Control of microstructure and properties in a high strength Al-Li alloy

Tite, Caroline Norma Jane (1988) Control of microstructure and properties in a high strength Al-Li alloy. University of Southampton, Doctoral Thesis.

Abstract

The new series of lightweight Al-Li-Cu-Mg alloys currently being developed, rely upon the coprecipitation of δ'(Al_3Li) and S(Al_2CuMg) phases to achieve the mechanical properties necessary for aerospace applications. The present research has used a combination of microscopical, microanalytical and thermal techniques to study the microstructures developed in the high strength alloy Al-2.5% Li-2.0% Cu-0.8% Mg conforming to the 8091 specification. It has been shown that a homogeneous distribution of S phase can be achieved in the T6 condition but that precipitation of S phase requires a critical combination of free vacancy concentration and copper and magnesium supersaturation. Lithium limits the free vacancy concentration due to the high binding energy between lithium atoms and vacancies. During δ' precipitation vacancies are released and aid the formation of S phase, thus making the precipitation of δ' and S interdependent. The existence of precursors to S phase within the 8091 alloy has been established, and their contribution to S phase precipitation in Al-Li-Cu-Mg alloys is discussed. Attempts have been made to optimise solution treatment and subsequent age-hardening treatments to encourage the formation of a homogeneous distribution of S precipitates. In addition to δ' and S phases, it is necessary to understand other microstructural development within the Al-Li based alloy. Besides providing sites for the heterogeneous formation of δ' phase, dislocation loops form at the β'(Al₃Zr) particles and act as sites for additional particle formation; θ(Al_2Cu), the stable phase of the Al-Cu system has been identified at these sites, and its presence is discussed in terms of the infiltration of copper atoms into the dislocation loops. (D76771)

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