Lattice changes of iron-carbon martensite on aging at room temperature
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I.
INTRODUCTION
IRON-carbon and iron-nitrogen martensites can, on average, be described on the basis of a body-centered tetragonal (bct) lattice of iron atoms. In these structures, the positions of the iron atoms vary due to a random occupation of a fraction of the octahedral interstitial sites by the carbon or nitrogen atoms. The displacements of the individual iron atoms affect the diffraction-peak intensities as compared to the unstrained condition. On aging the martensites, a number of processes, such as segregation, clustering, and/or ordering o f interstitials, may occur before actual precipitation of the transition carbide/nitride takes place. These processes, gathered under the heading preprecipitation, tL2] are associated with lattice changes of the martensite. Hence, diffraction analysis, in particular, can contribute to the understanding of these processes in interstitial iron-based martensites. Iron-carbon and iron-nitrogen martensites appear to be similar in many respects (e.g., comparable Ms temperatures t31 and lattice parameterst4]). However, distinct differences may occur on aging. For example, the Fe sublattice of the transition nitride (a") tSJ is isostructural with that of the parent martensite; i.e., a" can be conceived as an "ordered" martensite, implying that for coherent precipitation, transport of nitrogen atoms alone suffices, and indeed, it precipitates at room temperature for aging times of the order of only a d a y . [7] On the other hand, the Fe sublattice of the transition carbide is not isostructural with that of the parent martensite, t61 and precipitation of the transition carbide therefore involves transport of both carbon and iron atoms: nonisothermal
LIU CHENG, Graduate Student at the time of this work, N . M . van der PERS, Research Assistant, A. BOTTGER, Scientist, T h . H . de KEIJSER, Senior Scientist, and E.J. MITTEMEIJER, Professor, are with the Laboratory of Metallurgy, Delft University of Technology, 2628 A L Delft, The Netherlands. Manuscript submitted June 15, 1990. METALLURGICAL TRANSACTIONS A
analysis of the kinetics suggests that in this case, aging times of the order of a year would be required for precipitation at room temperature, t2j M6ssbauer spectroscopy, tS] transmission electron microscopy, r9] differential scanning calorimetry, tl~ and dilatometry t1~ have been employed to study the aging process of iron-carbon martensite at room temperature. In this article, the aging behavior at room temperature of Fe-C martensite containing 5.1 at. pct C (5.3C/100Fe; i.e., 5.3 atoms C per 100 atoms Fe) is investigated employing X-ray diffractometry. The analysis 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 earlier qualitative conclusions, t~~ Related preceding work is provided by References 11 through 13. However, these articles deal with Fe-Ni-C martensite (18 to 25 wt pct Ni), and it should be realized that the Ni affects the precipi
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