Chemistry and distribution of phases produced by solid state sic/nicrai reaction
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INTRODUCTION
THERE is at the present time great interest in the use of silicon-base structural ceramics as components in hot machinery such as gas turbines, where the ceramic will be in intimate contact with metal parts under reducing conditions at temperatures of at least 1000 ~ Under these conditions, chemical reactions may occur which will severely degrade the properties of both the metal and ceramic. Previous work ~ has shown that extensive metal-ceramic reactions do occur at temperatures greater than 700 ~ particularly in the case of SiC, and this has limited the use of this material in metal matrix composites. Recently, the reaction of SiC, Si3N4, and reaction-bonded SiC with commercial superalloys under conditions simulating those encountered in hot machinery has been studied) '6'7 Although significant reaction was noted, these studies were hampered by the complex chemistry of the superalloys. In the first systematic study of the details of the reaction between silicon-base ceramics and metals, SiC was reacted with a model superalloy at temperatures between 700 ~ and 1150 ~ s'9'~~The model superalloy consisted of Ni, Cr, and A1, with the Cr representing the solid solution strengthening elements generally added to commercial superalloys, and A1 representing the 7' (nominally Ni3A1, L12 structure, ordered fcc) forming elements. The simple chemistry of the model superalloy allowed phase equilibria concepts to be applied to the reaction. The results of this investigation can be summarized as follows: 1. Reaction of SiC with a Ni-20 at. pet Cr-10 at. pct A1 alloy occurred readily in the temperature range 700 ~ to 1150 ~ A parabolic relation is followed for the growth of reaction products on either side of the original metal/ ceramic interface. 2. Discrete zones containing reaction products are formed during the reaction. In the SiC, a banded zone is formed; in E. L. HALL, M. R. JACKSON, and R. L. MEHAN are Staff Scientists at the General Electric Corporate Research and Development Center, Schenectady, NY 12301. Y. M. KOUH is an Undergraduate Student at Cornell University, Ithaca, NY 14853 in the Department of Materials Science and Engineering, and during the course of this work was employed at the General Electric Corporate Research and Development Center as part of the Cornell University Engineering Cooperative Program. Manuscript submitted June 18, 1982. METALLURGICALTRANSACTIONS A
the metal at the original interface, a transition reaction zone (TRZ) is seen, as well as a metal reaction zone (MRZ) between the TRZ and metal matrix. Significant amounts of silicon are found in the superalloy matrix after the reaction. 3. The bands in the SiC reaction zone consist of alternating layers of 6-Ni2Si plus graphite followed by 6-Ni2Si. The TRZ contains primarily ~'-Cr3Ni2SiC. The metal reaction zone is an extremely complex phase mixture, with the phases present depending on reaction temperature. At 1150 ~ y', fl-NiA1, a-Cr, and ~,, a Ni-Si-AI ternary phase, were found. Qualitative X-ray spectroscopy in the analyti
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