Scheil reaction scheme

The transition reaction Ui (Fig. 14) appears at 2568 K. The deepest eutectic temperature is calculated to be 1657 K. It is designated as degenerated reaction D2 in the Scheil reaction scheme of Fig. 14. It is connected to the binary eutectic reaction 63 (1 = Si + SiBg) but B4C does not participate in the ternary reaction. The experimental value is 1658 K [4]. Three ternary transition reactions and three degenerated reactions, respectively, occur. According to the calculation six three-phase fields establish the equilibria at room temperature. 

In agreement with experimental information at a pressure of 1 bar the tieline Si3N4-BN exists up to the Si3N4 decomposition temperature of 2114 K. The Scheil reaction scheme as derived from the calculation is given in Fig. 16. Six invariant ternary reactions are calculated. All of them are of a degenerated type because boron nitride, formed at 2767 K, does not participate in the ternary reactions. At room temperature six three-phase equilibria are detected. 

A Scheil reaction scheme was given by [117] and later modified with regard to the reaction temperatures by [234]. The Scheil reaction scheme for the Si-C-N system (valid for P = 1 bar) is shown in Fig. 18. Three degenerated reactions and one transition reaction occur. The reactions (1) and (2)... 

Fig. 18. Scheil reaction scheme for the Si-C-N system (/i-SiC/a-SiC and a-Si3N4/jS-Si3N4 transformations not indicated) [234]...

The ternary system was calculated by extrapolation from the binary subsystems (Kasper, 1996) [33]. The calculations cover phase equilibria at one bar and do not assume any solubilities as no experimental evidence for stable sohd solutions between B4+5C and BN or a-BN and graphite exist. The section between graphite and boron nitride including the invariant reactions Uj, Dj and U2 (Fig. 23) is shown in Fig. 22. A calculated potential phase diagram (logpN2-T) can be found in [244], The complete Scheil reaction scheme (P = 1 bar) is shown in Fig. 23. 

The Scheil reaction scheme for the quaternary system valid for a total pressure of 1 bar is shown in Fig. 26. It is consistent with the ternary Scheil-reaction schemes as shown in Figs. 14, 16, 18 and 23. Twelve quaternary invariant reactions occur Besides die maximum (M) all of the reactions are degenerated reactions where the fifth phase does not participate in the reaction. At room temperature nine four-phase equilibria occur. 

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