Elastic moduli of MoSi 2 -based materials
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We prepared MoSi 2 , two-phase MoSi 2 /Mo 5 Si 3 , and (Mo,W)Si 2 solid-solution alloy powders by mechanically alloying mixtures of elemental molybdenum, silicon, and tungsten. These powders were consolidated by hot-pressing them at 1500 °C in graphite dies. We measured the elastic moduli of these alloys by a recently developed technique for non-contact ultrasonic spectroscopy. Second-phase M05S13 additions to MoSi2 result in decreased values for G and E, whereas alloying MoSi2 with WSi 2 results in increased values for G and E. An analysis of these Young's moduli and of data from the literature for various intermetallic alloys suggests that for any given alloy system, the Young's moduli for its various intermetallics as a function of density fall on a straight line. Further, the lines for the different alloy systems are approximately parallel.
The excellent resistance of MoSi2 to oxidation at high temperatures has been known for more than 80 years,1'2 and extensive research has focused on developing oxidation-resistant coatings based on MoSi 2 . 3 More recently, MoSi2 and MoSi2-based compounds have been investigated as high-temperature structural materials, especially as matrices for metal and ceramic composites.4'5 MoSi2 has a brittle-to-ductile transformation at 900 °C, a characteristic that presents two problems: brittleness at temperatures below 900 °C and the loss of strength for temperatures well above 900 °C. SiO 2 inclusions that form preferentially at the grain boundaries during alloy synthesis also contribute to the decrease in high-temperature strength. Eliminating these inclusions requires careful control of all processing steps. Recent work has concentrated on reinforcing MoSi2 by alloying and by second-phase additions. For example, MoSi2 and WSi 2 form a continuous solid solution, which is significantly stronger at high temperatures than singlephase MoSi 2 . Additions of M05S13 also increase the high-temperature strength of M0S12.5'6 These developments have led to greatly increased research activity in MoSi2-based materials.7 The knowledge of elastic moduli is important in the development of structural materials. There is, however, a serious lack in the literature of elastic constants data for intermetallics.8 Values of Young's modulus of polycrystalline MoSi2 and Mo5Si3 have been reported.8 Recently, the second-order elastic moduli of MoSi2 and WSi 2 were measured in single crystals.9 The elastic moduli of M0S12 have also been calculated from first principles using the linear-muffm-tin-orbitals method in the atomic sphere approximation.10 There are, however, no moduli data for polycrystalline two-phase materials based on MoSi 2 . We report here room-temperature elastic moduli of polycrystalline MoSi 2 , two-phase M0S12/M05S13, and 1610 http://journals.cambridge.org
(Mo, W)Si 2 . We also report the use of a newly developed, non-contact ultrasonic technique for measuring elastic moduli in samples of small dimensions. We used mechanical alloying (MA), a high-energy ball-milling process, to prepare Mo
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