Effect of internal stresses on the fracture toughness of a TiAl-based alloy with duplex microstructures

A series of fine duplex microstructures with different volume ratios of equiaxed gamma and lath colonies (alpha(2) + gamma) were achieved from an isothermally forged Ti-46.5Al-2Cr-2Nb alloy. The internal stress determination using X-ray diffraction techniques and plane-strain fracture toughness tests were carried out at room temperature, and the effects of the internal stresses on the fracture toughness were investigated. It was found that the internal stresses play an important role in determining the fracture toughness of the TiAl-based alloy. The compressive internal stresses in gamma phase and the tensile internal stresses in alpha(2) phase may be responsible for the delamination along the gamma/alpha(2) boundaries. For the microstructures with different volume fraction of lamellar grains, the fracture toughness increases with the internal stresses, which facilitate the formation of shear ligaments; and the effects of the alpha(2) phase and equiaxed gamma grains on the internal stresses and the fracture toughness were discussed. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.