Alloying effects on creep and oxidation resistance of austenitic stainless steel alloys employing intermetallic precipitates

Microstructure evolution during creep testing at 750 °C and 100 MPa of Fe-20Cr-30Ni-2Nb (at.%) alloys with and without 0.4 Si, 0.2 Zr or 5.0 Al additions has been studied, in order to explore the viability of Fe-rich austenitic stainless alloys strengthened by intermetallic phases. Fine Fe₂Nb Laves phase dispersions with the size of less than 1 μm within the γ-Fe matrix were obtained in the base and Si-modified alloys after aging at 800 °C. The addition of Si helped to refine and stabilize the size of particles, resulting in finer and denser Fe₂Nb dispersion than that in the base alloy. The alloys with only solution heat-treatment exhibited superior creep resistance to the aged ones, which is due to dynamic precipitation of the Fe₂Nb Laves phase during creep testing with a size of 300-400 nm, resulting in more effective pinning of dislocations. The base alloy also showed the meta-stable γ″-Ni₃Nb phase with a size of 50 nm during the transient state of the creep testing. The Zr-modified alloy achieved significant improvement of creep properties. This is hypothesized to be due to the stabilization of δ-Ni₃Nb phase relative to Fe₂Nb, resulting in the formation of multiple fine dispersions of stable intermetallic compounds of δ and Fe₂Nb within the γ-Fe matrix. A small amount of a {Ni, Zr and Nb}-enriched unidentified phase was also observed. The addition of Al significantly improved the oxidation resistance because of the formation of protective alumina scales on the surface. The alloy also showed superior creep resistance to that of the base alloy due to the formation of a dense dispersion of spherical Ni₃Al, typically 30 nm in diameter. Collectively, these findings provide the alloy design basis for creep and oxidation-resistant austenitic stainless steel alloys via intermetallic precipitates.