Date of Award

12-2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Yanfei Gao, James R. Morris, John Landes

Abstract

Ferritic superalloys strengthened by coherent ordered NiAl B2-type precipitates are promising candidates for ultra-supercritical steam-turbine applications, due to their superior resistance to creep, coarsening, oxidation, and steam corrosion as compared to Cr ferritic steels at high temperatures. Combined computational and experimental tools have been employed to investigate the interrelationships among the composition, microstructure, and mechanical behavior, and provide insight into deformation micromechanisms at elevated temperatures.

Self and impurity diffusivities in a body-centered-cubic (bcc) iron are calculated using first-principles methods. Calculated self and impurity diffusivities compare favorably with experimental measurements in both ferromagnetic and paramagnetic states of bcc Fe. The calculated impurity diffusivities of W and Mo are larger than the self diffusivity of Fe, mainly owing to the lower activation energies.

The microstructural attributes of NiAl-type B2 precipitates are investigated in several designed ferritic superalloys. Ultra-small-angle X-ray scattering in conjunction with transmission electron microscopy is employed to quantify the average size, size distribution, inter-particle spacing, and volume fraction of the primary precipitates. It is observed that as the Al amount increases from 4 to 10 mass%, there is a decrease in the average inter-particle spacing and average particle diameter. An alloy with 6.5 weight percent Al exhibits the optimal creep resistance and an associated maximum Orowan stress at 973 K. The dislocations-particle interaction mode during the secondary creep regime is identified as a combination of Orowan looping and dislocation climb.

In-situ neutron diffraction experiments during tensile and creep deformations reveal the intergranular and interphase load-sharing mechanisms during plastic deformation at elevated temperatures. The change of deformation mechanisms from dislocation slip below 773 K to power-law creep above 873 K is well captured by the evolution of the different lattice strains. High-temperature deformation above 873 K is possibly assisted by the relaxation processes, e.g., grain-boundary sliding and/or diffusional flow along grain boundaries and matrixparticle interfaces. The evolution of lattice strains during high-temperature deformation is further verified by crystal-plasticity finite-element simulations.

The significant findings in the present work provide the crucial baseline information for further alloy optimization and improvement in high-temperature creep resistance of ferritic superalloys.

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