Directional Solidification of Mo 5 Si 3 -MoSi 2 Eutectic
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Directional Solidification of Mo5 Si 3-MoSi 2 Eutectic S.M. Borowicz*, L. Heatherly**, R.H. Zee*, and E.P. George** *Materials Engineering Program, Auburn University, Auburn, AL 36849 "**Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT The Mo-Si phase diagram exhibits a Mo5Si3-MoSi 2 eutectic at the 54% Si composition. Since the terminal phases have comparable melting points and are equidistant from the eutectic composition, there is the possibility of obtaining lamellar microstructures in this system. In addition, if the alloys are directionally solidified, there is the further possibility of obtaining aligned lamellae. In this study, a high temperature (xenon-arc-lamp) optical floating zone furnace is utilized to directionally solidify Mo-Si alloys of the eutectic composition. Growth conditions are systematically varied to investigate their effects on the solidification microstructure. Growth rates and rotation speeds are identified that result in lamellar microstructures. INTRODUCTION Intermetallics have received considerable attention for use as high temperature structural materials. New structural applications will require higher operating temperatures that will be difficult to meet with nickel-base superalloys. To meet this demand alloys based on refractory elements with superior high temperature creep strength will be necessary. Silicides of molybdenum are attractive because they have high melting points (2000'C) and generally exhibit good oxidation and corrosion resistance compared with other refractory silicides and intermetallic compounds. Refractory metals (Mo, W, Ta) form only a few binary compounds with silicon. Figure 1,shows the molybdenum-silicon phase diagram [1], which consists of small terminal solid solubility ranges and three intermediate compounds (Mo 3Si, Mo5 Si3, MoSi2). Mo 5 Si3 has a homogeneity range of about 3 at.% silicon compared to MoSi2 which is a line compound. A high melting eutectic is formed between Mo5 Si3 and MoSi 2 at 54 at.% silicon. Of the three compounds, MoSi 2 is well known for excellent oxidation resistance in the 7000C 17000C range [21. However it is brittle at ambient temperatures and exhibits high creep rates at temperatures above 10000C [3]. MoSi 2 has the C1l1b tetragonal crystal structure (a = 3.205A, c = 7.845A) (space group 14/mmm, space group number 139) and a melting temperature of 20200 C [4]. MoSi 2 is used as furnace heating elements and coating material on molybdenum and other refractory metals but has so far not been exploited as a structural material. Mo5Si3, in the absence of impurities, has the D8m tetragonal crystal structure (space group 14/mcm, space group number 140) with twenty Mo atoms and twelve Si atoms in the unit cell [4]. Mo5Si3 has a melting temperature of 2180'C [4], which is higher than that of MoSi 2. Our measurements indicate that the density of monocrystalline samples of Mo5Si3 and MoSi 2 are 8.16 and 6.3 g/cc, respectively. Lamellar microstructures are associated with phase diagrams in which the t
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