Steel superalloys are of interest because some present creep and oxidation resistance similar to Ni-based superalloys, at far less cost.
Gamma (γ): Fe-based alloys feature a matrix phase of austenite iron (FCC). Alloying elements include: Al, B, C, Co, Cr, Mo, Ni, Nb, Si, Ti, W, and Y. Al (oxidation benefits) must be kept at low weight fractions (wt.%) because Al stabilizes a ferritic (BCC) primary phase matrix, which is undesirable, as it is inferior to the high temperature strength exhibited by an austenitic (FCC) primary phase matrix.
Gamma-prime (γ): This phase is introduced as precipitates to strengthen the alloy. γ-Ni3Al precipitates can be introduced with the proper balance of Al, Ni, Nb, and Ti additions.