Deformation-Induced Phase Transformation in a Quaternary Zirconium Alloy
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Zirconium alloys are used extensively in the nuclear industry, largely due to the very low neutron capture cross-section of zirconium. Alloying of zirconium with substitutional solutes, like Sn, Mo or Nb, is common to assist in strengthening the material, through either direct solute strengthening, or enhancing control of the microstructure through the introduction of the β phase. One such alloy, Zr-3.2Sn-0.8Mo-0.8Nb, known as Excel, has been the subject of recent study as a possible replacement for the more common Zr-2.5wt%Nb alloy. This dissertation presents a body of research investigating the metastability of the β phase within this alloy, and exploring the decomposition of this phase under mechanical and thermal treatment. This thesis is presented in manuscript format, composed of six manuscript chapters. Chapter 3 presents work investigating the thermal α + β to β phase transformation, establishing the transus temperature and the nature of solute redistribution during cooling from the fully β microstructure. Chapter 4 investigates the nature of retention of a metastable β phase to room temperature, with a focus on understanding the effect of cooling rate and providing a framework for understanding why this process is achievable in this particular alloy system. Chapter 5 provides evidence for a mechanically-induced β to α phase transformation, with a combination of neutron and electron diffraction data, supported by a simple and effective new model that directly relates the experimental data to crystallographic properties of the phase transformation. The next two chapters present studies that further probe the nature of this phase transformation, with a focus on temperature and loading orientation in Chapter 6, and the effect of strain and dislocation dynamics in Chapter 7. Both chapters further develop the model and demonstrate the unique behaviour of the deformation-induced phase transformation. Chapter 8, the final manuscript chapter, presents a study on the thermal decomposition of the metastable β phase. The six manuscript chapters are followed by Chapter 9, which presents a summary of the major conclusions of each work, a discussion on how each chapter is related, and recommendations for future research.