The Study Of Numerical Simulation And Experimental Research Of Shock Phase Transformation Of NiTi Shape Memory Alloy Based On SHPB

Shape memory alloy(SMA)refers to a type of material that can be restored to its original shape by heating to a certain temperature,and can have a large recoverable elastic deformation when unloaded.Ni Ti shape memory alloy is a typical shape memory alloy.Ni Ti shape memory alloy is widely used in aerospace and medical fields due to its excellent shape memory effect,high strength and good bioco MPatibility.In Ni Ti shape memory alloys,temperature and stress-induced reversible phase transformations between B2 austenite and B19’ martensite cause shape memory effects and superelasticity,respectively.The existence of different phases(such as austenite and martensite)in Ni Ti shape memory alloy determines its application.The shock phase transformation of Ni Ti shape memory alloys has always received extensive attention,and it is also a hot issue to reveal the phase transformation mechanism of Ni Ti shape memory alloys.Therefore,this paper studies the effect of different shock velocity on the shock phase transformation of Ni Ti shape memory alloys.When the shock velocity is large enough,the tensile wave generated by unloading will cause micro-defects and fractures in the metal material.Therefore,this paper studies the shock of the fracture of the defective metal and the grain boundary on the shock phase transformation of Ni Ti shape memory alloys.Because temperature will also cause phase transformation of Ni Ti shape memory alloy,this paper studies the influence of initial temperature on the shock phase transformation of Ni Ti shape memory alloy.The main contents are as follows:The SHPB experiment and MD method were used to study the effect of loading velocity on the shock phase transformation of Ni Ti shape memory alloy.The experimental study shows that when the initial loading temperature is 300 K,the martensitic transformation stress increases with the increase of shock velocity.After the shock loading is completed,the residual strain of Ni Ti shape memory alloy increases with the increase of shock velocity.The MD study shows that When the initial loading temperature is 300 K,when the shock velocity is less than or equal to 10 (?)/ps,as the shock velocity increases,the volume fraction of the B19′ martensite phase after the martensite transformation is completed will increase.,And(100)compound twins appear when the shock velocity is 6(?)/ps,8 (?)/ps,and 10 (?)/ps.As the shock velocity continues to increase,the volume fraction of the B19’ martensite phase after the martensite transformation is not significantly increased co MPared to that at 10 (?)/ps,and no obvious twinning behavior is observed.The temperature and phase transformation stress of Ni Ti shape memory alloy during phase transformation will increase with the increase of shock velocity.The effect of grain boundary on the shock phase transformation of Ni Ti shape memory alloy was studied by MD method.The results show that the existence of grain boundaries will affect the shock phase transformation process of Ni Ti shape memory alloy.Under the load of shock wave,the B2 austenite phase gradually transforms into the B19’martensite phase.When the grain boundary exists,the nucleation of the B19’ martensite phase will concentrate on the grain boundary.With the increase in the number of grain boundaries,the B19’ martensite phase is increasingly concentrated at the grain boundaries.After the transformation is completed,the volume fraction of B19’ martensite phase decreases with the increase of grain boundaries.The molecular dynamic simulation was conducted to analyze the influence of twist angle of twist boundary(TB)and orientation on fracture of nano-nickel bicrystal with micro-defects under uniaxial tensile loading.The result shows that with the increase of twist angle,nucleation and emission of dislocation at grain boundaries gradually increase,the cracks propagate forward more quickly,and the deformation of the void becomes smaller,so that the fracture failure caused by void growth at 0° changes to the fracture caused by propagation of the cracks at 45°.For nano-nickel bicrystal with twist boundaries in different crystal directions,(110)twist boundary nickel bicrystal has a greater crack growth rate co MPared to(010)and(111)twist boundary,the concentration of plastic deformation also makes the void growth not obvious,.However,the crack propagation of the(010)and(111)twist boundary nickel bicrystal is more difficult,and the void growth is more obvious,so it has better ductility and malleability.The SHPB experiment and MD method were used to study the effect of loading initial temperature on the shock phase transformation of Ni Ti shape memory alloy.The experimental study shows that with the increase of temperature,the martensitic transformation stress of Ni Ti shape memory alloy will become higher and higher;at the same time,the strain produced by the Ni Ti alloy after loading will also increase with the increase of the initial temperature of loading.The MD study shows that when the initial temperature of loading is greater than Af,the volume fraction of martensite phase obtained when the martensite transformation is completed decreases with the increase of the initial temperature of loading.In the case of 300 K,(100)compound twins appeared,but did not appear at other temperatures.When the initial temperature of loading is less than Af,(100)compound twins appear after the shock loading is completed.After the unloading is completed,the reverse martensite transformation is completed,and the volume fraction of B19’ martensite phase increases as the temperature decreases.