GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Tuesday, August 2, 2016

12:00 PM
in Love 295

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Alex Bryant

"Shock Compression Induced Phase Changes in Cerium-Based Metallic Glass"

Committee Members:

Prof. Naresh N. Thadhani, Advisor, MSE

Prof. Faisal Alamgir, MSE

Prof. Arun M. Gokhale, MSE

Prof. Josh Kacher, MSE

Dr. Christopher Wehrenberg, LLNL

Abstract:

The proposed research is aimed at investigating pressure-induced phase changes in a Ce-based metallic glass (MG) through the use of laser-driven shock experiments and atomic resolution structural characterization. MGs exhibit very high strength, have intrinsically low density, and deform by shear banding. At high pressures and high strain rates, MGs can undergo phase changes that not only absorb energy, but also transform the structure into a higher density configuration with superior mechanical properties. However, the structural evolutions associated with shock compression induced phase transitions are not well understood. Understanding the atomic structure changes concomitant with these phase transitions is the motivation for this work.

The proposed research will involve performing laser-driven shock experiments at varying pressures on melt-spun Ce3Al MG and crystalline ribbons. The extreme pressure (~5 - 250 GPa) response of the MG will be investigated using the high energy Omega laser facility at the Laboratory for Laser Energetics. The lower pressure (~10 GPa) experiments will be performed using Nd:YAG laser driven shock compression and time-resolved velocity interferometry to determine the equation of state for the MG and its various phases. In addition, specimens will be recovered for post-impact structural analysis to gain information about the phases formed, transition states, and the mechanisms for the shock-induced phase changes. Structural analyses will be performed using synchrotron X-ray scattering, comparing macroscopic and atomic structure via X-ray diffraction (XRD), pair distribution function (PDF), and Extended X-ray Absorption Fine Structure (EXAFS). The ultimate goal will be to correlate the structural changes with shock compression conditions. Preliminary PDF results from recovery experiments at pressures > ~9.5 GPa indicate a shock-induced phase change resulting in a higher density amorphous structure. XRD measurements of recovered samples from ~120 and ~75 GPa experiments indicate shock-induced crystallization.

The overall objective of this research is to determine the phase changes and structural evolution occurring in the Ce3Al MG under shock compression and correlate those with the structural stability as a function of shock loading conditions. The significance of the proposed research is the advancement of the fundamental understanding of pressure-induced phase changes in metallic glasses.