Atomic displacements due to C in Fe Ni C martensite
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P. C. CHEN, former student at Purdue University, is now with SCM Corp., Cleveland, OH 44106. B. O. HALL was with Argonne National Lab, Idaho Falls, ID 83401 and is presently with Westinghouse Research and Development Center, Pittsburgh, PA 15235. P. G. WINCHELL is Professor of Materials Engineering, Purdue University, W. Lafayette, IN 47907. Manuscript submitted November 15, 1979.
which is essentially all due to movements of metal atoms caused by interstitial carbon. Insofar as displacements due to different carbon atoms are additive and independent of the carbon a t o m neighborhood, a constant m e a n strain is obtained for various atomic rearrangements such as clustering short of precipitation but the same atomic rearrangements m a y significantly increase the m e a n square displacement and hence the peak attenuation. EXPERIMENTAL PROCEDURE The X-ray measurements were carried out on the 4-circle diffractometer described previously ~5,16using Cr K , radiation detected by an energy dispersive detector. An 8 m m long line X-ray source and an equally high receiving slit were used without solar slits so that a range of _+ 1.7 deg in X values could diffract and the specimen was rocked so that a range of _+ 1.8 deg in o~ could diffract. Martensite X-ray peaks were step counted either for 40 s in the case of prior austenite single crystals or for 200 s in the case of prior austenite polycrystals at intervals of 0.04 deg 0. F r o m each count a linearly interpolated background was subtracted and the difference was divided by the Lorentz-polarization factor and the square of the structure factor and was computer plotted without smoothing. The integrated intensity was calculated by summing the plotted counts, and its value was corrected for changes in r o o m air adsorption from arbitrary standard value. In this correction air was assumed to be an ideal gas a n d f r o m the four or five temperature and pressure m e a s u r e m e n t s recorded per run, the mean temperature and pressure was calculated using the observed count rate as a weighting function. R o o m temperature deviated f r o m the standard value by less than 4 ~ and r o o m pressure deviated from the standard value by less than 27 k P a (20 m m Hg), and the calculated correction of intensity was always less than two percent and usually less than one percent. In samples tempered above 100 ~ the sample c h a m b e r was not evacuated. In these runs an additional air adsorption correction was employed. It was estimated theoretically and checked experimentally on each sample and introduced negligible error. The
ISSN 0360-2133/80/0811-1323500.75/0 METALLURGICAL TRANSACTIONS A 9 1980 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 11A, AUGUST 1980--1323
counting error of the integrated intensity for peaks obtained from transformed single crystals was about 0.4 pct and that for the transformed polycrystals was about 1 pct, but the latter peaks were separated into three peaks and the errors in the smaller of these peaks was about 6 pct
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