On the microstructure and mechanical behavior of melt-spun Fe-24 pct Ni-0.5 pct C Ribbons: Effect of austenite grain siz
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I.
INTRODUCTION
A number of studies have been carried out on the retention of austenite in Fe-Ni alloy cooled with rates varying between 4 x 106 to 107 Ks -1 . 1,2 According to these studies, austenite is retained to room temperature in splat quenched Fe-Ni alloys with Ni > 20 wt pct. This is because the Ms temperature is depressed by a small as-solidified austenite grain size. The depression of Ms increases with increasing Ni content. On annealing in the austenite region, the austenite grain size increases and M, returns to the conventional solid state quenched value. Maclin and Cohen, 3 who studied the formation of martensite in Fe-Ni and Fe-Ni-C solid state quenched alloys, arrive at the conclusion that the amount of transformed phase is independent of the austenitizing temperature, grain size being maintained constant. In sharp contrast, Entwisle and Feeney 4 have pointed out that the austenitizing treatment is the dominating factor controlling the start of transformation. From the hardness point of view, increasing the Ni content increases the microhardness of splat-quenched martensitic Fe-Ni-C to a maximum of - 9 0 0 K g / m m 2 at 20 wt pct Ni. Retention of austenite produces a great drop in the microhardness to - 1 0 0 K g / m m 2 in fully austenitic alloys. 2 For Fe-Ni alloys between 0 and 15 wt pct Ni, the splat-quenched alloys have a constant Vickers microhardness of --700 Kg/mm 2, whereas in solid state quenched alloys, the microhardness increases from - 7 0 Kg/mm z for pure iron to --250 Kg/mm 2 at 25 wt pct Ni. However, information concerning the tensile properties of Fe-Ni and Fe-Ni-C melt quenched ribbons is Somewhat lacking. The objective of the present investigations was to use a combination of X-ray diffractometry, optical and TEM metallography, microhardness measurements, and tensile tests in order to determine the role of carbon on the retention and decomposition of austenite in a melt-quenched Fe-24 wt pct Ni-0.5 wt pct C alloy made by the melt-spinning method. According to the above-mentioned schools of thought, the present melt-quenched ribbon should retain the austenite even at temperatures close to the liquid nitrogen temperature. This increases the possibility of studying systemFAWZY H. SAMUEL, on leave of absence from The Central Metallurgical Research and Development Institute, National Research Centre, Dokki, Cairo, Egypt, is Associate Professor, Laboratoire de Physique du Solide, Ecole des Mines, Parc de Saurupt, 54042 Nancy Cedex, France. Manuscript submitted September 14, 1984. METALLURGICALTRANSACTIONS A
atically the initial stages of the ferrite and martensite formation and their morphologies on annealing the asquenched ribbons in the (ferrite + austenite) and austenite phase fields. The corresponding changes in the mechanical properties were correlated with the variation in the microstructure.
II.
EXPERIMENTAL PROCEDURE
A high purity Fe-24 wt pct Ni-0.5 wt pct C alloy was prepared by melting in an induction furnace under a dynamic argon atmosphere and cast into metallic mol
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