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Title: The effects of zirconium on the microstructure and mechanical properties of the Al-Li-Cu-Mg alloy 8090
Author: Smith, William Daniel
ISNI:       0000 0001 3469 2920
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 1997
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The effects of differing Zr content on the microstructure and mechanical properties of sheet form Alloy 8090 have been studied. The research has involved a detailed study of the direct and indirect effects Zr on the grain size, texture and precipitation sequences, and the subsequent effect on the tensile properties and fracture toughness of the material. Whilst the direct effects of Zr in inhibiting recrystallisation and providing a refined grain structure are well known, this work has identified additional changes in the microstructure. This has been enabled further explanation of some of the changes in the balance of mechanical properties with respect to the composition and heat treatment practices. The equilibrium precipitate known as the I-phase has been identified using Transmission Electron Microscopy, by its characteristic diffraction patterns, and using Energy Dispersive X-ray Spectroscopy. The I-phase has been shown to form at grain boundaries during the quench from solution treatment, and with increasing abundance as the quench rate is decreased. Differential Scanning Calorimetry has enabled the temperature range for I-phase formation to be accurately predicted, and the work has shown that rapid I-phase formation occurs during cooling between 450 and 420°C. Back-scattered Electron Imaging and Image Analysis techniques have enabled the area fraction of the I-phase to be measured for different heat treatment conditions. The work has also demonstrated that the abundance of I-phase is significantly increased in 8090 compositions with high Zr content. Although some of the increase in heterogeneous grain boundary precipitations can be explained in terms of the higher density of grain boundary sites in the high Zr alloys, a direct interaction between I-phase particles and Al₃Zr dispersoids has been observed, I-phase particles have been seen in close proximity to the Al₃Zr dispersoids and on numerous occasions Al₃Zr dispersoids were observed encompassed within I-phase particles.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Sheet form alloy