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Title: An Analysis of Microstructure Evolution in Hot Worked Aluminium Subjected to Non-linear Deformation
Author: Lopez-Pedrosa, Magdalena
ISNI:       0000 0001 3612 7052
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2007
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Accurate predictions of the microstructure evolution of thennomechanically processed metals and alloys are generally only achieved when the defonnation process is nominally linear. This is not surprising since most microstructure prediction models are only described in tenns of strain, strain rate and temperature and the testing methods (e.g. axisymmetric compression, plane strain compression and torsion testing) used to calibrate the underlying equations are linear unidirectional tests. However, there are many reported examples of microstructure evolution being strain path sensitive, particularly changes in recrystallisation response and final grain size. On the other hand the microstructure analysis of metals subjected to non-linear strain paths under hot working conditions is quite sparse. . In this thesis, the effects of strain path reversal on the macroscopic orientation of microbands and subsequent static recrystallisation in AA1200 and AA5052 aluminium alloys has been studied. The strain path testing was perfonned using the sophisticated and revolutionary Arbitrary Strain Path II (ASP II) torsion-tension machine, whilst microstructure analysis was undertaken using fully quantitative high resolution Electron Backscatter Diffraction (EBSD). Defonnation was carried out using mainly linear and non-linear strain paths via forward and reverse torsion tests at a constant temperature of 300°C and strain rate of Is·1 to a total equivalent strain ranging from 0.25 to 0.75. The orientation of the microbands was defined with respect to two principal directions, namely the macroscopic maximum principal stress and strain directions. The maximum principal stress lies at 45° to the longitudinal direction, whilst the maximum principal stress when the strain is reversed lies at -45°. The principal strain direction is rotated during defonnation and was calculated for each test condition. Microbands were observed in the majority of grains subjected to linear strain path tests such as forward torsion to an equivalent strain of 0.25 (i.e. 0.25F), forward torsion in two defonnation steps to a total equivalent strain of 0.5 (Le. 0.25F+0.25F) and in the same way 0.25F+0.25F+0.25F test condition in the AA5052 and 0.25F+0.25F condition in the AA1200 alloy. In all linear defonnation cases two sets of microbands were observed: 1) parallel to the rotation plane and 2) at an angle to the longitudinal direction of the specimen with the angle increasing as a function of strain. For non-linear tests when the net strain is zero, such as 0.25F+0.25R microbands were more randomly orientated. However, for tests where the final reversal strain was greater than the initial forward strain, such a 0.25F+0.5R condition, microband detection was again very simple and strongly clustered in a similar way to the linear tests suggesting that microbands fonned in the forward defonnation have dissolved and all new microbands fonned have developed from the onset of reversal and are orientated with respect to the new defonnation axes. Analysis of the microband angle with respect to the two principal directions indicates that microband angles are symmetrical about the principal strain direction suggesting that macroscopic shape change is the critical factor in detennining microband angle.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available