Interfacial instability and creep damage evolution in a 5 wt. % Al alumina-forming austenitic steel at 700 °C

The development of 700 o C advanced ultra-supercritical power plants has created a critical demand for structural materials that bridge the performance gap between conventional stainless steels and costly superalloys. The purpose of this investigation was to evaluate the correlation among precipitate evolution, deformation behavior, and creep damage at 700 o C of a 5 wt.% Al alumina-forming austenitic (AFA) steel. Creep tests conducted under stresses ranging from 120 to 250 MPa yielded an apparent stress exponent of 5.77. The creep deformation is consistent with a dislocation climb-controlled mechanism, a process significantly influenced by interactions between precipitates and dislocation. However, the alloy exhibits a conspicuous microstructural trade-off at this elevated aluminum level. The NiAl phase undergoes a progressively coarsening during creep, while the Fe 2 (Nb, Mo)-type Laves phase remains relatively stable. The formation driving force of the σ phase is calculated to increase as the volume fraction of the NiAl phase increases. Moreover, the precipitation of these brittle σ phases contributes to a reduction in matrix stability. The ensuing interfacial incompatibility has been demonstrated to promote local stress concentration and damage accumulation, a finding that is consistent with the measured creep damage tolerance factor (λ = 9.15). Consequently, the ultimate failure can be more reasonably interpreted as a damage process associated with microstructural degradation and cavity coalescence. The aforementioned results suggest that the addition of 5% aluminum (Al) does not effectively balance antioxidant properties and structural stability during the creep. However, systematic compositional validation remains imperative in the future.