Self-propagating high-temperature synthesis microalloying of MoSi 2 with Nb and V
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U. Anselmi-Tamburinia) Department of Physical Chemistry, and IENI/CNR, University of Pavia, Viale Taramelli 16 27100 Pavia, Italy, and Department Chemical Engineering and Materials Science, University of California, Davis, California 95616
Z.A. Munir Department Chemical Engineering and Materials Science, University of California, Davis, California 95616 (Received 22 January 2003; accepted 9 May 2003)
Microalloying of MoSi2 to form Mo(1−x)MexSi2 (Me ⳱ Nb or V) was investigated by the self-propagating high-temperature synthesis method. With alloying element contents up to 5 at.%, a homogeneous C11b solid solution was obtained. For higher contents of alloying elements, the product contained both the C11b and the hexagonal C40 phases. The relative amount of the C40 phase increases with an increase in the content of alloying metals in the starting mixture. The alloying element content in the hexagonal C40 Mo(1−x)MexSi2 phase was nearly constant at a level of about 12 at.% for all starting compositions. In contrast, the content of the alloying elements in the tetragonal phase is considerably lower (around 4 at.%) and increases slightly as the Me content in the starting mixture is increased.
I. INTRODUCTION
Increased interest in high-temperature structural materials over the past several years has been largely motivated by the search for alternatives to Ni-based superalloys.1 In this regard intermetallic compounds have become the focus of this search.2 Of these, transition metal silicides are of particular interest because of their high melting points, good oxidation resistance, and low density.3,4 In this category, MoSi2 is by far the most investigated and has already found application as a hightemperature heating element.5 Its application as structural material, however, is limited primarily by three significant shortcomings. It has poor low-temperature ductility (1200 °C).6,7 It also suffers from poor oxidation resistance in the range between 500 and 800 °C (usually referred to as pest oxidation).8,9 Early attempts to overcome these limitations through the addition of a second phase have been partially successful. For example, ductile-phase toughening through the addition of a metallic phase (e.g., Nb) is limited due to the reactivity of
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J. Mater. Res., Vol. 18, No. 8, Aug 2003 Downloaded: 14 Mar 2015
metals with the silicide at high temperatures.7,10 On the other hand, reinforcement with ceramic phases like SiC or zirconia produced modest improvements in plasticity and toughness.11 Since bonding in MoSi2 is partially metallic in character, the possibility of overcoming the shortcomings by alloying presents an interesting alternative. Recent ab initio calculations have shown that alloying of MoSi2 with other elements such as Mg, Al, Nb, V may result in improved mechanical properties in the tetragonal C11b structure.12 Recent work has demonstrated this prediction by incorporating the most promising of t
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