MICROSTRUCTURE TOUGHNESS RELATIONSHIPS IN FULLY LAMELLAR GAMMA-BASED TITANIUM ALUMINIDES

The fracture toughness of the gamma-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 mu m was found to have little effect on the fracture toughness value. By subjecting the material to different cooling rates from the alpha-phase field, specimens were produced with different lamellar lath thicknesses. A variation in the lamellar lath thickness from 0.43 to 1.68 mu m 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 degrees C min(-1) resulted in a random distribution of the four types of boundary possible, whereas for cooling rates of 5 degrees C min(-1) or greater, the boundaries were mainly of the 0 degrees and 180 degrees types. Since the fracture toughness specimen of the former cooling rate exhibited more pronounced interlamellar failure, it was suggested that 60 degrees and/or 120 degrees boundaries may possess a slightly lower interlamellar interfacial strength.