THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

MASTER OF SCIENCE

on Thursday, November 2, 2017

10:00 AM - 12:00 PM
in LOVE 295

will be held the

MASTER’S THESIS DEFENSE

for

Tanvi Dave

"Effect of Hot-rolling and Intermittent Annealing on the texture evolution of monotectoid, BCC Zr-Nb"

Committee Members:

Dr. Hamid Garmestani, Advisor, MSE

Dr. Chaitanya Deo, NRE

Dr. Preet Singh, MSE

Abstract:

In the case of illicit trafficking of nuclear materials or interdicting nuclear debris, it is necessary to have the capabilities to intercept, collect, and analyze the nuclear material and determine its origin in a timely fashion.  Nuclear forensics aims to determine robust and predictive signatures of nuclear materials in order to determine the origin of the material.  Isotopic signatures, microstructure morphology, and chemical composition all contribute to identification of the material and its origin.  The data that is collected is then compared to empirical data from various databases or is computationally interpolated from existing models of tested material systems.

The incorporation of material science in nuclear forensics allows for the analysis of microstructure-processing relationships of interdicted materials in order to determine the material’s provenance.  The processing steps of actinide alloys can introduce discriminating microstructural elements such as phase or morphological changes which can act as signatures that connect the microstructure to its process path.  Texture analysis of a material at each stage in its process path may provide distinct signatures which can be input into an inverse processing model and reverse engineer a fabrication process for a given final microstructure.

In this work, the first step is taken towards predicting a material’s processing history by understanding the structure-processing relationship of an actinide alloy surrogate material as it undergoes thermo-mechanical processing.  The chosen system for this study was the Zr-18wt% Nb which has applications in the nuclear industry and is a suitable surrogate for U-6wt% Nb.  The as-received Zr-18wt% Nb alloy was β-quenched to retain the high temperature BCC phase during subsequent processing.  The sample was hot rolled to height reductions of 10%, 20%, 30% and 40% and intermittently annealed.  The microstructure evolution was captured by characterizing each stage of this process path using optical microscopy, X-ray diffraction (XRD), and electron back scattered diffraction (EBSD).  The texture and microtexture of the β-phase Zr-Nb alloy is analyzed by calculating the orientation distribution functions (ODF) and discussed in terms of the rolling and recrystallization textures of BCC metals and alloys. The typical rolling and recrystallization textures are well understood for BCC materials that undergo such processing over one pass however the overarching effect on final texture of a material that is repeatedly rolled and annealed has been less frequently studied.  Changes in the initial texture of the Zr-18wt% Nb alloy as it undergoes each hot-rolling and annealing step leads to unconventional patterns of texture evolution as sample reduction is increased.  This work aims to report and explain how such patterns may arise and how these patterns differ when comparing texture to microtexture.