Abstract
The deformation and fracture mechanism of titanium cast into a phosphate-bonded silica alumina investment mold was examined by means of 3-point bending test, acoustic emission(AE)analysis, and metallographic examination.The α case formed on the casting increased the elastic limit and the maximum load, and so the removal of the surface by pickling decreased them.For the as-cast, sharp cracks propagated through the Si, P-rich layer into the acicular structure in the elastic deformation stage and then opened in the plastic deformation stage.As with the room-temperature mold casting, low-amplitude AEs were generated at a constant rate throughout the entire deformation process, in which crack formation, propagation, and opening occurred in the tension-side surface ranging from the center to both the end supports.As with the 800℃ mold casting, low-amplitude but high-energy AEs of a continuous type were generated during the elastic deformation, and they suggested simultaneous propagation of the primary crack.In the latter half stage of plastic deformation, the load and the AE rate were remarkably reduced.This is because a few main cracks near the center propagated continuously within the internal structure which possesses little resistance to the crack propagation.As with the pickled casting, AEs were generated not in the elastic deformation stage but in the plastic deformation stage, thus releasing high energy.The cause of such high-amplitude AEs is obscure.
