The effect of aging on the microstructure and mechanical behavior of the alumina-forming austenitic stainless steel Fe–20Cr–30Ni–2Nb–5Al

The effect of aging on the microstructure and mechanical behavior of an alumina-forming austenitic stainless steel, Fe–20Cr–30Ni–2Nb–5Al (at%) has been investigated. The alloy was fully solutionized after a 1250 °C, 24 h heat treatment, and the precipitation of B2 and Laves phases was studied after aging at 800 °C for up to 1325 h. While after 24 h the Laves phase precipitates in the matrix were 205 nm in diameter, they showed a further increase in diameter of only 50 nm even after aging for 1325 h. In contrast, the B2 precipitates in the matrix grew at a faster rate: after first being observed after aging for 24 h at an average diameter of 194 nm, they more than doubled in size from 330 to 734 nm as the aging time increased from 240 h to 1325 h. Both the Laves and B2 precipitates in the grain boundaries grew at a faster rate and were larger than matrix precipitates. The grain boundary coverage at 2.4 h (Laves 192 nm, NiAl 126 nm) was 56% with Laves phase initially making up the bulk of the precipitates, but after 2.4 h Laves phase and B2 precipitates alternated on the grain boundaries and total coverage reached 93% after 1325 h. An increase in the volume fraction of precipitates in the alloy was accompanied by an increase in the yield strength from 205 MPa after the solutionizing treatment up to 383 MPa after aging at 800 °C for 1325 h. After aging for 1325 h, even with extensive intermetallic grain boundary coverage, the alloy showed a room temperature elongation of 19%.