THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Monday, November 7, 2016

1:00 - 3:00 PM EST

(10:00 AM – Noon PST)

in MRDC 3515

will be held the

DISSERTATION DEFENSE

for

Rene Diaz

"Dynamic Deformation of Titanium-Based Bulk Metallic Glass Composites"

Committee Members:

Prof. Naresh Thadhani, Advisor, MSE

Dr. Douglas Hofmann, NASA JPL

Prof. Surya Kalidindi, ME

Prof. Arun Gokhale, MSE

Prof. Mo Li, MSE

Abstract:

This work sought to understand the role of the microstructure of titanium-based bulk metallic glass (BMG) and bulk metallic glass matrix composites (BMG-MCs) under dynamic deformation. BMG-MCs provide enhanced toughness and ductility in contrast to monolithic BMGs through in-situ formed crystalline dendrites. The BMG and BMG-MC system investigated in this work is the titanium-based ``DVX'' system consisting of Ti-Zr-V-Cu-Be with varying size, morphology, and distribution of the second phase dendrites. The focus of this work was to determine the influence of the glass-composite structure of titanium-based bulk metallic glass matrix composites with in-situ precipitated dendrites of varying composition, crystallinity, and morphology in the dynamic deformation response compared to monolithic titanium-based bulk metallic glasses.

A comprehensive assessment of the microstructural response on the dynamic yielding and spall response through controlled plate impact experiments. The experiments consisted of simultaneous impact of two samples with one being probed using VISAR interferometry and the other being recovered for post-mortem fractography and characterization. The dynamic properties observed focused primarily on the dynamic compressive yielding, referred to as the ``Hugoniot Elastic Limit'', and the dynamic tensile strength referred to as the ``spall strength'', were determined using VISAR interferometry from experiments performed at impact pressures from 6.0 - 17.3 GPa. The spall strength and HEL were also correlated as a function of strain rate from decompression, peak pressure, and subsequent recompression states after spallation. The decompressive strain rate sensitivity provides insight on the resistance to spall fracture and showed the DV1-SSF alloy, to have the highest resistance to spall fracture. The recompression characteristics after spallation were indicative of the role of microstructure on dynamic fracture characteristics. The recompressive strain rate sensitivity showed that the DV1-SSF results in the most ductile fracture response compared to the other DVX alloys.

Post-mortem microstructural characterization done on the recovered samples provided a good correlation with the observed dynamic fracture characteristics seen during recompression. The dynamic fracture of the BMG composites was seen to be directly dependent on the glass content, dendrite size measured using the mean linear intercept through the dendrite, interdendritic spacing measured through the mean free path through the matrix, interfacial surface area, two-dimensional matrix connectivity, and microhardness of the dendrite. The experimentally measured microstructural parameters including glass volume fraction, surface area per unit volume, and mean linear intercept through the dendrite, were utilized to develop a stereologically based empirical model to project the spall strength of metallic glass composites across varying strain rates. The projections reveal that composites with minor amounts (few to 30%) of softer in-situ formed dendrites in a majority content of the stronger metallic glass matrix provides the maximum spall strength.