Regulation Mechanism Of Residual Stress Distribution In ?-TiAl Alloy And Its Effect On Fracture Behavior

?-TiAl alloys have become promising high temperature lightweight structural materials in aerospace and other fields due to their excellent high temperature properties.However,because of its brittleness at room temperature,residual stress is easily introduced in workpiece processing,which restricts its application in aerospace and other fields.The existence of residual stress is often ignored by people for it is not easy to detect comprehensively and the poor intuitiveness and so on.Different distribution forms of residual stress seriously affects the machining accuracy,geometric dimensional stability,static strength and fatigue strength of workpiece.When the external load and residual stress are coupled during the service process,the defects such as microvoids and microcracks in the materials evolve continuously to form cracks.And the instability propagation of cracks is one of the main reasons for material failure.In order to avoid the harmful residual stress,it's essential to effectively regulate and control the distribution of residual stress after processed.It ought us to understand the control mechanical of residual stress under different process parameters and the effect of residual stress on mechanical properties in ?-TiAl alloys.However,it is hard to observe the whole process of dynamic evolution of micro-defects in ?-TiAl alloys when under loading failure and annealing to eliminate residual stress via experiments,so it is necessary to study it by means of computer simulation on micro scale.Molecular dynamics method(molecular dynamics,MD)is employed in this paper to study the mechanism of regulation residual stress distribution in polycrystalline ?-TiAl alloy by relief annealing and the effect of residual stress on the fracture behavior of ?-TiAl alloys with ?3(111)twin boundaries.The main findings are as follows:The regulation mechanical of residual stress distribution in the polycrystalline?-TiAl is simulated.After annealing with different parameters,the average residual stress along the X direction increases slightly and the fluctuation is very small.The higher the annealing temperature is,the smaller the average residual stress after annealing in the Y direction,and lower average residual stress can be obtained at slower cooling rate at the same temperature.The change of residual stress after relief annealing is mainly due to the change of the distribution of residual stress at grain boundaries.During the cooling process,the grain boundaries are shrunk,severeplastic deformation occurs and the stress is released,finally lead the residual tensile stress concentration at boundaries to be reduced.The stress concentration phenomenon exists at the grain boundaries of polycrystalline ?-TiAl alloy after Z-direction prepressing deformation.A large number of dislocation distributions at grain boundaries after the introduction of residual stress by prepressing deformation,and there are a few point defects and dislocations in the grains.The average grain size of polycrystalline ?-TiAl alloy increased slightly after annealing with 700K/900K/1100 K,there is no phase transition occurred during annealing,but the atomic distortion occurred with the change of temperature.During the heating process,the grain boundaries volume increases,and the precipitated atoms enter the grain interior to form the point defects.The concentration of point defects increases with the increase of temperature,and vice versa.The higher the annealing temperature,the less point defects after annealing.The residual stress distribution in X direction of ?-TiAl alloys with ?3(111)twin boundaries is symmetrical.After the residual stress is introduced into the materials through prepressing deformation,the residual tensile stress at the grain boundary increases,and the residual stress distribution in the grains is more uniform,1% and2% prepressing deformation is almost identical.The stiffness of the material is not affected by the residual stress.By observing the evolution of microdefects such as dislocation nucleation,emission and grain boundaries migration,it is found that the initiation of cracks at the grain boundaries is promoted by the presence of residual tensile stress at the grain boundaries which is much higher than the stress level in grains.On the contrary,since the residual compressive stress can reduce the local stress level,it has a shielding effect on crack initiation.The fracture mode of ?-TiAl alloys with ?3(111)twin boundaries is crack initiation at the boundaries,propagation along the grain boundaries interface,and fracture at the grain boundaries.The ?3(111)twin boundaries has good thermal stability,the stress distribution remains symmetry in the X direction after heating from 1K to 300 K.The residual compressive stress begin to release step by step when the temperature increased,it is completely released at 1050 K,and the amplitude of residual tensile stress increases.The yield strength reduce since the point defects which formed by precipitated atoms during the high temperature,and the workpiece exhibits good ductility at 1050 K.