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Characterizing the brittle fracture and the ductile to brittle to ductile transition of heat-treated binary aluminum-lithium alloys

Fragomeni J. M.

Engineering Transactions

2001

Vol.49, iss.4

573-598

An aluminum alloy containing 2.6wt.% Li and 0.09wt.% Zr exhibited a very low value in tensile ductility consistently prior to the peakaged strength independent of thermal treatment. A transition was characterized by very low ductility in the slightly underaged condition up to the near peakaged condition, then followed by a substantial increase in ductility with aging after the peakaged treatment. In order to better understand the deformation and fracture, a scanning electron microscopy study of the fracture surfaces of Al-2.6wt.% Li-0.09wt.% Zr tensile samples solution heat-treated and artificially aged was performed to relate the mechanical behavior to microstructure in the precipitation hardened Al-Li alloy. SEM analysis of the surface features and fracture morphology of the alloy was performed to understand the mechanisms of fracture in relation to the ductile-to-brittle transition that resulted in the alloy from precipitation hardening. TEM analysis was also performed to characterize the deformation behavior, and revealed the distribution of precipitates (both Al3Li (a') and Al3Zr-Al3Li) in the microstructure at very high magnifications as well as the dislocation subgrain structure of the alloy at lower magnification. It follows from this study that the presence of a' particles in the matrix promotes intense planar slip which was believed to be responsible for the ultra-low ductility prior to the peakaged temper. Based on a detailed quantitative microscopy study, it was proposed that the increase in the ductility of the alloy after aging was a consequence of particle coarsening with aging and thus resulting in the Orowan process due to the transition from dislocation particle shearing to dislocation particle bypassing.