Intermetallic compounds based on refractory metals such as Nb, Mo, and W possess properties conducive to high temperature applications. Many of these intermetallics have very high melting points (above 1700"C) and retain their strength and stiffness to a high temperature (> 1000°C). A serious impediment to the application of these materials is their low room temperature fracture toughness. A viable solution to this problem is to form in-situ composites of a brittle intermetallic compound and metallic phase. Room temperature toughness is improved in such composites by the ductile phase ligaments bridging cracks [1]. However, since the deformation of the continuous phase controls the overall deformation behavior of the composite, the role of the intermetallic matrix is also important, especially at temperatures where the intermetallic phase undergoes plastic deformation.

Composites based on Nb - niobium silicide can be used as a model system for this class of materials. The room temperature fracture toughness of niobium silicide can be improved substantially by forming in-situ composites with metallic niobium [2]. The mechanisms that control the plastic deformation both at room and elevated temperatures are not completely understood. This paper presents the microstructure of cast and deformed Nb3Si phases in the Nb - 10 a/o Si in-situ composite.