Mechanisms of discontinuous yielding in a Nb-Ti-Al alloy

Alloys based on the composition Nb-40Ti-15Al (at%) are being investigated for possible use in high-temperature corrosive environments. These B2-matrix alloys show an interesting combination of mechanical properties, including high yield strength maintained through 800 degrees C in the heat-treated condition, and room temperature ductility exceeding 30% in the as-recrystallized condition. On aging this alloy at intermediate temperatures, a yield-point is observed on subsequent room-temperature tensile deformation. Also, discontinuous yielding is observed upon testing at intermediate temperatures. These phenomena have been investigated from both a mechanical and microstructural standpoint, and both areas will be addressed here. On testing at intermediate temperatures, a critical strain for the appearance of serrated yielding and a negative strain-rate sensitivity are observed, consistent with a Portevin-Le Chatelier effect. The microstructural changes during yielding have been studied using transmission electron microscopy (TEM). Evidence will be presented which suggests that it is the dynamic precipitation and subsequent dissolution of the omega-phase which is responsible for the reported effects. This work has been supported by the US ONR, Dr. George Yoder as Program Manager.