GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Monday, February 18, 2019

11:00 AM
in Love 295

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Sepideh Parvinian

“Effect of Combined Addition of B and Nb on the Microstructure and Toughness of Maraging Steel”

Committee Members:

Prof. Hamid Garmestani, Advisor, MSE

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

Prof. Naresh Thadhani, MSE

Prof. Steven Liang, ME

Prof. Joshua P. Kacher, MSE

D. Elliott Sievers, The Boeing Co.

Abstract:

Developing a new generation of high-strength steels for gear train applications can be achieved by expanding on existing processing paradigms to include alloys with different compositions. Requirements of new alloys demand a 20% increase in specific power by achieving a superior strength and high wear resistance. Maraging steels with a combination of high strength and damage tolerance are known to be good candidates for this purpose. The main drawback of very high-strength alloys, such as Maraging steel 350 is a low ductility, leading to a low toughness. The primary objective of this research is to characterize a new microalloyed maraging steel in order to understand the influence of microalloying elements in its microstructural evolution and toughness. In the proposed work, three compositionally different maraging steel alloys are fabricated based on varying amounts of boron and niobium elements. The first part of the proposed research involves investigation of the nature of the precipitates and their strengthening effect. For this purpose, the aging parameters during which precipitate hardening occurs are varied for several time durations and temperatures. An advanced high-resolution Transmission electron microscope will be utilized to investigate the composition, morphology, and percentages of the formed precipitates in the microstructure. The influence of the precipitates on the strengthening will be investigated by a nanoindentation instrument. Finally, to study the effect of Nb and B addition on the retained/reverted austenite, and its influence on the materials ductility, two different annealing temperatures will be used. The microstructure of alloys in terms volume fraction of reverted/retained austenite at different annealing temperatures and compositions will be studied by Electron Back Scatter Diffraction (EBSD). The bulk mechanical properties will be measured using uniaxial tensile and Charpy impact tests. In addition, more insight on the contributions of the reverted austenite and precipitates on the fracture behavior of the alloys can be revealed by using a high-resolution electron microscopy at the fracture surface of the sample form bulk mechanical testing. The proposed work will have a profound impact on gear train applications by providing detailed information on the newly developed alloy with high toughness and high yield strength. The broader impact of this study is on economical and environmentally-friendly improvements in the aerospace industries by replacing parts with materials that have higher strength and more reliability, but with less processing.