Occurrence of incoherent α"-Fe16N2at room temperature in iron-nitrogen martensite observed by neutron and X-ray diffraction

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Abstract
Aging behaviour of iron-nitrogen martensite (about 5 at.% N) at room temperature is investigated by neutron and X-ray diffraction experiments. An attempt is made to reveal the configuration of the nitrogen atoms during the aging processes especially by means of neutron diffraction, since the scattering lengths of iron and nitrogen atoms are almost the same. Two successive processes are distinguished: a redistribution of nitrogen atoms in the iron matrix and the formation of incoherent Fe16N2. During about the first 40 h of aging a decrease in the integrated intensity (10-20%) of the neutron and X-ray (002) martensite reflection is observed. This reduction is ascribed to the change in mean square displacement of iron atoms close to occupied interstices within nitrogen-enriched regions. After only about 1 d of aging, incoherent Fe16N2 has already formed. The changes in the integrated intensities of the precipitate and martensite reflections indicate that, during the precipitation process, Fe16N2 and ferrite are formed locally while the nitrogen content of the remaining martensite is unchanged.