Materials Science and Engineering A289 (2000) 91 – 98 In-situ TEM study of fracture mechanisms of polysynthetically twinned (PST) crystals of TiAl alloys Y.H. Lu a,b, *, Y.G. Zhang b , L.J. Qiao a , Y.-B. Wang a , C.Q. Chen b , W.Y. Chu a a Department of Materials Physics, Beijing Uniersity of Science and Technology, Beijing 100083, PR China b Department of Materials Science and Engineering, Beijing Uniersity of Aeronautics and Astronautics, Beijing 100083, PR China Received 10 December 1999; received in revised form 22 March 2000 Abstract The fracture mechanisms of polysynthetically twinned (PST) crystals of Ti – 49at.% Al in different lamellar orientations have been investigated by using in-situ transmission electron microscopy (TEM). The results indicated that the fracture mechanisms and crack propagation behavior depended strongly on the lamellar orientation of PST crystals. When the tensile axis was nearly parallel to the lamellae, the main crack propagated discontinuously by nucleation, growth and linkage of microcracks formed ahead of crack tip. Slip band decohesion, fracture along prism plane of  2 plates and twinning-induced microcracks are three modes of nucleation of microcracks. When the loading axis was perpendicular to the lamellae, the crack parallel to the lamellae propagated by tearing and shear of shear ligament. Interfacial microcrack usually occurred ahead of the main crack tip. Sometimes, the main crack propagated along (111) cleavage plane within  lamellae. © 2000 Elsevier Science S.A. All rights reserved. Keywords: TiAl; Polysynthetically twinned (PST) crystals; In-situ transmission electron microscopy (TEM); Fracture mechanism; Crack www.elsevier.com/locate/msea 1. Introduction TiAl based alloys have received increasing attention especially for advanced applications in heat propulsion and protection because of the low density, a relatively high modulus of elasticity and good resistance to oxida- tion. It has been recognized that the mechanical proper- ties of two-phase TiAl alloys at room temperature are strongly dependent on microstructures [1]. Among the four types of structures prepared through various ther- momechanical processing routes, duplex (DP) and fully lamellar (FL) structures are two typical microstructures and have been subjected to the most investigation. In general, the DP microstructures yield high ductility, but low fracture toughness, and FL microstructures exhibit high fracture toughness, but low ductility. Such inverse relations between tensile properties and fracture tough- ness and the unbalanced properties are one of the key shortcoming of the current properties [2,3]. Many re- searchers have investigated the fracture behavior of the lamellar structure [4–8]. Chan and Kim [4–6] have studied the fracture mechanism of lamellar structure by scanning electron microscopy (SEM) and the results showed that the FL microstructure had higher fracture toughness than other microstructures because of high crack-tip plasticity and an anisotropic composite-like fracture characterized by yielding a tortuous crack path, shear ligament toughening and an improved resis- tance-curve behavior. The fracture mechanism of FL microstructures was characterized by multi-steps frac- ture ahead of the crack tip, and microcrack nucleation and propagation along slip bands. The studies of PST crystals have indicated that strength, elongation-to-fail- ure and fracture toughness, are highly anisotropic [7,8]. For example, when the notch is parallel to the lamellae, the fracture toughness is low (K IC =4.0 MPa m ). On the other hand, when the notch is perpendicular to the lamellae, a much higher fracture toughness (K IC =25 MPa m ) can be attained. So far, however, the effect of lamellar orientation on fracture behavior has not been fully understood. In this respect, Lu et al. have studied macroscopically the effects of lamellar orienta- tion on fracture mechanisms using in-situ SEM tech- nique [9]. The objective of current paper is to study * Corresponding author. 0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0921-5093(00)00903-5