Effect of hafnium micro-addition on precipitate microstructure and creep properties of a Fe-Ni-Al-Cr-Ti ferritic superalloy

Abstract The Fe-Ni-Al-Cr (FBB8) ferritic alloy contains B2-ordered NiAl precipitates which, upon minor addition of Ti to the alloy, display L21-ordered Ni2TiAl sub-precipitates; this improves creep resistance, consistent with an increase in coherency strains from the hierarchical NiAl/Ni2TiAl precipitates. Here, we study the effect of a small addition of Hf (0.5 wt%) to FBB8-1.5wt.%Ti alloy on precipitate structure and creep properties. The main microstructural changes with Hf addition are relatively minor: (i) decrease of mean radius of Ni2TiAl/NiAl hierarchical precipitates from 84 ± 14 to 78 ± 13 nm, (ii) increase of volume fraction of these precipitates from 18 ± 2 to 19 ± 2%, (iii) increase of their number density from 9 ± 0.3 × 10−19 to 11 ± 0.3 × 10−19 m−3, and (iv) decrease of precipitate edge-to-edge distance from 187 ± 57 to 160 ± 48 nm. The larger volume fraction, higher number density and smaller edge-to-edge distance of B2/L21 precipitates in the FBB8-1.5Ti-0.5Hf alloys, are all favorable to higher creep resistance for a strengthening mechanism based on precipitate climb bypass. However, the threshold stress for creep at 700 °C decreases upon Hf addition to FBB8-1.5Ti, from 156 to 122 MPa. The lower creep resistance is explained by a decrease of the lattice misfit between the Hf-enriched B2-precipitates and the bcc-matrix, and by a decrease of the volume fraction of L21-Ni2TiAl sub-precipitates within B2-NiAl precipitates from 16 ± 4% to 10 ± 3% without and with Hf, respectively.