Effect of small amount of nitrogen on carbide characteristics in unidirectional Ni-base superalloy
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I.
INTRODUCTION
WHEN nickel-base superalloys are used in the investment casting of gas-turbine components, scrap (such as gates, runners, sprues, and scrap castings) is a by-product of the process, thereby giving rise to revert generation[1]; the revert material sometimes contains a higher level of nitrogen than the base alloy.[2] Some investigators attributed the microporosity in the revert alloy to the large amount of nitrogen.[3,4] Painter and Young[4] considered that microporosity was not a function of total nitrogen content, but of the form in which the nitrogen was present. Additions of high chromium nitride increased microposity, while additions of TiN reduced microporosity. Whether the presence of titanium nitride would deleteriously affect properties was uncertain with respect to the cast superalloy IN100.[4] It was reported that carbides of the MC type, which formed during solidification of Ni-base superalloys, contribute to strengthening of grain boundaries at elevated temperature. Fatigue life in alloys is usually related to the carbide shape, size, and content.[5] We have found that the characteristics of carbide changed greatly in revert melts compared with those in virgin melts. All those works focus on as-cast polycrystalline superalloys and seldom touch on unidirectional ones. In the present work, the influence of small amounts of nitrogen on the characteristics of carbides has been examined by making deliberate additions at levels greater than those experienced in normal production.
II.
EXPERIMENTAL
A. Preparation of Specimens The virgin alloy was remelted and cast into bars with varying amounts of nitrogen by making deliberate additions of chromium nitride. The compositions of all bars are given in Table I. The amount of N was determined by a TC-436 N/O analyzer (LECO Co. Ltd., St. Joseph, MI), C by infrared absorption spectroscopy, V and B by inductively coupled plasma–Auger electron spectroscopy, and others by colorimetric analysis. The bars were machined to 8 mm in diameter and directionally solidified using the apparatus shown in Figure 1. The sample was placed on a watercooled chill, heated to 1773 K for 10 minutes, and pulled downward at a constant rate of 2.5, 17, 125, or 200 mm/s in high-purity argon flow. The thermal gradient in the liquid in front of the solid-liquid interface was about 15 K/mm. The sample was pulled 30 mm and then quenched quickly into water. B. Microstructural Examination The longitudinal section of the specimen was polished and etched so as to examine carbide morphology. Scanning electron microscopy (SEM) together with an image analyzer was used to investigate the distribution (area percentage), mean size, and morphology of carbide. Electron probe microanalysis was employed to determine the phase composition. The mean size of carbide is expressed by the diameter of the circle with equal area: d5
XUEBING HUANG, Postdoctoral Student, YUN ZHANG, YULIN LIU, Professors, and ZHUANGQI HU, Professor, and Academician of the Chinese Academy of Engineering, are with the I
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