Growth of The Mo 5 SiB 2 Phase in A Mo 5 Si 3 /Mo 2 B Diffusion Couple
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Growth of The Mo5SiB2 Phase in A Mo5Si3/Mo2B Diffusion Couple Sungtae Kim1, R. Sakidja1, Z. F. Dong1, J. H. Perepezko1 and Yeon Wook Kim2 1 Department of Materials Science and Engineering, University of Wisconsin-Madison, 1509 University Ave, Madison, Wisconsin 53706, USA 2 Department of Materials Science and Engineering, Keimyung University, Taegu, KOREA
ABSTRACT 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.
INTRODUCTION The challenges of a high temperature environment (T>1400°C) impose severe material performance constraints in terms of melting point, oxidation resistance and structural functionality. Even though ceramic materials, intermetallic compounds and refractory metals with high melting temperature above 1500°C are available material choices, these materials as a single component rarely satisfy all the above requirements because of brittleness of ceramic materials and intermetallic compounds at low temperatures and oxidation problem and poor creep resistance of refractory metals at high temperatures. In order to address these issues multiphase designs such as composites or multicomponent alloys have been demonstrated to represent an effective approach. In this regard, the recent identification of refractory metal – refractory metal intermetallic compound combination based on Mo-Mo5SiB2 (T2) two-phase equilibrium in the relevant ternary phase diagram [1,2], has attracted considerable interest for high temperature application. Based upon the electron probe microanalysis (EPMA) examination of phase compositions of the long-term annealed as-cast samples and rapidly solidified samples and x-ray diffraction determination of phase identity, the Mo-Si-B isothermal section at 1600°C is under continuing study. Figure 1 [2] shows the current determination for the Mo-MoB-Mo5Si3 region (excl
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