Phase equilibria and kinetics in the solid state reaction between silicon and NiCrAI
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I.
INTRODUCTION
THE use
of structural silicon-base ceramics is being considered for certain components in hot machinery where the ceramic must come into contact with a metal, in many cases a Ni-base superalloy. The joint details are unimportant at the present time except for one fact: the ceramic and metal will be in intimate contact in order to eliminate vibrationinduced stress. Under these conditions, it is possible for solid state ceramic-metal reactions to take place, leading to a degradation in mechanical properties of both the metal and the ceramic. The study of such reactions has been carried out in the past primarily in connection with metal matrix composites, and it has been shown 1-4 that SiC filaments are badly degraded during fabrication, of necessity conducted under reducing conditions. The situation is similar in the metal-ceramic contact area discussed above, because the tight coupling between the two materials tends to exclude oxygen and leads to a reducing atmosphere. Recent work in this laboratory5'6'7 with hot-pressed SiC and reaction-bonded SiC (which contains free silicon and will henceforth be referred to as Si-SiC) in contact with a model superalloy has confirmed that such reactions take place at temperatures as low as 700 ~ and possibly lower. The reaction increases in severity with increasing temperature. For the case of SiC, the increase in rate is linear with the reciprocal of temperature up to the highest temperature studied, 1150 ~ For the case of ceramics containing free Si, however, temperatures in excess of about 900 ~ to 950 ~ lead to liquid phase formation and the degree of reaction becomes extensive and unacceptable for any engineering application. Diffusion barriers are considered necessary for the prevention of SiC and Si-SiC/metal reaction, and several approaches have recently been discussed.8 One goal of the study of the reaction of unprotected silicon-base ceramics with a model superalloy consisting of Ni, Cr, and A1, discussed in References 5 through 7, was to obtain an understanding of the fundamental mechanisms and phase relations controlling these reactions. However, even with a relatively simple five component system (Si, C, Ni, Cr, and AI), the mechanisms and phase relations were found E.L. HALL, R.L. MEHAN, and M.R. JACKSON are Members of the Technical Staff of the General Electric Corporate Research and Development Center, P.O. Box 8, Schenectady, NY 12301. Manuscript submitted December 13, 1982. METALLURGICALTRANSACTIONS A
to be exceedingly complex and very much dependent on the presence or absence of free Si. For example, in the SiC/model superalloy system, 14 individual phases were identified and more are known to be present. A similar situation exists for the Si-SiC/metal system. The impetus behind the present work was to simplify further the reaction system to a four component one: Si, Ni, Cr, and A1. The objective was to obtain additional understanding of the phase relations involved between silicon-base ceramics and a simple metal. Furthermore, it was hoped
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