1968 Volume 54 Issue 1 Pages 34-48
We have measured the Mossbauer effect of 57Fe in iron-carbon martensite and its change after ternpering. The resonant absorption spectrum in martensite was resolved into several components arising from iron atoms, which are the 1st, 2nd, 3, 4th neighbors to the carbon atoms and remnant iron atoms. A carbon atom causes localized changes of electronic states of the surrounding iron atoms, especially affecting the 1st nearest neighboring iron atoms.
The internal magnetic field at nuclei of the 1st neighbors is 265 KOe, which is far less than 330 KOe of pure iron. Variation of the isomer shift and appearance of a large positive quadrupole effect were also observed. It is suggested from the MOssbauer measurement that the bonding between the carbon atoms and the 1st neighboring iron atoms in martensite may be accompanied, possibly, by the covalent admixture of wave functions.
Internal magnetic fields and isomer shifts of the 2nd and 3, 4th neighbors also deviate slightly from those of pure iron, but the sign of the deviation is opposite to those of the 1st neighbors. Their quadrupole effects were nearly zero, which means that the positive charge of the carbon ion is almost screened out within a distance of the order of 2Å by the additional electron clouds.
The localized nature of the effect of the carbon atom permits a parallel discussion between the ironcarbon martensite and the α-solid solution.
From the changes of the Mössbauer spectrum after tempering of martensite, we have concluded that the bonding between the carbon atoms and the iron atoms in the coherent ε-carbide formed at the stage I is essentially the same as the bonding in the martensite or the a-solid solution.
We have also evidence that the χ-carbide is formed at the stage III a of tempering, the formation of which has not been fully proved in the previous x-ray or magnetic measurements. The mean internal magnetic fields after the heat treatments change from 194 KOe due to mainly χ-carbide (at 220°C) to 208 KOe due to cementite (at 520°C).
The reduction of the internal field of the 1st neighboring iron atom in various Fe-C systems, irrespective of their crystal structure such as the solid solution, the ε-phase, the χ-phase, the θ-phase (cementite), is roughly proportional to the number of their nearest neighboring carbon atoms. Decreases in the internal field of an iron atom per carbon atom are about 60 KOe. These localized and additive behaviors suggest that the bonding nature between the carbon atoms and the neighboring iron atoms in the above structures can be understood on a common ground.