Research On The Microstructure Evolution Characteristics And Formation Mechanism Of NiTi Alloy Under Impact Compression/tension Load

As a kind of functional material with excellent performances,NiTi alloy has been ex-tended to the fields of aerospace,spacecraft telescopic components,building seismic protection devices,etc.These applications are faced with complex environments with high pressure and high strain rate under high velocity impact.Researching and mastering the responses of NiTi alloy under dynamics load can systematically recognize its physical and mechanical proper-ties,which can provide guidance for the optimization design of industrial applications.The topic of this thesis is "Research on the microstructure evolution of Ni52Ti48 alloy under shock compression/tensile loadings",from aspects of experiment and molecular dynamics simulation,we systematically study the dynamic responses of polycrystalline Ni52Ti48 alloys under shock compression and tensile loadings,including microstructural features,evolutionary laws and their formation mechanisms.The main research progress and results obtained are as follows:(1)Based on pulsed power generator CQ-4,using the method of magnetical drive flyer shock compression,the experimental techniques of momentum trapping sample recovery and electron backscatter diffraction(EBSD)metallographic microscopic analysis,Ni52Ti48 under shock pressure of 34 GPa was studied.The results show that after high pressure shock wave compression,microstructures such as high-density dislocations,twins and new grains are found in the recovered samples,the proportion of small-angle grain boundaries is obviously increased,and the sample undergoes obvious plastic deformation.The phase transition of austenite to martensite was not observed in the EBSD results,which is consistent with the experimentally measured wave profiles and shock wave velocity-particle velocity curves.(2)Based on the validated EAM potential function,the molecular dynamics simulation method was carried out by LAMMPS,and the microstructure characteristics of Ni52Ti48 alloy under experimental conditions were reproduced.On this basis,the microstructure characteris-tics and evolution of Ni52Ti48 alloy at different initial ambient temperatures of 300 K and 500 K and different impact velocities of 0.6 km/s,0.8 km/s and 1.0 km/s were systematically stud-ied.The formation mechanism of deformed twins and new grains under shock compression is discussed.The main understanding and conclusions are:·The {112} twin of Ni52Ti48 alloy can be formed by continuous slip of the 1/3[111]dislo-cation on the {112} plane.Twins may generate at the triple junction of grain boundaries,interaction region of different types dislocations,and jogs of the same type of disloca-tion.Twinning may occur during the shock compression process or the sparse wave unloading process.The two initial ambient temperatures only affect the twinning during shock compression,and twins generated during the unloading process are not affected,which may be caused by the difference in the shock temperature rise much higher than that of the initial ambient temperature(200 K).·A series of dislocations with the same slip system excited at the large angular bound-aries or small angle grain boundaries can cause grain rotation.When large angle grain boundaries and small angle grain boundaries exist simultaneously inside the grain,the interactions of dislocations within the grains may result in the creation of new grains.The higher initial ambient temperature(500 K)has a significant effect on the formation of new grains,which may lead to softening of the boundary and increase the difficulty of generating dislocations at large angular boundaries.Therefore,at the initial ambient temperature of 500 K,the generation of new grains requires a higher shock speed.(3)Using the techniques of magnetical drive flyer shock loading-unloading and soft re-covery,the spall strength of the Ni52Ti48 alloy and the microstructure characteristics of the recovered samples were studied.The experimental shock loading pressure was increased from 3 GPa to about 13 GPa.The wave profile results show that the Ni52Ti48 alloy has a spall strength of about 3±0.2 GPa and Hugoniot elastic limit of 3.0 GPa.The EBSD results of the recovered samples show that,unlike the compressed samples,the grains in the spall zone of the tensile sample are completely elongated and broken,forming many tiny grains,the orientation changes greatly,and the probability of occurrence of twin features is reduced.During the spalling pro-cess,it undergoes recrystallization during the large deformation process,which is similar to the compression condition,and forms substructures(subgrains)at positions of high dislocation density.As the loading stress increases,the grain fragmentation in the fracture zone is severe,and a large number of tiny grains are formed.The orientation changes greatly,reflecting that plastic deformation and recrystallization were prefer to undergone in the fracture zone.(4)Molecular dynamics simulation was used to systematically study the spall strength and melting characteristics of Ni5-2Ti48 alloy at different initial ambient temperatures(300 K-1000 K)and impact velocities(0.4 km/s,0.6 km/s and 0.8 km/s).The main understanding and conclusions are:·Compared with the experimental results,the spall strength of Ni52Ti48 alloy has obvious strain rate effect.The temperature rise of shock compression of Ni52Ti48 alloy is much lower than that of melting point caused by pressure increase,so Ni52Ti48 will not melt in the process of impact compression,and the melting point of Ni52Ti48 is lower than the initial melting point in the process of impact tension.·The spall strength decreases with the increase of initial ambient temperature(300 K-1000 K),and the spall zone width increases gradually with the increase of impact veloc-ity.