EFFECT OF FABRICATION AND MICROSTRUCTURE ON THE FRACTURE INITIATION AND GROWTH TOUGHNESS OF AL-LI-CU ALLOYS

Fracture toughness properties of three microstructural variants of alu minum-lithium alloy 2020 (4.6 Cu, 1.1 Li-0.5 Mn-0.2 Cd), were analyzed . The first variant is the base alloy, REX-2020, which is completely r ecrystallized, The second variant was produced by thermomechanically p rocessing the REX-2020 and the third variant was obtained by replacing Mn in the REX-2020 by Zr, as the grain refining element. These two ve rsions of the alloy labeled as TMP-2020 and Zr-2020, resulted in an un recrystallized microstructure. Fracture toughness data were analyzed i n terms of crack initiation toughness K-Jc, and the crack growth tough ness parameter, T-R (tearing modulus). The three variants of the alloy 2020 exhibit minimum differences in both the fracture initiation toug hness, K-Jc, and the crack growth toughness, T-R, in the peak aged con dition. However, a considerable improvement in K-Jc is observed in the thermo mechanically processed (TMP-2020) and the Zr containing 2020 ( Zr-2020) in the underaged condition. The improvement in K-Jc in the TM P-2020 and Zr-2020 variants over the REX-2020, is attributed to a high er plasticity attainable at the crack tip and finer grain size. The te aring modulus, T-R, is however higher in the REX-2020 due to planar sl ip and conversely the lower T-R in the TMP-2020 and Zr-2020 is due to a lower degree of planar slip than the REX-2020 alloy. The low toughne ss at the peak age condition in the three alloys is due to grain bound ary fracture from the presence of grain boundary precipitates. There i s some recovery in this lost toughness in the overaged condition where plasticity is gained via the Loss in the yield strength of the alloys . (C) 1998 Elsevier Science S.A.