Preferential distribution behavior of B and its enhancement on creep strength of alumina-forming austenitic heat-resistant steel

The addition of a trace amount of 20 ppm B significantly enhanced the creep performance of the 2.5Al-AFA alloy at 650 °C, demonstrating sustained improvement even after prolonged thermal aging. Boron (B) preferentially distributed in the Laves phase within the matrix, rather than in the NiAl or Cr - rich phases. APT analysis revealed M23C6 may serve as a reservoir of B, gradually allowing B diffusing into the surrounding Laves phase at grain boundaries (GBs). Creep cracks propagated mainly along GBs, with the σ phase serving as initiation sites. The average volume fraction of σ phase exhibited a linear correlation with the reciprocal of creep rupture time. B accelerated the precipitation of Laves with a larger size and higher volume fraction, which played a dual role of improved GB strengthening and more effective suppression of σ phase growth.