The formation mechanism, growth, and effect on the mechanical properties of precipitate free zones in the alumina-forming austenitic stainless steel Fe-20Cr-30Ni-2Nb-5Al during creep

The mechanisms of formation and growth of L12 precipitate-free zones (PFZs) in the alumina-forming austenitic stainless steel Fe-20Cr-30Ni-2Nb-5Al during creep were studied using scanning electron microscopy and energy dispersive x-ray spectroscopy. The PFZs formed following the dissolution of L12 precipitates due to depletion of nickel and aluminum. Little PFZ growth occurred until the grain boundaries were substantially covered by Laves phase and B2 precipitates large enough to deplete the surrounding grain of nickel and aluminum. This resulted in the dissolution of the L12 precipitates. Additionally, micro-cracks observed in the PFZ at long creep times suggest that the PFZ is ultimately the weakest point in the microstructure, and that fracture initiates there.

Automated summary

The mechanisms of formation and growth of L12 precipitate-free zones in Fe-20Cr-30Ni-2Nb-5Al austenitic stainless steel during creep were studied. PFZs are formed after the dissolution of L12 precipitates due to depletion of nickel and aluminum, and little growth occurs until grain boundaries are covered by Laves phase and B2 precipitates leading to the depletion of surrounding elements. The presence of micro-cracks in PFZs suggests their weakness in the microstructure and their role as initiation points for fracture.