Microstructural evolution of single Ni₂TiAl or hierarchical NiAl/Ni₂TiAl precipitates in Fe-Ni-Al-Cr-Ti ferritic alloys during thermal treatment for elevated-temperature applications

Precipitate features, such as the size, morphology, and distribution, are important parameters determining the mechanical properties of semi- or fully-coherent precipitate-hardened alloys at elevated temperatures. In this study, the microstructural formation and evolution of recently-developed Fe-Ni-Al-Cr-Ti alloys with superior creep resistance have been systematically investigated using transmission-electron microscopy (TEM), scanning-electron microscopy (SEM), and atom-probe tomography (APT). These alloys were designed by adding 2 or 4 wt percent (wt. %) Ti into a NiAl-hardened ferritic alloy with a nominal composition of Fe-6.5Al-10Cr-10Ni-3.4Mo-0.25Zr-0.005B in wt. %. These alloys were, then, subjected to a homogenization treatment at 1473 K for 0.5 h, followed by aging treatments at 973 K for 1–500 h. In the homogenization-treated case, both alloys contain a primary L2₁-type Ni₂TiAl precipitate, but with the distinct size and morphology of the precipitates and precipitate/matrix interface structures. In the subsequent aging treatments, the 2 wt % Ti alloy establishes a hierarchical-precipitate structure consisting of a fine network of a B2-type NiAl phase within the parent L2₁-type Ni₂TiAl precipitate, while the 4 wt % Ti alloy retains the single Ni₂TiAl precipitate. It was found that the hierarchical structure is more effective in remaining the coherent interface during the growth/coarsening of the precipitate. The formation of the different types of the precipitates, and their effects on the microstructural evolution are discussed, and the driving forces for these features are identified from the competition between the interface energy and elastic interactions due to the lattice misfit and misfit dislocations.