Fracture behavior and toughness of PST crystals of TiAl

Abstract

Fracture behavior and toughness of PST crystals of TiAl have been investigated through three-point bend tests of specimens which are notched, fatigue-precracked and side-grooved. Bend tests have been conducted in air at room temperature. Fracture surface morphology and toughness of notched PST crystals of TiAl strongly depend on their notch orientation with respect to the lamellar boundaries and crystal orientation of γ lamellae. In this study, we have tested specimens with five different notch orientations, i.e. types I-〈110〉, I-〈112〉, II-〈110〉, II-〈112〉 and III, where I, II and III are crack arrester, crack divider and crack delamination orientations, respectively, and 〈 hkl〉 indicates the direction normal to the notch plane, which is given with respect to crystal orientation of γ lamellae. Specimens of type III failed by cleavage-like mode on a macroscopic habit plane parallel to the lamellar boundaries and exhibited a very low fracture toughness value. Specimens of types I and II exhibited considerably rougher fracture surfaces and much higher fracture toughness than type III. The five notch orientations give increasing fracture toughness values in the sequence: III, II-〈112〉, I-〈112〉, II-〈110〉, I-〈110〉. The relative magnitudes of fracture toughness and microstructural characteristics of fracture surfaces for these orientations are discussed in terms of crystallography and toughening mechanisms available in PST crystals.