THE CLUSTERING AND COARSENING OF CARBON MULTIPLETS DURING THE AGING OF MARTENSITE FROM MOSSBAUER-SPECTROSCOPY - THE PREPRECIPITATION STAGE OF EPSILON CARBIDE

Moessbauer spectra of as-quenched and aged martensite (9. 5 at. pct C) are analyzed in terms of seven Fe environments, the abundances of which evolve with aging time. The carbon multiplet model, based on interstitials residing exclusively in octahedral sites, an elastic stability of the C-C 1/2 LT AN BR 111 RT AN BR doublet and a diffusion path for carbon in (001)// alpha // prime , can explain all the features observed. A first step is attributed to the clustering of isolated multiplets, and a second step to the coarsening into extended multiplets which leads to the emergence of a new ordered iron carbide (B2 monoclinic with parallelepiped cell of 5. 73, 6. 74, and 8. 60 A) which transforms in situ into epsilon -carbide (Fe//9C//4) ( eta carbide as long as it is coherent with the matrix). The positions of carbon atoms in Fe//9C//4 are specified and a synopsis with related phenomena is presented.