Microstructure and Creep Deformation Behavior of a Hierarchical-Precipitate-Strengthened Ferritic Alloy with Extreme Creep Resistance

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.