Mossbauer effect in iron-carbon and iron-nitrogen alloys

Abstract

Interstitial solid solutions of carbon and nitrogen in iron have been analyzed by Mossbauer spectroscopy. In both allotropie forms, austenite (gamma) and martensite (alpha), electric quadrupole effects have been observed, arising from a repopulation of electrons among the d-orbitals of iron atoms adjacent to the interstitial atoms. In carbon-austenite, ¦e 2q Q 2 ¦=0.625 mm/sec for iron atoms which are first neighbors of carbon atoms. In nitrogen-austenite, the quadrupole effects are smaller but an appreciable positive isomer shift is observed for iron atoms which are first neighbors of nitrogen atoms. This suggests that these iron atoms have more d-electrons due to some covalent bonding with nitrogen as exists in the isomorphous Fe 4N phase. Carbon shows a positive and nitrogen a negative ion behavior in both martensite and in austenite. The c-axis is a direction of difficult magnetization in both Fe-C and Fe-N martensites. In carbon-martensite, two six-peak spectra are clearly resolved: a high intensity spectrum, with an unresolved fine structure, and an average effective field slightly larger than in α-iron; and a low intensity spectrum, with a quadrupole effect of ¦e 2q Q 2 ¦=− 0.8 mm/sec and an effective field 80% that of the main spectrum. This low-intensity spectrum, which disappears on tempering, is attributed to the two iron nuclei pushed apart along the c-axis by the carbon atom. In nitrogen-martensite, the same low-intensity spectrum appears, though not as well resolved, and shows an effective field larger than in carbon martensite and a quadrupole effect of opposite sign. The indications are that a strong σ bond develops between carbon and iron along the “dipole distortion” but not between nitrogen and iron. This is in agreement with estimates from crystal field theory, since for the two iron atoms of the “dipole” the d z 2 orbital should be in the lowest state in carbon-martensite, but not in nitrogen-martensite.