Fracture behaviour of lamellar TiAl is studied by means of experiments and numerical simulations using a cohesive model. The fracture process can be described by two cohesive parameters: traction, T 0 , and separation work, Γ 0 . These are identified for polysynthetically twinned crystals (PST) and a polycrystalline TiAl alloy by comparison of the numerical simulations with experimental fracture tests. The appropriate shape of the traction-separation law for quasi-brittle fracture was identified. The simulation of the PST crystal deformation is based on crystal plasticity including the specific lamellar orientation relations. In a PST crystal, fracture occurs by inter-and translamellar fracture. In an extruded polycrystal, the lamellae of the colonies are randomly aligned in radial direction but more uniformly in extrusion direction for which different fracture toughness values are obtained. For the polycrystalline γTiAl conventional J 2 plasticity is applied, which seems appropriate to predict macroscopic effects either for stable crack extension or unstable failure. The cohesive separation, δ 0 , can be interpreted as a characteristic length for a representative microstructural volume.

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Quasi-brittle fracture of fully lamellar two phase (α2+γ)TiAl is investigated both experimentally and numerically. Fracture tests are conducted at room temperature, which fail in a quasi-brittle and unstable manner but exhibit significant variations in crack initiation and propagation prior to unstable failure. Fractographic investigations are performed which elucidate the micromechanical causes of the macroscopic behaviour. The observed deformation and fracture behaviours of the specimens are simulated by a finite element model containing cohesive elements for modelling the material separation. In order to capture the scatter of the macroscopic behaviour, a stochastic approach is chosen, in which local variations of cohesive parameters are taken into account. The model can describe and explain the physical phenomena of the specific material.