EFFECTS OF LAMELLAE SPACING AND COLONY SIZE ON THE FRACTURE-RESISTANCE OF A FULLY-LAMELLAR TIAL ALLOY

The fracture resistance in a lamellar TiAl-base alloy was studied as a function of lamellae spacing and colony size. Fully-lamellar microstructures with several lamellae spacings and colony sizes were obtained by heat-treatment. Fracture resistance curves were generated by performing standard J-testing at ambient temperature, while the corresponding fracture and toughening mechanisms were identified by characterizing the crack-tip region and crack-wake ligaments using optical and scanning electron microscopies. Results of this study were compared with published data to elucidate the role of lamellae spacing and colony size in fracture of lamellar TiAl alloys. The comparison revealed that both the initiation toughness, K-lC, and crack growth toughness, K-s, of lamellar TiAl alloys increase with decreasing lamellae spacing, but depend upon the colony size in a more complex manner. In general, fracture toughness (K-lC or K-5) increases with colony size, but may become independent of colony size when the crack-tip plastic zone is embedded within an individual colony, which occurs at approximate to 600 pm in the alloys used for present experiments. For colony sizes exceeding approximate to 600 mu m, fracture toughness appears to be controlled by the lamellae spacing and the colony orientation through their influence on the size of shear ligaments formed between the main crack and the microcracks.