FEM Simulations Of The Effect Of Orientation And Grain Boundary On The Activation Of Dislocation Slip And Twinning Of γ-TiAl Alloys

There are inevitably crystal defects in metal materials in practical application.The crystal defects(such as dislocation, grain boundary, voids and the others) have been a key factor to determine the mechanical properties of materials in the deformation process. Computational materials science contributes to penetration insight into the deformation mechanisms, especially for the processes that incapable or hardly reachable experimentally; meanwhile it exhibits a combination of high efficiency and convenience. Orientation of grains and grain boundaries play an important role in the plastic deformation of metal materials. In this paper,finite element simulation of crystal plasticity method is used to study the effects of orientation and grain boundary on the activation of slipping and twinning systems.TiAl based alloys are considered to be a kind of perfect structural material for aerospace industry which can work under high temperature. γ-TiAl phase is the main component of TiAl based alloy, and it has several plastic deformation mechanisms. Rate dependent crystal plasticity finite element constitutive model is established, in which it is considered that ordinary dislocations, super dislocations and twinning. The tensile process of gamma-TiAl grains with different size and orientation are simulated. Response(e.g. strains and stresses) to deformations of monocrystalline γ-TiAl alloy is anisotropic with the various orientations of grains.The activation sequence of slipping and twinning systems depended strongly on the orientations of grains. The results indicates that it is relatively easy for ordinary dislocations and twinning to start, however, super dislocation systems are not activated during the deformation process. The simulation results of tensile deformation behaviors of γ-TiAl bicrystal with perpendicular grain boundary indicates that deformation mainly occurred inside soft grains with bamboo shaped boundaries, and stresses usually concentrate on these boundaries. The number and sequence of activated slipping and twinning systems change with the misorientation angle of the bicrystal. Plastic deformation of γ-TiAl tricrystal mostly occurred inside the grains. There was relatively small amount of strain at the triple junction of grain boundaries. In the meanwhile, stress concentration uncertainly appeared at the triple junction of grain boundaries. It could be connected with grain orientations and the interaction between grain and boundary,and which needs a further study.The polycrystalline model of γ-TiAl alloy and crystal plasticity constitutive relationship on coupling twinning was established by Voronoi algorithm. The uniaxial tension of polycrystalline γ-TiAl alloy was simulated by finite element method. It was analyzed that activation of slipping and twinning systems and their effect on the tensile deformation process by observing the startup of slipping and twinning systems with various orientations in polycrystalline model. The results show that there exist dramatic differences in startup situations of slipping and twinning systems with various orientations in polycrystalline γ-TiAl alloy. The activation of ordinary slip systems with greater Schmid factor was first, and the activation of super dislocation and twinning was relatively later. The activated numbers of slip systems increased with the increase of the loading stress. Ordinary dislocations have the greatest influence upon deformation of polycrystalline γ-TiAl alloy, while super dislocations and twinning the less.