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Title: An investigation into the texture development during hot-rolling of dual-phase zirconium alloys
Author: Daniel, Christopher
ISNI:       0000 0004 7965 2932
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2018
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Dual-phase alpha plus beta Zr-Nb alloys have a higher strength and fracture toughness than single-phase alpha alloys and develop different crystallographic orientations (textures) during thermo-mechanical processing. The textures developed at manufacture are particularly important in determining the life-limiting in-reactor behaviour of nuclear components. Dual-phase Zr alloys tend to form a strong transverse (TD) texture of the basal pole, the origin of which is poorly understood and cannot be predicted by crystal plasticity texture evolution models. This is because the microstructure and texture evolution of these dual-phase alloys arises from complex interactions between the alpha (hexagonal-close-packed, hcp) and beta (body-centred-cubic, bcc) phases, during both deformation and phase transformation. The work presented here is an investigation of the texture evolution during high temperature rolling of an industrially used Zr-2.5Nb alloy, along with the hot-rolling and uniaxial compression of two model dual-phase Zr-Nb alloys (Zircaloy-4 with 2.5 wt.% Nb and Zircaloy-4 with 7 wt.% Nb). The aim was to determine the relative roles of plastic strain partitioning between phases, the activity of the different deformation modes and phase transformation on the final texture. The effect of temperature (700C to 825C), reduction ratio (50% to 87.5%) and strain rate, along with the influence of starting texture, was characterised using time-of-flight (TOF) neutron diffraction and EBSD techniques. The alpha transverse texture component, with prismatic alignment {11-20}, strengthens with greater rolling reduction at the higher temperatures, accompanied by a weakening of basal part orientations with 0002 in ND. Software reconstruction of EBSD orientation maps, using the Burgers relationship, shows how the strength of the transverse texture component varies across the material depending on the orientation of large prior-beta grains. A more detailed characterisation of the high temperature deformation and phase transformation behaviour was made on a hot-rolled Zircaloy-4 with 7 wt.% Nb alloy. Since a greater proportion of metastable beta-Zr is retained to room temperature, a snapshot of the material before beta to alpha phase transformation can be captured, distinguishing high temperature primary alpha grains from the nucleation and growth of secondary alpha variants. By analysing these structures in 3D, using a plasma focused ion beam (PFIB) and taking sequential EBSD slices, it was found that the degree of breakup is affected by the distribution of primary alpha laths within each beta-grain. Further analysis shows that the orientation of the primary alpha influences the breakup behaviour of the beta-grains. Softer alpha orientations, with 0002 in TD, are favoured through a slip compatibility with the beta-matrix. Harder alpha grain orientations develop a much higher misorientation, with a greater stored energy to undergo a dynamic transformation, during deformation. These findings suggest new ways in which the current models can be developed, to enable the successful prediction of hot-rolling textures in these alloys.
Supervisor: Preuss, Michael ; Quinta Da Fonseca, Joao Sponsor: Not available
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: deformation processing ; dynamic recrystallization (DRX) ; processing maps ; phase transformation ; compatible slip ; flow softening ; uniaxial compression ; load partitioning ; reactor core materials ; mechanical testing ; aerospace materials ; alpha/beta processing ; CPFEM modelling ; transverse texture component ; recrystallization ; nuclear reactor ; zirconium ; rolling ; thermo-mechanical processing ; texture ; crystallographic orientation ; EBSD ; neutron diffraction ; titanium ; fast acquisition ; slip ; niobium ; high temperature ; dynamic transformation ; dual-phase alloy