An atom probe study of the aging of iron- nickel- carbon martensite
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I.
INTRODUCTION
O V E R the last few years much interest has been shown in the aging and tempering behavior of ferrous martensites. Many different techniques have been employed to gain understanding of the processes occurring. These techniques have included X-ray diffraction, dilatometry, electrical resistivity and hardness measurements, transmission electron microscopy, electron diffraction, and Mrssbauer spectroscopy. These investigations have led to the proposal of a number of different views on the effects occurring during the aging process. ~-9 The atom probe provides a useful technique for direct study of the composition fluctuations that occur during the aging of martensites. This instrument combines the atomic resolution of the field ion microscope with single-atom chemical analysis sensitivity for all elements, including the light elements such as carbon. In this paper we report an extension of our earlier atom probe investigation of low- and high-carbon martensites 1~to the processes occurring during the early stages of aging of iron-nickel-carbon martensites of subzero M~ temperature. In these alloys the experimental difficulties of observing the virgin material and the preliminary stages of aging are minimized.
II.
EXPERIMENTAL
The materials selected for this investigation were Fe-Ni-C alloys with an M~ temperature of approximately - 5 0 ~ The two alloys used had slightly different nickel levels (23 or 29 pct Ni) and the same carbon content, 0.42 wt pct C (2.08 at. pct). The results obtained from the two alloys were essentially identical and will be discussed together.
M. K. MILLER and S. S. BRENNER, Associate Research Consultant, are both with U.S. Steel Research, 125 Jamison Lane, Monroeville, PA 15146. P. A. BEAVEN is with GKSS, 2054 Geesthacht, Federal Republic of Germany. G. D. W. SMITH is with the Department of Metallurgy and Science of Materials, University of Oxford, Oxford, United Kingdom. This paper is based on a presentation made at the "Peter G. Winchell Symposium on Tempering of Steel" held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees. METALLURGICALTRANSACTIONS A
All specimens, in the form of 1-mm bar, were austenitized at 950 ~ for 20 minutes and then were waterquenched. Those specimens that were to be aged were immediately transformed to martensite by further quenching into liquid nitrogen. This heat treatment produces a structure of predominantly twinned lenticular plates of martensite with a nonuniform twin distribution and only a small volume of retained austenite. The specimen blanks were aged for various times at temperatures of 22, 40, 100, and 250 ~ The field ion specimens were then prepared by standard electropolishing techniques, l~ Particular care was taken to ensure that the temperature of the specimen did not exceed 20 ~ during the electropolishing procedure. All specimens were also given a short back polish to remove any deformation that might
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