Investigation of the surface oxidation behavior of iron-based alloys using recoilless nuclear gamma resonance

The recoilless nuclear gamma resonance method, also known as Mössbauer spectroscopy, is a non-destructive method of studying the solid state. This technique can be used to determine the phase composition, local atomic environments, magnetic properties, and the nature of chemical bonds. The experiment requires an iron-containing material to observe the effect of resonant absorption of gamma quanta. One advantage of Mössbauer spectroscopy is that it enables the study of materials both in thin surface layers and in bulk. Mössbauer spectroscopy makes it possible to register different decay products of an excited nucleus. Conversion electron Mössbauer spectroscopy (CEMS) provides information on the structure of materials at a depth of 0.3 𝜇m. On the contrary, Conversion X-ray Mössbauer spectroscopy (CXMS) has a probing depth of around 10 𝜇m. This study examined the impact of high-temperature treatment on the formation of oxide layers in steels and iron-based alloys used in additive manufacturing. The samples studied were produced by selective laser melting (SLM) of austenitic 316L stainless steel and 1.2709 maraging steel powder. Following 3D printing, the surface of the material was subjected to mechanical polishing and sandblasting, after which it was thermally annealed in air at a temperature ranging from 500 to 1000 °C. The surface of samples was studied using scanning electron microscopy (SEM), energy dispersion analysis (EDS), X-ray diffraction (XRD) and grazing incidence X-ray diffraction (GIXRD), in conjunction with the Mössbauer spectroscopy method. Following exposure to temperature, an austenitic iron phase (𝛾-Fe) is present throughout the bulk of 316L stainless steel. The CXMS singlet corresponding to the 𝛾-phase exhibits slight quadrupole splitting indicating the presence of alloying element atoms in the FCC lattice. The CEMS method revealed phase transformations 𝛾→𝛼 in a thin surface layer measuring around 0.2-0.3 𝜇m. The presence of an oxide phase containing iron was also indicated by the detection of a Fe3+ doublet. An increase in the annealing temperature and exposure time of the samples in the laboratory furnace resulted in an increase in oxide scale, which exhibited an inhomogeneous structure. A combination of MS and other methods (XRD, EDS) revealed that the surface of 316L stainless steel is covered with trigonal mixed oxides (Fe-Cr)₂O₃ and Mn-rich oxides Mn₂(Fe-Cr)O₄ after temperature impact. For 1.2709 maraging steel, the method of surface treatment affects the oxidation process. Using the CEMS and GIXRD methods, it was found that surface treatment involving mechanical polishing and heating leads to the formation of 𝛼-Fe2O3, whereas 𝛾-Fe2O3 forms on surfaces treated with sandblasting. GIXRD also revealed that the surface of the 1.2709 maraging steel was inhomogeneous at different incident angles. CEMS showed that at a depth of approximately 0.2 𝜇m the sandblasted and heated (at 500 °C for 2 hours) surface contains of the 𝛾-Fe2O3 of and 𝛼-Fe2O3. According to CEMS, the polished and heated (at 500 °C for 2 hours) surface contains of retained austenite and 𝛼-Fe2O3.