Co-dependent microstructural evolution pathways in metastable delta-ferrite in cast austenitic stainless steels during thermal aging

Cast austenitic stainless steels (CASS) are excellent alloys because they combine high corrosion resistance with high strength and toughness. However, they are susceptible to embrittlement upon long-term thermal aging at elevated temperatures. Thus, the microstructural evolution pathways during thermal aging need to be understood to predict and potentially prevent embrittlement. Atom probe tomography was used to identify and quantify the microstructural evolution pathways of the delta-ferrite in different CASS alloys aged for up to 10,000 h at temperatures between 290 degrees C and 400 degrees C. The four steels - CF8, CF8M, CF3, and CF3M - which vary by Mo and C concentration, each experienced spinodal decomposition of the delta-ferrite, and precipitation of G-phase clusters and Cu clusters attached to the G-phase. There were large differences in the extent of these features due to their Mo and C concentration. Using radial distribution function analysis, the interactions of constituent elements was found to determine the evolution of these features, with Mo and C specifically influencing the movement of Cr, Ni, Si, Mn, and Cu atoms due to their relative miscibility with these elements. The results will help inform predictive models for the use of duplex stainless steels for extended operation at high temperatures. (C) 2018 Elsevier B.V. All rights reserved.