Document Type



Master of Science (MS)


Materials Science and Engineering

First Advisor's Name

Yu Zhong

First Advisor's Committee Title

Committee Chair

Second Advisor's Name

Zhe Cheng

Second Advisor's Committee Title

Committee Member

Third Advisor's Name

Jichua Chen

Third Advisor's Committee Title

Committee Member


Grade 91 steel, Creep resistance, Ferritic-Martensitic steels, Welding microstructure, Computational thermodynamics, Secondary phase, Alloy composition

Date of Defense



Grade 91 (Gr.91) is a common structural material used in boiler applications and is favored due to its high temperature creep strength and oxidation resistance. Under cyclic stresses, the material will experience creep deformation eventually causing the propagation of type IV cracks within its heat-affected-zone (HAZ) which can be a major problem under short-term and long-term applications. In this study, we aim to improve this premature failure by performing a computational thermodynamic study through the Calculation of Phase Diagram (CALPHAD) approach. Under this approach, we have provided a baseline study as well as simulations based on additional alloying elements such as manganese (Mn), nickel (Ni), and titanium (Ti). Our simulation results have concluded that high concentrations of Mn and Ni had destabilized M23C6 for short-term creep failure, while Ti had increased the beneficial MX phase, and low concentrations of nitrogen (N) had successfully destabilized Z-phase formation for long-term creep failure.




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