On the α+γ⇆γ-phase boundary in nickel and in manganese containing stainless steel alloys

The alpha -ferrite-phase in nickel and in manganese containing stainless steel alloys is very brittle. Its formation is connected with a huge decrease in volume, which also causes stresses in the alloys that are very dangerous if the materials cannot accommodate them. The alpha -ferrite-phase is formed in stainless steel alloys only if nucleation sites are provided. alpha ' -martensites are nucleation sites for the formation of alpha -ferrite, even at temperatures at which alpha -ferrite is not stable. If alpha ' -martensite is dissolved during an anneal, no new alpha -ferrite is formed, and the remaining alpha -ferrite transforms back into gamma -austenite. In "pure" nickel or in "pure" manganese containing stainless steel alloys the martensitic temperature is above 100 degreesC and thus alpha ' -martensite is always present at ambient temperature in these materials, giving rise to the formation of alpha -ferrite during a subsequent anneal. It was established in the present work that the gamma <----> gamma+alpha -phase boundary in iron-chromium-nickel alloys is also almost independent of the temperature as in iron-chromium-manganese alloys. Thus the existing phase diagram for nickel containing stainless steels has to be revised. The various elements added to "pure" stainless steel alloys, as we find them in EUR-316L, US-316L, US-PCA, and in AMCR, cause a drastic decrease of the martensitic temperature so that neither alpha ' -martensite nor alpha -ferrite is found in these alloys. However, in all these four alloys alpha -ferrite is formed readily during irradiation with high energy particles and a tentative gamma <----> gamma+alpha -phase boundary, valid during irradiation with high energy particles, is derived. The amount of alpha -ferrite formed during irradiation increases with decreasing irradiation temperature and with decreasing applied stress. The alloys EUR-316L, US-316L, US-PCA, and AMCR do not survive one reactor cycle, if irradiated at 100 degreesC.