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 aluminum–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 recrystallized. The second variant was produced by thermomechanically processing 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 versions of the alloy labeled as TMP-2020 and Zr-2020, resulted in an unrecrystallized microstructure. Fracture toughness data were analyzed in terms of crack initiation toughness KJc, and the crack growth toughness parameter, TR (tearing modulus). The three variants of the alloy 2020 exhibit minimum differences in both the fracture initiation toughness, KJc, and the crack growth toughness, TR, in the peak aged condition. However, a considerable improvement in KJc is observed in the thermomechanically processed (TMP-2020) and the Zr containing 2020 (Zr-2020) in the underaged condition. The improvement in KJc in the TMP-2020 and Zr-2020 variants over the REX-2020, is attributed to a higher plasticity attainable at the crack tip and finer grain size. The tearing modulus, TR, is however higher in the REX-2020 due to planar slip and conversely the lower TR in the TMP-2020 and Zr-2020 is due to a lower degree of planar slip than the REX-2020 alloy. The low toughness at the peak age condition in the three alloys is due to grain boundary fracture from the presence of grain boundary precipitates. There is some recovery in this lost toughness in the overaged condition where plasticity is gained via the loss in the yield strength of the alloys.