MD Study On The Interaction Of Grain Boundaries And Voids During The Plastic Deformation Of ?-TiAl Based Alloy

y-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.It is popular of the study on the mechanical properties of y-TiAl based alloys by using experimental and simulation methods at present.As a representative of atomic simulation method,the dynamic tracking of material microstructure can be realized via molecular dynamics(MD)method by solving the Newton's motion equations of each particle.MD is an effective means of computer-aided materials research.In this paper,the present discusses the influence of intracrystalline nano void on tensile deformation of ?-TiAl polycrystalline with double voids employing MD method.The mechanisms of plastic deformation and fracture of y-TiAl polycrystalline were studied through analysis of microstructural evolution during low temperature tensile deformation with various distributions of inner voids from atomic scale.The main contents are as follows:The stress-strain curve of ?-TiAl polycrystalline shows as a typical elastic-plastic characteristics.Through the observation of atomic configuration,grain boundaries severely distort during the tensile deformation with the strain less than 0.043,but no dislocation nucleating,which indicating that grain boundary sliding is the main mechanism.Furthermore,the dislocations only emit at triple grain boundary and slip inside the grains,which is a coordination of the grain boundary sliding.Dislocation emission is later than the grain boundary sliding,so grain boundary sliding at the early stage of tensile deformation provides orientation and shear stress conditions for dislocation emission.With the increasing of the tensile stress,more dislocations emit from grain boundary,which leads to increasing of dislocation density and severe stress concentration at the grain boundaries.Then,micro-cracks nucleate at the boundaries when the strain is about 0.075 and grow perpendicular to the tensile axis till the fracture of the y-TiAl polycrystalline combined with necking.Two spherical voids are preset in the central grain of the perfect y-TiAl polycrystalline cell,and radius is 0.5 nm/1 nm with a distance of 1.5 nm/3.3 nm between the two voids.Then the tensile deformation of the polycrystalline cell with voids is simulated by MD method.The simulation result shows that the variation tendency is similar with the perfect polycrystalline,but there is difference on the peak stress of cell and its corresponding tensile strain.Among the y-TiAl polycrystalline with double voids,the peak stress of the polycrystalline is the maximum one with void radius R of 0.5 nm and distance D of 1.5 nm between the two voids and the peak stress of the polycrystalline is the minimum one with void radius R of 0.5 nm and distance D of 3.3 nm between the two voids.The peak stress has more close relationship with the distance D than that of the void radius R.In the tensile deformation of polycrystals with void radius R of 0.5 nm and distance D of 1 nm between the two voids,the main deformation mechanism is dislocation nucleation and expansion near the grain boundary of the voids.During the subsequent deformation,the dislocation emitted by the grain boundary extends to the surface of the voids and interacts with the voids.The voids grows and expands near the grain boundary until it breaks.At this time,the plastic strain is much larger than that of the perfect polycrystalline,which indicates that the existence of the micro-voids could improve the plasticity of ?-TiAl alloys to certain extent.Comparing.the deformation of the polycrystalline cell with different void radius and distances(R=1 nm,D=3.3 nm;R=0.5 nm,D=3.3 nm),the dislocation nucleation is more prior with the increasing of the radius and distance,which is a primary deformation mechanisms of dislocation nucleating from the boundaries near the voids.What is more,the voids are gradually coarsening with the dramatic interaction of dislocation and void,which leads to the material failure eventually.