Increasing the creep resistance of Fe-Ni-Al-Cr superalloys via Ti additions by optimizing the B2/L21 ratio in composite nano-precipitates

Abstract The Fe-10Cr-10Ni-6.5Al-3.4Mo-0.25Zr-0.005B (wt.%) ferritic FBB8 superalloy shows good creep resistance due to the presence of B2-NiAl precipitates, created upon aging. When titanium is added to the alloy, L21-Ni2TiAl sub-precipitates are developed within the B2-NiAl main precipitates. The microstructural evolutions of these B2/L21 composite precipitates - radius, number density, volume fraction, edge-edge distance, and B2/L21 phase fraction - are studied here for Ti additions spanning up to 4 wt%. As Ti increases from 0 to 3.5 wt%, the alloy strength at ambient temperature rises due to an increase of the L21 sub-precipitate volume fraction within the B2 precipitates, which enhances their lattice misfit with the matrix up to ∼1.26%, and increases coherency strengthening. The alloy strength drops sharply for 4 wt% Ti, consistent with the precipitates losing (i) their composite structure (by becoming a fully L21 phase), (ii) their coherency with the matrix (and showing high dislocation density at their interfaces), and (iii) their coarsening resistance (increasing abruptly in size). Creep resistance at 700 oC follows a similar trend (raising from 0 to 3.5 wt% Ti and dropping sharply at 4 wt% Ti); this trend is consistent with the lattice misfit between the coherent B2/L21 precipitates and the matrix increasing with the L21 fraction, thus producing a stronger elastic stress field, which makes the climb bypass of the precipitates by the matrix dislocations more difficult.