Carbon – Molybdenum – Silicon

Abstract

The C-Mo-Si system is the base of materials for the high temperature applications because of their good oxidation resistance. The C-Mo-Si phase diagram is of a great importance for the development, exploitation and technology of these materials. The main features of the C-Mo-Si phase diagram were established by [1954Now] who determined the phase relations in the system using optical microscopy and X-ray diffraction analysis of the alloys. The alloys were prepared by high temperature pressing of the respective powder mixtures consisting of Mo, Si and soot (>99% C) or Mo2C, MoSi2 and SiC followed by annealing at 1600°C for 12 h. The chemical analysis of the alloys, their density and melting points were determined also. The investigation indicated the existence of one ternary phase in the system having crystal structure of the D88 type and the composition near Mo5Si3 with small contents of carbon. The ternary phase was characterized by a rather wide homogeneity range. The isothermal section at 1600°C, liquidus surface and vertical section SiC-Mo of the C-Mo-Si phase diagram were constructed. The ternary phase was established to be in equilibrium at 1600°C with most of the solid phases of the relevant binary systems, C-Mo, Mo-Si and C-Si. The C-Mo-Si phase diagram constructed by [1954Now] was reported to be in essential agreement with the results of the investigation by [1956Bre]. The C-Mo-Si phase diagram by [1954Now] was, however, partially revised by [1981Loo, 1982Loo] who analyzed ternary alloys equilibrated at 1200°C and diffusion couples by means of optical microscopy, micro-probe and X-ray diffraction methods and constructed the isothermal section of the phase diagram at this temperature. [1981Loo, 1982Loo] confirmed, in general, the phase relations in the solid state established by [1954Now] except for the existence of an equilibrium between the ternary phase and MoSi2. According to [1981Loo, 1982Loo], the equilibrium between the ternary phase and MoSi2 was absent at 1200°C in contrast with the isothermal section at 1600°C by [1954Now]. The absence of the equilibrium between the ternary phase and MoSi2 corresponded to the existence of the equilibrium between the binary silicide Mo5Si3 and SiC at 1200°C by [1981Loo, 1982Loo]. Another different feature established by [1981Loo, 1982Loo], as compared with [1954Now], was the smaller homogeneity range of the ternary phase. Trying to explain the difference between the isothermal sections at 1600°C [1954Now] and at 1200°C [1981Loo, 1982Loo] on the existence of the equilibrium between the ternary phase and MoSi2, [1992Boe] supposed a possibility of the invariant four-phase reaction MoSi2 + ternary phase Mo5Si3C + SiC between 1600 and 1200°C. However, in a later work of [1994Cos] the equilibrium between the ternary phase and MoSi2 by [1954Now] was confirmed at 1600°C and established also at 1200°C in contrast to [1981Loo, 1982Loo]. The existence of the equilibrium between the ternary phase and MoSi2 was confirmed also at 1300°C [1975Kut] and at 1500°C [1998Suz]. A thermodynamic calculation performed by [2000Fan] showed the existence of the ternary phase + MoSi2 +SiC equilibrium and, respectively, equilibrium between the ternary phase and MoSi2 below 1594°C down to at least 1227°C (1500K). A number of works was devoted to the composition and crystal structure of the ternary phase [1954Now, 1954Sch, 1955Par, 1956Bre, 1956Kie, 1965Par, 1974Cev, 1981Loo, 1982Loo, 1993Gar, 1998Suz, 2002Bha]. The ternary phase was established to melt congruently with a flat maximum [1954Now]. Its homogeneity range at 1600°C was outlined in the first work [1954Now]. According to [1981Loo, 1982Loo], the homogeneity range of the ternary phase at 1200°C is slightly smaller, than that at 1600°C [1954Now], but is in the limits of the latter. A close extension of the homogeneity range of the ternary phase was established at 1500°C in [1998Suz]. Some differences of the homogeneity ranges established in [1954Now, 1981Loo, 1982Loo] and own results [1998Suz] were explained by the different temperatures used in the experiments.