Doctoral Dissertations

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Yanfei Gao, Hahn Choo, Hairong Qi


Hierarchical NiAl [nickel-aluminium compound]/Ni2TiAl [nickel-titanium-aluminum compound] or single Ni2TiAl-precipitate-strengthened ferritic alloys have been developed by adding 2 or 4 weight percent [wt. %] of Ti [titanium] into a previously-studied NiAl-precipitate-strengthened ferritic alloy. A systematic investigation has been conducted to study the interrelationships among the composition, microstructure, and mechanical behavior, and provide insight into deformation micro-mechanisms at elevated temperatures.

The microstructural attributes of hierarchical or single precipitates are investigated in the Ti-containing ferritic alloys. Transmission-electron microscopy in conjunction with the atom-probe tomography is employed to characterize the detailed precipitate structure. It is observed that the 2-wt.-%-Ti alloy is reinforced by a two-phase NiAl/Ni2TiAl precipitate, which is coherently distributed in the Fe [iron] matrix, whereas the 4-wt.-%-Ti alloy consists of a semi-coherent single Ni2TiAl precipitate. The creep resistance of the 2-wt.-%-Ti alloy is significantly improved than the NiAl-strengthened ferritic alloy without the Ti addition and greater than the 4-wt.-%-Ti alloy.

The microstructural evolution of precipitates during heat treatment at 973 K is investigated in the 2-and 4-wt.-%-Ti alloys. Transmission-electron microscopy and atom-probe tomography are used to study the precipitate evolution, such as the size, morphology, composition of the precipitates. It reveals that the hierarchical structure within the precipitate of the 2-wt.-%-Ti alloy evolves from the fine two-phase-coupled to agglomerated coarse structures, as the aging time increases. Moreover, the transition from the coherency to semi-coherency is concomitant with that of hierarchical structure within the precipitate.

In-situ neutron-diffraction experiments during tensile and creep deformations reveal the interphase load-sharing mechanisms during plastic deformation at 973 K. The evolution of lattice strains during high-temperature deformation is further verified by crystal-plasticity finite-element simulations. In-situ neutron-diffraction experiments during stress relaxation at 973 K exhibits the load, which is transferred from the matrix to precipitate is relaxed, which indicate the occurrence of the diffusional flow along the matrix/precipitate interface.

These results could provide a new alloy-design strategy, accelerate the advance in the development of creep-resistant alloys, and broaden the applications of ferritic alloys to higher temperatures.

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