MFM imaging of expanded austenite formed on 304 SS and CoCrMo alloys

New data related to the magnetic nature of the expanded austenite layers on CoCrMo and austenitic stainless steel by nitrogen plasma immersion ion implantation (PIII) are presented. Implantations were performed in the temperature range between 300 and 550 °C for a fixed processing time of 1 h. Magnetic properties, nitrogen distribution, implanted layer phases, and surface topography were studied with a combination of experimental techniques involving magnetic force microscopy, SIMS, XRD, SEM and AFM. As a function of the processing temperature, phase evolution stage for both alloys follows the same trend: (1) initial stage of the expanded phase formation, γN; (2) its full development; and (3) its decomposition into CrN precipitates and the Cr-depleted matrix, fcc γ-(Co, Mo) for CoCrMo and bcc α-(Fe, Ni) for 304 SS. MFM imaging reveals distinct, stripe-like ferromagnetic domains for the fully developed expanded austenite layers both on CoCrMo and 304 SS alloys. Weak domain structures are observed for the CoCrMo samples treated at low and high processing temperatures. The images also provide strong evidence for grain orientation dependence of magnetic properties. The ferromagnetic state for the γN phase observed here is mainly linked to large lattice expansions due to high N content.