Lithium-containing aluminum alloys: cyclic fracture
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Table II.
Peak-Aging Alloy Treatment A 64 hrs at 160 ~ B 18 hrs at 191 ~ C 32 hrs at 191 ~
A. K. VASUDI~VAN and S. SURESH Lithium-containing aluminum alloys have been the subject of increasing research activity over the past few years (e.g., References 1, 2). This is in view of their revived potential for extensive use in aerospace applications, stemming primarily from the improvements in modulus and density which accompany lithium additions to aluminum alloys. Early studies of the fatigue crack growth resistance of peakaged (T651) aluminum alloy 2020 (A1-4.5Cu-l.lLi-0.52 Mn-0.2Cd) were carried out by Sanders and co-workers. 34 They found that this alloy exhibited superior crack growth resistance as compared to the widely used 7075-T651 in the Paris regime of crack propagation. Subsequent work by Vasud6van et al.5 revealed that such beneficial fatigue behavior of this commercial 2020 alloy is even more pronounced in the near-threshold regime, where the propagation rates are more than two orders of magnitude slower than conventional aluminum alloys of the 2XXX and 7XXX series. Such initial work has shown 3-6 that the improved resistance to cyclic crack advance in the A1-Li system can be attributed to highly inhomogeneous slip behavior (and the resulting nonlinear crack profile) induced by the ordered 6' (A13Li) precipitates. The intent of this article is to document some results on the cyclic crack growth response of high purity A1-Li-Cu-X alloys to controlled variations in the relative amounts of lithium and copper. The three high purity aluminum alloys investigated in this study mainly contain lithium (1.1 to 2.9 wt pct) and copper (1.1 to 4.5 wt pct). The nominal compositions (in wt pct) of the alloys are listed in Table I. The laboratory-fabricated plates of these materials were solution treated for 30 minutes at 515 ~ (Alloy A) or 551 ~ (Alloys B and C), following which they were cold water quenched, stretched (by 2 pct), and artificially aged to peak strength. The aging treatment and room temperature mechanical properties (L-T orientation) of the alloys are provided in Table II. Laue transmission X-ray analyses of the three materials revealed predominantly unrecrystallized microstructures (subgrain size 2 to 3 /xm, as inferred from TEM analyses) with some (partially) recrystallized grains. Optical microscopy showed the presence of high angle grain boundaries with a spacing of 50 to 100 Fzm in the direction of crack advance. X-ray Guinier de Wolfe analyses revealed that in addition to 6' Table I.
Alloy A B C
Li 1.13 2.00 2.90
Cu 4.55 2.80 1.10
O-y ouTs El. Mod. (MPa) (MPa) (GPa) 531 593 76.5 490 544 79.2 434 524 82.7
Elong. (Pct) 12 10 6
(A13Li), all the alloys contained a T-type (AlxCuyLiz) phase with platelet structure similar in morphology to that observed in the case of O precipitates in the A1-Cu system. The exact precipitation characteristics of these A1-Li-Cu-X alloys are currently under investigation; detailed microstructural descriptions from similar systems are available in the literature. 7"8
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