In-situ composites, consisting of a brittle intermetallic matrix and a ductile metallic phase, have been of considerable interest as high temperature structural materials. Some of these materials have been shown to have considerable room temperature fracture toughness. The ductile metallic phase plays a major role in toughening the composite by bridging cracks in the brittle matrix [1]. Nb-10 a/o Si alloy, which consists of a ductile metallic niobium phase in a Nb3Si intermetallic matrix, is being studied as a model material for ductile phase toughened in-situ composites. The material microstructure can be modified by thermomechanical processing, which in turn affects the level of toughening provided by the ductile phase. A study of the microstructural changes occurring during high temperature deformation is, therefore, of great importance.

Recent work on the high temperature deformation of Nb-10 a/o Si alloy showed that the material undergoes flow softening during deformation. Flow softening is manifested by the decrease in flow stress with increasing strain, and is typically observed during deformation when extensive dislocation annihilation is taking place. Flow softening has been observed in two-phase alloys, such as AI-Mg alloys [2], Zr-2.5 Nb [3] and titanium alloys [4]. This paper discusses the mechanism of flow softening occurring during high temperature deformation of Nb-10 a/o Si in-situ composite.