Abstract
Fracture behaviour of a two phase TiAl alloy was investigated based on observations of fracture surfaces of three point bending (3PB) precracked specimens and detailed observations of configuration changes at precrack tips, based on which new findings are presented. The higher toughness of coarse, near fully lamellar microstructure as compared to that of finer duplex microstructure is attributed to the path of crack propagation. In coarser NFL microstructure, the crack should cross more large grains with unfavourable orientation or bypass them by much more tortuous path, which causes a much rougher fracture surface than that observed in the finer duplex microstructure. The inverse relationships, showing a low tensile ductility but a higher fracture toughness of the coarser NFL microstructure, are explained by the difference in sampling volume between tensile testing and toughness testing. In the precracked 3PB specimens, the sampling volume is restricted to the vicinity along the precrack direction. The coarser NFL microstructure shows the higher toughness. However, in tensile tests, where a much higher sampling volume includes the entire region within the gauge, the cleavage fracture of the coarser NFL microstructure is able to take the weakest route via the high number of interlamellar cracks. The toughening mechanisms, which make the main crack difficult to propagate or cause it to be stopped, are attributed to reducing the tensile stress perpendicular to the interface between lamellae.
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