Why to use Mössbauer spectroscopy?
Mössbauer spectroscopy is a nuclear analytical tool for material research. The results obtained by Mössbauer spectroscopy show high resistance to the arrangement, preparation, and macroscopic texturing of the sample. Since both paramagnetic and magnetic phases (austenite – ferrite) are well distinguished in the Mössbauer spectra, it is possible to perform basic evaluation of retained austenite in a fast way. Taking into consideration that alloying elements and significant amounts of retained austenite exhibit more complex Mössbauer spectra, they need to be analyzed by more precise approach. In basic concept, all magnetic phases that exhibit sextet (including the martensite as a supersaturate solution of carbon in ferrite) are assigned to the ferritic phases. This method is nondestructive and could find a wide application in industrial use. In the precise mode the method is able to distinguish all iron-bearing phases (ferrites, oxides, carbides, etc.).
Mössbauer spectroscopy is a nuclear resonance spectroscopic technique based on the nuclear emission and resonant absorption of gamma rays. This experimental technique provides qualitative and quantitative analysis of materials (e.g., structural, phase, and magnetic information) containing specific elements. The 57Fe isotope shows the most favorable parameters for Mössbauer spectroscopy. Backscattering geometry allows to analyze surfaces of bulk materials. Hence, 57Fe Mössbauer spectroscopy is considered as crucial experimental method in steel characterization. Scattering method utilizes the conversion X-rays registration which analyzes material surface up to the depths of 1–20 µm.
Mössbauer spectroscopy employes electric and magnetic hyperfine interactions between electrons of the iron atom and Mössbauer-active nucleus (in the source). The hyperfine parameters are isomer shift, quadrupole splitting, and hyperfine magnetic splitting. The isomer shift is a result of the Coulomb interaction between nuclear/nuclei charge and electrons charge. The charges distributed asymmetrically around the atomic nucleus (electrons, ions, and dipoles) increase the electric field gradient, which differs from zero on the site of nucleus. These electric quadrupole interactions cause a splitting of the excited nuclear level and provide information about bond properties and the local symmetry of iron site. The third hyperfine parameter is magnetic splitting. This magnetic field can originate within the atom itself, within crystals via exchange interactions, or it can be external one. The magnetic field (nuclear Zeeman effect) splits the nuclear states.
References:
- Olina, A.; Píška, M.; Petrenec, M.; Hervoches, C.; Beran, P.; Pechoušek, J.; Král, P.: Assessment of Retained Austenite in Fine Grained Inductive Heat Treated Spring Steel, Materials 12(24)4063, doi:10.3390/ma12244063
- Pechoušek, J.; Kouřil, L.; Novák, P.; Kašlík, J.; Navařík, J.: Austenitemeter – Mössbauer spectrometer for rapid determination of residual austenite in steels, Measurement 131, 671–676, doi:10.1016/j.measurement.2018.09.028
- Kouřil, L.; Pechoušek, J.; Novák, P.; Navařík, J.; Kohout, P.: Toroidal proportional gas flow counter for conversion X-ray Mössbauer spectroscopy, Nuclear Instruments and Methods in Physics Research B 432, 55–59, doi:10.1016/j.nimb.2018.07.020
- Pechoušek, J.; Kuzman, E.; Vondrášek, R.; Olina, A.; Vrba, V.; Kouřil, L.; Ingr, T.; Král, P.; Mašláň, M.: Successive Grinding and Polishing Effect on the Retained Austenite in the Surface of 42CrMo4 Steel, Metals 12(1)119, doi:/10.3390/met12010119
