Date of Award

5-2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Yanfei Gao, Mariya Zhuravleva, Hairong Qi

Abstract

To create and design novel structural materials with enhanced creep-resistance, fundamental studies have been conducted on high-entropy alloys (HEAs), using (1) thermodynamic calculations, (2) mechanical tests, (3) neutron diffraction, (4) characterization techniques, and (5) crystal-plasticity finite-element modeling (CPFEM), to explore future candidates for next-generation power plants.

All the constituent binary and ternary systems of the Al-Cr-Cu-Fe-Mn-Ni and Al-Co-Cr- Fe-Ni systems were thermodynamically modeled within the whole composition range. Comparisons between the calculated phase diagrams and literature data are in good agreement. The AlxCrCuFeMnNi HEAs have disordered [face-centered-cubic (FCC) + body-centered-cubic (BCC)] crystal structures. Excessive alloying of the Al element results in the change of both microstructural and mechanical properties in AlxCoCrFeNi HEAs. After homogenization, the Al0.3CoCrFeNi material is a pure FCC solid solution. After aging at 700 °C for 500 hours, the optimal microstructure combinations, the FCC matrix, needle-like B2 [ordered BCC] phase within grains, and granular σ [sigma] phase along the grain boundary, is achieved for Al0.3CoCrFeNi. The chemical elemental partitioning of FCC, BCC, B2, and σ phases at different temperatures, before and after mechanical tests, in Al-Cr-Cu-Fe-Mn-Ni and Al-Co-Cr-Fe-Ni systems are quantitatively characterized by both synchrotron X-ray diffraction, neutron diffraction with levitation, scanning-electron microscopy (SEM), advanced atom-probe tomography (APT), and transmission-electron microscopy (TEM). In-situ neutron-diffraction experiments were conducted to study the strengthening effect of the B2 phase directly. The results show that the creep behavior of Al0.3CoCrFeNi is superior to conventional alloys, and the heat treatment introduces the secondary B2 phase into the FCC matrix, which increases the yielding strength, decreases the ductility, and diminishes the serrated flow during compression tests at high temperatures.

In summary, the outcomes of the development of the HEAs with creep resistance include: (1) Suitable candidates, for the application to boilers and steam and gas turbines at temperatures above 760 °C and a stress of 35 MPa. (2) Fundamental understanding on the precipitate stability and deformation mechanisms of both single-phase and precipitate-strengthened alloys at room and elevated temperatures, and (3) The demonstration of an integrated approach, coupling modeling and focused experiments, to identify HEAs that outperform conventional alloys for high-temperature applications.

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