Doctoral Dissertations

Date of Award

5-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Hahn Choo, Yanfei Gao, John D. Landes

Abstract

In order to improve the thermal efficiency and decrease the greenhouse gases emission, it is required to increase the steam temperature and pressure in fossil-energy power plants. In the United States, research has been performing in order to push steam temperature to 760 Celsius degree and steam pressure to 35 MPa. However, the highest operational temperature for current commercial heat-resistant ferritic steels is ~ 620 Celsius degree. In this sense, new advanced ferritic alloys with better creep resistance are needed, considering such service conditions in next-generation ultra-supercritical fossil-energy power plants.

Coherent B2-ordered NiAl-type precipitates have been employed to reinforce the body-centered-cubic iron for high-temperature applications in fossil-energy power plants. One NiAl-strengthened ferritic alloy, designated as FBB8 (Fe-6.5Al-10Ni-10Cr-3.4Mo-0.25Zr-0.005B, weight percent), has been investigated in this study. This study mainly focuses on three critical issues in the development of advanced NiAl-strengthened ferritic alloys: (1) the stability of NiAl-type precipitates at expected service temperatures (e.g., 700, 800, and 950 Celsius degree); (2) the poor ductility and fracture mechanisms at room temperature; and (3) the creep resistance and underlying deformation mechanisms.

The present study achieves a systematical understanding of the microstructure-mechanical properties relationship in NiAl-strengthened ferritic alloys. It furthers our fundamental understanding of the phase stability and deformation mechanisms in precipitate-strengthened alloys, and provides new insights into developing new advanced materials.

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