EFFECT OF RATE ON THE FRACTURE MECHANISM OF TIAL

This study investigated the effect of microfracture mechanisms on the fracture toughness of bath fully lamellar and duplex TiAl. It is known that the static fracture toughness of TiAl is improved by a lamellar microstructure and also depends on the grain size. The effect of loading rate on the fracture toughness is very important. In this paper, we measured the static fracture toughness of TiAl with different grain sizes of lamellae and various volume fractions of gamma phase, and also estimated the effect of loading rate on the fracture toughness. The static fracture toughness increased with an increase in grain size of the lamellae and with a decrease in volume fraction of gamma phase. Many acoustic emission (AE) signals were detected before the final fracture, and microcracking was observed. The toughening due to microcracking and sequential shear ligaments was demonstrated to be a major mechanism for improvement of the toughness, which was confirmed by our toughening model. Our advanced AE technique also demonstrated that it took several microseconds to generate microfracture. The dynamic fracture toughness decreased with an increase in grain size of the lamellae and with a decrease in volume fraction of gamma phase. It was concluded that the toughening mechanism under static conditions could not occur under dynamic conditions and that another toughening mechanism operates under dynamic conditions.