Influence mechanism of cerium addition on precipitation behaviour of super austenitic stainless steel S32654

The influence mechanisms of Ce addition on the precipitation behaviour of σ phase and nitrides were systematically investigated. The results demonstrated that the addition of Ce significantly promoted the nucleation and slightly inhibited the growth of intergranular and intragranular σ phases. Meanwhile, Ce addition also accelerated the precipitations of π phase and Cr2N. Ce promoted the nucleation of intergranular σ phase mainly through: (i) leading to a linear increase in the driving force for σ phase and Cr activity; (ii) resulting in apparent grain refinement and providing more favorable nucleation sites. Besides increasing driving force and Cr activity, Ce addition also promoted the nucleation of intragranular σ phase by: (i) leading to lattice distortion, which not only caused local element concentration, but also reduced σ/γ interface energy, jointly increasing nucleation sites; (ii) providing other nucleation sites (Ce-bearing inclusions) for σ phase; (iii) enhancing Cr and Mo diffusion coefficients, which accelerated the supply of elements required for nucleation. Furthermore, Ce addition enhanced the driving force for Cr2N, Cr activity as well as Cr and N diffusions, and it also promoted the formation of more favorable nitride nucleation sites around σ phases. These combined actions accelerated the precipitations of π phase and Cr2N.