The influence of Al and Y contents on tensile properties of 18Ni–14Cr-(2.5–3.5)al-based stainless steels under thermal aging

The microstructural stability and mechanical performance of alumina-forming austenitic (AFA) steels under prolonged thermal exposure are critical for their application in advanced nuclear energy systems. This study systematically investigates the individual and synergistic effects of aluminum content (2.5 vs. 3.5 wt%) and yttrium microalloying (0.05 wt%) on the tensile properties and microstructural evolution of Fe–18Ni–14Cr–(2.5–3.5)Al-based AFA steels during thermal aging at 500 °C for durations up to 1500 h. Uniaxial tensile tests were conducted from RT to 300 °C. Results demonstrate that increasing Al content from 2.5 wt% to 3.5 wt% significantly enhances strength, primarily attributed to a substantial increase in the volume fraction of B2–NiAl precipitates. The addition of 0.05 wt% Y further refines the grain structure and promotes the precipitation of both B2–NiAl and NbC phases, yielding the highest strength among the studied alloys. However, this strength enhancement is coupled with a pronounced degradation in ductility, particularly after extended aging (1500 h), where the Y-containing alloy exhibited a marked reduction in elongation at 300 °C to 20.3 %. Microstructural analysis reveals that prolonged aging leads to coarsening of B2–NiAl particles and their preferential aggregation at grain boundaries, transitioning the fracture mode from ductile dimple rupture to brittle cleavage. A definitive composition-microstructure-property relationship is established, elucidating the competing mechanisms of precipitation strengthening and over-aging. This work provides vital insights for designing AFA steels with balanced mechanical properties for long-term high-temperature service.