GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Tuesday, December 18, 2020

9 AM

via

Bluejeans Video Conferencing

https://bluejeans.com/341423848

will be held the

DISSERTATION DEFENSE

for

Sepideh Parvinian

“High Throughput Exploration of Process and Chemistry Space in 18% Ni Maraging Steels (C350)”

Committee Members:

Prof. Hamid Garmestani, Advisor, MSE

Prof. Surya R. Kalidindi, Advisor, ME/MSE

Prof. Naresh Thadhani, MSE

Prof. Steven Liang, ME

Prof. Josh Kacher, MSE

Abstract:

Maraging steels are ultra-high strength steel alloys used in critical aerospace applications such as landing gear, and rocket motor cases. Current maraging steel alloys have been developed with different compositions and heat-treating processes, yet there is potential to advance a new generation of these alloys for weight reduction, performance improvement, and improved processing efficiencies offered by further exploring new compositions and/or more complex heat-treating processes. The design and development process of such advanced structural materials is challenged by the large design spaces in potential chemical compositions and processing histories (e.g., heat treatment). Exploration of this alloy design space requires not only systematic and reliable techniques for characterization of material microstructures and their mechanical properties but also high throughput assays providing savings in cost and time. This thesis focuses on developing and demonstrating high throughput experimental assays to explore materials design space, including both process history and chemistry spaces in 18% Ni maraging steels (C350).

First, we develop and demonstrate the viability of high throughput experimental assays for rapid exploration of thermomechanical process space (e.g., aging temperature and time) and formulating practically useful process-property (P-P) linkages in commercial 18% Ni maraging steels. Second, the viability of high throughput experimental assays for fast screening of the thermomechanical process and chemistry spaces are investigated in three compositionally different hot-forged 18% Ni maraging steels. We specifically targeted the influence of B and Nb microalloying elements on the aging response of maraging steels employing microindentation stress-strain protocols. Furthermore, the evolution of size distribution of martensite block, volume fraction of austenite and crystallographic texture in selected maraging steel samples was evaluated using various microstructure characterization techniques including SEM, EDS, EBSD, and XRD. Lastly, the fidelity and the high throughput nature of protocols employed in this work were presented by comparing them with conventional mechanical testing in terms of estimated yield strength measurements as well as the cost per material condition.

This work aims to have a broad impact on the acceleration of development of new maraging steel alloys by reducing the time and energy spent in mechanical characterization as well as providing useful insights into the microstructure evolution of microalloyed steels.