Evolution of Precipitate Phases During Long-Term Isothermal Aging at 1083 K (810 °C) in a New Precipitation-Strengthened Heat-Resistant Austenitic Stainless Steel

Evolution of precipitate phases in a precipitation-strengthened high nitrogen 15Cr-15Ni-4Mn-3Cu-1.5Mo-0.46Si-Nb-C-N-Al-B austenitic heat-resistant stainless steel during 10,000 hours of isothermal aging at 1083 K (810 °C) was investigated. The major precipitate phases found in microstructure after 330 hours of aging are coarser residual primary Nb(C,N) precipitates, Z-phase precipitates, (Nb,Cr)N precipitates with fcc crystal structure, Cu precipitates, and coarser Cr₂N precipitates. No significant changes were noted in the chemical compositions, crystal structure of the precipitate phases, or the phase species after 10,000 hours at 1083 K (810 °C). The coarsening behavior of Z-phase, (Nb,Cr)N, and Cu precipitates was the only observed notable change in the microstructures during the performed aging. Z-phase and (Nb,Cr)N precipitates exhibited superior dimensional stability and morphology with extremely low coarsening rates, while Cu precipitates coarsened at the fastest rate. In addition, Z-phase particles showed a remarkable lower coarsening rate as compared to that of (Nb,Cr)N precipitates. The abnormal slow coarsening kinetics of Z-phase can be explained by the nucleation energy barrier (NEB) limited coarsening mechanism applied for faceted ceramic crystals.