Microscopic and macroscopic aspects of fracture in lithium-containing aluminum alloys

Abstract

The effects of alloy composition and aging treatment on plane strain fracture initiation toughness ( K lc , J lc ) and crack growth resistance (characterized by the tearing modulus, T R ) were investigated in high purity alloys belonging to the Al-Li-Cu-Zr system. The Li Cu ratio (expressed in terms of atomic fractions) was varied from 2.2 to 25.2 and in each of these alloys, microstructures spanning the very underaged to the severely overaged conditions were studied. In peak-aged and over-aged alloys of higher lithium content, intergranular fracture arising from voids initiated around grain boundary particles dictates fracture initiation toughness and crack growth resistance. In such cases, both K lc and T R are insensitive to aging beyond the peak strength. When the amount of grain boundary precipitates is limited, the existence of ordered δ′ (Al 3Li) particles in the matrix promotes intense slip planarity. While there are reports of experimental evidence indicating a detrimental effect of planar slip on fracture toughness upon artificial aging, there is also a concomitant beneficial (geometrical) effect on crack growth due to severe bifurcation of the crack during quasi-static fracture. We demonstrate with the aid of branched crack models that such bifurcation can account for an important part of the microstructural effects on fracture toughness in some underaged alloys. The magnitude of microstructurally-influenced crack deflection is systematically correlated with the volume fraction of the precipitates using quantitative transmission electron microscopy. It is shown that microstructural characteristics, which are generally considered undesirable for fracture initiation toughness, can in fact lead to marked improvements in the tearing modulus. We rationalize such results based on the observed variations in crack path. In order to separate out the individual effects of Li and Cu and to address some key issues pertinent to the micromechanisms of failure, fracture experiments have also been performed in a high purity Al-3 wt% Li alloy containing only Mn (used for grain size control). Furthermore, results of heat treatments designed to produce microstructural reversion are discussed to identify the individual contributions to fracture from matrix deformation and grain boundary failure. An attempt is made to present, as quantitatively as possible, an overall mechanistic perspective on fracture initiation toughness and crack growth resistance in lithium-containing aluminum alloys.