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 1h. Magneti...

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Bibliographic Details
Published in:Surface & coatings technology 2014-10, Vol.256, p.15-22
Main Authors: Öztürk, O., Fidan, M., Mändl, S.
Format: Article
Language:English
Subjects:
Alloys
Austenite
Austenitic SS
Austenitic stainless steels
CoCrMo alloy
Cross-disciplinary physics: materials science
rheology
Exact sciences and technology
Expanded austenite structure
Ferromagnetism
Imaging
Magnetic properties
Materials science
MFM
Phases
Physics
PIII
Secondary ion mass spectrometry
Surface treatments
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Summary: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 1h. 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. •Expanded austenite layers on CoCrMo and 304 SS are found to be ferromagnetic.•Expanded phase magnetic properties change from grain to grain.•At high PIII temperatures, expanded phase layers on 304 SS and CoCrMo decompose.•The decomposition phases are CrN, Cr2N and the Cr-depleted matrix.
ISSN:0257-8972
1879-3347
DOI:10.1016/j.surfcoat.2013.11.045