POSAP ANALYSIS OF THE OXIDE ALLOY INTERFACE IN STAINLESS-STEEL

A recently built position-sensitive atom probe (POSAP) was used to visualize the 3D atom-by-atom microstructures of the oxide-alloy interface of stainless steel. The oxide was formed in oxygen at a pressure of 10(-4) Torr on the clean field-evaporated surface of a type 316 stainless steel. The oxidation was carried out at temperatures of 350-degrees-C and 475-degrees-C for 5 min. The composition of the oxide and the microstructure of the interface were determined by the POSAP, and were compared with the oxide film on the ferritic phase of a duplex stainless steel formed at 600-degrees-C. The oxide films formed at 350-degrees-C and at 475-degrees-C consist of mostly iron oxide, whereas in the topmost part of the film at 475-degrees-C, chromium oxide is predominant. The oxide on the duplex stainless steel at 600-degrees-C consists mainly of chromium oxide. The interface is very sharp at 300-degrees-C. It becomes slightly rough at 475-degrees-C, and a rough interface is observed at subnanometer scale at 600-degrees-C. The oxide film tends to grow only in the region just above where molybdenum does not exist.