MICROSTRUCTURAL CHARACTERIZATION AND MECHANICAL PROPERTIES OF EXCEL ALLOY PRESSURE TUBE MATERIAL
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Microstructural characterization and mechanical properties of Excel (Zr-3.5%Sn-0.8%Mo-0.8%Nb), a dual phase αZr-hcp and βZr-bcc pressure tube material, is discussed in the current study which is presented in manuscript format. Chapter 3 discusses phase transformation temperatures using different techniques such as quantitative metallography, differential scanning calorimetry (DSC), and electrical resistivity. It was found that the αZr → αZr+βZr and αZr+βZr → βZr transformation temperatures are in the range of 600-690°C and 960-970°C respectively. Also it was observed that upon quenching from temperatures below ~860°C the martensitic transformation of βZr to –hcp is halted and instead the microstructure transforms into retained βZr with ω hexagonal precipitates inside βZr grains. Chapter 4 deals with aging response of Excel alloy. Precipitation hardening was observed in samples water-quenched from high in the αZr+βZr or βZr regions followed by aging. The optimum aging conditions were found to be 450°C for 1 hour. Transmission electron microscopy (TEM) showed dispersion of fine precipitates (~10nm) inside the martensitic phase. Energy dispersive X-ray spectroscopy (EDS) showed the chemical composition of precipitates to be Zr-30wt%Mo-25wt%Nb-2wt%Fe. Electron crystallography using whole pattern symmetry of the convergent beam electron diffraction (CBED) patterns together with selected area diffraction (SAD) polycrystalline ring patterns, suggests the -6m2 point group for the precipitates belonging to hexagonal crystal structure, with a= 2.936 Å and c=4.481 Å, i.e. c/a =1.526. Crystallographic texture and high temperature tensile properties as well as creep-rupture properties of different microstructures are discussed in Chapter 5. Texture analysis showed that solution treatment high in the αZr+βZr or βZr regions followed by water quenching or air cooling results in a more random texture compared to typical pressure tube texture. Variant selection was observed upon water quenching while partial memory effect and some transformation texture with variant selection was observed in the air-cooled sample. The results of creep-rupture tests suggest that fully martensitic and aged microstructure has better creep properties at high stress levels (>700 MPa) while the microstructure from air cooling from high in the αZr+βZr region is less sensitive to stress and shows better creep properties compared to the as-received annealed microstructure at lower stresses (<560 MPa).