Molecular Dynamics Of Stress Relief Annealing On The Regulation Of Cutting Residual Stress In γ-TiAl Alloy

γ-TiAl alloys have become one of the most competitive lightweight high-temperature structural materials in aerospace and automotive fields because of their excellent properties such as low density,high specific strength and high-temperature oxidation resistance,but their room-temperature brittleness and low ductility make it easy to introduce defects and residual stresses during processing,which seriously affect their service performance.As an effective way to eliminate residual stresses by stress relief annealing process,it is not clear the connection between cutting residual stresses and microscopic defect evolution in stress relief annealing.To this end,this paper established a nano-cutting and stress relief annealing model to study the evolution of subsurface defects and residual stress distribution during cutting,analyzed the evolution of microscopic defects and cutting residual stress under different stress relief annealing parameters,and elaborated the mechanism of stress relief annealing to regulate the residual stress distribution,and finally used nanoindentation simulation to study the effect of stress relief annealing process on the mechanical properties of γ-TiAl alloy.The paper focuses on the following research:1.Molecular dynamics simulation of the nano-cutting process of single-crystalγ-TiAl alloy was used to analyze the evolution of microscopic defects and the evolution of cutting residual stresses during the machining process.The residual stress distribution on the machined surface and subsurface during the annealing process was derived by different annealing temperatures,and the effect of microscopic defect evolution on the residual stress was investigated.It is found that the high temperature and pressure generated during the cutting process reduces the energy potential of dislocation nucleation and dislocation emission.The regulation of cutting residual stresses is related to the dislocation reaction and lattice distortion.Stress transformation exists on the machined surface of the specimen during the annealing process,and the best cutting residual stress regulation is achieved when the annealing temperature is 900 K.2.The evolution of microscopic defects inside the workpiece at different heating and cooling rates during the annealing process was studied,and the evolution of cutting residual stresses before and after annealing was compared.It is concluded that the subsurface dislocations slide upward to the surface of the workpiece during the heating-up phase,and some dislocations continue to slide downward to the interior of the workpiece.The regulation of residual stresses in the heating stage was dominated by dislocation annihilation.In the cooling stage,the low cooling rate promotes the formation of stacking fault tetrahedra,and the residual stresses are released mainly through the dislocation reaction.The cutting residual stresses are well regulated at a cooling rate of 1 K/ps.3.The variation of residual stresses in the nanoindentation simulation was analyzed,and the relationship between microscopic defect evolution,residual stresses and mechanical properties of the workpiece was investigated.It was found that during loading,the residual compressive stress increased along with the depth of the indentation,but the stress recovery of the workpiece after annealing was less than that before annealing.Subsurface defects affect the surface mechanical properties of the workpiece.The decrease in dislocation density makes it easier to release the residual stresses in the workpiece,and the decrease in dislocation density leads to a decrease in surface hardness.