Study of Interface-Related Mechanisms in the Early Stage Precipitation of σ Phase in Hyper Duplex Stainless Steels

This work aims to computationally explore the diffuse prerequisites for σ phase precipitation in high-alloyed duplex steels. The diffusion control for the early precipitation stage of the σ phase was studied by the Calphad thermodynamic modeling combined with the DICTRA software. Different nucleation sites, α/γ, and α/α and γ/γ grain-boundaries, were studied using thermodynamic and mobility databases developed by the present authors in previous works for high-alloyed duplex stainless steel. Simulation results were compared to experimentally obtained microstructures of aged samples. The study of the concentration profiles of the elements Cr, Mo, and Ni revealed that the precipitation is most favorable within the α phase due to fast mobility and strong thermodynamic driving force favoring the formation of the σ phase. Based on the present results, the role of the different heterogeneous sites for the precipitation of the σ phase in hyper duplex steel is discussed.