A CEMS study of surface oxidation of Fe--Ni alloys

Abstract

A study by Conversion Electron Mössbauer Spectroscopy (CEMS) carried out by using a Parallel Plate Avalanche Counter with samples of Fe--Ni alloys (50 and 65 at.% Fe) is reported. Each sample was analyzed without oxidation and after heating it under an oxygen atmosphere at 200°C. All CEMS measurements were carried out at room temperature. In both samples (50 and 65 at.% Fe), without oxidation and after oxidation, the Mössbauer spectra showed a six line magnetic spectrum according to their ferromagnetic character, with a broad Hyperfine Field Distribution (HFD), according to the disordered character of the alloys. The obtained Mean Hyperfine Field (MHF) for the sample 50 at.% Fe was 30.9 T, meanwhile for the invar composition (65 at.% Fe) was 25.5 T, which is close to values previously reported by Transmission Mössbauer Spectroscopy (TMS). Results from the treated samples (with oxidation at 200°C) showed a difference in the surface composition as a result of this process. In the 50 at.% Fe sample, additionally appeared a doublet that could be assigned to an oxihydroxide of Fe³⁺. Otherwise, the 65 at.% Fe sample (invar) presented ferromagnetic oxides (α-Fe₂O₃ and Fe₃O₄) with a large relative area (82.5%).