QUANTITATIVE-EVALUATION OF TOUGHENING MECHANISMS OF TIAL WITH LAMELLAR STRUCTURE

Fully-lamellar TiAl has higher fracture toughness than the duplex one. Some toughening mechanisms of fully-lamellar TiAl have been proposed to explain this difference, but only few quantitative evaluations of fracture toughness have been carried out. This paper has elucidated the mechanism of fracture toughness of fully-lamellar TiAl quantitatively. The following knowledge could be revealed by the fracture test of various grain size fully-lamellar TiAl and the modeling of fracture mechanism based upon the observation of crack propagation. Fracture toughness of fully-lamellar TiAl increases in accordance with grain size coarsening. Fracture toughness of this material could be divided into the matrix fracture toughness, the microcrack stress shielding contribution and the shear ligaments contribution with which fracture toughness increase could be mainly explained. Shear ligaments are the unconnected part between microcracks in front of main crack and formed in the process zone as the result of mismatched crack planes of deflected cracks. This fracture toughness increase is mainly responsible for shear ligaments toughening, which is indicated by a fracture model given by observation of the side view of crack propagation.