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Microstructural characterisation and mechanical testing of zirconium alloys and hydrides

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Title: Microstructural characterisation and mechanical testing of zirconium alloys and hydrides
Authors: Wang, Siyang
Item Type: Thesis or dissertation
Abstract: Microstructure and formation mechanisms of δ-hydrides in rolled and recrystallised fine grain (grain size ~11 μm) and ‘blocky alpha’ large grain (grain size >200 μm) Zircaloy-4 have been studied using electron backscatter diffraction (EBSD). A unique {0001}α ||{111}δ;<112 ̅0>α ||<110>δ matrix-hydride orientation relationship (OR) was found which supports the hydride formation mechanism through partial dislocation gliding on alternate matrix basal planes. Hydride stringers in the fine grain material were found to form through sympathetic hydride growth across grain boundaries. The macroscopic alignment of hydride stringers perpendicular to the plate normal direction (ND) is due mainly to the matrix material texture while relates weakly to the grain boundary characters. Micropillar compression tests reveal that the properties of <a> basal slip in Zircaloy-4 vary strongly with temperature between 298 K and 623 K and the change in critical resolved shear stress (CRSS) with temperature follows a thermally activated constitutive law. At 623 K, the addition of hydrogen solutes has led to more homogeneous <a> basal slip and negligible strain hardening. The interactions between crystal slip and hydrides in Zircaloy-4 have been studied using micropillar compression tests of single crystal specimens (where hydrides are intragranular) and macroscale tensile tests of polycrystal specimens (where hydrides are intergranular). Results suggest that local shear along some of the hydride-matrix interfaces is favoured over slip in the matrix nearby, causing localised deformation. Slip bands in the matrix, when reaching the hydride-matrix interfaces (nearly) perpendicularly, can either get arrested at the interfaces or result in shear within the hydride. Using high angular resolution EBSD (HR-EBSD), these complicated slip-hydride interactions have been observed to give rise to significant geometrically necessary dislocation (GND) pile-up along the hydride-matrix interface, indicating the risks of local damage accumulation and the tendency of hydrogen diffusion towards the dislocated area at high temperature triggering delayed hydride cracking (DHC).
Content Version: Open Access
Issue Date: Oct-2019
Date Awarded: Feb-2020
URI: http://hdl.handle.net/10044/1/95972
DOI: https://doi.org/10.25560/95972
Copyright Statement: Creative Commons Attribution NonCommercial NoDerivatives Licence
Supervisor: Britton, Thomas Benjamin
Giuliani, Finn
Sponsor/Funder: Rolls-Royce Group 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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