Microstructural evolution of Fe–20Cr–30Ni–2Nb–5Al AFA steel during creep at 760 °C

Abstract

The microstructural evolution of an alumina-forming austenitic stainless-steel Fe–20Cr–30Ni–2Nb–5Al (at. %) was investigated after creep at 760 °C. It was found that the secondary creep rate was correlated with changes in the microstructure. During creep, both Laves phase and B2 precipitates in the matrix both coarsened and increased in volume fraction. In addition, nano-sized L1 2 -structured Ni 3 Al precipitates nucleated and then coarsened and increased in volume fraction during creep. Grain boundary coverage by Laves phase and B2 precipitates increased during creep, and a precipitate free zone (PFZ) formed along the grain boundaries at creep times great than 2000 h. The creep rate decreased for times up to ~2000 h primarily due to strengthening from the L1 2 precipitates, with grain boundary strengthening playing a minor role. At longer times, there was a slight increase in creep rate, due to the coarsening of precipitates in the matrix and the formation of the PFZ. • The secondary creep rate is shown to depend on the microstructural evolution during creep testing of an AFA steel. • The coarsening of the large Laves phase and B2 precipitates in the matrix during creep increases the creep rate. • The nucleation and increase in volume fraction of nano-sized L1 2 precipitates during creep decreases the creep rate. • The Laves phase and B2 precipitates grain boundary precipitates appear to strengthen the material.