Microstructural effects on the slip and fracture behavior of isotropic aluminum-lithium-copper-X alloys.

摘要

Although Al-Li-Cu alloys showed initial promise as lightweight structural materials, implementation into primary aerospace applications has been hindered due in part to their characteristic anisotropic mechanical and fracture behaviors. The Air Force recently developed two relatively isotropic Al-LiCu-X alloys with $2.1/ow$Li and $1.73/ow$Li designated AF/C-489 and AF/C-458, respectively. The elongation at near peak strength was less than the design requirement of 5% for the $2.1/ow$Li variant, but greater than 10% for the $1.73/ow$Li alloy. The objectives of this investigation were to first, identify the mechanisms for the large difference in ductility between the near peak aged AF/C-489 and AF/C-458 alloys, and then develop an aging schedule to optimize the microstructure for high ductility and strengths.; Duplex and triple aging practices were designed to encourage homogeneous deformation by minimizing grain boundary precipitation while promoting matrix precipitation of the T₁ strengthening phase. Duplex aged treatments for the AF/C-489 alloy showed significant increases in ductility by as much as 64% with a only small decrease of 4% and less than 2% for the yield and ultimate tensile strengths, respectively. In contrast, no significant variations were found through duplex, triple, and quadruple aging practices for the AF/C-458 alloy, which suggests a large processing window. Quantitative TEM precipitate density measurements indicate that the tensile property variations between single and duplex aged treatments were due to dissimilar precipitation responses between the AF/C-458 and AF/C-489 alloys.; Grain boundary T₁ precipitation was found in all single and duplex aged treatments and, thus, determined not to be the major cause for the lower ductility of the AF/C-489 alloy. Further TEM and HREM investigations demonstrated that matrix δ′ and T₁ precipitates were sheared in all single and duplex aged alloys while grain boundaries with misorientations of greater than 11° and 11–18° for the AF/C-458 and AF/C-489 alloys, respectively, were determined to arrest slip. Strain localization due to the shearing of both δ• and T₁ lead to fine planar slip characteristics for the AF/C-458 alloy while coarse planar slip was evident in the AF/C-489 alloys. The intensity of this planar slip was predicted through slip intensity calculations utilizing the precipitate density measurements, dislocation-particle TEM data, and grain boundary misorientation-slip continuity statistics and finally correlated to actual AFM measured slip heights. These slip intensity predictions related quite well to the actual physically AFM measured slip heights from single aged 2% plastically strained tensile samples for both alloys.; In summary, our results support the hypothesis that the low ductility of the AF/C-489 in comparison to the AF/C458 alloy is most strongly attributed to the ∼3 times greater slip length and to a much lesser extent the increased volume fraction of shearable δ• precipitates. The much larger slip length greatly increased the intensity of planar slip impinging upon a grain boundary, thereby, promoting the low-energy intergranular fracture of the AF/C-489 alloy. Therefore, our results suggest the low ductility of the AF/C-489 alloy is not intrinsically related to the higher (2.05wt%) Li content, but, due primarily to the much larger slip lengths represented by the much larger grain size.