The effect of inhomogeneous plastic deformation on the ductility and fracture behavior of age hardenable aluminum alloys

The role of alloy composition, grain structure, precipitate microstructure, and precipitate dislocation interactions on the plastic deformation characteristics and the resulting fracture behavior of two isotropic Al-Li-Cu-X alloys designated AF/C-458 (1.8 w/o Li) and AF/C-489 (2.1 w/o Li) was examined. Inhomogeneous deformation due to strain localization from the shearing of the δ′ (Al3Li), θ′ (Al2Cu), and T1 (Al2CuLi) precipitates lead to fine and coarse planar slip for the AF/C-458 and AF/C-489 alloys, respectively. The intensity of this planar slip was predicted through slip intensity calculations using precipitate density measurements, dislocation particle interactions, and grain boundary misorientation-slip continuity statistics. The slip intensity predictions were corroborated through atomic force microscopy (AFM) measured slip height offsets on the polished surface of single aged and 2% plastically strained tensile samples. Our results suggest that the low ductility of AF/C-489 in comparison to AF/C-458 is primarily due to the much larger slip lengths, i.e. grain size, which increased the strain localization and stress concentrations on grain boundaries, thus promoting low-energy intergranular fracture.