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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Microstructure and creep properties of a commercial aluminium lithium alloy.

Author: TETLOW, Philip

Year: 1991

Supervisor(s): Craig Davies

The creep properties, and the effect of long term ageing and of creep on microstructure, were determined for a commercial aluminium-lithium alloy (Alcan 8090) initially in the peak aged T8 condition. The as-received material was aged on at the commercially recommended ageing temperature of 463K (190oC) for periods up to 20Ms (320 days) and post peak ageing was also carried out at a range of lower temperatures down to 395K (122oC) for periods up to 3375ks (39 days). Transmission electron microscopy and X-ray diffraction were used to study the evolution of the microstructure during ageing and the kinetics of particle growth. Creep tests were carried out at constant stresses from 200MPa to 450MPa at temperatures of 425K (152oC) and 445K (172oC) on material in the T8 condition. At 425K tests were also carried out on material in a post peak aged condition. The temperature and stress dependence of the various creep properties were determined. As with other alloys hardened by coherent precipitate particles, creep strain rates were found to be very sensitive to stress at high stresses. Attempts were made to fit the creep curves (strain/time data) to an empirical equation (Theta Projection) but this did not prove to be successful. The effect of creep on dislocation structure and the precipitated phases was studied using transmission electron microscopy. Particle cutting by pairs of dislocations was observed in some ranges of stress but Orowan looping was not identified with certainty. Optical microscopy and scanning electron microscopy were used to study creep fracture behaviour. The creep behaviour was discussed in terms of the interactions between particles and dislocations and the internal stresses produced thereby.