Katsuyuki Yanagihara

High temperature oxidation and pesting of Mo(Si,Al)2

Abstract Molybdenum aluminosilicide, Mo(Si,Al)2 with the C40 structure is a promising material at elevated temperatures. High temperature oxidation of Mo(Si,Al)2 gives a protective Al2O3 scale at temperatures below 1868 K of which is the eutectic temperature in the SiO2-3Al2O32SiO2 system. Above the eutectic temperature, the scale is a liquid of SiO2–Al2O3 with the Al2O3 content being richer than the eutectic composition. The cooling of the hypereutectic liquid scale suppresses the formation of β-crystobalite and gives an amorphous scale with an excellent adherence to intermetallics. The disintegration of MoSi2 (pesting) due to oxidation is severe around 773 K. The addition of Al markedly reduces pesting. Amorphous Mo–Si–Al–O forms in initial cracks and voids. This amorphous oxide is probably more plastic than the amorphous Mo–Si–O formed on MoSi2 and decreases the stress generation at the crack which leads to disintegration.

Effect of third elements on the pesting suppression of Mo-Si-X intermetallics (X = Al, Ta, Ti, Zr and Y)

The pesting behaviour of Mo-Si-X (X = Al, Ta, Ti, Zr and Y) intermetallics produced by arc-melting has been examined in air at temperatures between 673 and 973 K. MoSi2 disintegrates severely at 773 K. The selective oxidation of Si to SiO2 should occur at the grain boundaries of MoSi2 because the affinity to oxygen of Mo is much smaller than that of Si. The volume expansion accom- panying selective oxidation of Si is calculated to be about +85.6%. This significant volume expansion generates a large internal stress at the grain boundaries and results in disintegration of MoSi2. The volume change (4.9 vol%) in the oxidation of Mo(Si,Al)2 is small, and the pesting of Mo(Si,Al)2 is suppressed. In the case of third elements whose affinities to oxygen are larger than that of Si (i.e. Al, Ti, Zr and Y), the third elements oxidize selectively at the grain boundaries. When the volume expansion accompanying the internal oxidation is small, the pesting phenomenon is suppressed.

The role of microstructure on pesting during oxidation of MoSi2 and Mo(Si,Al)2 at 773 K

The pesting behavior of MoSi2 and Mo(Si,Al)2 has been examined in air at 773 K to clarify the origin and mechanism of pesting phenomena and the effect of aluminum on pesting phenomena. The initial cracks play a much more important role than the grain boundaries and the initial oxide layer in pesting. Mo and Si oxidize to amorphous Mo-Si-O simultaneously with about a 200% volume expansion. Therefore, large stress appears at the cracktips and induce many new cracks. MoO3 vaporizes from the Mo-Si-O layer on the external surface and crack surfaces causing the oxides in the initial cracks to become porous. Oxygen has a short-circuit path to enter the sample in the cracks. Therefore, the partial pressure of oxygen is sufficiently high to allow oxidation of Mo in the materials. The platelet-like MoO3 grows on the external surface and also in the cracks. Finally, the sample distintegrates into powder. Pesting of Mo(Si,Al)2 occurs in the same way, however, its rate is much lower than that of MoSi2. The role of Al is to decrease the initial crack density of the samples from the melt. Other effects of Al might be to decrease the oxygen flux toward the oxide-intermetallic interface and to increase the plasticity of the amorphous oxide being formed in the cracks.

Microscopic Features in the Transition from External to Internal Oxidation in an Fe–6 mol.% Si Alloy Annealed Under Various H2O–H2 Atmospheres

The occurrence of external or internal oxidation in Fe–Si alloys is strongly affected by oxidation conditions. In the present study, X-ray diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy, secondary-ion mass spectrometry, and glow-discharge optical-emission spectrometry were used for characterizing the microscopic features of oxide layers formed on the (011) surface of an Fe–6 mol.% Si alloy. The starting materials were annealed at 1473 K under dry hydrogen gas and were subsequently annealed at 1123 K under a 75% H2–25% N2 atmosphere with various partial pressures of water vapor. The results show that the microscopic morphology and elemental distribution in oxide layers strongly depend on oxidation conditions. The surface was found to become rough by annealing in higher partial pressures of water vapor. This phenomenon may be induced by internal oxidation. Corresponding to the morphological changes of the surface, changes in the distribution of alloying elements have systematically been characterized in surface layers. These experimental results are discussed in conjunction with thermodynamic data on oxidation of elements.

High temperature oxidation of intermetallic compounds of Mo(Si1-xAlx)2

Abstract High temperature oxidation of Mo(Si 1- x Al x ) 2 and MoSi 2 was examined at 1823 and 2048 K in air. At 1823 K, Mo(Si 1- x Al x ) 2 gave a scale of alumina and the scale on MoSi 2 was composed of amorphous silica which partly crystallizes into crystobalite. At 2048 K, the scales on both MoSi 2 and Mo(Si 1- x Al x ) 2 were in a liquid state. On cooling, the scale on MoSi 2 completely crystallized into crystobalite, while that on Mo(Si 1- x Al x ) 2 remained in an amorphous state because the dissolution of alumina suppressed the crystallization. The oxidation rate was higher for Mo(Si 1- x Al x ) 2 than for MoSi 2 . The scale adherence was excellent for Mo(Si 1- x Al x ) 2 although the scale easily spalled off in MoSi 2 .

Formation of Intermetallic Compounds on Refractory Metals in Aluminum-Silicon Liquid

Publisher Summary This chapter explores that for energy serving and ecological issue, an increase in operation temperature of gas turbine systems has been required to increase energy conversion efficiency. New high temperature materials have been required with higher applicable temperatures than that of Ni based superalloys. Disilicides such as MoSi 2 has a promising material because of excellent resistance for high temperature oxidation. Since MoSi 2 has low creep strength and high temperature strength, the intermetallic compound is reasonable for application of coating to prohibit high temperature Oxidation. Dipping the refracto U metals, Mo and Nb, into molten A1 liquid saturating Si, aluminodisilicides were formed on the surface of the metals. The existence of A1 and Si at the grain boundary of the intermetallic layer and the interface between the refractory metals and the intermetallic layer was revealed from the results of detail observation and electron probe microanalysis (EPMA). The supply of AI and Si is carried out by passing the A1-Si liquid at the grain boundary. The reaction for the formation of alumino-disilicide occurs at the A1-Si liquid at the interface between the refractory metals and the intermetallics.