Initial stages of Fe-C martensite decomposition

Abstract

The decomposition of Fe–C martensite (α') was studied using conventional transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffractometry and differential scanning calorimetry. The ageing and tempering of α' at temperatures between room temperature and 423 K result in changes in the atomic structure of α' and subsequently precipitation of iron carbide. In an early stage of decomposition, carbon enrichments develop in the iron matrix. At temperatures below about 363 K. this development of carbon-enriched and carbon-depleted zones leads gradually to the formation of a modulated structure, as revealed by the observation of specific diffraction phenomena. The carbon enrichments possess variable carbon content. Their misfit with the matrix is, at least initially, accommodated elastically. At temperatures above about 363 K. the transition carbide precipitates. It could be shown that the transition carbide has the η-carbide crystal structure and the orientation relationship with the matrix was determined.