Modeling of Lamellar Fracture of Polysynthetically Twinned (PST)-TiAl Crystals Using Cohesive Zone Models and XFEM in a Two-scale FE Approach

Abstract

The mechanical behavior of intermetallic TiAl alloys is influenced by the morphology of single phase (gTiAl) grains and two-phase (a2Ti3Al+gTiAl) lamellar structures. To understand the role of the lamellar structures for the deformation and fracture behavior of TiAl alloys, a crystal plasticity based FE model (CPFEM) has been developed that takes important microstructural information and anisotropic phase properties into account and considers local fracture behavior within the lamellar phases. The lamellar microstructure consists of relatively thin a2Ti3Al and thick gTiAl plates. Between two a2Ti3Al plates many gTiAl lamellae are located. For modeling purpose the lamellar microstructure has been described in a FE unit cell using representative volume elements (RVE) based on an ideal description of the lamellar phases with the average vol.% of the phases and their respective orientations. Two types of lamellar fracture have been described in this unit cell. One is the interlamellar fracture that occurs due to interface failure at the a2/g or g/g interfaces, and the other is the translamellar fracture that occurs due to cleavage of the bulk gTiAl phases. For the numerical description of interlamellar fracture the phase boundaries are designed with cohesive interfaces to capture debonding damage. On the other hand, the translamellar fracture has been modeled using an XFEM approach combined with cohesive damage behavior which allows one to model material splitting due to cleavage. This unit cell has been embedded in a macro-scale FE model for describing overall response of the lamellar structure. First order homogenization approach is used to couple these two models. For the macroscopic damage behavior a modified homogenization approach for cohesive damage has been assumed where the discrete crack has been modeled in the unit cell and in macro FE elements. The micromechanical deformation of the phases has been described using a classical CPFEM model. The model parameters for the crystal plasticity have been estimated fitting the stress-strain response obtained from tensile experiments on lamellar polysynthetically twinned (PST)-TiAl crystals. The cohesive model parameters are estimated by adjusting the numerical fracture behavior with the experimental fracture response for this PST-TiAl alloy. In this presentation we will demonstrate the models ability to predict interlamellar and translamellar fracture of PST crystals under different lamellar orientation condition. Furthermore, macroscopic failure will be predicted based on the unit cell calculations.