Texture evolution during beta-quenching of a zirconium alloy

Zirconium alloys are widely used by the nuclear industry as fuel cladding and structural materials. Many physical and metallurgical properties of zirconium alloys, that are important for their performance in nuclear reactors, are affected by crystallographic texture due to the strong anisotropy of individual crystals. Irradiation assisted growth is one example. Zirconium crystals deform anisotropically under irradiation, which in the presence of strong textures (like the ones observed in cold-rolled sheet) causes undesirable deformation of components during service. For this reason, the nuclear industry is interested in developing thermomechanical processes that produce random textures, taking advantage of the allotropic phase transformation undergone by zirconium, from the low temperature hcp alpha-phase to the high temperature bcc beta-phase. One of these processes is beta-quenching, which has showed certain success in weakening strong rolling textures. However, there is no consensus about the fundamental mechanisms involved.The aim of this work is to study the evolution of the texture of the zirconium alloy Zircaloy-2 during beta-quenching, in order to gain understanding on the mechanisms involved on texture development and evolution during the alpha-to-beta and beta-to-alpha phase transformations. Firstly industrially beta-quenched samples were characterised using well known techniques such as laboratory X-ray diffraction (LXRD) and electron backscatter diffraction (EBSD), which revealed a relationship between peak temperature and the inherited alpha texture. An in situ synchrotron X-ray diffraction (SXRD) experiment provided, for the first time, information of texture evolution of zirconium during rapid changes and at non-ambient conditions. Different peak temperatures and stress/strain conditions were tested. Detailed post mortem EBSD characterisation of samples studied in situ provided insight on the relationship between the microstructure and the texture. Finally, laboratory furnaces were used to beta-quench samples at very high temperature. It was found that there is selection of orientation variants during beta-quenching of zirconium, but while the selection during the alpha-to-beta transformation is almost negligible, depending on the texture evolution of the beta-phase (affected by grain growth and/or plastic deformation), diverse mechanisms of variant selection act during the beta-to-alpha phase transformation. The inherited textures observed result from the combination of these mechanisms. Some of the results of this work can be transferred to other systems such as titanium and the alpha-gamma-alpha phase transformation in steel.EThOS - Electronic Theses Online ServiceWestinghouse Electric Sweden ABGBUnited Kingdo

The effect of aluminium on twinning in binary alpha-titanium

The deformation mechanisms of binary Ti–Al model alloys (0–13.1 at.% Aluminium) have been investigated with respect to the twinning activity using in-situ loading in combination with neutron diffraction as well as detailed post mortem electron backscatter diffraction analysis. A consistent starting grain size and texture was generated for all alloys promoting tensile twinning during compression testing. Long-wavelength neutron diffraction and selected area diffraction transmission electron microscopy analysis were carried out to detect evidence of Aluminium ordering and Ti3Al formation.It was found that raising the Aluminium content in Titanium does first slightly enhance twinning, with {10View the MathML source2} tensile twinning being by far the dominant type, while the critical residual intergranular strains for twin initiation decreases. This suggests that either the lowering of stacking fault energy by Aluminium or its solute solution strengthening effect are important factors. At around 7 at.% Aluminium a turning point in twinning activity was noticed and a further increase in Aluminium did result in a dramatic loss of twinning activity particularly when the material had been exposed to an additional low temperature age. The dramatic decrease of twinning activity is strongly correlated with increasing evidence of short range ordering and also early signs of Ti3Al-formation in case of the highest Aluminium content. In addition, electron backscatter diffraction analysis revealed that the formation of Aluminium ordered zones do severely hinder growth of twin boundaries