Formation and stability of radiation-induced phases in neutron-irradiated austenitic and ferritic steels

Abstract

Ni 3Si (γ'), G, and in many cases M 2P or M 3P type phosphides are phases induced by irradiation in austenitic stainless steels. Radiation-induced segregation (RIS) of alloying elements (i.e., strong enrichment of Ni and Si, and often a relative depletion of Cr and/or Mo) appears to be the major factor affecting their formation and subsequent stability in such steels. These phases are also sensitive to variations of alloy composition, thermomechanical pretreatment, irradiation temperature, and He/dpa ratio. In some austenitic stainless steels, RIS and precipitation can lead to transformation of the matrix into ferrite during irradiation. In martenstic/ferritic 9Cr and 12Cr steels, irradiation can induce formation of M 6C(η), χ, α', and in some cases G, Cr 2X, σ, Cr 3P or MP phases. These phases are sensitive to irradiation temperature, and also vary somewhat with alloy composition, but not as much as in the austenitic steels. The RIS phenomenon in the ferritic steels appears to involve minor elements like Si, Ni, P and possibly V, judging from the compositions of the radiation-induced phases. As most of these phases are also Cr-rich, Cr supersaturation or segregation during irradiation may also play a role.