 plates increases steadi ly during ageing. The following crystallographic relationship between the matrix and T-1 precipitates was observed: [00.1]alpha\\[(1) over b ar 2(1) over bar0$]T-1 and(110)alpha\\(1 0(1) over bar0$)T-1. (C) 1997 Elsevier Science S.A.