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I.

INTRODUCTION

BOTH iron-carbon and iron-nitrogen martensites can, on average, be described as a body-centered tetragonal (bct) lattice of iron atoms; the interstitial atoms, carbon and nitrogen, respectively, are randomly distributed over the c-type octahedral interstices. Because these structures are metastable, redistribution of interstitials will take place upon aging and tempering* of the mar*The terms aging and tempering will be used such that aging refers to storage of the martensitic specimen at room temperature (RT) and that tempering refers to heating of the specimen above RT.

tensites. The first stage of tempering leads to the precipitation of the transition carbide (e/~7)/nitride (a"), the second stage involves the decomposition of retained austenite, and the third stage of tempering corresponds with the formation of the "equilibrium" carbide (0)/nitride (y ') [1,2,3] On aging the martensites, processes take place that precede the actual precipitation of the transition carbide/nitride. These processes are gathered under the heading preprecipitation and can be studied by, for example, X-ray diffraction analysis. Previous work on, in particular, iron-carbon and ironnitrogen martensites has been presented in References 2 and 4 through 7 and References 3 and 8 through 12, respectively. Although these interstitial martensites are rather similar in some respects (for comparison of the M, temperatures, see Reference 13, and for comparison of the lattice parameters, see Reference 14), there are clear differences in the aging and tempering processes of both martensites. On aging at RT, for instance, local enrichments of interstitials develop in both carbon and nitrogen M.J. VAN GENDEREN, Graduate Student, A. BOTTGER, Scientist, and E.J. MITTEMEIJER, Professor, are with the Laboratory of Metallurgy, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands. R.J. CERNIK, X-ray Diffraction Project Team Leader, is with the Science and Engineering Research Council Daresbury Laboratory, Warrington WA4 4AD, United Kingdom. Manuscript submitted January 22, 1993. METALLURGICAL TRANSACTIONS A

martensites. In FeN martensites, these enrichments appear to exhibit an ordered arrangement of nitrogen atoms according to the c~"-Fel6N2 structure, tl2J The iron sublattice of this transition nitride is isostructural with that of the parent martensite a ' ;[8] so, diffusion of interstitials suffices to produce the enrichments. In FeC martensites, an a"-like ordering of interstitials has not been observed (yet) and it has been suggested that the carbon enrichments can be conceived as clusters of interstitial carbon atoms, containing different amounts of carbon interstitials distributed randomly over the c-type octahedral interstices of the bct Fe sublattice, tvj However, it has also been proposed that an (imperfect) ot"-Fel6C2 s t r u c t u r e would develop in FeC martensite t~5] (this suggestion is based on a re-evaluation t~Sj of Mrssbauer data, but it is not corroborated by the existing diffraction d