The role of microstructure on pesting during oxidation of MoSi{sub 2} and Mo(Si,Al){sub 2} at 773 K
- Tokyo Inst. of Technology (Japan)
- Univ. of Mining and Metallurgy, Cracow (Poland)
The pesting behavior of MoSi{sub 2} and Mo(Si,Al){sub 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. MoO{sub 3} 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 MoO{sub 3} grows on the external surface and also in the cracks. Finally, the sample distintegrates into powder. Pesting of Mo(SiAl){sub 2} occurs in the same way, however, its rate is much lower than that of MoSi{sub 2}. 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.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 518359
- Journal Information:
- Oxidation of Metals, Journal Name: Oxidation of Metals Journal Issue: 3-4 Vol. 47; ISSN OXMEAF; ISSN 0030-770X
- Country of Publication:
- United States
- Language:
- English
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