An experimental and analytical study of the mechanisms influencing the fracture behavior of an extruded aluminium-lithium-zirconium alloy

The mechanisms influencing the fracture behavior of a Al-2.6Li-0.09Zr alloy were investigated by varying the aging condition, the specimen thickness and the extrusion parameters which affect grain structure, namely extrusion ratio (9:1, 36:1 and 73:1) and extrusion geometry (round rod and flat bar). This alloy does not exhibit the classic plane-stress to plane-strain thickness effect typically observed in conventional alloys. The notch sensitivity of the alloy is also considerably less than observed with other alloys. The fracture toughness exhibits some extrusion ratio sensitivity as evidenced by 25% lower values for the higher extrusion ratio product in the peakaged condition. Furthermore the difference in toughness between the rod and bar extrusions appears to be minimal. Fracture in the higher toughness underaged condition is characterized by crack tunneling and grain boundary microcracking confined to a zone surrounding the continuous crack surface. The fracture process for the overaged material is distinguished by large, intergranular delamination cracks perpendicular to the main fracture surface and extending deep into the plastic zone. This delamination cracking phenomena was investigated using linear, two-dimensional finite element simulations. A delamination-based fracture process is described for the alloy in the overaged condition whereby delaminations sequentially split the thickness of the specimen until the remaining ligaments are in plane-stress. Moreover the complexity and irregularity of the fracture surface and the changes in constraint caused by delamination cracking leads to serious questioning of the validity of conventional fracture toughness measures. Nevertheless the fracture toughness values in the underaged condition follow the functional form of the Garrett and Knott coalescence model whereas in the peak- and overaged condition the toughness can be described by the Bilby, Cottrell and Swinden plane-stress model. These toughness correlations are examined for this Al-Li-Zr alloy as well as other Al-Li-X alloys.