Lattice changes of iron-nitrogen martensite on aging at room temperature
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I.
INTRODUCTION
Tim aging behavior of interstitial
(carbon, nitrogen) ironbased martensites is highly complex. Recent work, devoted to iron-carbon martensites, in particular, tL2,3] demonstrated that a number of processes, such as segregation, clustering and ordering of interstitials, may occur before actual precipitation of the transition carbide/ nitride takes place. The first processes can be classified as preprecipitation processes (the latter one is usually denoted as the first stage of tempering). During aging at room temperature of iron-carbon martensite, no precipitation of the transition carbide (denoted as e or ,/) is possible for aging times of the order of a month, t4] but electron microscope, tS] X-ray diffraction, t21 atom probe, t6] M6ssbauer, [7] resistivity, tS] calorimetry, t31 and dilatometry t3j data obtained until now suggest a series of precursory processes which, at least in a quantitative sense, are difficult to analyze separately. Iron-nitrogen martensite has been much less extensively investigated, t9-~3] but the data obtained suggest that as compared with iron-carbon martensite, analysis of preprecipitation processes may be facilitated because the Fe sublattice of the transition nitride (ot"-Fel6N2) is isostructural with that of the parent martensite, in contrast to the transition carbide. The ot"-Fel6N 2 c a n be conceived as an "ordered" martensite, implying that for its coherent precipitation, transport of nitrogen atoms alone suffices, whereas precipitation of the transition carbide also involves transport of iron atoms. In this paper, the aging behavior at room temperature of Fe-N martensite containing 5.5 at. pet N ( ~ 5 . 8 N /
LIU CHENG, Graduate Student, N.M. van der PERS, Research Assistant, A. BOTTGER, Scientist, Th.H. de KEIJSER, Senior Scientist, and E.J. MITrEMEIJER, Professor, are with the Laboratory of Metallurgy, Delft University of Technology, 2628 AL Delft, The Netherlands. Manuscript submitted January 31, 1990. METALLURGICAL TRANSACTIONS A
100Fe, i.e., 5.8 atoms N per 100 atoms Fe) is investigated employing X-ray diffractometry. The present study of the martensite-lattice changes, as deduced from changes in line position, line shape, and integrated intensity, reveals hitherto unknown phenomena and allows quantification of our earlier qualitative conclusions based especially on transmission electron microscopy tl'l and combined calorimetry and dilatometry, t13] The understanding of preprecipitation processes thus achieved can be fruitfully used in a parallel investigation of the more complex preprecipitation processes in iron-carbon martensite. II.
EXPERIMENTAL PROCEDURES
A. Specimen Preparation The specimens were prepared from pure iron [0.005 wt pct C, 0.003 wt pct N, 0.01 wt pet O,
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