CPFEM Simulations Of The Effect Of Grain Orientation And Grain Boundary On The Plastic Behaviors Of Single Crystal And Dual-phase Lamellar TiAl Alloys

TiAl based alloys are widely applied in aerospace,shipping and automobile industry as one of the light-weight structural metals because of their low density,high specific strength,high fire retardant,excellent high temperature oxidation resistance and creep resistance properties.Lamellar structure is a typical microstructure in TiAl alloys,in which different grain boundaries and inter-phase interfaces with complex orientations exist.The co-existence of these complex boundaries has duplex effects on the mechanical properties.Crystal plasticity finite element method?CPFEM?is a model that incorporates the crystal plasticity theory into the framework of finite element method.The crystal plasticity constitutive law in the model is based on dislocation slips,deformation twin and lattice distortion.So it can give an essential description on the material plastic deformation compared to macroscopically isotropic constitutive law.In this paper,based on the theory of crystal plasticity and the deformation mechanism of TiAl alloy,the finite element models of single crystal,bicrystal and polycrystal lamellar structure were established.Using secondary development tools of ABAQUS/UMAT,the constitutive relation algorithm was embedded and simulated on the tensile plastic deformation of TiAl based alloy.It was mainly studied the meso-scale deformation mechanisms of the TiAl based alloy with the effect of grain orientation and interface?grain boundaries and phase boundaries?.The main research contents are as follows:In the tensile deformation of single crystal,the single crystals rotated along the central axis during the tensile deformation.Moreover,orientation has dramatic effect on the grain rotation.With tension along the ordinary slip orientation [1???0],less tensile resistance leads to easy activation of slip of the ordinary dislocation and has smallest rotation angle.However,the others?super slip orientation [???01],twin slip orientation [11???] and arbitrary orientation [???01]?have the more.With tension along the three orientations,the crystals should adjust the orientation factor to accommodating the deformation by grain rotation.Crystal plastic deformation was inhomogeneous due to the existence of the interface,and the stress concentration was formed at the interface during the tensile deformation of the TiAl bicrystals.The contribution of phase boundary was greater than that of grain boundary.In the meanwhile,priority activated slip systems with “soft” orientations supplied primary contribution to the plastic strain in those bicrystals with grain boundary and phase boundary.It was indicated that grain boundary misorientation has dramatic effect on the plastic behaviors.The stress concentration was not obvious at low angle grain boundaries during the tensile deformation,which shows better coordination of deformation.On the contrary,secondary hardening occurs in the region of severe stress concentration at high angle grain boundaries,which could easily lead to crack initiation.Compared with mechanical behaviors of TiAl bicrystals,there was better coordination in the plastic deformation of polycrystal lamellar structure due to the interaction of the grain boundaries and phase boundaries in the lamellar grains.Its deformation mechanisms was concurrent activation of multi-slip systems,and these shear deformation of multi-slip systems was not limited in “soft” and “hard” shear mode.There was a gradient distribution of stress concentration at the lamellar interface,and the stress at the triple junction interface was the largest.In the deformation of polycrystalline lamellar structure,the stress concentration mainly was formed inner the ?₂ grain with the triple junction boundary composed of ?₂ and ? grains.However,the stress concentration was formed inner each grain with the triple junction boundary composed of three ? grains.