Mn-C interaction in Fe-C-Mn steels: Study by thermoelectric power and internal friction
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I. INTRODUCTION
IN extra-mild steels, the substitutional atoms (S) strongly influence the microstructure and the mechanical properties, as they may interact with the dislocations and also with the interstitial atoms (I). In the latter case, the interaction is accompanied by the formation of “S-I dipoles” or “S-I complexes,” which influence the recrystallization texture of coldrolled sheets and also the mechanisms involved in the static strain aging of the annealed steels. From an experimental point of view, the interaction between the interstitial and substitutional atoms was clearly proved through internal-friction (IF) experiments.[1–12] The first experimental results obtained from the works of Wert[1] showed that for additions of less than a few atomic percent of a substitutional element such as Ni, Mo, Mn, Cr, or V in a binary Fe-C alloy, the shape of the carbon Snoek peak and the thermodynamic parameters associated with this peak are not modified. Later, other results were reported in the literature for the IF spectra of Fe-C-S, Fe-N-S, and Fe-C-N-S alloys. In fact, three kinds of behaviors were observed: (1) the absence of an effect, as in the case of the Fe-C-Co system,[10,11] (2) a decrease in the height of the normal Snoek peak without a significant broadening of this peak,[1,6,7,9–11] and (3) a decrease in the height of the normal Snoek peak and the appearance of two or more extra components at a temperature lower or higher than that of the ordinary Snoek peak.[2–5,8] The case of the Fe-C-Mn system has been particularly described in the literature.[1,9,11,15] In this system, the only significant visible modification of the IF spectra due to the presence of manganese atoms is a decrease in the height of the normal carbon Snoek peak when the manganese content in solution increases. This decrease can be interpreted as being due to the existence of quenched-in Mn-C dipoles (proposed by References 13 and 14), which do not contribute V. MASSARDIER, Assistant Professor, and J. MERLIN, Professor, are with GEMPPM, National Institute of Applied Science of Lyon, 69621 Villeurbanne, Cedex, France. Contact: [email protected] E. Le PATEZOUR, Postdoctoral Student, and M. SOLER, Engineer, are with the Arcelor Research Center, 57 283 Maizieres-Les-Mitz, Cedex, France. Manuscript submitted October 4, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
to the height of the normal carbon Snoek peak.[9–11,15,16] This explanation led several authors to consider that in Fe-C-Mn alloys, two populations of carbon atoms had to be distinguished: one constituted of single-carbon atoms which are free in the iron matrix, and one constituted of carbon atoms which are in interaction with one or several manganese atoms. For these two solute species, the interaction is attractive. As a consequence, the energy level of the interstitial sites around the manganese atoms is lowered, leading to the trapping of a given amount of carbon atoms by the manganese atoms.[17] From a theoretical point of view, several approaches were propo
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