On the formation of hierarchically structured L2₁ -Ni ₂TiAl type precipitates in a ferritic alloy

This study demonstrates a processing sequence used to generate coherent, two-phase L2₁ -Ni₂TiAl/B2-NiAl precipitates in a ferritic matrix, and documents their microstructural evolution. Dark-field and energy-filtered transmission electron microscopy in combination with energy-dispersive X-ray analysis are used to determine the structure and composition of the different phases in the course of the processing and aging treatment. Through rapid solidification, coherent L2 ₁ -Ni ₂TiAl type precipitates with an internal network of curved, isotropic 1/2〈100〉 antiphase boundaries form within the bcc-Fe matrix. The average width of the precipitates in the as-quenched state is 15 nm. During the subsequent aging heat treatment thin, anisotropic B2-NiAl zones are established, resulting in a three-tiered hierarchical microstructure. The L2 ₁ -Ni₂TiAl precipitates are coherent with the bcc-Fe matrix, while the fine B2-NiAl zones are coherently embedded in the Ni₂TiAl precipitates and aligned along the 〈100〉 directions. The L2 ₁ -Ni₂TiAl parent precipitates have a width of 40 nm in the heat-treated stage and the B2-NiAl zones are 3-7 nm wide. Hence, the addition of Ti to Fe-rich Fe-Ni-Al alloys leads to the formation of coherent L2₁ -Ni₂TiAl precipitates in the bcc-Fe matrix with an internal network of fine, anisotropic B2-NiAl zones arranged in a hierarchical manner. These microstructures are of potential interest for designing and optimizing the mechanical properties of precipitation-strengthened ferritic alloys.