The effect of nb morphology on the cyclic oxidation resistance of MoSi 2 /20 vol pct Nb composites
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I.
INTRODUCTION
THE toughness of a variety of brittle matrices has been improved through the addition of a ductile phase. [l-7] A few of these composites appear to be ideally suited for applications as structural materials at elevated temperatures in aggressive environments, as the ductile phase provides for toughness and the matrix provides for oxidation resistance. However, many of the ductile phases added have been refractory metals, which do not possess chemical stability in oxygen at elevated temperatures. Therefore, it is imperative to determine the effect of the addition of a ductile refractory phase on the oxidation resistance of a brittle matrix. With the exception of work by Meschter, [8; little has been reported on the oxidation resistance of ductile phase toughened composites. Meschter studied the 500 ~ and 1200 ~ cyclic oxidation resistance of a MoSi2/20 vol pct Nb particulate composite. However, recent research indicates that the toughness of MoSi2 and other intermetallic matrices reinforced with Nb or other ductile phases is greatly affected by the morphology of the ductile reinforcement (i.e., size, shape, and orientation), t9-~3] Thus, it is important also to determine the effect of the morphology of a ductile phase on the oxidation resistance of the matrix. This article describes the effect of the Nb morphology (i.e., particles vs random short fibers vs continuously aligned fibers) on the cyclic oxidation resistance of MoSi2/20 vol pet Nb composites at 1200 ~
not intended to be an all inclusive review. Above 800 ~ the surface reaction is as follows: 5MoSi2 + 702 ~ M05Si3 + SiO2 The SiO2 layer is adherent and protects the underlying MoSi2, as the kinetics of the reaction follow a parabolic rate law. Once formed, the SiO2 layer protects the underlying MoSi2 even during thermal cycling at lower temperatures, where MoSi2 has been found to undergo the pest reaction (between 400 ~ and 600 ~ Pesting in MoSi2 is associated with the formation of nonprotective MoO3 crystals at grain boundaries during oxidation. The pesting reaction is 2MoSi2 + 702---> 2M003 + 4SIO2 In general, the pesting phenomenon disintegrates exposed material along grain boundaries. However, pesting is not inevitable, as it depends on the porosity, presence of pre-existing cracks, and Si content of the material. [22,23]
B. Oxidation of Nb The oxidation of Nb also has been well characterized. tz4'25'261 A Nb205 scale has been found to form on the surface of Nb upon oxidation at elevated temperatures. The process to form Nb205 occurs in three stages, with two transient oxides forming prior to Nb2Os. The reaction is as follows: Nb + O {stage I} ~ NbO + O {stage II} 2NbO2 + O {stage III} --~ Nb205
II.
BACKGROUND
A. Oxidation of MoSi2 The oxidation behavior of MoSi2 has been well characterized; ~ the following is a brief description and D.E. ALMAN, formerly Graduate Student, Materials Engineering Department, Rensselaer Polytechnic Institute, is Research Engineer, United States Bureau of Mines, Albany Research Center, Albany, OR 97321.
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