4^th International Aluminium Lithium Conference

J. Phys. Colloques 48 (1987) C3-447-C3-453
DOI: 10.1051/jphyscol:1987351

CALORIMETRIC EVALUATION OF THE MICROSTRUCTURE OF Al-Cu-Li-Mg 2091 ALLOY

S. ABIS¹, E. EVANGELISTA², P. MENGUCCI² et G. RIONTINO<sup>3

1</sup>  Istituto Sperimentale Metalli Leggeri, Aluminia S.p.A. , Via Fauser, 4, I-28100, Novara, Italy
²  Universita di Ancona, Dipartimento di Scienze dei Materiali, Via Brecce Bianche, I-60131, Ancona, Italy
³  Universita di Torino, Istituto di Chimica Generale, Facolta di Farmacia, Via Giuria, 9, I-10125, Torino, Italy

Abstract
The precipitation of δ', T1 and S' phases during ageing at 160 °C after quench, has been studied in a commercial 2091 alloy by using Differential Scanning Calorimetry (DSC) technique, 2091 samples have been solubilized, quenched and aged at 160 °C for times ranging from 10 minutes up to 90 hours ; DSC traces have been then obtained during continuous heating ; for each temper several temperature rise speeds have been used. At short times of ageing (10 min) three phases precipitated during ageing have been detected, causing three endothermic effects of dissolution in the DSC traces. For larger times of annealing the peak at lower temperature tends to disappeare and after 2 hours it can never been detected, whilst another thermal effect at higher temperature becames more pronounced and rises up to 90 hours. The peak temperatures depend on the temperature rise speed (thermally activated peaks), and this effect has been used to determine the apparent activation energies of dissolution according to the Ozawa plot. Transmission Electron Microscopy (TEM) and Selected Area Diffraction Pattern (SADP) have been performed for microstructural characterization of precipitated phases. It has been concluded that for short ageing times (up to 2 hours) three phases are present : the δ' (Al₃ Li) T1 (Al₂ CuLi) and 5' (Al₂ CuMg) ; the T1 precipitate has been always detected on defects (dislocations and grain boundaries). For larger times of annealing the T1 phase tends to redissolve while the δ' and S' precipitates grow up to 90 hours.