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Texture evolution in Ti-6AI-4V

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Title: Texture evolution in Ti-6AI-4V
Authors: Wyatt, Simon James
Item Type: Thesis or dissertation
Abstract: Crystallographic orientation and microstructural morphology control properties in engineering materials. Titanium alloys are used extensively in commercial turbofan aircraft engines, due to their high strength and excellent corrosion resistance. The formation of preferred crystallite orientations during manufacturing must be understood in order to maximise component lifespan and avoid failure. In this thesis, I present a methodology which generates virtual 3D microstructures representing a material, conforming to an approximation of a 2D reference surface characterised by electron backscatter diffraction (EBSD). The subsurface grains of this microstructure are instanced using statistical information taken from the map, controlling grain size and texture. The subsurface texture is controlled through optimisations of an orientation distribution function (ODF) and misorientation distribution function (MDF). The influence of this control is shown through simulating deformation within the DAMASK crystal plasticity fast Fourier transform (CP-FFT) solver, to demonstrate the effect of subsurface texture on the stress and strain partitioning on the reference surface. The textures of Ti-6Al 4V formed through hot-rolling at temperatures between 750 and 950 °C are characterised by EBSD. As this method measures spatial and orientation information describing a 2D surface the material, I investigate the mechanisms through which lattice orientations of crystallites evolve during processing. EBSD maps are segmented by preferred orientation to demonstrate the spatial distribution of texture fibres. By measuring phase composition through direct backscatter spectroscopy (DBS) and elemental composition through energy dispersive x-ray spectroscopy (EDS), I demonstrate the influence of the β phase on the formation of texture fibres during rolling, with weak evolution of all existing texture fibres as the Ti-6Al-4V bar plastically deforms through slip. Through CP-FFT simulations of synthetic textured polycrystals in DAMASK, using the orientations of the texture fibres observed by EBSD, I simulate the texture evolution during deformation by hot rolling. Through examination of lattice rotation, slip shear rates and twinning shear rates, I demonstrate that the texture evolution resulting from plastic deformation at high temperatures is conducted entirely by crystal slip, resulting in only small lattice rotations and weak texture evolution. This is in agreement with the textures obtained through EBSD of hot-rolled Ti-6Al-4V.
Content Version: Open Access
Issue Date: Nov-2019
Date Awarded: Mar-2020
URI: http://hdl.handle.net/10044/1/96474
DOI: https://doi.org/10.25560/96474
Copyright Statement: Creative Commons Attribution NonCommercial ShareAlike Licence
Supervisor: Britton, Thomas
Dunne, Fionn
Sponsor/Funder: Engineering and Physical Sciences Research Council (EPSRC)
Rolls-Royce plc
Department: Materials
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Materials PhD theses

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