THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Thursday, April 23, 2020

10:00 AM

via

BlueJeans Video Conferencing https://primetime.bluejeans.com/a2m/live-event/wapzjccp

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Jahnavi Desai

"In-Situ Optical Microscopy Corrosion Experiments to Study the Influencing Microstructural Parameters on β Precipitation in AA 5456"

Committee Members:

Prof. Josh Kacher, Advisor, MSE

Prof. Hamid Garmestani, MSE

Prof. Preet Singh, MSE

Prof. Mathew McDowell, ME/MSE

Prof. Chaitanya Deo, MSE/NRE

Abstract:

Aluminum 5xxx series alloys are non-heat treatable, moderate strength wrought alloys with 3-5% of magnesium. They are widely used as structural components for marine, military and other transportation applications due to their excellent corrosion resistance, low density and good mechanical properties. However, at moderate and high temperatures (50-250°C) these alloys are vulnerable to sensitization, which is precipitation of β phase (Al3Mg2) preferentially on the grain boundaries (GBs). This β phase is more electrochemically active compared to the rest of the Al matrix. As a result, the β phase is preferentially dissolved when the samples are exposed to corrosive environments, making them susceptible to intergranular corrosion and environmentally induced stress corrosion cracking. The primary objective of this research is to characterize and understand the most influential microstructural parameters that dictate β phase precipitation and thus, local intergranular corrosion (IGC) susceptibility of aluminum 5456 in corrosive environments.

My objective is to understand the influence of microstructural features on β phase precipitation, with a focus on extrinsic characteristics like dislocation density and the intrinsic characteristics, misorientation angle and GB plane. For dislocation density study, AA5456 alloy microstructure is characterized using electron backscatter diffraction (EBSD) which facilitates the measurement of the geometrically necessary dislocation density, GB misorientation angle, and various other microstructural features.  After characterization, the sample is subjected to in-situ optical microscopy corrosion experiments, where the sample is immersed in 1% phosphoric acid while the corrosion events occurring on the surface are recorded. These videos are then analyzed to track the progress of IGC, and the corresponding corrosion maps are extracted from them. Image analysis tools and direct comparisons are used to obtain spatial correlations from corrosion maps and maps of microstructural features. To obtain information on GB plane, another microstructural feature of interest, a serial sectioning method will be used. This method consists of a series of mechanical polishing and EBSD mapping cycles to reconstruct a 3D microstructure from 2D microstructural data. This is facilitated by using fiducial markers on the surface and by manually aligning the 2D scans with the help of these markers. The grain boundary plane information will then be determined using an appropriate software. The influence of intrinsic and extrinsic characteristics of GBs on β phase precipitation will be analyzed using data analytics tools. Gaining a better understanding of the interplay of differing microstructural features on grain boundary precipitation of AA5456 alloy will open the door for microstructure sensitive design for enhanced corrosion resistance properties.