Study On Phase Stability,plastic Deformation And Damage Mechanism Of NiTi Alloy Under High Rate Loads

NiTi alloys are typical shape memory alloys with complex multiphase transformations,temperature/component/stress sensitivity,and close macro-micro linkage characteristics.Its discovery has accelerated the popularization and application of shape memory alloys in biomedical,aerospace and other engineering and has made many progress.The shape memory effect and superelasticity related to the martensitic transformation of NiTi alloys under quasi-static state have been widely studied.The tensioncompression asymmetry,"lüders" deformation,shape memory effect and superelasticity through strained glass state(?)martensitic transformation expansion have also been studied.However,there is still a problem that the experiment and theoretical calculation are inconsistent in the determination of the stable phase of NiTi alloy at room temperature.In addition,the dynamic research of NiTi alloy under transient strong shock loading(birds,space debris,micro meteorite,etc.)mainly focuses on the controversy of whether austenite→martensite transformation will occur at high strain rate,little is known about the plastic deformation and damage mechanism of NiTi alloy under high strain rate loading.Therefore,it is of great scientific significance to carry out relevant research,which is conducive to the safe design and service of NiTi alloy key components(aviation actuators,protective structures of satellites and spacecraft,etc.)in extreme environments.Therefore,this paper first expounds the dispute over the stable phase of martensite in NiTi alloy in experiment and theoretical calculation,summarizes the explanations given by the current academic communities based on the analysis of the existing literature,further guesses other possible factors causing the dispute in view of the limitations of these explanations.Based on the first principle density functional theory,a universal explanation for the existence of NiTi martensitic stable phase B19’ in experiments is given.Then,on the basis of understanding and mastering the mechanical response and micro deformation mechanism of NiTi alloy under quasi-static state,guide further research on the dynamic behavior of NiTi alloy under extreme shock load,and analyze and extract the microstructure characteristics of recovered samples under shock compression and shock tension with the help of conventional micro characterization means such as EBSD(Electron Backscattered Diffraction)analysis.These microstructure characteristics are captured/reproduced in the nonequilibrium molecular dynamics shock loading simulation to analyze their generation and evolution mechanism,and then study the plastic deformation and spallation damage mechanism of NiTi alloy under extreme shock compression and tensile load.The main conclusions and innovations of this paper are as follows:(1)Based on the high-resolution TEM experimental results of(001)martensitic twins found in the samples prepared by chemical method after annealing of fully amorphous NiTi alloy by Waitz[1-3],the specific structure of(001)martensitic twin interface of NiTi alloy was determined by density functional theory,i.e.the interface corresponding to TSC-1(twinned supercell-1).Combined with the structural similarity between the(001)twin composed of two B19’ monoclinic lattices and B33 structure,we use the nudged elastic band method to predict the minimum path(MEP)from the(001)martensitic twin with twin interface "TSC-1" to B33 structure and the potential barrier(～4 meV/atom)between them.Martensitic(001)twins interface can not only provide additional strain for martensitic deformation,but also promote the stable existence of B19 ’phase with higher energy than B33 phase in the experiment.(2)The recovered samples from the shock compression experiment of NiTi alloy based on pulsed power generator CQ-4 were microscopically characterized and analyzed.There may be no phase transition in the near atomic ratio NiTi alloy with the initial phase of austenite phase under shock compression(strain rate>104 s-1),and its plastic deformation modes were mainly dislocation slip,mechanical twinning and recrystallization,and the evidence of the existence of {112} austenite twins and new grains is provided.Based on this,the shock compression simulation of columnar polycrystalline NiTi alloy is carried out with the help of non-equilibrium molecular dynamics simulation method.The main plastic deformation characteristics of NiTi alloy measured in the experimental recovered samples are captured/reproduced in the simulation,and the generation and evolution mechanism of {112} austenite twins and new grains under shock load is expounded:{112} austenite twins can be formed by continuously sliding a/3[111]on the(211)plane;New grains can be formed by nanoscale rotational deformation caused by nanoscale collective ideal shear events at different types of grain boundaries on both sides of the grains under shock loads at very high strain rates.(3)Based on the pulsed power generator CQ-4,the NiTi alloy shock tensile spalling experiment was carried out.The microscopic characterization analysis of the recovered samples revealed that the plastic deformation mechanism of the NiTi alloy spalling region under low-speed shock stretching was dominated by recrystallization.And with the increase of shock loading velocity,the tendency of recrystallization in the spall region increases while the tendency of twinning decreases.The shock tensile spalling characteristics of nano-polycrystalline NiTi at higher strain rates were studied by nonequilibriummolecular dynamics simulation.According to the time-position-density nephogram and the inflection point characteristics of the density and velocity profiles,three types of spalling modes in nano-polycrystalline NiTi alloys were identified,classic spallation,multi-layer spallation,and microspallation.Through the thermal path analysis,radial distribution function(RDF)analysis and Voronoi tessellation(VT)analysis of the spalling region,the microscopic damage mechanism of different spalling modes of nano-polycrystalline NiTi alloy was further studied,and the critical shock loading velocity Up between different spalling modes is given semi-quantitatively.Increase the initial ambient temperature To will reduce the spallation strength of nano-polycrystalline NiTi due to the initial ambient temperature T0 affect the thermodynamic path and grain boundary diffusion rate in spallation region.