Etude de la precipitation des phases S'/S etT1 dans un alliage quaternaire a base d’aluminium vieilli a differentes temperatures. identification par diffraction electronique a transmission..

Abstract

Differential Scanning Calorimetry (DSC), X-Ray Diffraction (XRD), microhardness and Transmission Electron Microscopy (TEM) are used to study the precipitation of Al2CuMg (S’/S) and Al2LiCu (T1) phases which lead to structural strengthening of Al-2,7Li-1,4Cu- 1Mg-0,11Zr (wt%) alloy (8090). DSC results for aged samples for various times at 160, 190 and 210°C, show that threshold-times for (S’+T1) precipitation are 2h, 30mn and 15 mn respectively. These results are confirmed by XRD. DSC analysis, for the water quenching state, carried out with various scanning rates, permits to evaluate the activation energy of (GPBS’+δ’) formation, activation energy of their dissolution and activation energy of (T1 + S’/S) precipitation. TEM diffraction revealed extra-reflexions from the orthorhombic S’/S and hexagonal T1 phases in addition to those of α matrix and L12 structure δ’ and β’ phases. Theoretical study, based on orientation relationships precipitate/matrix, shows that 12 different systems S’/S and 12 variants T1 can simultaneously diffract with α matrix. Two diagrams   100 and 112  are plotted on which the S’/S and T1 reflexions are indexed. The obtained results for the theoretical patterns are in good agreement with those observed experimentally.