Doctoral Dissertations

Date of Award

5-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Sudarsanam Suresh Babu

Committee Members

Hahn Choo, Yanfei Gao, Steven Zinkle

Abstract

With the demand for materials at higher operation temperature for fossil energy to improve efficiency and reduce the emission of CO2, current ferritic martensitic steels are not comparable due to the upper temperature limit and Type IV cracking in the weldment. Therefore, a carbon free Fe-30Cr-3Al alloy system with C14 Laves phase precipitate as the strengthening phase was designed with thermodynamic calculations (ThermoCalc®) at 700 °C for creep applications. Previous studies showed the minimum creep rate decreases from 1.7×10-8 s-1 to 1.3×10-9 s-1 as the Laves phase fraction increases from 1.5 to 3.38 mole percent. Precipitate free zones (PFZs) adjacent to grain boundaries were observed and hypothesized to be the potential creep failure mechanism. A grain boundary zone strength factor (GBZSF) was proposed to quantify the behavior of PFZs. In this study, to increase the Laves phase fraction to obtain a lower minimum creep rate and minimize PFZ, the addition of elements that tend to form Laves phase (e.g. Nb, Ti, W, Zr, and Ta) were considered. After considering the calculated Laves phase mole percent, nucleation driving force, and BCC solvus temperature for manufacturing feasibility, alloy compositions were narrowed down and aged at the target service temperature 700 °C. Aging studies up to 3000 hours (still going up to 10000 hours) were conducted to understand the microstructural evolution of Laves phase and PFZ. The Fe-30Cr-3Al-1Nb-0.2Si-1Ti alloy has the highest microstructural stability among all the tested alloys after aged for 3000 hours. The Fe-30Cr-3Al-1Nb-0.2Si-6W alloy exhibited a rapid nucleation and postponed coarsening behavior compared to the other tested alloys. Thus, we hypothesized the Laves phase in the 6W alloy has a core-shell structure with different diffusivity that causes the postpone coarsening. Advanced characterization atom probe tomography (APT) and in-situ transmission electron microscope (TEM) were proposed to understand the fundamental mechanism for the aforementioned behavior.

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