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Microstructure and crystallographic texture evolution in TIMETAL® 6-2-4-6 billet.

Nasseri, Nima (2010) Microstructure and crystallographic texture evolution in TIMETAL® 6-2-4-6 billet. PhD thesis, University of Sheffield.

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Abstract

The effects of thermomechanical processing via open die forging on the microstructure and crystallographic texture of three different diameter billets of the α+β titanium TIMETAL® 6AI-2Sn-4Zr-6Mo were analysed. This work included a quantitative study of primary alpha (αp) within the microstructure of the three billets with an objective of determining if any variation within the cross-section of the billet could be related back to the macroscopic deformation history during breakdown from the Vacuum Arc Remelting (VAR) ingot. The metallographic observation within two of the α+β TIMETAL® 6AI-2Sn4Zr-6Mo billets (313 mm and 213 mm diameter), that were manufactured using standard forging routes, revealed a bimodal microstructure consisting of relatively equiaxed αp within an acicular transformed β matrix throughout the whole cross-section. The results showed, with the exception of the extreme edge to a depth of 10 mm, that the microstructure, at least at the scale at which it was quantified, is homogeneous regardless of its location with respect to the deformation axes. However, despite the microstructure being homogeneous radially, analysis of the microstructure at the centre of the larger billet revealed continuous ex grains in addition to a slight increase in the total percentage of αp volume. Another difference between these two billets was variation in the percentage of αp volume. Although the total percentage of αp volume in the larger billet was generally uniform, there was a higher percentage of αp volume in the smaller sized billet when compared to the larger billet. A third alternatively forged billet was also analysed. This billet underwent a major difference in its forging route when compared to the standard route in that it was α+β forged following the ingot breakdown above β transus temperature therefore resulting in termination of the prestrain and β recrystallisation stages of the standard route. Results showed a significant difference in the αp size distribution meaning a higher percentage of αp having a smaller grain size. In fact, the actual scale of the alternatively forged billet microstructure was smaller; however, there were some larger grains demonstrating an inhomogeneous microstructure. In addition, the grains were longer and thinner in shape. This analysis was then further extended so as to determine whether the same could be said for crystallographic texture using the Electron Back Scatter Diffraction (EBSD) technique. It was shown that the textures of both large and small diameter billets were strongly dependent on the strain imposed by the deformation process. Macrozones (regions of closely aligned grains) were also observed throughout all three billets stretching along each billet's longitudinal axis. At the edge of the larger billet, however, the widths of the macrozones were wider in comparison to those of the smaller billet. In general, the billets' textures were quite weak, however, there were some noticeable differences in the textures from the edge towards the centre of each billet. At the edge of the billet the crystallographic texture was aligned with the compression direction and, unlike the centre of the billet, had a strong influence of the variant selection mechanism was observed. Deformation symmetry was also particularly noticeable at the centre of the billets. Within the alternatively forged billet, however, it was observed that the strong texture seen at the centre of both previous billets had been replaced by a less intense fibre like texture along the longitudinal direction. In order to analyse the global texture of the billet using EBSD, it was necessary to examine a number of samples from a variety of locations within the billet. There were, however, disadvantages to this procedure as there was a size limitation per sample and, although many samples were analysed, the results were not complete. For these reasons, heat tinting was used as an alternative method. During this research, results obtained from the heat tinting experiments were subject to adjustment in Corel PHOTOPAINT and then compared directly to the actual texture measurements obtained by EBSD. The obvious disadvantage of heat tinting in comparison to EBSD was that it could not identify texture at a very specific location or give a very detailed analysis. However, when comparing heat tinting and EBSD images, it was determined that the colours generated by heat tinting gave some degree of texture information as the basal (0001) and pyramidal {10-11} planes could be detected based on the colour of their oxide. Additional advantages of heat tinting were that there was no size limit to the sample (providing that it was possible to grind and polish the sample and a laboratory furnace was available) and the procedure was less time consuming. Heat tinting was also used to confirm the presence of two separate deformation regimes located at the centre and edge of the billets.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Materials Science and Engineering (Sheffield)
Other academic unit: Department of Engineering Materials
Identification Number/EthosID: uk.bl.ethos.725766
Depositing User: EThOS Import Sheffield
Date Deposited: 09 Sep 2019 14:46
Last Modified: 10 Sep 2019 09:48
URI: http://etheses.whiterose.ac.uk/id/eprint/21886

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