Effect of small amounts of nitrogen on properties of a Ni-based superalloy
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INTRODUCTION
NICKEL -based superalloys have long been recognized as important engineering materials because of their excellent resistance to high-temperature deformation. When they are used in the investment casting of gas-turbine components, scraps (such as gates, runners, sprues, and scrap castings) are a by-product of the process, thereby giving rise to revert generation. The revert materials sometimes contain a higher level of nitrogen than the virgin alloy.[1] The performance of castings with significant amounts of revert material is frequently inferior to that of the virgin alloy, and components often show extensive microporosity,[2,3] especially in the conventionally cast form. In the case of the unidirectionally solidified castings, microporosity can be reduced to a relatively low level if the solidification is along a temperature gradient from bottom to top. However, in such cases, nonmetallic inclusions such as nitride, carbonitride, and oxide are expelled into interdendritic regions, which will result in the decrease in transverse mechanical properties. Fracture of the alloy sometimes initiates from the cracking of inclusions[4,5] during the deformation process, because of the marked difference in ductility between the brittle inclusions and the matrix. This effect probably contributes to the fluctuation of the properties of revert alloys, which contain some large carbonitrides. Durber et al.[2] investigated the influence of small amounts of nitrogen on MAR-M002 and found that increasing the nitrogen content would result in a change in carbide morphology from that of a “Chinese script” to one of blocky XUEBING HUANG, formerly Postdoctoral Student, Institute of Metal Research, Chinese Academy of Sciences, is Postdoctor, Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208. YUN ZHANG, Professor, and ZHUANGQI HU, Professor and Academician, Chinese Academy of Engineering, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China. Manuscript submitted June 9, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
form and in increased microporosity. The increased nitrogen content was associated with a significant decrease in the rupture life at 760 8C/695 M N m22. Painter and Young[6] also investigated the influence of nitrogen, added as CrN and TiN, respectively, on the microstructure in IN100; it was found that microporosity increased with increasing amounts of CrN, but decreased with increasing amounts of TiN. However, these works were confined to conventional castings. The effect of nitrogen on a directionally solidified (DS) superalloy has not been reported before. The purpose of the work described in this article is to examine the effect of small changes in nitrogen content, which are characteristic of revert superalloys, on the microstructural and mechanical properties of both conventionally and directionally cast nickel-based alloy specimens. II. EXPERIMENTAL The virgin alloy, with a mass of 20 kg, was melted in an alu
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