Growth of The Mo5SiB2 Phase in A Mo5Si3/Mo2B Diffusion Couple


The high melting temperature and oxidation resistance of the Mo5SiB2 (T2) phase and multiphase microstructures incorporating the T2 phase in the Mo-Si-B system have motivated further studies for applications in very high temperature environments. Since the long term microstructural stability is determined by diffusional processes, diffusion couples consisting of binary boride and silicide phases have been examined in order to evaluate the kinetics of T2 phase development and the relative diffusivities controlling the kinetics. Long term annealing (500 hrs) of the Mo5Si3/Mo2B diffusion couple yields the phase sequence of Mo5Si3/Mo3Si/T2/Mo2B at 1600°C. This indicates that the T2 phase initiates and grows from the Mo2B side to a thickness of about 32µm and the Mo3Si phase initiates and grows from the Mo5Si3 side to a thickness of about 15µm. Other annealing treatments allow for an analysis of the diffusion kinetics based upon the layer thickening and composition profile measurements. To identify the crystallographic growth direction of T2 on Mo2B, a wedge shaped TEM sample with very thin leading edge was prepared. Microstructure images indicate that the growth mode of the T2 phase is columnar. There is a clear tendency for the growth of T2 to be approximately normal to c-axis.

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The support of the AFOSR (F49620-00-1-0077) is gratefully acknowledged. We thank Dr. J. Fournelle for expert guidance with EPMA measurements.

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Correspondence to Sungtae Kim.

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Kim, S., Sakidja, R., Dong, Z.F. et al. Growth of The Mo5SiB2 Phase in A Mo5Si3/Mo2B Diffusion Couple. MRS Online Proceedings Library 646, 74–79 (2000).

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