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Natural Aging in Al-Cu-Li high performance alloys

By Pascal Neibecker (25.02.2013)

Due to their high strength, high Young's modulus and low density, Aluminum-Copper-Lithium (Al-Cu-Li) alloys are of special interest in aerospace industry. A major application of these alloys is for instance as structural material in the fuel tanks of the European Ariane Rocket as well as in airplane components example given of the A380.


[Bildunterschrift / Subline]: Figure 1: Booster Tanks of the European Ariane Rocket produced by MT Aerospace AG (Copyright: MT Aerospace AG)
[Bildunterschrift / Subline]: Figure 2: Aluminum-Copper-Lithium (Al-Cu-Li) as structural material in the fuel tanks of the European Ariane Rocket (Copyright: MT Aerospace AG)

Al-Cu-Li alloys mainly acquire their strength from precipitation processes occurring during aging of the alloy. These precipitation processes are highly complex and not very well understood up to this point. Depending on annealing time and temperature, usually two stages of hardening/ strengthening are observed whereas for both of them their origin is not unambiguously revealed yet. The first hardening stage seems to be controlled by so called precursor phases namely the formation of Guinier-Preston(GP)-zones (one atomic Cu layers on the {100}-planes) and presumably formation of short range order; the second stage by the formation of the T1-phase (Al2CuMg), accompanied by δ'(Al3Li)- and ϴ'(Al2Cu)-precipitates. The phase mainly responsible for the strength of the alloy is the T1-Phase which has to be present in a high concentration. This is usually achieved by rather complex thermo-mechanical treatments. It is believed that in the precursor phases might lie a key to the processing of high-strength Al-Cu-Li alloys only by thermal treatments and thus increasing processing efficiency and reliability. In order to understand the formation of T1-precipitates by precursor phases, the precursor phases have to be studied in room temperature aging experiments. Mainly, it is important, to understand the competing phases in this process and the corresponding kinetics. An experimental method suitable to trace precipitation and aging processes in metals is to measure the electrical resistivity of the material. This method is the only approach allowing an in-situ observation of precipitation formation. However, the implementation of an in-situ electrical resistivity experiment is quite challenging since rather complex instrumentation is needed in order to improve the signal-to-noise ratio in these measurements.

In the presented research, a modern Al-Cu-Li alloy (2195) was investigated. This alloy is extremely promising because of its superior strength in comparison to other 2xxx series alloys, both at room temperature and at cryogenic temperatures. Additionally, a Li-free Al-Cu alloy (2219) used for similar applications was taken into account in order to distinguish Li and Cu influences on the precursor phases. The compositions of the studied alloys are Al-4%Cu-1%Li-0.3%Ag-0.4%Mg-0.14%Zr (wt%) and Al-6%Cu-0.36%Mn-0.13%Si-0.22%Fe-0.18%Zr-0.12%V (wt%), for 2195 and 2219 respectively.

It was found that both Li and Cu participate in the room temperature aging process. Their contributions thereby behave independently. This implies that both Li and Cu form their own precursor phases. It is known from Literature and Transmission Electron Microscopy measurements that Cu forms as mentioned above Guinier-Preston zones. The kinetics of the formation of these zones in 2195 and 2219 could be determined. Whereas Copper forms long range ordered GP-zones, the Lithium contribution is presumably a short range ordering/ clustering process.  Relative to the atom-percentage of alloying elements, Lithium short range ordering contributes less to the overall change in resistivity.
The overall curve shape of the relative resistivity increase as seen in Figure 2 can be described using Formula 1.

[Bildunterschrift / Subline]: Formula 1: Formula describing the kinetics of the relative change in resistivity for room temperature aging processes in Al-Cu-Li alloys, with Δρ being the relative change in resistivity, t being the time, k being the half time of the increase and n being an additional fitting parameter.

The half lives of the resistivity increase upon natural aging in 2195 and 2219 were found to be 3x10^3s and 3x10^4s in 2219 and 2195, respectively (Figure 2b). It is concluded that Lithium with a high vacancy binding energy of 0.26 eV traps the excess vacancies and decelerates the precipitation process of Cu.
Furthermore, an orientation dependence of the natural aging curves in cold-rolled 2195 sheets was found. As seen in Figure 2a), samples cut from two different orientations within the cold-rolled sheet show drastic discrepancies in the relative change in resistivity. It could be shown with corresponding simulations that this effect is based on the orientation dependence of the GP-zones and the overall change in resistivity can be predicted quite reliably using geometric models for the distribution of GP-zones within the sample with respect to the direction of electric current flow.

[Bildunterschrift / Subline]: Figure 2: Relative change in resistivity vs. Time in 2195 (a) and 2195/2219 (b). Figure 2a) shows the orientation dependence of the natural aging process in 2195 cold-rolled sheets. Figure 2b) shows the increase in resistivity in 2195 in comparison to the Li-free 2219 alloy

The presented research was performed in the group of Prof. Ferdinand Haider (Experimental Physics I) at Augsburg University. The pictures were gratefully provided by MT Aerospace©.

  • 10/2011 – 10/2013
  • „Advanced Materials Science“ Master’s Program, TU Munich, University of Munich, University of Augsburg
  • 07/2011
  • B.Sc. (with distinction) Materials Science and Mechanical Engineering, Saarland University, Germany
  • 06/2011
  • B.Sc. (with distinction) Mechanical Engineering, Oregon State University, USA

Research Experience
  • 2012-2013
  • Department of Physics, University of Augsburg, Germany, Group Prof. Haider, Research Topic: High performance Al-Cu-Li alloys
  • 2011
  • Department of Mechanical Engineering and Materials Science, Yale University, USA, Group Prof. Schroers, Research Topic: Mechanical Properties of Bulk Metallic Glasses
  • 2010-2011
  • Department of Materials Science, Oregon State University, USA, Group Prof. Cann, Research Topic: Construction of an aerosol deposition device for ceramic thin films
  • 2008-2010
  • Department of Materials Science, Saarland University, Germany, Group Prof. Busch, Research Topic: Bulk Metallic Glasses