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

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.