Microstructural Evaluation and Thermal Creep Behavior of Zr-Excel
Ahmmed, Kazi Foyez
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Dual phase (α-β) Zr-Excel Pressure Tube (PT) material has been heat treated in the (α+β) or β-phase regime to generate variable microstructures. These heat-treated microstructures revealed significant modification in the inital microstructure. The microstructural changes by heat-treatment will have a profound influence on their deformation behavior; characterizing those properties is the main goal of this study. In this dissertation, the experimental results are presented in a manuscript format, which is divided in three technical chapters. Chapter 3 discusses the effect of heat treatment on texture modification; where, as received (ASR) PT materials were heated to a range of temperatures and cooled either in water or in air. It has been observed that due to the orientation relationship between α and β-phase, the ASR microstructure has been significantly altered during heating and cooling. The extent of this alteration strongly depends on the solution temperature and cooling rate. Although, variant selection is observed during texture modification, significant randomization is noticed in the room temperature texture. In Chapter 4, line profile analysis technique has been used to quantitatively analyze the microstructural details of the heat-treated materials. Diffraction pattern analyses demonstrated significant peak broadening in the heat-treated material; which is attributed due to the increase of volume fraction of martensitic α and alteration in the dislocation structures. Line profile analyses also revealed that primary α consists with large sub-grains and correlated dislocations but the martensitic α are highly dislocated. Finally in Chapter 5, thermal creep behavior of the heat treated materials has been studied. Microstructural analyses were also conducted in the pre- and post-creep materials to understand the creep mechanism. Creep anisotropy of the heat treated materials has been investigated by correlating the experimental creep data with their associated texture. The ASR PT material shows very anisotropic creep behavior due to the strong texture. However, the water-quenched-895 treatment substantially decreased the anisotropy by creating a balance between the texture and microstructure. Nevertheless, texture randomization at higher solution temperatures might have changed this balance and made the material being anisotropic again.