Fracture in lamellar TiAl simulated with the Cohesive Model

Abstract

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, T0, and separation work, Γ0. 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 J2 plasticity is applied, which seems appropriate to predict macroscopic effects either for stable crack extension or unstable failure. The cohesive separation, δ0, can be interpreted as a characteristic length for a representative microstructural volume.