Characterization of Silicide Precipitates in Primary Nb Phase in Nb-10% Si In-Situ Composites

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Intermetallic compounds based on refractory metals such as Nb, Mo and W have high melting points (>1700°C) and retain their strength and stiffness to a very high temperature (> 1000°C) making them candidate materials for very high temperature applications. Intermetallic compounds are extremely brittle, and successful implementation of these materials, therefore, depends on improved toughness both at room temperature and at elevated application temperatures. One possible method of increasing toughness is to form in-situ composites of the intermetallic compound and a metallic phase. A study of the interaction between the brittle intermetallic phase and the ductile metallic phase is important because these in-situ composites attain their toughness from the morphological distribution the two phases. The Nb- Si binary system has been chosen as a model system for this study. The equilibrium phase for alloys over the composition range 0 to 37.5 (atomic) percent Si, at temperatures from room temperature to 1700°C are the terminal metallic Nb phase and the intermetallic compound Nb5Si3. The solubility of Si in Nb is small (<0.1%) below 1200°C. There is an increase in solubility up to the eutectoid temperature above which the compound Nb3Si is stable. The maximum solubility of Si in Nb occurs at the eutectic temperature. This paper deals with the identification and characterization of the precipitates in the primary Nb phase in Nb-10 (atomic) percent Si alloy.

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