Effects of loading rate on damage and fracture behavior of TiAl alloys

Based on the results of tensile tests and notch 3PB tests with various loading rates and the observation of fracture surfaces, the effects of loading mode and rate on damage and fracture mechanisms of fully lamellar (FL) and duplex phase (DP) TiAl alloys are indicated: (1) For the FL specimen fractured in tensile test, a number of interlamellar cracks occur before final fracture, which is produced by the cracking of the area remained between the existing cracks on a most weakened cross-section. However the DP specimen in tensile test is fractured by the propagation of a crack with a critical length acting as a Griffiths crack in brittle materials. In the 3PB tests of notched specimens the fracture mechanism is different with that in the tensile tests. Crack initiates at the notch root and propagates along a strip around the center line where the normal stress is highest. For FL specimen a more tortured path through low-resistance-interlamellar cracks can be taken at a low loading rate. Because of the low resistance and the rate-dependence of the interlamellar cracking, the loading rate affects significantly the fracture mode. (2) Based on the variation of the fracture mechanisms, the reason why the FL alloy shows inferior tensile properties but matching even superior fracture toughness to the DP alloy is explained further incorporating the effects of loading rate. (3) The tensile strength and fracture toughness show a decreasing trend with lowering loading rate and it is associated with rate-dependent interlamellar cracks involving in the fracture process. (C) 2007 Published by Elsevier B.V.