CPFEM Simulation Of Grain Boundary Mechanical Behavior Of TiAl Based Alloys

As a new type of lightweight high-temperature structural material,TiAl-based alloy has the characteristics of low density,high specific strength,high specific modulus,excellent oxidation resistance,fire resistance and creep resistance,etc.And it has broad application prospects in aviation,aerospace and automotive.In general,although the grain boundary thickness of the polycrystal polycrystal is small,it tends to have a large influence on the mechanical properties of metallic materials.In this research,the grain boundary and its influence area are combined to be regarded as an interface layer with a certain thickness.The grain boundary is considered as a region that is different from the internal part of the grain to withstand deformation and stress,and at the same time a specific constitutive model is given to the grain boundary.Then a computational simulation system is established to describe the mechanical behavior of grain boundaries effectively.Furthermore,this study uses the crystal plastic finite element method(CPFEM),considering dislocation slip,deformation twinning and grain boundary effect,the ?-TiAl bicrystal model has been established with different grain boundary thickness and different grain boundary angles.And?-TiAl tricrystal model has been established with different grain boundary type.by Abaqus/UMAT platform,numerical implementation and subroutine writing are carried out.The effects of grain boundary and grain orientation on the plastic microscopic deformation mechanisms of bicrystal and tricrystal are investigated.The main contents are as follows:The tensile deformation of ?-TiAl bicrystal is affected by grain orientation and grain boundaries at room temperature.When grain orientation is constant,it is effectively coordinating of the plastic deformation of adjacent grains on both sides of grain boundary with increasing grain boundary thickness,and the overall plastic deformation ability is great improved.When the influence area of grain boundary is fixed,the small-angle grain boundary is better than the high-angle grain boundary,and it shows better coordination in both the inner grain and the grain boundary,and the plastic deformation of the double crystals is more uniform.In this paper,biaxial tensile simulation is carried on ?-TiAl tricrystals by considering two types of grain boundaries(geometric grain boundaries and physical grain boundaries).The results show that when considering the geometric grainboundary,stress concentration does not necessarily occur at the triple junction of grain boundary during the tensile deformation of the tricrystals.Stress concentration occurs at the triple junction of grain boundary of the tricrystals with the physical grain boundary.In the model of physical grain boundaries,it is found that the plastic deformation of tricrystals not only comes from crystals,but also occurs in the grain boundaries.As an indispensable part of polycrystalline materials,the calculation model considering the properties of grain boundaries can be used to describe the microscopic plastic behaviors of ?-TiAl polycrystals more effectively.In the bicrystals,the initiation state of slip systems is greatly related to the soft and hard orientations due to the influence of the Schmid factors.Compared with bicrystals,the activation of slip systems in tricrystals depends not only on the Schmid factors of the slip systems.The shape of crystals,the relative position of crystals and the interaction between slip systems in the same crystal will effect the current strength of the slip system and the resolved shear stress of the slip system,thus affecting the plastic behavior of the crystal.