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Title: Strain-induced precipitation during the thermomechanical processing of AA6111 alloy
Author: Song, Yinggang
ISNI:       0000 0001 3471 1757
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2007
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It is well known that the finishing temperature of 6xxx alloys determines the resultant texture, which clearly has a strong effect on formability. Specifically, the texture is determined by whether the finishing temperature is above or below the ß transus. This study was initiated in order to directly determine the mechanisms that take place during the hot deformation of AA6111 to directly explain these observations. Accordingly, the effect of inter-pass time during thermomechanical processing of AA61111 on flow behaviour and microstructure evolution has been investigated. This was achieved using plane strain compression testing undertaken on the Sheffield Thermomechanical Compression (TMC) facility, using the hit-hold-hit-quench approach at temperatures of 320°C, a strain rate of 85 s'1 to an initial strain of 0.5, unloaded and held for delay times of 0,6,60,600 and 6000 seconds, followed by the second deformation. Hardening of the alloy was observed, the extent of which was dependent on the hold time. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used in the investigation and a comparison of the substructural characteristics obtained by these techniques was made. TEM identified precipitation, predominantly ß and Q phases, on dislocation lines, the size of which was a function of the hold time. In contrast to conventional ageing studies, the Q phase was found to be the majority precipitate phase. The coarsening rate of the Q during the hold period of the precipitates was considerably faster than for coarsening following a conventional precipitation treatment. The size of the microband structure at the end of the double deformation was a function of the hold time, suggesting that coarsening of the precipitates during the hold had altered the Zener pinning potential. Texture analysis through EBSD revealed that the texture was mainly composed of the α and β fibres, indicating a classic hot deformation texture with no evidence of a recrystallisation texture. This, the EBSD maps themselves, optical and transmission electron microscopy indicated that no recrystallisation had occurred for any inter-pass delay time. Small changes in deformation texture were observed with changes in inter-pass delay time, however, such differences were believed to be small. Deformation substructure increased in size (e.g. microband width) with increased inter-pass time, as expected, which with the increase in precipitate size led to the softening observed at 600 and 6000 seconds. The implications of these observations are discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available