Microstructure toughness relationships in fully lamellar γ-based titanium aluminides

The fracture toughness of the γ-aluminide alloy Ti-48.4Al-1.9Mn-2.0Nb (at.%) was determined at room temperature for fully lamellar microstructures with randomly oriented colonies. Variation of the lamellar colony size in the range 400–1800 μm was found to have little effect on the fracture toughness value. By subjecting the material to different cooling rates from the α-phase field, specimens were produced with different lamellar lath thicknesses. A variation in the lamellar lath thickness from 0.43 to 1.68 μm produced a very small increase in the fracture toughness value as the lath thickness was reduced. However, this increase was not significant at the 95% confidence level. Many specimens exhibited “pop-in” behaviour during testing. Transmission electron microscopy (TEM) analysis of the boundary types present in the material revealed that cooling at 1 °C min−1 resulted in a random distribution of the four types of boundary possible, whereas for cooling rates of 5 °C min−1 or greater, the boundaries were mainly of the 0° and 180° types. Since the fracture toughness specimen of the former cooling rate exhibited more pronounced interlamellar failure, it was suggested that 60° and/or 120° boundaries may possess a slightly lower interlamellar interfacial strength.