Mechanism for the multi-stage precipitation of Fe-Ni based alloy

This work lays great emphasis on the distribution evolution of sigma (σ) phase in Fe-Ni based N08028 alloy during aging process, the result of which could provide new insights into phase change and precipitation mechanism. It is found that the σ phase, in any case, tends to separate out with a granular shape at grain boundaries (GBs) primarily, and with the increment of time, it is obliged to precipitate in grain interiors (GIs) with a lamellar structure. The mechanisms for the evolving volume fraction and morphology of σ phase are discussed, and a model appropriate for the multi-stage behavior transforming from intergranular to intragranular precipitation is derived, as well as revealing the thermodynamics and transformation kinetics corresponding to this process. The results reveal that the occurrence of multi-stage precipitation is correlated with the redistribution of solute atoms and with the difference in coupling effect of thermodynamic driving force and kinetic activation energy between the intergranular and intragranular precipitation.