Heterogeneous Nucleation of the T 1 Phase on Dispersoids in Al-Cu-Li Alloys
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DOI: 10.1007/s11661-015-2894-4 The Minerals, Metals & Materials Society and ASM International 2015 – Modern Al-Cu-Li alloys are finding increasing use in aircraft due to their superior specific properties compared to other conventional Al alloys that were being used so far in such applications. The interaction between three types of particles that are of high significance in these systems are being investigated here: T1, Al3Zr, and Al20Cu2Mn3 dispersoids. The first is the major strengthening phase in Al-Cu-Li alloys with Li levels up to 1.3 wt pct and with a pre-stretch applied before aging.[1] The second is used to prevent recrystallization and homogenize slip.[2,3] The last one is employed to homogenize slip, weaken texture, reduce in-plane anisotropy, and improve fatigue resistance.[2,4–6] However, it deteriorates recrystallization resistance when added together with Zr,[2,7] with a subsequent detrimental effect on fracture toughness.[8] The T1 phase has been studied thoroughly and a satisfactory level of understanding has been achieved so far. A crucial topic that is still under scrutiny is the nucleation mechanisms of this phase. Several nucleation sites have been reported up to now in the literature and are summarized below. – Dislocations.[9–12] Homogeneous T1 distribution is attained by introducing a large number density of dislocations in the microstructure, due to preferential nucleation within the strain field of the latter. A small percentage of pre-stretch is typically required for this, which leads to an improvement in aging
DIMITRIOS TSIVOULAS, Research Associate, is with the School of Materials, The University of Manchester, Manchester M13 9PL, U.K. Contact e-mail: [email protected] Manuscript submitted December 19, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
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kinetics, yield strength, tensile strength, and elongation.[13] In particular, T1 nucleation is favored by dislocation loops surrounding Zr-containing dispersoids,[14] despite contradictory data claiming that this is not possible.[15] Recent atom probe studies have demonstrated that the T1 phase nucleates on dislocations (including loops) that contain clusters of Cu and Mg, with lower amounts of Li and Ag.[16] However, this is not always the case since for certain particles there are no evident nucleation sites. Stacking faults.[15,17,18] Combined addition of Mg and Ag, as well as low supersaturations, reduces the stacking fault energy and leads to the formation of stacking faults on {111}Al planes. Low-angle grain boundaries.[9,11,17,19–23] Even with the application of a pre-stretch, heterogeneous nucleation on subgrain boundaries cannot be avoided. Most often at the peak-aged condition, T1 precipitation is quite intense on LAGBs where the phase forms as short, thick plates. When the precipitate habit plane is parallel to the boundary’s plane, it can then grow longer. Some methods of reducing the amount of precipitates on subgrains are either increasing the Li content (from 0.5 to 1.6 wt pct[19]), or applying multi-
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