Graduate Studies, UNL
Embargoed Master's Theses
First Advisor
Bai Cui
Second Advisor
Juan Macchi
Committee Members
Li Tan, Guillaume Hachet
Date of this Version
12-2025
Document Type
Thesis
Citation
A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science
Major: Mechanical Engineering and Applied Mechanics
Under the supervision of Professor Bai Cui
Lincoln, Nebraska, December 2025
Abstract
Hydrogen energy in the form of internal combustion engines [ICEs] dates to the invention of ICEs themselves. Current efforts to reduce carbon emissions of automobiles are highly invested in hydrogen ICEs, and so investigation into the materials needed for this application is highly important. One problem is hydrogen embrittlement.
Most metals fail through ductile fracture. Hydrogen embrittlement is characterized by a transition from ductile to brittle fracture. This transition results in a lower yield stress and a lack of warning before failure in the absence of significant elastic deformation. A cruicial step in this process is the dissociation of the H2 molecule into two H atoms, which dramatically lowers the size and allows diffusion through the matrix.
This project investigated a proprietary alloy used in engine valves for hydrogen ICEs. The alloy is a majority nickel with a high chromium content and lesser amounts of alloying elements such as titanium, aluminum, etc. A protective layer of CrN is present on the combustion surface. These valves were subjected to an intense exposure regimen in a high temperature hydrogen environment and then analyzed. Analysis includes SEM-EDS, TEM imaging and ACOM, micro- and nano-hardness, and Raman spectroscopy.
Advisor: Bai Cui
Comments
Copyright 2025, Ryan A. Wall. Used by permission