The clustering and coarsening of carbon multiplets during the aging of martensite from mössbauer spectroscopy: The preprecipitation stage of epsilon carbide

Mössbauer 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. Besides the atoms of the matrix (A), Fe atoms which are first nearest neighbors of an isolated interstitial can only be distinguished by their quadrupole shift depending upon whether they lie in a direction perpendicular (B _a) or parallel to thec axis (B _c); classC atoms have two carbon neighbors whileD orE belong to an ordered phase; classF atoms have three carbon neighbors. The carbon multiplet model, based on interstitials residing exclusively in octahedral sites, an elastic stability of theC-C 1/2«111” doublet and a diffusion path for carbon in (001)α' can explain all the features observed: a first step 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 Å) which transformsin situ into ε-carbide (~Fe₉C₄) (η carbide as long as it is coherent with the matrix). The positions of carbon atoms in Fe₉C₄ are specified and a synopsis with related phenomena is drawn.